Substituted silanones

Kudo and Nagase found that fluorine substitution reverses the order of stability of silanone and of the corresponding silylene, so that at MP3/6-31G //3-21G, HSiOF is by 126.5 kcal mol-1 less stable than FHSi=0217b. This result could have been anticipated considering the dramatic differences in the strengths of the Si-F and O-F bonds. 

The potential energy surface for the head-to-tail dimerization of silanone (equation 25) was also studied137. The fascinating structure of the product 33 was discussed in Section IV.C.2.b. The dimerization is calculated to be highly exothermic (109.4 kcal mol-1 at 

FIGURE 36. Energy profiles (in kcal mol-1) for the addition of H20 to H2Si=0 (full line) and H2C=0 (dotted line), at MP3/6-31G //6-31G. Reprinted with permission from J. Phys. Chem., 88, 2833 (1984). Copyright (1984) American Chemical Society2173. 

MP2/6-31G //6-31G ) and to proceed with no barrier, via a stepwise mechanism. In contrast, the head-to-tail dimerization of formaldehyde involves a high energy barrier. The head-to-head dimerization of 72, in contrast to the head-to-tail process, does involve a considerable barrier137. 

The possible metathesis reaction 26 was also studied and at MP3/6-31G is exothermic by 25 kcal mol-1 182. This value was used to suggest that the C=0 bond is about 10 kcal mol-1 stronger than the Si=0 bond182. This reaction was also studied by Bachrach and Streitwieser136 and is discussed further in Section V.A.l.a.vi. 

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It is well known that thermal decomposition of allyl-substituted silanes proceeds by retro-ene reaction with formation of transient species having a Si=C bond, such as silaben-zene, silatoluene and dimethylsilaethylene4b e. The kinetic data on the gas-phase pyrolysis of a similar allyloxysilane derivative, (l,l-dimethylallyloxy)dimethylsilane (16), and the results on thermolysis of allyloxydimethylsilane (17) in a flow system both indicate the participation of an intermediate silanone, (CH3)2Si=0 (10), as shown in Scheme 523. 

According to ab initio calculations the stabilization of complexes of type 10 depends on the substitution pattern of the methyl groups (Scheme 5). Methyl groups at the silanone moiety destabilize the complex by 1-2 kcal/mol, while acetaldehyde is more strongly bound than formaldehyde by ca. 3 kcal/mol. 

In this section we will review the computational studies on reactions of silylenes, mainly addition and insertion reactions. Some reactions, in particular the following isomeriz-ations of silylenes to the corresponding multiply bonded species, were discussed above (a) to silaethylene in Section V.A.l.a.v, (b) to substituted silenes in Section V.A.l.b.iv, (c) to disilenes in Sections V.A.2.d and f, (d) to silanimines in Section Y.A.3, (e) to silanephos-phimines in Section V.A.4, (f) to silanones in Section V.A.5, (g) to silanethiones in Section V.A.6, (h) to silynes in Section V.B.l, (i) to disilynes in Section V.B.2, (j) to aromatic compounds in Section VI.A and to antiaromatic compounds in Section VI.D. 

We look forward to the addition of other chromophores substituted with heavy atoms to the short list compiled in this work. The preparation of gas phase telluro-ketones appears imminent. A variety of silicon substituted compounds have been reported including silanones (RR Si=0), silanethiones (RR Si=S), silaisonitriles (RNSi), silenes (R R"Si=CRjRj) and others. Phosphorus compounds such as phosphaallenes (RC=P=X), phosphaalkenes (RR C=PR") and diphosphenes (RP=PR ) have been synthesized by a number of groups A few compounds containing As, Sb, Bi, and Ge atoms have also been reported We are confident that many of these new species will have properties which are sufficiently at variance with those of conventional organic chromophores to make spectroscopic studies both novel and rewarding. 

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