Carbenes Derived from Four-Membered 5 Heterocycles

1 Carbenes Derived from Four-membered Heterocycles 

The simplest stable singlet carbene derived from a cyclic precursor is cyclo-propylidene. Like other carbocyclic carbenes it does not contain a 

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Stable free carbenes derived from four- (13), six-, or seven-membered heterocycles 14 are also known. Carbenes of type 13 can possess a phosphorus [19] or a boron atom [69] within the heterocycle. These carbenes exhibit extreme values regarding their NMR spectroscopic ( carbene-c 285 and 312.6 ppm) and structural properties (angle N-Ccarbene-N 96.72(13) and 94.0(2)°). 

Starting from (silyl)(phosphino)diazomethane derivatives, stable pseudo-diazoalkenes and carbenes have been obtained. The diazomethylenephosphoranes are very reactive dipolarophiles but are also synthetic equivalents of naked carbon C . The stable carbenes react in a similar manner to their transient analogues, but also exhibit new reactions the synthesis of unsaturated three-membered heterocycles is particularly noteworthy. Ring expansion reactions involving the latter are of special interest since they afford novel non-antiaromatic four-7t-electron four-membered heterocycles. 

Most of the known N-heterocyclic carbenes are derived from five-membered heterocyeles with additional oxygen, sulfur or phosphorus heteroatoms. Stable NHCs with up to four heteroatoms and saturated or unsaturated five-membered heteroeyeles are discussed in the following Section. 

Diverse N-heterocyclic carbenes containing Lewis-acidic boron atoms within the carbene ring have been described in the literature. Carbene 128 derived from a four-membered heterocycle (Figure 1.11) showed several similarities with the previously discussed carbene 77b (see Scheme 1.11). Substitution of the... 

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. 

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