Carbene complexes silicon-stabilized

Transition metals have been used to trap and stabilize many different types of reactive intermediates, such as carbenes. Reactive silicon intermediates have only recently yielded to this approach. In the case of alkenes, for instance, transition metal complexes are generally made by exposing the alkene to a transition metal bearing suitable leaving groups (e.g., carbonyl). Unlike carbon-based intermediates, however, silicon-based analogs have been very difficult to prepare until recently. Unless... 

Displacement of a good leaving group from silicon in a noncoordinating solvent (equation 5) allows solvent-free complexes (1) to be prepared when the substituents on silicon are bound via sulfur. As would be expected by analogy with carbene complexes, the Si NMR chemical shift for these complexes is at very low held (264.4 and 268.7 ppm, respectively, for R = S-/ -tolyl and SEt, respectively) and they also react readily with donor molecules such as MeCN to give four-coordinate, donor stabilized silylene species. (See Section 6 below for further details of transition metal silyl complexes.)... 

Very few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions. Ene products are the major components firom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 -t- 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The t-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

The orbitals involved in the formation of a metal-silicon multiple bond are shown in Figure 13.6. These orbital interactions are the same as those involved in the formation of a metal-carbon double bond in a carbene complex. However, back-donation from the metal d-orbital into the silicon p-orbital is much weaker in a metal-silylene complex than it is in a carbene complex. Therefore, the silicon remains Lewis acidic, and the silylene complexes are often stabilized by Lewis bases. 

In complex 155, the NHC ring was nearly orthogonal to the Si" heterocyle plane, consistent with electron donation from the carbene into an orbital of primarily p-character on the silicon center. Cui and co-workers were able to prepare the donor-acceptor stabilized silylones 157 and 158 by treating 154 with aldehydes. These reactions were reported to proceed through silicon heterocyclic ring expansion and insertion leading to 0x0 transfer from the aldehyde to silicon. Unlike Driess complexes 152 and 153, the formation of silylones 157 and 158 required the addition of the Lewis acid AICI3 to stabilize the Si=0 moiety (Scheme 5.26). " ... 

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