Chromium sulfoxide complexes

Sulfoxide adducts of chromium, molybdenum, and tungsten carbonyls have been studied as catalysts for the polymerization of monomers such as vinyl chloride (248). Simple adducts of the type [M(CO)5(Me2SO)] may be prepared by carbonyl displacement from the corresponding hexacarbonyl. Photochemical reactions are frequently necessary to cause carbonyl displacement in this manner, many carbonyl complexes of higher sulfoxides have been prepared (255, 256). Infrared (257) and mass spectral studies (154) of these complexes have appeared, and infrared data suggest that S-bonding may occur in Cr(0) sulfoxide complexes, although definitive studies have not been reported. 

No significant pseudo-contact shifts could be induced in the spectra of dibenzothiophene, its sulfoxide, or its sulfone, with Eu(dpm)3 although a marked shielding of the protons of one ring was observed in the spectrum of the chromium tricarbonyl complex of dibenzothiophene,presumably due to a contact shift mechanism. 

Sulfoxide complexes can be formed for a wide range of metal oxidation states, as shown by the chromium derivatives Cr02Cl2 3DMSO, CrCl3 3DMSO, CrCl2-2DMSO and Cr(CO)s DMSO. 

Thomas and co-workers59 have described the oxidation of methylthio-sub-stituted tricarbonyl-(ri6-arene)chromium(0) complex 26 with CHP in the presence of Ti(0-j-Pr)4/DET/H20 combination to give the corresponding sulfoxide in 90-95% ee, in a similar approach to that used by Kagan60 for the oxidation of aryl ferrocenyl sulfide 28 (Scheme 9). 

Oxidation of pentacarbonyl[amino(arylthioalkyl)carbine]chromium(0) complexes 131 with A -sulfonyloxaziridine 132 afforded the corresponding sulfoxides 133 in good to excellent yields <1995SL666>. The oxidation is chemoselective, affording only sulfoxide products whereas the use of dimethyloxirane resulted in overoxidation to sulfones. 

But-2-enedinitrile, 2-nitropropene, phenyl vinyl sulfoxide, styrene and but-2-enal do not form 1-methoxy-l -phenylcyclopropanes with the corresponding chromium-carbene complex. " On the other hand, methyl vinyl ketone gave a formal 1,4-adduct, which, due to its instability, was further transformed under acidic conditions. ... 

I. 10). Thus aryl ferrocenyl sulfoxides (36) (>99% ce) were prepared by CHP oxidation of (35), in presence of the titanium complex prepared under strictly defined conditions [72]. Similar oxidation was used by Gibson, nee Thomas, et al. for the preparation of sulfinyl substituted tricarbonyl (T -arene) chromium(O) complexes (38), with ee s of up to 86% [73]. It is remarkable that the conditions are mild enough to avoid overoxidation and destruction of the tricarbonylchromium moiety. 

Heating a sulfinyl substituted arene with an appropriate chromium tricarbonyl source does not afford the corresponding ti -arene chromium tricarbonyl complex. Instead, a complex mixture of products is obtained, arising from an initial reduction of the sulfoxide functional group. ... 

Complexes of butyl vinyl sulfoxide and iron, chromium and cobalt (III) nitrates were found to be unstable, the ferric salt (the least stable) exploding even as a 40 mol% solution in benzene. It is considered that other vinyl sulfoxide ligands will behave similarly. 

The above discussion has concentrated upon the reagents used, but it is equally of value to comment on the substrate, particularly in reactions for which other oxidation methods have been reported to fail. A good example is the oxidation of the iron-carbonyl complex (31) to the ketone (32 equation 14). The use of dimethyl sulfoxide activated with sulfur trioxide-pyridine complex gave a 70% yield of the product, in contrast to the use of the Pfitzner-Moffatt procedure (dimethyl sulfoxide-DCC) or the chromium... 

Although many oxidizing reagents remove the chromium tricarbonyl group, benzylic alcohols can be oxidized to benzaldehydes using dimethyl sulfoxide with acetic anhydride, trifluoroacetic anhydride, or sulfurtrioxide with minimal decomplexation. Asymmetric oxidation of aUcylthio-substituted complexes can be achieved using titanium tetraisopropoxide and an optically active tartrate ester (Scheme 108). Dimethyloxirane can also be used to oxidize sulfides to sulfoxides. 

In contrast, the chemistry of the oxidation of a primary alcohol to an aldehyde differs sharply from the oxidation of an aldehyde to a carboxylic acid (case (b)). Advantage, in this case, must be taken of the difference in the mechanisms of these steps. Among the reagents which can effectively oxidize alcohols and remain rather inert toward aldehydes are pyridinium chlorochro-mate (a chromium trioxide-hydrogen chloride complex of pyridine) or dimethyl sulfoxide-Lewis acid. 

Various complex and low yield procedures for the preparation of acetylallene have been described oxidation of homopropargytic alcohol with chromium trioxide in sulfuric acid,11 mild acid hydrolysis of conjugated ethoxyenyne,12 reaction of propargyltrimethylsilane with acyl halide,13 flash vacuum thermolysis of 0-keto trimathylsilyl enol ether14 and cydoelimination of p-silylethyl sulfoxide.15... 

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