Alkenes continued carbenes

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

Styrene cyclopropanation continues to attract much interest. Cationic complex CpFe(CO)2(THF) BF4" mediates carbene transfer from ethyl diazoacetate with high cis selectivity cis trans = 85 15) [38]. On the other hand, Tp Cu(C2H4), where Tp is hydrotris(3,5-dimethyl-l-pyrazolyl)borate, is one of the rare catalysts to promote carbene transfer from ethyl diazoacetate to alkenes and also to alkynes. While cyclopropanes are formed in high yield, cyclopropenes are obtained only in moderate yield [39]. The same complex also catalyzes nitrene transfer from PhI=NTs to alkenes to produce aziridines in high yields. 

Detailed investigations of the catalytic mechanism of metathesis revealed that transition metal carbenes and metallacyclobutanes represent key intermediates. " The metathesis reaction (Scheme 20.2) generally proceeds through a [2 -t 2] addition of an alkene to a transition metal carbene (A) to give a metallacyclobutane (B). The metallacyclobutane subsequently cleaves to reform a transition metal carbene (C), which can continue the catalytic cycle. Transition metal carbenes with metals in a high oxidation state are also called transition metal alkylidenes. 

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