Ketones carbon-chlorine bond formation

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

In the early twentieth century Leuchs reported a surprising example of the a-chlorination of chiral ketone 73, which gave optically active 74 in the absence of additional chiral sources.36 From a mechanistic point of view, however, there remains some ambiguity. Possible mechanisms for the formation of optically active 74 include (1) asymmetric chlorination via an enol intermediate (i.e., memory of chirality), (2) direct electrophilic chlorination of the C-H bond at the stereogenic carbon center, (3) complex formation of an achiral enol intermediate with optically active 73, (4) resolution of dl-74 by co-crystallization with optically active 73, and (5) simultaneous resolution of dl-74. 

An alternative method for the formation of aldehydes or ketones makes use of the complexes formed from a methyl sulfide with chlorine or A-chlorosuccinimide (NCS), in what is called the Corey-Kim oxidation. With dimethyl sulfide the salt 28 is generated and reacts with the alcohol to give the alkoxysulfonium salts and hence, on treatment with a base, the carbonyl compound. This reaction has fovmd particular application in the oxidation of 1,2-diols in which one alcohol is tertiary, to give a-hydroxy-aldehydes or ketones without rupture of the carbon-carbon bond. For example, the aldehyde 32 is formed in good yield from the diol 31 using dimethyl sulfide and NCS followed by addition of triethylamine (6.29). This transformation is thought to depend on the preferential five-membered transition... 

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