Carbonylative 1,3-silyl shift

As described in more detail in an earlier review2 simple acylsilanes, RtSiCOR, undergo two types of reaction when photolyzed. One involves the rapid reversible 1,2-silyl shift from carbon to oxygen of the carbonyl group leading to a siloxycarbene which can... 

However, a methyl group in a-position to the carbonyl group in 14e leads to methoxycarbene 13e, where the hydrogen shift can compete with the silyl shift. Since the hydrogen shift is irreversible, vinyloxysilane 16 is formed as the major product. 

A number of group VI carbonyl silyl substituted carbene and carbyne complexes have been reported114. The authors concluded that significant n interactions occur between the metal and the silicon atom either by hyperconjugation or participation of the silicon d orbitals. The chemical shifts for the carbene and carbyne complexes of the same metal were similar. For example, the chemical shfts of (CO)5W = C(OMe)(SiPh3) and (C5H5)(CO)2W=CSiPh3 are —18.3 and —26.7 ppm, respectively. 

Photochemical studies of several acyldisilanes have been carried out, initially to establish whether the carbonyl oxygen of the acyldisilane would attack the a-silicon (by a 1,2-silyl shift), analogously to the acylsilanes (see the above section), with the formation of a disilyloxycarbene, or whether reaction would be with the / -silicon (a 1,3-silyl shift), by analogy with the known (but thermal) behavior of / -ketosilanes110. In the case of the acyldisilanes the result of such a 1,3-silyl shift from silicon to oxygen would be the formation of a silene (equation 70). In the experiment it was evident that both modes of... 

Novel photochemical (and thermal) reactions of macrocyclic oxa-sila-acetylenic ring systems (expected to show unusual optical properties because of electronic effects arising from orbital overlap of the acetylenic n system with the silicon a bonds and the oxygen lone-pair electrons) were described. While thermolysis in the presence of a transition metal carbonyl compound gave cyclization to both benzenoid and fulvene species, photolysis in the presence of the transition metal carbonyl compound (which catalyzes 1,2-silyl shifts across a carbon-carbon triple bond) gave fulvene and vinylidene products, the latter being readily photolyzed to the fulvene 159 (equation 101). 

Although l,n-Brook rearrangements other than the 1,2-shift have been becoming increasingly important from a synthetic point of view, the main focus in this chapter will be on the 1,2-C-to-0 silyl shift because it uniquely enables a carbonyl carbon atom in acylsilanes 5 to act as a 1,1-dipole (electrophilic/nucleophilic character) 6 in combination with a nucleophile (Scheme 62). 

Murai and coworkers reported on operationally simple aldol reactions with lithium enolates generated from carbonylation of silylmethyl lithium species [57]. Upon 1,2-silicon shift, a-silyl acyllithium species can be stereo-selectively converted to (E) lithium enolates that undergo addition to aldehydes to give /3-hydroxy acylsilanes (Scheme 14). 

The JT-Jt transition of the carbonyl group in aryl acyl silanes produces an intense absorption band at around 250-260 nm, and the position and extinction coefficient of this transition are largely independent of the nature of the substituents on the silyl group. As would be expected for conjugated carbonyl group transitions, small red shifts are observed in polar solvents. 

This vanadium method enables the cross-coupling only in combinations of silyl enol ethers having a large difference in reactivity toward radicals and in their reducing ability. To accomplish the crosscoupling reaction of two carbonyl compounds, we tried the reaction of silyl enol ethers and a-stannyl esters based on the following consideration. a-Stannyl esters (keto form) are known to be in equilibrium with the enol form such as stannyl enol ethers, but the equilibrium is mostly shifted toward the keto form. When a mixture of an a-stannyl ester such as 45 and a silyl enol ether is oxidized, it is very likely that the stannyl enol ether will be oxidized preferentially to the silyl enol ether. The cation radical of 45 apparently cleaves immediately giving an a-keto radical, which reacts with the silyl enol ether selectively because of the low concentration of the stannyl enol... 

The first step of the mechanism involves the initial complexation of titanium tetrachloride to the carbonyl group of the electron-deficient alkene (enone) to give an alkoxy-substituted allylic carbocation. The allylic carbocation attacks the (trimethylsilyl)allene regiospecifically at C3 to generate vinyl cation I, which is stabilized by the interaction of the adjacent C-Si bond. The allylic Ji-bond is only coplanar with the C-Si bond in (trimethylsilyl)allenes, so only a C3 substitution can lead to the formation of a stabilized cation. A[1,2]-shift of the silyl group follows to afford an isomeric vinyl cation (II), which is intercepted by the titanium enolate to produce the highly substituted five-membered ring. Side products (III - V) may be formed from vinyl cation I. 

In 2006, Scheidt and coworkers [44] reported the first enantioselective direct nucleophilic addition ofthe silylated thiazolium salt 148, a precursor of the equivalent acyl anion, to nitroalkene 149 in the presence of tetramethylammonium fluoride (TMAF) and stoichiometric amounts of quinine-based thiourea 81b, producing the chiral [3-nitroketone 150 in 67% yield and with 74% ee (Scheme 9.51). The acyl anion equivalent 152 can be generated by the desilylation of 148 with TMAF, followed by the 1,2-H shift of the resulting alkoxide 151. The observed asymmetric induction indicates that there is a strong interaction between the thiourea and the nitroalkene during the carbonyl anion addition step. 

Komatsu et al. have developed unique methods for the generation of 1,3-dipoles from organosilanes (Scheme 10.222). Linder thermal conditions, N-(a-silylbenzyl) imines and -amides are converted, via 1,2- or 1,4-silatropic shift of the silyl group, into azomefhine ylides (153 from the amide) which react with dipolarophiles [578]. Similar thermal 1,4-silyl migrations of a-silylnitrosamines and S-a-silylben-zyl thioesters provide convenient routes to azomethine imines 154 [579] and fhio-carbonyl ylides 155 [580], respectively. 

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