Reverse double labeling experiment

Both 2H and 14C labeling experiments have shown that the reaction proceeds via the cleavage and reformation of the double bonds, i.e., it is a transalkylidenation process (5,6,7). The reaction is reversible, essentially thermoneutral, and attains a thermodynamic equilibrium typified by a statistical distribution of the alkylidene moieties. 

Mechanistic studies on the intramolecular hydroacylation by using deuterium-labeling experiments have been reported by several groups [95-100]. The results of their studies showed that the addition of the Rh-H bond to a carbon-carbon double bond takes place in syn fashion [95,96]. They also demonstrated that C-H bond cleavage, hydrid transfer to the double bond, and carbonyl deinsertion are all fast and reversible steps (Scheme 3) [99]. 

An alternative mechanism calls for hydrogen atom transfer by the reverse of eq 14 with no organome-tallic intermediates. Attempts to distinguish between these two possible pathways of hydrogen addition employed deuterium labeling. Stereoselectivity in the addition of LCoH to double and triple bonds was observed. Early work indicated Co—D cis addition to olefins and acetylenes.232 Later experiments showed... 

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