Carbene from thioureas

The reaction of thiourea derivatives with a metal complex to form NHC complexes is a combination of the NHC formation from thioureas with potassium or sodium [Eq. (23)] and the cleavage of electron rich olefins. For example, a lO-S-3-tetraazapentalene derivative is cleaved by Pd(PPh3)4 and [(Ph3P)3RhCl], respectively [Eq. (35)]. Other substitution patterns in the carbene precursor, including selenium instead of sulfur can also be used. ... 

This review tries to shed light on the early history of different methods for the generation of N-heterocyclic carbenes (NHCs), i.e., the extrusion of heterocumulenes (decarboxylations) from suitable mesomeric betaines, deprotonations of hetarenium salts, a-eliminations, tautomerizations of mesomeric betaines, and reductive desulfurizations of cyclic thioureas. Selected examples of acyclic and three- to eight-membered NHCs are presented, as well the generation of selected five- and six-membered anionic NHCs. 

The reduction of thiones can also be used to generate free carbenes (Scheme 1.6) [49]. A solution of thione is heated in the presence of potassium, and then the resulting solution can be filtered and worked up to yield carbene. While this route is typically very clean, it does require the use of elemental potassium, which can be hazardous. The thione compoimds can be prepared from the corresponding thioureas, for which a variety of synthetic routes are known. Again, the scale of this synthetic route is limited by the need to work in the glovebox. 

To date there have been no reported complexes of NHCs to oxygen centers and only one to a sulfur center. This is certainly due to the reactivity of the atoms involved and the tendency to form lu-ea or thiourea derivatives. The NHC-suliur complex was generated from a frustrated Lewis pair reaction of elemental sulfur, giving an NHC-S-B(C6p5)3 complex the selenium analog was also prepared [265]. There have been structures of carbene-alcohol complexes reported however, the coordination occurs between a carbon and a hydrogen via a hydrogen bond [266]. A/]Af-Diamidocarbenes have been exploited in the homonuclear bond-activation of peroxides, disulfides, bromine (vide infra), and even some C—C bonds in diones and cyclopropenones as mentioned earlier [174]. Cyclo-propenyl-l-ylidene-stabilized S(II), Se(II), and Te(II) mono- and dications have also been prepared and characterized [267]. 

Heck Reactions. The Heck reaction is a Pd-catalyzed olefi-nation usually performed between an aryl halide or triflate and an acrylate ester. While phosphines are traditionally used as ancillary ligands, new Pd(dba)2-mediated reactions have been performed with a variety of other ligand types. These include chelating Wheterocyclic carbene/phosphine ligands, benzimidazoles, and quinolinyl oxazolines. Air stable catalysts have been prepared from Pd(dba)2 and sterically hindered thiourea ligands (eq 24). An effective immobilized catalyst has been prepared from Pd(dba)2 and a dendritic phosphine-containing polymer. ... 

The rates of the thermal cleavage of 7V-alkyl groups from 1,3-dialkyl-imidazolium halides (484 R = R = alkyl, R =H, R = H, Ph, or NO2) have been measured. Deprotonation of 1,2,3-trimethylimidazolium salts (484 R —R = Me R = H) at the 2-methyl group has been investigated in a model study of thiamine catalysis. Photolysis of the 4-diazo-imidazole (485) in the presence of ethanol leads to a mixture of the carbene-insertion products (486) and (487). Treatment of 2-diazo-4,5-dicyanoimidazole (488) with tetramethyl-thiourea gives the stable ylide (489). The photochemical rearrangement of imidazole oxides (490) to 2-imidazolinones (491) has been described. 

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