C=Si double bonds

The relative energies at HF/6-31G //HF/6-31G of various isomers of monosilacyclobu-tadiene are given in Figure 22161. The global minimum on the C3SiFl4 PES is silylene 127, which is stabilized by the interaction of the vacant p-orbital on silicon with the C=C jr-bond to form a 27T-aromatic system. Four other silylenes 128-131 follow 127. These silylenes are all lower in energy than the isomeric structures which possess a C=Si double bond or strained rings, such as 132-137. This stability order contrasts with... 

The effects of the substituents on the thermodynamic stability of each of the isomeric silenes were evaluated by Apeloig and Kami by means of the isodesmic equations 16 and 17, in which the substituent R is transferred from a C=Si double bond to a C-Si (or C-C) single bond. The results are given in Table 24106. The substituent effects are relatively small, usually only a few kcal mol -19 especially when R is bonded to silicon. The largest effects are exerted by R = F (destabilizing by 7.8 kcal mol-1) and SiH3 (stabilizing by 6.0 kcal mol-1) when attached to carbon. 

Compounds with C=Si double bonds that formally are either silaaromatic (e.g. silabenzene) or silaantiaromatic (e.g. silacyclobutadiene) are discussed in Section VI. 

A silene-to-silylene isomerization by a 1,2-shift of a trimethylsilyl group from silicon to carbon was originally proposed in order to account for the formation of 1,1,3-trimethyl-1,3-disilacyclobutane during the pyrolysis of allylpentamethyldisilane (equation 93)214. Since silenes are planar and since the rc-component of the C=Si double bond is quite strong ( 40 kcal mol ) 19, m, it is not easy for the molecule to align the migrating SiH or SiSi bond with the carbon p-orbital that forms the new bond. 

The diastereoselectivity of the reaction may be rationalized by assuming a chelation model, which has been developed in the addition of Grignard reagents to enantiomerically pure a-keto acetals7,8. Cerium metal is fixed by chelation between the N-atom, the methoxy O-atom and one of the acetal O-atoms leading to a rigid structure in the transition state of the reaction (see below). Hence, nucleophilic attack from the Si-face of the C-N double bond is favored4. 

However, at elevated temperatures, the disilene (/-Uu (Si)(k)SiSi( R)(Si/-Bu () 721 undergoes isomerization to give the 1,2-disilyl benzene derivative 730, which can be rationalized in terms of a C-H addition to the Si-Si double bond (Scheme 97). 

It is possible to add reagants across the Si—Si double bond in some ways analogous to the C=C bond in alkenes ... 

Because carbon stands at the head of its group, we expect it to be different from the other members of the group. In fact, the differences are more pronounced in Group 14 than anywhere else in the periodic table. Some of the differences between carbon and silicon stem from the smaller atomic radius of carbon, which explains the wide occurrence of C=C and C O double bonds, compared with the rarity of Si=Si and Si=0 double bonds. Silicon atoms are too large for the side-to-side overlap of p-orbitals necessary for -ir-bonds to form between them. Carbon dioxide, which consists of discrete 0=C=0 molecules, is a gas that we breathe. 

The hexasila-Dewar benzene 13 is thermally stable at —150 °C, but it gradually reverted to the hexasilaprismane 1243. The half-life is 11/2 = 0.52 min at 0 °C in 3-methylpentane. The activation parameters for the isomerization of 13 to 12 are a = 13.7 kcalmol-1, A= 13.2 kcalmol-1 and A= — 17.8 cal K-1 mol-1. The small Ea value is consistent with the high reactivity of Si=Si double bonds. Most probably, the small HOMO-LUMO gap of 13 makes it possible that the Si=Si double bonds undergo a formally symmetry forbidden [2 + 2] thermal reaction. Hexasila-Dewar benzene is a key... 

TMS)sSiH adds across the C=C and C=0 double bonds of a variety of compounds under free-radical conditions. The propagation steps for these hydrosilylation processes are reported in equations 26 and 27. The available rate constants for the reaction of (TMS Si radicals with some ketones and alkenes (equation 26) are collected in Table 3. In the ketone series, the rate constants decrease in the series of quinone > diaryl ketone > dialkyl ketone50. On the other hand, the rate constants for the addition of (TMS)3Si radical to activated alkenes21 are close to 108 M 1 s 1. 

Tetrasila-1,3-diene 64 reacts with maleic anhydride to give the rather unusual tetracyclic adduct 216 [Eq. (102)]. A [2 + 2] cycloaddition of one of the Si = Si double bonds of 64 to a C = O group of the acid anhydride followed by the second [2 + 2] cycloaddition between the remaining Si = Si bond and the C = C double bond would afford the final product 216.143... 

The [2+2] cycloaddition of the Si=Si double bond of disilenes across a hetero double bond belongs to the most typical reactions for the preparation of disiletanes. Reaction of the supersilyl stabilized disilene 90 with PhHC=0 and Ph2C=S gave oxa- and thiadisiletanes 91 and 92, respectively (Scheme 15). The use of heterocumulenes 0=C=0 and 0=C=S in a similar cycloaddition reaction yielded oxa- and thiadisiletanes 44 and 31. The isolated disiletanes are colorless and oxygen, water, and thermostable compounds <2002CEJ2730>. 

A bicyclic adduct was obtained by a [2+2] cycloaddition of the C=0 double bond of a benzaldehyde across the endocyclic Ge=Si double bond of a five-membered silole <2004OM2822>. 

---