Transition metals from sulfide salts

The NS molecule is stabilized by coordination to a transition metal and a large number of complexes, primarily with metals from groups 6-9, are known.62,78 The most common preparative routes are reactions of (a) nitride complexes with a sulfur source, e.g., S8, propylene sulfide or sulfur halides, (b) (NSC1)3 with transition-metal complexes, and (c) [SN]+ salts with transition-metal complexes. 

From the reaction of divalent, first-row hexaaqua transition metal ions and MS4 (M = Mo, W), [Co(MS4)2] and [Ni(MS4)2] can be isolated, typically as quaternary phosphonium salts. The [MS4] ions act as bidentate ligands to the divalent metal ion, but the coordination geometries are different - tetrahedral for Co and square planar for Ni. These compounds model, in a structurally well-defined way, sulfide-bridged Co-Mo and Ni-Mo interactions and may serve as references for such interactions in CoMoS and NiMoS phases. 

The ylide, of general structure (4.132), is formed by reaction of the corresponding sulphide (4.133) with a metallocarbenoid resulting from transition metal salt-mediated decomposition of a diazo compound (4.134) or N-tosyl hydrazone salt (4.135). Reaction of the sulfonium ylide (4.132) with aldehyde (4.136) yields the trans-epoxide (4.137) as the major product and sulfide (4.133) which is then returned to the catalytic cycle (Figure 4.10). 

Fig. 5.25 Biological NO, chemistry adapted and simplified from Fukuto et al. (2000). Dotted arrow multistep process, Me transition metal ion, RSFl organic sulfide (-SF1 thiol group), RSSH disulfide, ONOO is an isomeric form of the nitrate radical NO3, ONOO (ONOOH) peroxonitrous acid, HON=NOH (H2N2O2) hyponitrous acid, N2O2 (0=N—N—0 ) radical anion of NO dimer, N3O3 (0=N—N—O—NO ) trimer radical anion (not to confuse with Angeli salt N304 [O2N—N—N02] ).

Evidence for the intermediacy of a-arylpalladium acetate complexes c was provided by the isolation of their trinuclear dialkyl sulfide adducts [11]. The two following steps, insertion of the alkene 4 and )8-hydride elimination, correspond to the classical Mizoroki-Heck reaction pathway. The resulting palladium(0) species, which is likely to be stabilized in the form of a hydridopalladium carboxylate e, is then reoxidized by molecular oxygen to the initial palladium(II) acetate (a) under liberation of water. The precise mechanism of this reoxidation is not yet fully understood, but it seems that, at elevated oxygen pressures, it is not rate-determining even in the absence of promoters. Mechanistic studies by Jacobs and coworkers [10] indicate that the beneficial effect of adding transition metal salts, originally intended to facilitate this oxidation step, in fact arises from an acceleration... 

Deprotonation of TosyIhyd razones. The deprotonation of to-sylhydrazones with LHMDS provides the corresponding lithium salts, which can be further decomposed into the diazo intermediates. The addition of late transition metal complexes leads to the formation of metal carbenoid species which undergo various reactions, such as cyclopropanation, aziridination, epoxidation, and C-H insertion. For instance, the lithium salt of tosylhydrazone 2, prepared from LHMDS, is reacted with an imine or an alkene in the presence of rhodium(II) acetate and a chiral sulfide to give respectively, the corresponding aziridine or cyclopropane derivatives (eqs 36 and 37). Under similar reaction conditions, the sodium salt prepared from NHMDS works equally well. 

In catalytic epoxidation reactions an alternative to the ylide generation method via alkylation/deprotonation is the transition metal-mediated carbene transfer from diazo compounds to sulfide catalysts. In 1994, Aggarwal and coworkers employed this method in the enantioselective catalytic epoxidation of aldehydes [25]. Using 20mol% of non-racemic sulfide 17 and lmol% of Rh2(OAc)4 together with the slow addition of PhCHN2, a 58% yield and 11% ee were obtained in the epoxidation of benzaldehyde (Scheme 20.10). The enantioselectivity was similar to the results obtained by Breau and Durst using preformed sulfonium salts [26]. 

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