Cyclopropanation stable singlet carbenes

The stereospecificity observed with (Z)- and ( )-2-deuteriostyrene presents convincing evidence for the concerted nature of the cyclopropanation reaction, and therefore the genuine singlet carbene nature of stable (phosphino)(silyl)carbenes (I). The diastereoselectivity is at first glance surprising. Indeed, it is clear that steric factors cannot govern the observed selectivity since the bis(amino)phosphino group... 

In the last ten years, it has been shown that although bis(amino)carbenes such as 25 do not behave as classical transient singlet carbenes (no cyclopropanation reaction, no 1,2-shift, no C-H insertion...), they are wonderful ligands for transition metals which have very high catalytic activities. [27] Their three center four k electron system makes them strong a-donor and poor Ji-attractor. In other words, they constitute a new class of Lewis base. Despites these facts, in the last two highlights of Angew. Chem. [28] devoted to stable carbenes, only bis(amino)carbenes were considered, without any mention of our work This prompted us, to work harder and harder to demonstrate that our carbene 17 was a true carbene, and that it was even possible to obtain other types of stable carbenes, which are closer to the transient species. 

Depending on the mode of generation, a carbene may be initially formed in either the singlet or triplet state, irrespective of its stability. Common methods used for the generation of carbenes include photolytic, thermal, or metal catalyzed decomposition of diazocompounds, elimination of halogenfrom gem-dihalides, elimination of Hx from CHX3, decomposition of ketenes, thermolysis of a-halo-mercury compounds and cycloelimination of shelf stable substrates such as cyclopropanes, epoxides, aziridines and diazirines. 

Laser flash photolysis has, as usual, illuminated the problem. Jones and Rettig photodecomposed 9-diazofluorene (188) in hexafluorobenzene and cw-4-methyl-2-pentene mixtures, and showed that the degree of stereoselectivity in the cyclopropane products depended on the concentration of the alkene. Laser flash photolysis showed that the first detectable intermediate in the photolysis reaction is the triplet carbene, and suggests that the product studies are consistent with initial formation of a singlet fluorenylidene which has an extremely short life (less than 5 ns) before forming the triplet. The singlet can be trapped only by high alkene concentrations while the more stable triplet is easily trapped. 

Stereospecificity (or lack of it) in the addition of a carbene to an alkene can be a good test of whether the carbene reacts as a singlet or triplet lack of stereospecificity in a carbene addition almost certainly indicates that a triplet carbene is involved, but the fact that an addition is stereospecific doesn t mean that the carbene must be a singlet. In some casest bond rotation may be quite slow, and spin-flipping rapid, leading to stereospecific addition. Notice that in this example the less stable cis (2) alkene was used the reaction will give the less encumbered frans-cyclopropane if it can. 

Carbenes come in two flavors —spin-paired singlets and spin-unpaired triplets. Often the triplet is the more stable form, but the singlet state is usually more reactive. Both singlet and triplet carbenes add to alkenes to give cyclopropanes. In the concerted reaction of the singlets, any stereochemistry present in the alkene is preserved. In the nonconcerted additions of triplets, stereochemistry is lost. 

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