Bicyclic disilenes

Among a number of review articles on stable disilenes published until now,6 9 the most comprehensive would be those written by Okazaki and West in 1996, which is hereafter abbreviated to review OW ,7 and by Weidenbruch in 2001.9 At the time when review OW was written, the number of known stable disilenes was 27 and the type was limited only to acyclic disilenes with rather simple substitution modes. During less than 10 years since then, the number of stable disilenes increased dramatically up to more than 70, and the types include not only acyclic disilenes but also endo- and exocyclic disilenes, bicyclic disilenes, and tetrasiladienes. Very recently, the syntheses of the first stable trisilaallene and disilyne were achieved. All these recent experimental achievements have stimulated renewed theoretical studies, and the interplay between theory and experiments has contributed to the innovation of the understanding of the bonding, structure, and reactivity of disilenes and to the development of the potentials of silicon chemistry. Since review OW appeared in a volume of Adv. Organomet. Chem. series, the present article is intended to be a supplemental review covering the studies performed during 1996-2004. 

Most of the fundamental reactions of disilenes have been studied and discussed extensively in previous review articles. During the last decade, many new types of reactions have been found for novel types of disilenes such as cyclic and bicyclic disilenes, and conjugated tetrasiladienes. Mechanistic studies have recently been performed on several fundamental reactions of disilenes both theoretically and experimentally, and have greatly deepened our understanding of their reaction pathways, their potential energy surfaces, the factors determining the rates and stereochemistry, and so on. 

Since the first isolation of tetramesityldisilene by West et al. in 1981, a number of stable disilenes have been synthesized, and their structures and properties have been extensively investigated up to date [1,2]. However, known types of stable disilenes had been limited to simple acyclic derivatives with one Si=Si double bond until 1996, when the first stable cyclotetrasilene was synthesized [3]. Recently, the number of known skeletal types of disilenes has increased remarkably, and studies of their structures and reactions have made astonishing progress. The chemistry of disilenes is now approaching a stage to be compared with the extensive and profound chemistry of olefins. Here, we discuss the synthesis, structure, and reactions of novel cyclic and bicyclic disilenes and a trisilaallene, the first compound with a formally sp-hybridized silicon atom. 

Kobayashi H, Iwamoto T, Kira M (2005) A stable fused bicyclic disilene as a model for silicon surface. J Am Chem Soc 127 15376... 

Kira and coworkers recently described the preparation of the first cyclic disilene, 396210. On photolysis at 420 nm 1,2-silyl rearrangement occurred to yield the bicyclic[1.1.0] system, 397 (equation 52), which on standing in the dark reverted to the relatively stabler disilene. 

Cyclotrisilene 49 reacts with 2 mol of phenylacetylene to give unique bicyclic product 203 in good yield [Eq. (96)].138 On the basis of the results of deuterium labeling experiments, compound 203 is proposed to form via two consecutive [2 + 2] cycloadditions of 49 to phenylacetylene, i.e., the initial [2 + 2] cycloaddition giving 204 followed by the isomerization to disilene 206 via 205, and then the second [2 + 2] cycloaddition of 206 [Eq. (97)]. The reaction of 1-disilagermirene 50 with phenylacetylene proceeds in a similar way.139... 

In the reaction with thiophene 32, the disilathiirane 38 was unexpectedly formed in about 50 % yield by way of sulfur extraction. The X-ray structure analysis of the disilathiirane 38 revealed a very short Si-Si bond length of 230.5 pm and an almost planar environment at the silicon atoms (angular sum C-Si-C + C-Si-Si + Si-Si-C = 358 7°) [10]. These features are typical for related three-membered rings of this type and were also observed for other disilathiiranes [13]. As illustrated by the isolation of the 1,2-disilacyclohexadiene 39, sulfur abstraction from 32 seems to be initiated by [2+4]-cycloaddition of disilene 3. The bicyclic compound 40 is most likely formed by photoisomerization of 39 [14]. 

Since maleic anhydride apparently exhibits a high reactivity towards Si-Si multiple bonds, we have also examined its reaction with the disilene 3, from which the bicyclic compound 10 was isolated. The driving force for the... 

Photolysis of 1 in the presence of hexa-2,4-diyne gave two compounds in high yield, of which one was easily identified as the propynylsilirene 13 (Scheme 4). Elucidation of the constitution of the 2 1 cycloadduct of the disilene 3 and hexa-2,4-diyne was more difficult finally use of a combination of NMR methods and X-ray crystallography demonstrated the formation of the bicyclic product 12. The mechanism of formation of 12 presumably begins with the addition of two molecules of disilene 3 to the C=C triple bond, followed by a thermally allowed sigmatropic 1,5-hydrogen shift. Silyl radicals could be formed by homolysis of the thus-formed cyclobutane ring and then two consecutive cyclization steps would furnish the isolated bicyclic product 12 [8]. 

Since the anhydride 28 apparently exhibits a high reactivity towards silicon-silicon multiple bonds, we have also examined its reaction with disilene 23, from which the bicyclic compound 32 was obtained in high yield (Scheme 6). Compound 32 can formally be considered as a [2+3] cycloadduct of the starting materials after a spontaneous 1,2-hydrogen shift. The driving force for the formation of 32 is assumed to be the oxophilicity of silicon. This leads to the dipolar addition product 30 that, in turn, affords the intermediate 31 through 1,4-addition. The last step of the sequence would then be a 1,2-proton shift to furnish the isolated compound [16]. 

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