Stepwise addition triplet carbenes

Electrophilic addition of carbenes to carbon-carbon double and triple bonds has been extremely useful synthetically. In many cases, the reaction goes with 100% stereospecificity, so that the stereochemistry about a double bond in the starting material is maintained in the product. Cases in which addition is not 100% stereospecific are rationalized on the basis of a triplet or diradical intermediate. If the triplet carbene is relatively unreactive, the formation of the two new carbon-carbon bonds may be a stepwise process that allows for rotation and, therefore, loss of stereochemistry in the intermediate. 

The spin state of the reacting acylcarbene has a decisive influence on the reactivity of this short-lived species (Houben-Weyl, Vol.4/5b, pp 1158-1257 Vol. E19b, pp 1052 and 1231 refs 67,100-103). As far as cyclopropanation is concerned, carbene addition to a (Z)- or ( )-alkene can occur with retention or with loss of the stereochemical relationship present in the alkene (stereospecific and nonstereospecific cyclopropanation, respectively). Singlet carbenes add to C-C double bonds in a concerted manner, and therefore, stereospecifically. In contrast, a triplet carbene undergoes stepwise addition via an intermediate triplet 1,3-diradieal. Sinee spin-in-version must occur before ring closure of the latter species, the extent to which stereospecificity... 

However, reaction of the triplet carbene is not stereospecific and the addition follows a radical pathway, and is stepwise, producing a mixture of diastereomeric... 

The reactions of triplet carbenes result from a stepwise addition involving the biradical intermediate shown in Scheme 10.11 B, which can undergo C-C bond rotation. Normally, radical combination is a very fast reaction, and closure to a cyclopropane is further facilitated because it is intramolecular. The reason that the rate of closure is slower than normal in this case is that an electron spin flip is required prior to closure (Eq. 10.56). The addition of a triplet carbene to an alkene first gives a triplet biradical that cannot cyclize. Instead, a spin flip must first occur, followed by radical combination to complete the ring closure. The stereochemistry of the alkene is typically not completely lost in the product, which indicates that the spin inversion and bond rotation rates must be comparable. 

The reactivity of the singlet and triplet states should be different, as proposed by SkelL The triplet state is a diradical, and would be expected to exhibit a selectivity in its reactions toward olefins similar to that of other species having unpaired electrons. The singlet state, with its unfilled p-orbital, should be electrophilic and exhibit reactivity toward olefins similar to that of other electrophilic species. In general, carbenes add to alkenes to give cyclopropane derivatives. Dibromocarbene is typical in this respect. This reaction could involve the singlet dibromocarbene in a concerted addition, or the triplet state in a stepwise process ... 

Carbenes are highly reactive species, and if an olefin or other addition partner is not available, carbenes will indiscriminately insert into C-H bonds. Singlet carbenes insert into C-H bonds in a concerted reaction that always proceeds with retention of configuration at the C-H bond undergoing insertion. Triplets undergo insertions via a stepwise process involving radical intermediates. Reactions of this type are covered in more detail in Chapter 11, where we discuss substitutions at saturated centers. 

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