Dimerization stable singlet carbenes

The simplest stable singlet carbene derived from a cyclic precursor is cyclo-propylidene. Like other carbocyclic carbenes, it does not contain a heteroatom within the ring and is therefore not further discussed here. The first cyclic carbene based on a four-membered heterocycle was described in 2004. Deprotonation of iminium salt 87a with KHMDS gave carbene dimer 88a=88a while the same reaction with 87b, bearing even bulkier N,N -substituents, led to the isolation of 88b (Scheme 1.11). Attempts to deprotonate 87a with KOt-Bu resulted in the opening of an endocyclic P-N bond, demonstrating the electrophilic character of the phosphorus atom in this compound. 

Jones, [61] Moss, [62] and Goodman [63] recently showed that transient singlet carbenes (such as chlorocarbenes) interact weakly with aromatics, but not with simple olefins. For example, this interaction led to an extension of the benzylchlorocarbene lifetime from 23 ns (isooctane) to 285 ns (benzene). [62] Similarly, we found that photolysis of the diazo precursor in hex-l-ene at -60°C led to the carbene dimer 42, while in toluene we obtained the carbene 41, which is stable up to -30°C. 

The results of these studies are summarised in Figure 5. For TS, the first intermediate observed is a dimer diradical, which can be converted thermally or by selective irradiation into a trimer, tetramer, etc.. Irradiation, however, also produces an asymmetric carbene trimer, which in turn can be converted into a series of oligomers. There is then a further side reaction which converts the asymmetric carbene trimer into a stable oligomer. Conversion of a diradical into an asymmetric carbene and a carbene into a stable oligomer occur only by the addition of a further monomer unit to the intermediate. These intermediates appear as the direct products of photo-polymerization below 80 K. Above K dicarbene radicals with singlet ground states and excited triplet and quintet states are observed. Some of these are sufficiently long for the interaction of the carbene radicals to... 

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