Carbene complexes sulfur-stabilized

Further improvements in activity of the ruthenium carbene complexes were achieved by incorporation of methyl groups in 3,4-position of imidazol-2-ylidene moiety. Introduction of sulfur in the trara-position to the N-heterocyclic carbene leads to increased stability of the resulting ruthenium complexes. The synthesis and the first applications of these new rathenium complexes are described herein. 

Displacement of a good leaving group from silicon in a noncoordinating solvent (equation 5) allows solvent-free complexes (1) to be prepared when the substituents on silicon are bound via sulfur. As would be expected by analogy with carbene complexes, the Si NMR chemical shift for these complexes is at very low held (264.4 and 268.7 ppm, respectively, for R = S-/ -tolyl and SEt, respectively) and they also react readily with donor molecules such as MeCN to give four-coordinate, donor stabilized silylene species. (See Section 6 below for further details of transition metal silyl complexes.)... 

The phosphorus and sulfur ylides are more stable than the nitrogen and oxygen analogs (66). The former species are comprised of carbenes complexed to soft donors, whereas in the latter the carbenes are not stabilized by the adjoining hard bases. The high stability of CH2I compared to that of CHjF is at variance with the classical theory of inductive effect, but it is exactly as predicted on the basis of HSAB principle if regarded as [X complexes. 

The most recent development concerns the heterocyclic (amino)(ylide)carbenes AYC. Such compounds have been known for some years [203] but so far had little impact compared to their diamino stabilized relatives. Both phosphorus ylide (86) and sulfur ylide (87) stabilized AYC ligands have been generated in situ and were stabilized at suitable metal centers (Fig. 27) [204, 205]. The palladium complex 88 with an anionic (amino) [bis(ylide)]carbene is also known [206]. 

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]. 

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