Triazolinylidene carbene

Almost 20 years after the initial report of the Stetter reaction, Ciganek reported an intramolecular variant of the Stetter reaction in 1995 with thiazolium precatalyst 74 providing chromanone 73 in 86% yield (Scheme 10) [64]. This intramolecular substrate 72 has become the benchmark for testing the efficiency of new catalysts. Enders and co-workers illnstrated the first asymmetric variant of the intramolecnlar Stetter reaction in 1996 utilizing chiral triazolinylidene pre-catalyst 14 [65]. Despite moderate selectivity, the implementation of a chiral triazolinylidene carbene in the Stetter reaction laid the fonndation for future work. 

Rovis and Liu have accomplished the desymmetrization of cyclohexadienones by using triazolinylidene carbene 75b (Scheme 15) [75, 76], Mnltiple hydrobenzo-furanones 103-106 were synthesized in good yields and excellent enantio- and diastereoselectivity. Generation of three co ntiguous stereocenters may be achieved in >99% ee and 80% yield. 

Fn and co-workers describe Umpolung reactivity of Michael acceptors catalyzed by triazolinylidene carbenes (Eq. 33) [168], Nncleophilic addition followed by tantomerization renders the [3 position of the Michael acceptor nncleophilic, which snbseqnently nndergoes alkylation. Compatible leaving gronps inclnde Br, Cl, and OTs. a,P-unsatnrated esters, nitriles, and amides all provide good to excellent yields of cyclized products. 

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

Barriers to carbene rotation in A -heterocyclic carbenes (NHCs) appear to be dominated by steric interactions, and they have been observed in square-planar complexes using the diastereotopic protons in benzyl, butyl, and ethyl derivatives. For example, 30 shows diastereotopic methylenes owing to a lack of a mirror plane through the Ir-C(carbene) plane, and averaging of the methylene protons at higher temperatures indicates a 67 kcal mol barrier to rotation." Steric interactions influencing barriers have also been observed in other cases and allow observation of rotamers with unsymmetrically substituted NHCs or triazolinylidenes. 

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