Selenium, carbene complexes

The complete series, thio-, seleno-, and telluroformaldehyde complexes of osmium [OsCl(NO)(r72-E=CH2)(PPh3)2] (E = S, Se, Te), was obtained by reaction of the carbene complex [OsCl(NO)(=CH2)(PPh3)2] with elemental sulfur, selenium, and tellurium, respectively.53... 

The synthesis of metal complexes of type 213 can be performed by reacting metal-carbene complexes with selenium sources such as alkyneselenolates 203.430 Also the stability of unstable selenocarbonyl compounds such as selenoaldehydes can be enhanced by coordination to metal carbonyls and the reactivity of such complexes has been studied. Complex 216 can react with methylthiohexyne and the product is a different complex 217 with the selenium atom still coordinating to the metal carbonyl fragment (Scheme 66).431... 

One of the synthetic procedures of metal complexes of type 53 is the reaction of metal-carbene complexes with selenium sources such as alkyneselenolates [109]. The stability of selenobenzaldehyde is enhanced by coordinating to metal carbonyls, and the reactivity of the complexes has been studied [110]. For example, the selenobenzaldehyde complexes reacts with methylthiohexyne even at - 30 °C to afford another type of complex where the selenium atom of the selenocarbonyl group is still coordinated to the metal (Eq. 29) [llOd]. 

The reaction of alkyl selenols occurs normally to give selenium-substituted carbene complexes in low yield (Fischer et al., 1973c). However, reaction of the more acidic selenophenol with carbene complexes leads to cleavage of the carbene ligand. Apparently, the carbon-metal bond in the tetravalent intermediate is protonated by CgHjSeH. 

Reaction of carbene complexes with sulfur or selenium gives moderate yields of thio and seleno esters (Fischer and Riedmuller, 1974). 

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. ... 

Naturally, reactions are especially interesting to us if the products obtained are not readily accessible by the methods of classic organic chemistry but are easily preparable with our complexes. We found one such example upon treatment of pentacarbonyl[methoxy(aryl)carbene]chrom-ium(O) complexes with oxygen, sulfur, or selenium (76). In this way one obtains conveniently the corresponding methyl esters of arylcarboxylic acids and the O-methyl esters of arylthio- as well as arylselenocarboxylic acids this seems to us to be synthetically useful in the last two cases ... 

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]. 

The reaction of ImMe27 Pr2 with tellurium tetraiodide yielded the carbene adduct of tellurium diiodide, implying reductive elimination of the halogen (Scheme 15.22) as had been observed for the selenium complexes of Dutton et al. When... 

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