Disilene hydrogenation reaction

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. 

In contrast to alkenes, disilene derivatives react very smoothly with various haloalkanes as shown in Eqs. (81) and (82). Tetramesityldisilene 1 reacts with tert-butyl chloride to give hydrogen chloride adduct 172 together with 2-methylpropene (173), while treatment of 1 with benzyl chloride affords l-benzyl-2-chlorodisilane 174.126 The reactions of tetrasilyldisilene 22 with haloalkanes proceed in a similar way to give l-halo-2-(haloalkyl)disilanes 175, whereas the reaction with carbon tetrahal-ides affords the corresponding 1,2-dihalodisilanes 176.127... 

Ene addition products have been isolated from reactions with various alkenes containing allylic hydrogen atoms compounds 49 and 50 are shown here as examples. Analogously, the reaction with a 1-alkyne furnishes the adduct 47 while styrene, in contrast, reacts to afford the [2 + 2] cycloaddition product 51. The latter mode of reaction, however, is no longer considered to be unusual since the tetraalkyldisilene 41 also forms [2 + 2] cycloadducts with various C=C double bond systems71-73. On the other hand, until very recently [2 + 2] cycloadditions of the tetraaryldisilene 9 were unknown. It has now been shown that 970, as well as 4171, can undergo cycloadditions with the C=C double bonds of styrene and 2-methylstyrene. [4 + 2] Cycloaddition reactions of disilenes with... 

It is possible in both cases that the reaction sequences begin with [2 + 2] cycloadditions of the disilene to the C=N multiple bonds. For the formation of 98 this is followed by a formal NR insertion whereas a 1,4-hydrogen shift with subsequent ring closure would be necessary for the formation of 99. 

The original report76 that the irradiation into the UV but not the visible absorption band of tetramesityldisilene in cyclohexane leads to 1,1,2,2-tetramesityldisilane, presumably by hydrogen abstraction, was later retracted and the observations were attributed to the initial use of crude reaction mixtures74. Such a side-reaction was not observed during the cis-trans isomerization studies of the two above disilenes. 

The X-ray structure analyses of the unsymmetrically substituted disilenes 22 and 23 revealed the presence of somewhat stretched Si=Si and Si-Si bonds. It is not clear as to why no 1,2-additions of the hydrogen halides to the double bonds of 6 occur in these reactions or why instead the previously unobserved 1,4-additions with formation of new double bonds take place. The closest precedent involves the action of small amounts of water on 6, which is presumably initiated by a 1,2-addition and subsequent 1,3-hydrogen shift to form compound 17. 

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]. 

It is well known from organic chemistry that butadienes undergo not only 1,2-additions but also 1,4-addition reactions. Very recently we have found that treatment of 24 with hydrogen halides, which were slowly generated from trichlorosilane or lithium bromide and trifluoroacetic acid, respectively, furnished unsymmetrically substituted disilenes as formal 1,4-addition products... 

The reaction mechanism and the stereochemical diversity of the addition of water to disilene has been studied at the MP2/6-311-h+G level. Two pathways are feasible leading to syn and anti-addition. The syn addition proceeds via nucleophilic attack by water oxygen with a barrier of ca. 12 kJ mol k anti-Addition proceeds via intramolecular electrophilic attack by water hydrogen in a weakly bound disilene/water complex with antarafacial approach, in accordance with the Woodward-Hoffmann rules, and leads to an activation barrier of ca. 22 kJ mol ... 

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