Reactions with disilenes

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]. 

Analogous adducts, generated by the reaction of cyanides with disilenes, are also known.44... 

Disilenes readily add halogens14,66 and active hydrogen compounds (HX), such as hydrogen halides,63,66 alcohols, and water,27 63 as well as hydride reagents, such as tin hydride and lithium aluminum hydride.66 These reactions are summarized in Scheme 9. The reaction of the stereo-isomeric disilene (E)-3 with hydrogen chloride and alcohols led to a mixture of E- and Z-isomers, but the reaction with chlorine gave only one of the two possible stereoisomers, thus indicating that the former two reactions proceed stepwise while the latter occurs without Si—Si rotation. 

Recently, however, Sekiguchi et al. reported that the transient disilene ( )- and (Z)-PhMeSi=SiMePh reacted with alcohols in a syn fashion with high diastereoselectivity, the extent of which depended on the concentration and steric bulk of alcohols used.31 These facts suggest that in the reaction with a sterically hindered disilene like (Z)-3, the addition of alcohols might also proceed diastereoselectively under appropriate reaction conditions. 

Disilene 21 can react with the C=N bond in a cyclic imine 73 and cumulenes such as carbodiimide75 and keteneimine74 (Eqs. 18-21). The reactions with cumulenes are not straightforward, although the initial products are most likely [2 + 2] cycloadducts. 

Thiadisiliranes are also formed in high yield in the reaction of disilenes with episulfides. Thus 1 with cyclohexene sulfide gave 5084 and the reac-... 

Although disilenes do not undergo Diels-Alder reaction with 1,3-dienes,68 they react with heterodienes like benzil,67-89 acylimines,90 and 1,4-diazabutadienes91 to give [4 + 2] cycloadducts (Eqs. 26-28). 

Quite unusual reactions take place between disilenes and the active forms of the group 15 elements, P4 and As4. In nearly all reactions of disilenes, the Si—Si w-bond is broken in the first step, leaving the products isolated from the reaction of disilenes 1,11, and 12 with P4 were found to have the [1.1. OJbicyclobutane structure 698-c.98,99 In these butterflyshaped compounds the silicon atoms are completely separated, although a P—P [Pg.266]

The highly hindered disilene 2 did not react with white phosphorus, even under forcing conditions. With disilene 3, which is more hindered than 1 but less so than 2, the reaction with P4 was more complicated. It proceeded slowly, producing small amounts of both stereoisomers of the bicyclobutane compounds 70 and 70. The major product, however, was a more complex compound containing four phosphorus and four silicon atoms, also obtained as a mixture of two stereoisomers. Two-dimensional 31P NMR spectroscopy established the probable structures to be 71.98... 

Compounds 71 are precursors to the bicyclobutanes, as shown in Scheme 18 heating of the reaction mixture led to their disappearance, with formation of 70 and 70. At the same time rearrangement of 70 to the exo-exo isomer 70 took place. This appears to be the only reaction of disilenes known to occur with inversion of configuration at silicon. 

At this time, although details remain to be elucidated, it seems that most of the main kinds of reactions of disilenes, have been discovered. These have led to numerous new classes of cyclic compounds, some with unprecedented and beautiful structures. In addition, the chemical bonding in disilenes has received serious study and is at least partially understood. 

While disilene 5 does not undergo Diels-Alder reactions with 1,3-dienes, the [4+2]-cycloaddition products are formed with heterodienes, e.g. 1,4-diazabutadienes [17] or a-ketoimines [19]. It can be deduced that the electron deficient properties of such dienes cause them to readily take part in hetero-Diels-Alder reactions, which have inverse electron demands. This is corroborated by theoretical calculations which predict an inverse electron demand of the Si-Si double bond it is strongly electron donating rather than electron accepting towards butadienes and other compounds [24,25]. 

Tokitoh et al. have synthesized a sterically hindered disilene TbtMesSiSiMesTbt 210 that is kinetically stable but thermally labile, thus providing the corresponding silylene TbtMesSi 211, whose reaction with isocyanides bearing bulky substituents has resulted in the formation of the first stable silylene isocyanide complexes TbtMesSiCNR (212, R = Tbt 213, R = Mes ) (Scheme 30).327... 

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