Formation of a Sterically Crowded 1,6,6αλ 4 -Triselenapentalene and 4 H -Selenopyran-4-selones Fused with Two Bornane Skeletons Through the Reaction of d -Camphor p -Toluenesulfonylhydrazone With a Base and Elemental Selenium

Abstract

Reaction of d-camphor p-toluenesulfonylhydrazone with t-butoxide and elemental selenium in dimethylformamide at an elevated temperature afforded a stable compound having a unique 1,6,6αλ⁴-triselenapentalene ring and 4H-selenopyran-4-selones along with dialkenyl diselenide, dibornylenes, and 1,2,5-triselenepin, and the structural confirmation of these products were carried out by X-ray crystallographic analysis. The sterically crowded 1,6,6aλ⁴-triselenapentalene ring fused with two bornane sleketons was stable enough under aerobic exposure and was inactive toward sodium borohydride reduction but was converted into 1,2-diselenole derivative through m-chloroperbenzoic acid oxidation.

Dialkenyl dichalcogenides have been widely recognized as the versatile synthetic precursors of various chalcogen-containing heterocycles and chalcogenocarbonyl compounds, ^{1 -9} and the recent synthetic interests have been concentrated to their higher-row derivatives. However, in spite of the potentiality of alkeneselenolate and alkenetellurolate ions as the synthetic equivalents of selenocarbonyl and tellurocarbonyl compounds, ^{10 -17} only limited studies on the generation of these alkenechalcogenolate ions have been carried out previously and the lack of general and convenient synthetic methods of these compounds has restrained the extension on the novel conversion of these compounds into chalcogen-containing heterocyclic compounds. In the course of our successive studies on the synthesis of higher-row chalcogenocarbonyl compounds, we have found a convenient synthesis of dialkenyl diselenides C through the reaction of p-toluenesulfonylhydrazones A, derived from ketones possessing an α-methylene group, with a base and elemental selenium. ^{18 -24} The reaction was assumed to proceed through the in situ generated alkeneselenolate ions B via Bamford–Stevens type reaction of p-toluenesulfonylhydrazones ^{25 -33} as shown in Scheme 1, and, therefore, the synthetic application of this sequence to the preparation of various selenium-containing heterocyclic compounds was keenly expected.

Scheme 1

Synthesis of dialkenyl diselenides C via the reaction of ketone p-toluenesulfonylhydrazone with a base and elemental selenium.

Especially, our interests have been concentrated to the application of the reaction sequence by using the bornane derivatives because d-camphor and its functionalized derivatives have been widely recognized as the versatile substrates for chiral auxiliaries, chiral ligands, and chiral building blocks for asymmetric syntheses due to their commercial availability with the structural stability and its chirality. ^{34 -37} However, in spite of the extensive research works on the functionalization of bornane skeletons within these decades, there found only limited studies on the selenium-functionalized bornane skeletons except for our previous works on the preparation and the conversion of substituted bornane-2-selones. Our previous attempts for the synthesis of dibornenyl diselenide using our procedure starting from d-camphor p-toluenesulfonylhydrazone (1) also resulted in the complex mixture containing dibornylene (3), 1,4-diselenin (4), 1,2,5-triselenepin (5), and a few uncharacterized compounds along with the formation of dibornenyl diselenide (2). ¹⁸ Therefore, we just started the investigation of the structural confirmation of these uncharacterized byproducts in order to obtain a key for the optimization of the reaction conditions, and after several efforts, we could isolate and characterize the structures of some of these byproducts involving a hitherto unknown purple-colored stable crystalline compounds, 1,6,6αλ⁴-triselenapentalene (6), and an isomeric mixture of greenish brown-colored 4H-selenopyran-4-selones (7, 8) fused with two bornane skeletons.

1,6,6αλ⁴-Triselenapentalenes D and their oxygen or nitrogen analogs are classified to be bicyclic 10-S-3 type sulfuranes having three sulfur atoms with a central hypervalent sulfur atom in the heteroaromatic ring system, and their structures, chemical reactivity, and their synthetic methods have been extensively investigated during these decades. ^{38 -50} However, in contrast to 1,6,6αλ⁴-trithiapentalenes D, only a few synthetic studies on their selenium analogs, such as 1,6,6αλ⁴-triselenapentalenes E, ^{51 -55} 1,6,6αλ⁴-thiadiselenapentalenes, and 1,6,6αλ⁴-dithiaselenapentalenes F, ^{56 -58} have been carried out to date may be due to the difficulty in the preparation of suitable 1,3,5-triselenoxo intermediates. The general structures of compounds D-F are shown in Figure 1. In this report, we would like to describe the isolation and characterization of new 1,6,6αλ⁴-triselenapentalene (6) and 4H-selenopyran-4-selones (7, 8) fused with two bornane skeletons as the minor products of the reaction of d-camphor p-toluenesulfonylhydrazone (1) with t-butoxide (t-BuOK) and elemental selenium at an elevated temperature. A novel oxidative selenium-oxygen replacement of 6 forming 1,2-diselenole (11) bearing a neighboring carbonyl group is also reported in this paper.

Figure 1

Structures of 1,6,6αλ⁴-trichalcogenapentalenes D, E, and F.

d-Camphor was at first converted into the corresponding p-toluenesulfonylhydrazones (1) according to the usual procedure. Subsequently, a hexamethylphosphoric triamide (HMPA) or a dimethylformamide (DMF) solution of d-camphor p-toluenesulfonylhydrazone (1) was treated with t-BuOK (2.5 mol amt.) and elemental sulfur or selenium (2.5 mol amt.) at 110°C-140°C for 3-24 hours followed by an aerobic exposure of the resulting reaction mixture at room temperature, ¹⁸ and only in the cases using elemental selenium, small amounts of several selenium-containing products were isolated besides dialkenyl diselenide (2), an E,Z-mixture of dimeric olefins (3), ^59,60 and a few uncharacterized products. On the other hand, the same reactions performed at a higher temperature over 160°C only afforded a complex mixture, and the similar reaction using elemental sulfur only afforded a complex mixture.

The mixture of less polar selenium-containing products obtained as orange oil gradually underwent decomposition with selenium extrusion during the workup procedure to form a separable mixture of relatively stable two compounds, pale yellow solids, and orange solids. All physical and spectral data for the former and the latter compound, involving mass spectrometry (MS), infrared (IR), ¹H nuclear magnetic resonance (NMR), ¹³C NMR, and elemental analysis, were fully consistent with 1,2- or 1,4-diselenin (4) and symmetrical 1,2,5-triselenepin (5), ¹⁸ respectively. Especially, the ¹H NMR spectra of both 4 and 5 reveal similar symmetrical patterns with only 3 singlet signals assignable to the methyl groups of the bornane skeletons, ie, 0.80 ppm (6 H, s), 0.81 ppm (6 H, s), and 1.00 ppm (6 H, s) for compound 4, and 0.82 ppm (6 H, s), 0.87 ppm (6 H, s), and 1.05 ppm (6 H, s) for compound 5, despite the difference in their parent ion peaks in the mass spectra, ie, m/z 428 (M⁺) for 4 and m/z 507 (M⁺) for 5, respectively. Furthermore, physical and spectral data of compound 5 were identical with those of 1,2,5-triselenepin 5 reported previously by us. ¹⁸

The other three compounds were also subjected to chromatographic separation, and all physical and spectral data for these compounds involving MS, IR, ¹H NMR, ¹³C NMR, and elemental analysis were also fully assignable to symmetrical 1,6,6αλ⁴-triselenapentalene (6) (purple crystals), symmetrical 4H-selenopyran-4-selone (7) (green crystals), and unsymmetrical 4H-selenopyran-4-selone (8) (green crystals). The ¹H NMR spectra of both 6 and 7 revealed similar symmetrical spectral patterns with only 3 methyl signals of two bornane skeletons, ie 0.68 ppm (6 H, s), 0.99 ppm (6 H, s), and 1.32 ppm (6 H, s) for 6, and 0.75 ppm (6 H, s), 0.96 ppm (6 H, s), and 1.28 ppm (6 H, s) for 7. Especially, the ⁷⁷Se NMR spectrum of 6 revealed a characteristic symmetrical Se-Se-Se patterns of 572.3 ppm (d, J _Se-Se = 294 Hz) and 1079.5 ppm (t, J _Se-Se = 294 Hz) attributed to the 1,6,6αλ⁴-triselenapentalene core. On the other hand, the ¹H NMR spectral pattern of compound 8 revealed an unsymmetrical spectral pattern with 6 methyl signals, ie, 0.74 ppm (3 H, s), 0.75 ppm (3 H, s), 0.94 ppm (3 H, s), 0.95 ppm (3 H, s), 1.26 ppm (3 H, s), and 1.74 ppm (3 H, s). It is worth noting that both 7 and 8 are green crystalline compounds with weak absorption peaks assignable to the n–π* transition of the selenocarbonyl functionality at 619 nm (ε = 370) for 7 and 723 nm (ε = 380) for 8, respectively, in the visible light region.

The structures of these compounds were finally determined by X-ray crystallographic analyses to be symmetrical 1,2,5-triselenepin 5, 1,6,6αλ⁴-triselenapentalene 6, and unsymmetrical 4H-selenopyran-4-selone 8, and their Oak Ridge Thermal-Ellipsoid Plot Program (ORTEP) drawings of 5, 6, and 8 are depicted in Figures 2–4, respectively. On the other hand, the X-ray analysis of 4 and 7 was not successful. However, 1,2,5-triselenepin 5 was gradually converted into compound 4 along with selenium extrusion by doping 5 in silica gel at room temperature for a long time as shown in Scheme 2, and these results strongly suggest the structure of compound 4 to be symmetrical 1,4-diselenin. Therefore, the alternative possible structures, such as 1,2-diselenin 4′ ^61,62 formed through [3,3] sigmatropic rearrangement of dialkenyl diselenide 2 and the isomeric unsymmetrical 1,4-diselenin 4″, are excluded.

Figure 2

An ORTEP drawing of 1,2,5-triselenepin 5.

Scheme 2

Conversion of symmetrical 1,2,5-triselenepin (5) into 1,4-diselenin (4).

Figure 3

An ORTEP drawing of 1,6,6αλ⁴-triselenapentalene (6).

Figure 4

An ORTEP drawing of 4H-selenopyran-4-selone (8).

According to the result of X-ray crystal data of 6, the 1,6,6αλ⁴-triselenapentalene ring system of 6 possesses a planar and symmetrical structure with a linear arrangement of Se(1)-Se(2)-Se(3) atoms and almost parallel 3 carbon–selenium bonds with the right Se-Se-C bond angles, ie, 88.2° for Se(1)-Se(2)-C(11), 88.8° for Se(3)-Se(2)-C(11), 87.0° for Se(2)-Se(3)-C(2), and 87.2° for Se(2)-Se(1)-C(13), respectively. Furthermore, the bond lengths of Se(1)-Se(2) and Se(2)-Se(3) were equal and the bond lengths of the central hypervalent Se(2) and the C(11) atom were also similar to those of Se(1)-C(13) and Se(3)-C(2) bonds, ie 1.82 Å for Se(1)-C(13), 1.92 Å for Se(2)-C(11), and 1.84 Å for Se(3)-C(2), respectively. In addition, the bond lengths of C(2)-C(1), C(1)-C(11), C(11)-C(12), and C(12)-C(13) were almost equal each other. These X-ray data indicated that the 1,6,6αλ⁴-triselenapentalene ring system possesses the heteroaromaticity. Therefore, the high field shift of 1 singlet signal of the methyl group (δ = 0.68 ppm) in the ¹H NMR spectrum of 6 was explained by the anisotropic effect of the 1,6,6αλ⁴-triselenapentalene ring, and the existence of the 3 sp² carbon signals (δ = 152.9, 169.2, and 195.2 ppm) in the ¹³C NMR spectrum of 6 was also explained by the central sp² carbon atom (=C(11)) of the heterocyclic ring of 6. The ⁷⁷Se NMR spectrum of 6 also revealed 2 selenium signals (δ = 573.2 and 1079.5 ppm) with a symmetrical Se-Se-Se coupling pattern, and the evidences for bond switching between the Se(1)-Se(2) and the Se(2)-Se(3) bonds of 6 through a tautomeric equilibration between 6′ and 6″ as shown in Scheme 3 were not observed from these NMR spectra. All the results suggest the heteroaromatic stabilization of 1,6,6αλ⁴-triselenapentalene ring of 6.

Scheme 3

Possible bond-switching tautomerization for compound 6.

In contrast, the heterocyclic ring system of 8 was confirmed to be an unexpected 4H-selenopyran-4-selone ring with unsymmeterically fused two bornane skeletons. Especially, the bond lengths of C(1)=Se(1) were almost equal to those of C(3)-Se(2) and C(4)-Se(2) of the core heterocyclic ring system, ie, 1.839 Å for C(1)=Se(1), 1.846 Å for C(3)-Se(2), and 1.81 Å for C(4)-Se(2), respectively, and the bond lengths of the carbon–carbon bonds of the ring system were also much similar to each other, ie, 1.39 Å for C(1)-C(2), 1.42 Å for C(1)-C(5), 1.37 Å for C(2)=C(3), and 1.39 Å for C(4)=C(5), respectively. In addition, the 4H-selenopyran ring was almost planar with small torsion angles of the 4 atoms of the ring, ie, −1° for Se(2)-C(3)-C(2)-C(1), 7° for C(2)-C(3)-Se(2)-C(4), −1° for C(3)-Se(2)-C(4)-C(5), and 11° for Se(2)-C(4)-C(5)-C(1), respectively. These results strongly suggested that the 4H-selenopyran-4-selone ring of 8 has a contribution of the tautomeric structure of heteroaromatic selenopyrylium ion 8′ as shown in Scheme 4.

Scheme 4

Structures of 4H-selenopyran-4-selone (8) and its tautomeric selenopyrylium ion (8′).

Selones 7 and 8 were unstable toward exposure to air and underwent gradual decomposition to afford the corresponding ketones quantitatively along with extrusion of elemental selenium. All physical and spectral data for these products involving MS, IR, ¹H NMR, ¹³C NMR, and elemental analysis were fully consistent to symmetrical and unsymmetrical 4H-selenopyran-4-ones 9 and 10, respectively. ^63,64 In contrast, compound 6 was stable enough toward the exposure to air, sunlight, heating, and sodium borohydride (NaBH₄) reduction. The stability of 6 toward such reagents might also be explained by the heteroaromatic character of the 1,6,6aλ⁴-triselenapentalene ring system. However, when a CHCl₃ solution of 6 was treated with m-chloroperbenzoic acid (mCPBA, 3.0 mol amt.) at room temperature for 0.5 hours, 1,2-diselenole (11) was obtained in 81% yield along with 4H-selenopyran-4-one (9) (trace) and some uncharacterized products (trace) along with the recovery of 6 (6%) as shown in Scheme 5. Therefore, the structure of selone 7 was confirmed to be a 4H-selenopyran-4-selone with symmetrically fused two bornane skeletons. The mass spectrum of 11 revealed the parent ion peak at m/e 456 as the base peak, and the IR spectrum of 11 reveals a strong C = O streching band at 1714 cm⁻¹. The carbonyl carbon signal was also observed at δ = 204.5 ppm in its ¹³C NMR spectrum, and all other physical and spectral data involving the ¹H NMR, ¹³C NMR, and elemental analysis data were fully consistent to the structure of 11 bearing a carbonyl group at the C-2 position of one bornane skeleton. Especially, the ¹H NMR and ¹³C NMR spectra of 11 revealed the unsymmeterical patterns, and the structure of symmetrical cyclic diselenide 11′ was excluded. It was assumed that the unusual selenium-oxygen replacement of 1,6,6aλ⁴-triselenapentalene ring forming 1,2-diselenole ring and a carbonyl group would proceed through oxidation of an electron-rich selenium atom, Se(1) or Se(3) of 6, via the plausible bond-switching tautomerization between 6′ and 6″. ^{38 -50} In addition, the structure of 11, bearing an isolated carbonyl group, strongly suggested that the formation of 1,6,6αλ⁴-oxadiselenapentalene ring would be unfavorable due to the weak attractive interaction between the selenium atoms of the 1,2-diselenole ring and the oxygen atom of the neighboring carbonyl group.

Scheme 5

Oxidation of 1,6,6aλ⁴-triselenapentalene (6) and 4H-selenopyran-4-selones (7, 8).

In contrast to the case of d-campor p-toluenesulfonylhydrazone (1), similar treatment of cyclopentanone p-toluenesulfonylhydrazone with t-BuOK and elemental selenium at 110°C for 3 hours only afforded the corresponding dialkenyl diselenide in 35% yield along with the formation of a complex mixture of uncharacterized products, and neither 1,6,6αλ⁴-triselenapentalene nor 4H-selenopyen-4-selones was found at all in the mixture. This result strongly suggested that the steric protection by the bulky bornane substituents was much effective for the stabilization of selenium-containing products 5 -8. However, attempts for trapping of the precursors of 6 -8 by applying various trapping agents were not successful at all even in the case starting from d-camphor p-toluenesulfonylhydrazone (1).

The reaction pathway for the formation of products 4 -8 remains unclear. However, dibornenyl diselenide 2 was mainly obtained and 4 -8 were not found at all in the crude product when the reaction of 1 with t-BuOK and elemental selenium at 140°C for 3 hours, and the prolong reaction time for the same reaction, in turn, afforded 4 -8 along with lowering of the yield of 2 as presented in Table 1. These experimental results suggested that 4 -8 are just the secondary products formed from borneneselenolate ion B and the alternative pathway via 1,2,3-selenadiazole and/or selenirene intermediates from 1 would be negligible. Therefore, it is assumed that the products 4 and 5 would be afforded through a plausible pathway involving the in situ generation of β-selenoxoselenolate ion G formed through the reaction of borneneselenolate ion B with elemental selenium, the subsequent formation of unstable cyclic polyselenides H, and the final thermal or oxidative selenium extrusion to result in the formation of relatively stable symmetrical 1,4-diselenins 4 and symmetrical 1,2,5-triselenepins 5. Though we could not isolate their isomeric products, 4′ and 5′, these compounds would also be formed as the minor components through this pathway and would be contained in the uncharacterized mixture of selenium-containing products. 1,3,5-Trithiones are generally recognized as the synthetic precursors of 1,6,6aλ⁴-trithiapentalenes. ^{38 -50} Therefore, it was assumed that the precursor of 1,6,6aλ⁴-triselenapentalene 6 should be 1,3,5-triselone or its alkeneselenolate-type intermediates J formed through homologation of borneneselenolate ion B at the C-3 position. Actually, treatment of compound 1 with t-BuOK and elemental selenium in HMPA afforded only 2, 3, and 5, and neither 6, 7, nor 8 were found at all in the crude products as shown in Table 1. This result strongly suggests that DMF, used as the solvent of the reactions, behaves as the C₁ source for the formation of 6 -8. Formation of selenocarbamate ions (I) by treating DMF with a base and elemental selenium has been reported previously, ^{15,65 -69} and, therefore, it was assumed that formylation or selenoformylation at the C-3 position of the bornane skeleton ^70,71 would proceed through the reaction of alkeneselenolate ion B with selenocarbamate ion (I) or its related derivatives under the strong basic conditions to form 1,3,5-triselenoxo-type intermediate J ⁷² as shown in Scheme 6. However, addition of C₁ sources, such as potassium carbonate (K₂CO₃), dimethyl carbonate, or CHCl₃ in the reaction mixture did not affect the results at all, and introduction of CO₂ gas to the reaction atmosphere only showed a slight improvement on the combined yield of 6 -8. All the results of the attempts for optimization of the reaction condition are summarized in Table 1. The formation mechanism of unsymmetrical 4H-selenopyran-4-selone 8 also remained unclear at this time, but 8 was obtained as the minor component in all cases in contrast to 7, and 8 was assumed to be formed through a different pathway involving the reaction of 2-bornene or its carbene-type precursor generated in situ through Bamford–Stevens reaction of substrate 1. ⁷³

Scheme 6

Plausible pathway for the formation of 4-7.

Table 1

Reaction of d-Camphor p-Toluenesulfonylhydrazone (1) with t-BuOK and Elemental Selenium.

Solvent	Additive(mol amt.)	Atmosphere	Temp (°C)	Time (h)	Yield (%) a
					2	3 b	4	5 b	6	7	8
HMPA	-	Ar	120	3	64	30	0	5	0	0	0
DMF	-	Ar	140	3	46	18	0	0	2	4	1
DMF	-	Ar	140	24	10	15	17	15	5	3	Trace
DMF	-	Ar --> CO2 c	140	2	40	20	0	0	5	6	2
DMF	K2CO3 (2.5)	Ar	140	24	7	32	0	0	6	2	1

DMF, dimethylformamide;HMPA, hexamethylphosphoric triamide.

^aIsolated yields.

^bAbout 3:2 mixture of E- and Z-olefins 3.

^cA DMF solution of 1 was treated with t-BuOK and elemental selenium under an argon atmosphere and then dry CO₂ gas was introduced to the reaction atmosphere along with an additional heating for 1.5 hours.

In conclusion, we found a new method for the synthesis of 1,6,6αλ⁴-triselenapentalene (6) and 4H-selenopyran-4-selones (7 and 8). Especially, our new findings on the short-step preparation of sterically crowded 1,6,6αλ⁴-triselenapentalene (6) are recognized to be synthetically useful by providing us a new motif of potential redox-permeable selenium compounds for the practical uses such as the components of organic electronic devices, and further studies on the reactivity of selenurane 6 involving their redox behaviors are expected in our laboratory.

Experimental

Instruments

The melting points were determined with a Büchi 535 micro melting point apparatus. ¹H NMR spectra were recorded on a Bruker AC-400P (400 MHz) spectrometer, and the chemical shifts of the ¹H NMR spectra are given in δ relative to internal tetramethylsilane. ¹³C NMR spectra were recorded on a Bruker AC-400P (100 MHz). ⁷⁷Se NMR spectra were recorded on a Bruker AC-400P (76 MHz). Mass spectra were recorded on a Hitachi M-2000 mass spectrometer with electron-impact ionization at 20 or 70 eV using a direct inlet system. IR spectra were recorded for thin film (neat) or KBr disks on a JASCO FT/IR-7300 spectrometer. Elemental analyses were performed using a Yanagimoto CHN corder MT-5.

Materials

Column chromatography was performed using silica gel (Merck, Cat. No. 7734 or 9385) without pretreatment. Dichloromethane and chloroform were dried over phosphorus pentoxide and were freshly distilled before use. N,N-DMF and HMPA were dried over calcium hydride and were freshly distilled before use. Ethanol was dried over magnesium oxide and was freshly distilled before use. All substrates and reagents including cyclopentanone, d-camphor, p-toluenesulfonylhydrazide, acetic acid, potassium t-BuOK, elemental sulfur, elemental selenium, anhydrous potassium carbonate (K₂CO₃), anhydrous sodium sulfate (Na₂SO₄), sodium thiosulfate (Na₂S₂O₃), NaBH₄, and mCPBA were commercially available reagent grade and were used without any pretreatment.

General Procedure for the Reaction of Ketone p-Toluenesulfonylhydrazones with Potassium t-Butoxide and Elemental Selenium

A 30 mL DMF solution of p-toluenesulfonylhydrazone 1 (5.00 mmol) was treated with elemental selenium (1.00 g, 12.5 mmol) and potassium t-butoxide (1.40 g, 12.5 mmol) at room temperature, and the reaction mixture was heated at 100°C-150°C under an argon atmosphere for several hours. The reaction was then cooled to room temperature and was exposed to air for a few hours. The reaction was quenched with water and extracted with benzene. The organic layer was washed with water and dried over anhydrous Na₂SO₄ powder. After removing the solvent in vacuo, the crude product was purified using column chromatography on silica gel to afford dialkenyl diselenide 2, E/Z mixture of dimeric olefins 3, and several minor components (4, 5, 6, 7, and 8).

Physical and Spectral Data for Dialkenyl Diselenide 2 ¹⁸

2: Orange oil.

IR (neat): 2951, 2870, 1449, 1385, 1155 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.81 (6H, s), 0.85 (6H, s), 1.08 (6H, s), 1.21-1.82 (8H, m), 2.33-2.40 (2 H, m), 5.73 (2H, d, J = 2.1 Hz).

¹³C NMR (CDCl₃) δ: 12.9 (q), 18.9 (q), 19.1 (q), 25.5 (t), 31.3 (t), 54.5 (d), 60.3 (s), 62.2 (s), 129.7 (d) 137.4 (s).

MS (m/z): 430 (M⁺; 4%, ⁸⁰Se), 215 (M⁺/2; 7%, ⁸⁰Se), 41 (bp).

Calcd for C₂₀H₃₀Se₂: C, 56.08; H, 7.06%. Found: C, 55.93; H, 7.03%.

Mixture of Dimeric E- and Z-Olefins 3 (Approximately 3:2 Mixture) ^18,59,60

Colorless oil.

MS (m/z): 272 (M⁺; bp).

IR (KBr): 2951, 2866, 1468, 1448, 1386, 1372, 1365 cm⁻¹.

Calcd for C₂₀H₃₂: C, 88.16; H, 11.84%. Found: C, 87.46; H, 11.10%.

Plausible E-Isomer 3

¹H NMR (CDCl₃) δ: 0.78 (12H, s), 1.01 (6H, s).

¹³C NMR (CDCl₃) δ: 14.6 (q), 18.6 (q), 19.8 (q), 27.8 (t), 34.0 (t), 37.1 (t), 44.7 (s), 47.4 (s), 52.4 (s), 135.5 (s).

Plausible Z-Isomer 3

¹H NMR (CDCl₃) δ: 0.70 (6H, s), 0.78 (6H, s), 1.09 (6H, s).

¹³C NMR (CDCl₃) δ: 17.0 (q), 19.6 (q), 20.5 (q), 28.5 (t), 35.7 (t), 40.1 (t), 43.8 (s), 50.6 (d), 137.7 (s).

Physical and Spectral Data for 1,4-Diselenin 4

Pale yellow needles.

MP: 122.0°C-123.0°C

IR (KBr): 2987, 1544, 1386, 1111, 817 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.80 (6H, s), 0.81 (6H, s), 1.00 (6H, s), 1.55-1.60 (4H, m), 1.79-1.85 (4 H, m), 2.31 (2H, d, J = 3.6 Hz).

¹³C NMR (CDCl₃) δ: 12.5 (q), 19.1 (q), 19.5 (q), 25.6 (t), 32.5 (t), 57.3 (s), 57.9 (d), 58.6 (s), 128.5 (s), 133.4 (s).

MS (m/z): 428 (M⁺; bp, ⁸⁰Se), 214 (M⁺/2; 12%, ⁸⁰Se).

Calcd for C₂₀H₂₈Se₂: C, 56.34; H, 6.62%. Found: C, 56.18; H, 6.77%.

Physical and Spectral Data for 1,2,5-Triselenepin 5 ¹⁸

Orange solids.

MP: 162.0°C-163.0°C

$[\alpha {]}_{\mathrm{D}}^{20}$ = −191.3° (c 0.1, CHCl₃).

IR (KBr): 2952, 1528, 1386, 1107, 813 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.82 (6H, s), 0.87 (6H, s), 1.05 (6H, s), 1.12-1.30 (4H, m), 1.51-1.59 (2 H, m), 1.78-1.85 (2H, m), 2.38 (2H, d, J = 3.7 Hz).

¹³C NMR (CDCl₃) δ: 12.9 (q), 18.9 (q), 191. (q), 25.6 (dd), 31.3 (t), 54.4 (s), 60.3 (d), 62.2 (s), 137.3 (s), 145.2 (s).

⁷⁷Se NMR (CDCl₃) δ: 357.0, 527.2.

MS (m/z): 508 (M⁺; 17%, ⁸⁰Se), 105 (bp).

Calcd for C₂₀H₂₈Se₃: C, 47.54; H, 5.59%. Found: C, 47.30; H, 5.59%.

X-Ray Crystallographic Data for 5

Colorless prism, monoclinic, P2₁ (#4), a = 7.857(3) Å, b = 14.637(5) Å, c = 9.005(4) Å, β = 97.09(3)°, V = 1027.7(6) ų, Z = 2, D _calc = 1.633 g/cm³, μ(MoK_α) = 53.68 cm⁻¹, R = 0.025, R _W = 0.026. The data were deposited in Crystallographic Data Center (CCDC-1954439).

Selected bond lengths (Å), bond angles (deg), and torsion angles (deg) of compound 5: Se(1)-C(1), 1.894(5); Se(1)-C(8), 1.880(5); Se(2)-Se(3), 2.3452(8); Se(2)-C(2), 1.892(4); Se(3)-C(9), 1.887(5); C(1)-C(2), 1.337(7); C(8)-C(9), 1.351(7); C(1)-Se(1)-C(8), 113.0(2); Se(3)-Se(2)-C(2), 100.0(2); Se(2)-Se(3)-C(9), 100.3(1); Se(1)-C(1)-C(2), 138.1(4); Se(1)-C(1)-C(5), 114.4(3); C(2)-C(1)-C(5), 107.2(4); Se(2)-C(2)-C(1), 127.9(4); Se(2)-C(2)-C(3), 123.9(3); C(1)-C(2)-C(3), 107.5(4); Se(1)-C(8)-C(9), 136.0(4); Se(1)-C(8)-C(12), 116.5(3); C(9)-C(8)-C(12), 106.9(4); Se(3)-C(9)-C(8), 128.2(4); Se(3)-C(9)-C(10), 123.3(4); C(8)-C(9)-C(10), 107.6(4); Se(1)-C(1)-C(2)-Se(2), −1.3(9); Se(1)-C(1)-C(2)-C(3), −171.8(5); Se(1)-C(1)-C(5)-C(4), −151.6(4); Se(1)-C(1)-C(5)-C(6), 102.2(4); Se(1)-C(8)-C(9)-Se(3), 0.0(9); Se(1)-C(8)-C(9)-C(10), −169.8(5); Se(1)-C(8)-C(12)-C(11), −153.5(4); Se(1)-C(8)-C(12)-C(13), 100.1(4); Se(2)-Se(3)-C(9)-C(8), −57.3(5); Se(2)-Se(3)-C(9)-C(10), 111.0(4); Se(2)-C(2)-C(1)-C(5), 171.9(4); Se(2)-C(2)-C(3)-C(4), 154.0(4); Se(2)-C(2)-C(3)-C(7), −100.6(5); Se(2 - 2)-C(3)-C(17), 26.6(7); Se(3)-Se(2)-C(2)-C(1), −53.9(5); Se(3)-Se(2)-C(2)-C(3), 115.3(5); Se(3)-C(9)-C(8)-C(12), 170.8(4); Se(3)-C(9)-C(10)-C(11), 155.6(3); Se(3)-C(9)-C(10)-C(14), −100.2(4); Se(3)-C(9)-C(10)-C(20), 26.9(7); C(1)-Se(1)-C(8)-C(9), 19.8(7); C(1)-Se(1)-C(8)-C(12), −150.3(4); C(1)-C(2)-C(3)-C(4), −34.9(6); C(1)-C(2)-C(3)-C(7), 70.4(5); C(1)-C(2)-C(3 - 17), −162.3(5); C(2)-Se(2)-Se(3)-C(9), 81.8(2); C(2)-C(1)-Se(1)-C(8), 16.9(7); C(2)-C(1)-C(5)-C(4), 33.3(6); C(2)-C(1)-C(5)-C(6), −72.9(5); C(5)-C(1)-Se(1)-C(8). −156.0(4); C(8)-C(9)-C(10)-C(11), −34.1(5); C(8)-C(9)-C(10)-C(14), 70.2(5).

Physical and Spectral Data for 1,6,6aλ⁴-Triselenapentalene 6

Dark red needles.

MP: 173.0°C-176.0°C

$[\alpha {]}_{\mathrm{D}}^{20}$ = +280.0° (c 0.1, CHCl₃).

UV (C₂H₅OH) λ_max: 287 nm (ε = 37100), 536 nm (ε = 3460), 596 nm (ε = 2100).

IR (KBr): 2953, 2922, 2867, 1435, 1283, 1104 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.68 (6H, s), 0.99 (6H, s), 1.26-1.34 (4H, m), 1.32 (6H, s), 1.69-1.75 (2H, m), 2.17-2.24 (2 H, m), 3.43 (2H, d, J = 3.8 Hz).

¹³C NMR (CDCl₃) δ: 13.7 (q), 19.8 (q), 20.3 (q), 26.1 (dd), 29.7 (t), 32.0 (dd), 55.7 (d), 57.7 (s), 61.9 (s), 152.9 (s), 169.2 (s), 195.1 (s).

⁷⁷Se NMR (CDCl₃) δ: 572.3 (d, J _Se-Se = 294 Hz), 1079.5 (t, J _Se-Se = 294 Hz).

MS (m/z): 520 (M⁺; 10%, ⁸⁰Se), 290 (bp).

Calcd for C₂₁H₂₈Se₃: C, 48.76; H, 5.46%. Found: C, 48.89; H, 5.48%.

X-Ray Crystallographic Data for 6

Red needle, monoclinic, P2₁ (#4), a = 15.668(1) Å, b = 8.3478(8) Å, c = 16.072(3) Å, β = 96.371(3)°, V = 2089.0(4) ų, Z = 4, D _calc = 1.607 g/cm³, μ(MoK_α) =52.84 cm⁻¹, R = 0.045, R _W = 0.049. The data were deposited in Cambridge Crystallographic Data Center (CCDC-1954440).

Selected bond lengths (Å), bond angles (deg), and torsion angles (deg) of compound 6: Se(1)-Se(2), 2.590(2); Se(2)-Se(3), 2.585(2); Se(1)-C(13), 1.82(1); Se(2)-C(11), 1.928(10); Se(3)-C(2), 1.84(1); C(1)-C(2), 1.36(1); C(1)-C(11), 1.40(1); C(11)-C(12), 1.40(1); C(12)-C(13), 1.38(1); Se(2)-Se(1)-C(13), 87.2(3); Se(1)-Se(2)-C(11), 88.2(3); Se(2)-Se(3)-C(2), 87.0(3); Se(1)-Se(2)-Se(3), 176.98(7); Se(3)-Se(2)-C(11), 88.8(3); Se(3)-C(2)-C(3), 127.5(8); Se(2)-C(11)-C(12), 117.1(7); Se(1)-C(13)-C(12), 123.8(7); Se(2)-C(11)-C(1), 116.0(7); Se(1)-Se(2)-Se(3)-C(2), 9(1); Se(1)-Se(2)-C(11)-C(1), −177.3(7); Se(1)-Se(2)-C(11)-C(12), 3.6(7); Se(1)-C(13)-C(12)-C(11), 2(1); Se(1)-C(13)-C(12)-C(16), 178.1(7); Se(1)-C(13)-C(18)-C(14), −107(1); Se(1)-C(13)-C(18)-C(17), 148.3(8); Se(1)-C(13)-C(18)-C(19), 19(1); Se(2)-Se(1)-C(13)-C(12), 1.0(8); Se(2)-Se(1)-C(13)-C(18), 176.6(9); Se(2)-Se(3)-C(2)-C(1), −2.1(8); Se(2)-Se(3)-C(2)-C(3), 172.5(9); Se(2)-C(11)-C(1)-C(2), −5(1); Se(2)-C(11)-C(1)-C(6), −177.0(8); Se(2)-C(11)-C(12)-C(13), −4(1); Se(2)-C(11)-C(12)-C(16), −179.8(7); Se(3)-Se(2)-Se(1)-C(13), −12(1); Se(3)-Se(2)-C(11)-C(1), 2.2(7); Se(3)-Se(2)-C(11)-C(12), −176.9(7); Se(3)-C(2)-C(1)-C(6), 178.6(7); Se(3)-C(2)-C(1)-C(11), 5(1); Se(3)-C(2)-C(3)-C(4), −108(1); Se(3)-C(2)-C(3)-C(7), 150.3(8); Se(3)-C(2)-C(3)-C(10), 20(1); C(2)-Se(3)-Se(2)-C(11), −0.2(4); C(11)-Se(2)-Se(1)-C(13), −2.2(4).

Physical and Spectral Data for Symmetrical 4H-Selenopyran-4-Selone 7

Green needles.

MP: 180.7°C-181.4°C (dec.).

$[\alpha {]}_{\mathrm{D}}^{20}$ = +108° (c 0.01, CHCl₃).

UV (C₂H₅OH) λ_max: 207 nm (ε = 23700), 234 nm (ε = 17550), 319 nm (ε = 17980), 456 nm (ε = 33210), 619 nm (ε = 370).

MS (m/z): 440 (M⁺; bp, ⁸⁰Se), 425 (M⁺-CH₃; 41%, ⁸⁰Se), 410 (M⁺−2CH₃; 30%, ⁸⁰Se).

IR (KBr): 2938, 2868, 1518, 1467, 1442, 1380, 1229, 1142, 1065, 739 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.75 (6H, s), 0.96 (6H, s), 1.28 (6H, s), 1.77-1.84 (4H, m), 2.06-2.13 (4 H, m), 3.89 (2H, d, J = 3.8 Hz).

¹³C NMR (CDCl₃) δ: 12.4 (q), 19.1 (q), 19.3 (q), 24.2 (dd), 33.2 (dd), 56.1 (s), 57.4 (d), 60.8 (s), 153.1 (s), 158.4 (s), 188.4 (s).

Calcd for C₂₁H₂₈Se₂: C, 57.54; H, 6.44%. Found: C, 57.10; H, 6.23%.

Physical and Spectral Data for Unsymmetrical 4H-Selenopyran-4-Selone 8

Metallic green plates.

MP: 170.5°C-172.2°C (dec.).

$[\alpha {]}_{\mathrm{D}}^{20}$ = +297 ° (c 0.01, CHCl₃).

UV (hexane) λ_max: 210 nm (ε = 25760), 274 nm (ε = 3130), 313 nm (ε = 14450), 445 nm (ε = 30410), 723 nm (ε = 380).

MS (m/z): 440 (M⁺; bp, ⁸⁰Se).

IR (KBr): 2987, 2957, 2923, 2871, 1498, 1473, 1459, 1434, 1301, 1131, 1020, 751, 682 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.74 (3H, s), 0.75 (3H, s), 0.94 (3H, s), 0.95 (3H, s), 1.26 (3H, s), 1.74 (3H, s), 1.75-1.83 (4H, m), 2.01-2.13 (4 H, m), 2.82 (1H, d, J = 3.9 Hz), 3.74 (1H, d, J = 3.9 Hz).

¹³C NMR (CDCl₃) δ: 12.7 (q), 14.4 (q), 18.8 (q), 19.2 (q), 19.3 (q), 19.5 (q), 24.3 (dd), 26.9 (dd), 30.6 (t), 34.0 (dd), 54.3 (s), 56.1 (s), 57.5 (d), 58.1 (d), 58.2 (s), 60.5 (s), 148.9 (s), 149.3 (s), 156.5 (s), 160.3 (s), 193.1 (s).

⁷⁷Se NMR (CDCl₃) δ: 473.2, 1134.9.

Calcd for C₂₁H₂₈Se₂: C, 57.54; H, 6.44%. Found: C, 57.36; H, 6.53%.

X-Ray Crystallographic Data for 8

Green platelet, orthorhombic, P2₁2₁2₁ (#19), a = 6.779(3) Å, b = 13.334(4) Å, c = 21.610(7) Å, V = 1953(1) ų, Z = 4, D _calc = 1.491 g/cm³, μ(MoK_α) =37.85 cm⁻¹, R = 0.061, R _W = 0.027. The data were deposited in Cambridge Crystallographic Data Center (CCDC-1954441).

Selected bond lengths (Å), bond angles (deg), and torsion angles (deg) of compound 6: Se(1)-C(1), 1.839(9); C(1)-C(2), 1.39(1); C(1)-C(5), 1.42(1); C(2)-C(3), 1.37(1); C(4)-C(5), 1.39(2); Se(2)-C(3), 1.846(9); Se(2)-C(4), 1.81(1); Se(1)-C(1)-C(2), 118.2(7); Se(1)-C(1)-C(5), 123.5(8); C(2)-C(1)-C(5), 118.3(9); C(1)-C(2)-C(3), 127.8(9); C(1)-C(2)-C(6), 128.5(9); C(3)-C(2)-C(6), 103.5(9); Se(2)-C(3)-C(2), 124.9(7); Se(2)-C(3)-C(9), 125.0(7); C(2)-C(3)-C(9), 109.8(9); Se(2)-C(4)-C(5), 127.7(8); Se(2)-C(4)-C(17), 124.1(8); C(5)-C(4)-C(17), 108.2(9); C(1)-C(5)-C(4), 123.8(9); C(1)-C(5)-C(14), 131(1); C(4)-C(5)-C(14), 103.7(9); C(3)-Se(2)-C(4), 95.2(4); Se(1)-C(1)-C(2)-C(3), 166(1); Se(1)-C(1)-C(2)-C(6), −9(2); Se(1)-C(1)-C(5)-C(4), −160.3(8); Se(1)-C(1)-C(5)-C(14), 4(2); Se(2)-C(3)-C(2)-C(1), −1(2); Se(2)-C(3)-C(2)-C(6), 175.6(8); Se(2)-C(3)-C(9)-C(8), −104(1); Se(2)-C(3)-C(9)-C(10), 151.4(8); Se(2)-C(3)-C(9)-C(13), 22(1); Se(2)-C(4)-C(5)-C(1), −11(2); Se(2)-C(4)-C(5)-C(14), −178.9(7); Se(2)-C(4)-C(17)-C(16), 106.4(9); Se(2)-C(4)-C(17)-C(18), −147.1(8); C(1)-C(2)-C(3)-C(9), −174(1); C(1)-C(2)-C(6)-C(7), 103(1); C(1)-C(2)-C(6)-C(10), −151(1); C(1)-C(5)-C(4)-C(17), 167(1); C(1)-(C5)-C(14)-C(15), −93(1); C(1)-C(5)-C(14)-C(18), 161(1); C(1)-C(5)-C(14)-C(21), 33(2); C(2)-C(1)-C(5)-C(4), 18(2); C(2)-C(1)-C(5)-C(14), −178(1); C(2)-C(3)-Se(2)-C(4), 7(1); C(2)-C(3)-C(9)-C(8), 69(1); C(2)-C(3)-C(9)-C(10), −35(1); C(2)-C(3)-C(9)-C(13), −165(1); C(2)-C(6)-C(7)-C(8), 69(1); C(2)-C(6)-C(10)-C(9), −50(1); C(2)-C(6)-C(10)-C(11), −171(1); C(2)-C(6)-C(10)-C(12), 64(1); C(3)-Se(2)-C(4)-C(5), −1(1); C(3)-Se(2)-C(4)-C(17), −178.8(8); C(3)-C(2)-C(1)-C(5), −12(2); C(3)-C(2)-C(6)-C(7), −73(1); C(3)-C(2)-C(6)-C(10), 32(1); C(3)-C(9)-C(8)-C(7), −68(1); C(3)-C(9)-C(10)-C(6), 49.5(9); C(3)-C(9)-C(10)-C(11), 168(1); C(3)-C(9)-C(10)-C(12), −66(1); C(4)-Se(2)-C(3)-C(9), 179.0(9); C(4)-C(5)-C(14)-C(15), 73(1); C(4)-C(5)-C(14)-C(18), −32(1); C(4)-C(5)-C(14)-C(21), −161(1); C(4)-C(17)-C(16)-C(15), 70(1); C(4)-C(17)-C(18)-C(14), −50.3(9); C(4)-C(17)-C(18)-C(19), −169.6(9); C(4)-C(17)-C(18)-C(20), 65(1); C(5)-C(1)-C(2)-C(6), 172(1); C(5)-C(4)-C(17)-C(16), −72(1); C(5)-C(4)-C(17)-C(18), 35(1); C(5)-C(14)-C(15)-C(16), −69(1); C(5)-C(14)-C(18)-C(17), 49.5(9).

Procedure for mCPBA Oxidation of 1,6,6aλ⁴-Triselenapentalene 6

A 30 mL CHCl₃ solution of 1,6,6aλ⁴-triselenapentalene 6 (52 mg, 0.1 mmol) was treated with mCPBA (70%, 74 mg, 0.30 mmol) at room temperature for 30 minutes. Then, the reaction was quenched by addition of an excess amount of saturated Na₂S₂O₃ solution, and saturated NaHCO₃ solution was added to the reaction mixture. The reaction mixture was extracted with CH₂Cl₂, and the organic layer was washed with water and dried over anhydrous Na₂SO₄ powder. After removing the solvent in vacuo, the crude product was purified using column chromatography on silica gel to afford 1,2-diselenole 11 (37 mg, 81% yield) as orange crystals along with 4H-selenopyran-4-one 10 (trace) as a pale yellow oil.

General Procedure for Aerobic Oxidation of 4H-Selenopyran-4-Selones (7, 8)

A CHCl₃ solution of symmetrical or unsymmetrical 4H-selenopyran-4-selone (7 or 8, 1.00 mmol) was standing at room temperature for 1 week, and the reaction mixture was filtered to remove the elemental selenium. Then, the solvent of the filtrate was removed in vacuo. The crude product was purified using column chromatography on silica gel to afford the corresponding 4H-selenopyran-4-one 9 or 10, respectively, in quantitative yields.

Physical and Spectral Data for 1,2-Diselenole 11

Orange needles.

MP: 164.8°C-165.6°C

$[\alpha {]}_{\mathrm{D}}^{20}$ = −17.6° (c 0.1, CHCl₃).

UV (CHCl₃) λ_max: 248 nm (ε = 18550), 438 nm (ε = 9590), 464 nm (ε = 11450).

MS (m/z): 456 (M⁺; bp, ⁸⁰Se), 41 (46%).

IR (KBr): 2957, 2869, 1714, 1631, 1496, 1340, 1260, 1221, 1108, 1076 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.73 (3H, s), 0.83 (3H, s), 0.93 (3H, s), 0.95 (3H, s), 1.15 (3H, s), 1.22 (3H, s), 1.24-1.26 (4H, m), 1.56-1.84 (2 H, m), 1.92-2.17 (2H, m), 3.03 (1H, d, J = 3.7 Hz), 3.05 (1H, d, J = 3.8 Hz).

¹³C NMR (CDCl₃) δ: 9.1 (q), 12.9 (q), 19.0 (q), 19.6 (q), 20.1 (q), 20.3 (q), 25.4 (dd), 26.3 (dd), 30.4 (dd), 32.9 (dd), 51.1 (s), 51.3 (d), 54.3 (d), 56.7 (s), 58.6 (s), 60.5 (s), 129.5 (s), 147.3 (s), 151.8 (s), 173.2 (s), 204.5 (s).

⁷⁷Se NMR (CDCl₃) δ: 402.2, 889.2.

Calcd for C₂₁H₂₈OSe₂: C, 55.51; H, 6.21%. Found: C, 55.33; H, 6.45%.

Physical and Spectral Data for Symmetrical 4H-Selenopyran-4-One 9

Pale yellow oil.

MS (m/z): 376 (M⁺; 31%, ⁸⁰Se), 320 (M⁺-CO-C₂H₄; 8%, ⁸⁰Se), 81 (bp).

IR (KBr): 2955, 2924, 2854, 1732, 1595, 1461, 1377, 1269, 1123 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.74 (6H, s) 0.92 (6H, s), 1.21 (6H, s), 1.76-1.81 (4H, m), 1.98-2.03 (4 H, m), 3.32 (1H, d, J = 3.7 Hz).

Calcd for C₂₁H₂₈OSe: C, 67.19; H, 7.52%. Found: C, 66.81; H, 7.34%.

Physical and Spectral Data for Unsymmetrical 4H-Selenopyran-4-One 10

Pale yellow solid.

MP: 117.9°C-118.7°C

MS (m/z): 376 (M⁺; bp, ⁸⁰Se), 348 (M⁺-CO; 68%, ⁸⁰Se).

IR (KBr): 2955, 2925, 2873, 1736, 1579, 1458, 1385, 1371, 1102 cm⁻¹.

¹H NMR (CDCl₃) δ: 0.76 (3H, s) 0.77 (3H, s), 0.87 (3H, s), 0.91 (3H, s), 1.19 (3H, s), 1.51 (3H, s), 1.71-1.78 (4H, m), 1.97-2.03 (4 H, m), 2.76 (1H, d, J = 3.9 Hz), 3.24 (1H, d, J = 3.7 Hz).

Calcd for C₂₁H₂₈OSe: C, 67.19; H, 7.52%. Found: C, 66.77; H, 7.38%.