Dual carbene-based

The following mechanism is proposed The catalytic entity is a dual site consisting of a carbenium ion locked up in the zeolite close to an aluminium ion on T-site, and an induced basic site on one of the oxide ions surrounding the A1 ion. This provides a dual acid/base site which is necessary for the carbene mechanism. The absence of basic sites on fresh H-ZSM-5 has been a major argument for rejecting the carbene mechanism. A formal description of the mechanism is given in schemes 1 and 2. 

Izquierdo, J., Orue, A., Scheldt, K. A. (2013). A dual Lewis base activation strategy for enantioselective carbene-catalyzed annulation. Journal of the American Chemical Society, 135, 10634-10637. 

In 2007, Cordova and coworker [17] first found that amino and heterocyclic carbene catalysis could be combined in one-pot for the direct catalytic conversion of a,P-unsaturated aldehydes into valuable P-functionalized esters in good to high yields with excellent enantioselectivities (Scheme 43.8). The one-pot combination of chiral amine and heterocyclic carbene catalysis opens up the possibility of bio-mimetic asymmetric dual organocatalysis based on carbene catalysis. 

However we rationalized that using silicon as a migrating group could result in a unique stabilization of the energy surface of olefin isomerization. This rationalization was based on silicon s well known ability to stabilize both a-carbanions and p-carbocations. Thus a hypothetical "dual-stabilized" zwitterion would be produced by a 90° twist of a vinyl silane, and a following 1,2-shift of Silicon would produce a singlet carbene possessed of the same hyperconjugative stabilization as in a P-silyl cation (Eq. 5). 

The isolation of IPrSiCb provided a unique platform for the preparation of a wide variety of novel silicon compounds (Scheme 15.15). The SiCb fragment is ambiphilic in nature, able to accept a lone pair of electrons (from a carbene for instance), and able to donate its own pair as a Lewis base. This dual-bonding... 

Recently, the catalytic activities of combined amines and N-heterocyclic carbenes (NHCs) have been recognized in various reactions [18]. For example, in 2009, Rovis and coworkers successfully developed a novel one-pot, asymmetric formal [3-1-2] reaction between 1,3-dicarbonyl compound 43 and a,P-unsaturated aldehydes 44 based on the combinational use of chiral diarylproUnol silyl ether 46 and NHC catalyst 47 (Scheme 43.9) [19]. The dual catalytic process involved a chiral... 

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