Nickel thioester acylation

In conclusion, the [NiS] mediated formation of thioesters from alkyl, CO, and thiol groups lends support to an acetyl-CoA formation pathway that comprises CO insertion into a Ni Me and an intramolecular S -C bond formation between nickel-bound acyl groups and thiolate ligands. These reactions are favored at square-planar nickel complexes that enable two-electron redox reactions and readily add fifth ligands. 

A comparison of Eq. 54 and Scheme 36 reveals that the nickel center performs two functions. First, the nickel center acts as mediator of the two electron-transfer reactions during which S—C bonds are cleaved and formed (36 —> 37 and 39 —> 40). Second, it facilitates the formation of an acyl group from an alkyl group and CO (38 —> 39). This reaction is expected to be favored when the nickel center has a coordination number lower than 5. A low coordination number of nickel also facilitates the final release of the thioester in the consecutive reaction of 39 with excess CO, because intermediates such as complex 42 can readily form. 

Metal complexes are known to insert CO into carbon-sulfur bonds [118], even catalytically [119], Stoichiometric precedents exist for the formation offhioesters from nickel-alkyls, CO, and thiols [120], For example, NiMe2(bipy) reacts with thiols to afford mefhylnickel(ll) fhiolates, which carbonylate to afford acetyl-nickel(ll) fhiolates. These acetylnickel(II) thiolates reductively eliminate fhioester in the presence of CO [121], More biologically relevant is the reactivity of nickel acyls toward fhiolates, which gives fhe thioester concomitant with reduction to Ni(0) (Eq. 12.10) [122]. Thiolates are known to reduce Ni(II) to Ni(0) under an atmosphere of CO [123]. 

Aliphatic nitriles react slowly with phenols and phenyl ethers in the presence of trifluoromethanesulphonic acid to give ketones after hydrolysis, in a variation of the Houben-Hoesch reaction. The crystalline complex of copper(i) triflate and benzene induces the acylation of aromatic substrates with selenol esters, affording a transition-metal mediated version of the Friedel-Crafts reaction. Aromatic carboxylic acids can be converted into symmetrical diaryl ketones in good yield by treatment of their 5-(2-pyridyl)thioesters with bis-(l,5-cyclo-octadiene)nickel [equation (15)]. In contrast to other methods for preparing symmetrical aromatic ketones, this method allows their preparation from a single starting material. 

Analogous decarbonylation reactions were observed with acyl halides [18], esters [19], acyl cyanides [20], thioesters [21], selenoesters [21b], acylphospho-nates [22], and even ketones (preceding chapters). Acid anhydrides underwent a fragmentation reaction upon treatment with a nickel catalyst to afford alkenes,... 

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