NMR Studies Revealed the Reaction Mechanism

we prepared Mo complex 70 having four carbon monoxides from (nob) Mo(CO)4 with ligand 64. 

Mo complex 70 (2 mole) were reacted with 1 mole of carbonate L-C to generate lmole of tt-allyl Mo complex 71, lmole of free ligand 64, lmole of Mo(CO)6, 

2 mole of Mo complex 70 and 1 mole of branched carbonate (either S or R carbonate or racemate B-C). It is noted that nucleophilic attack of the Mo complex jifrom the cleanly open opposite side of the Mo atom should provide the R-adduct, instead of the experimentally observed S-adduct  

Additional Studies for Confirmation of the Retention-Retention Mechanism 

Stoichiometric reaction with matched S-carbamate having the D atom in the Z-position 733) in the presence of S,S-ligand 64 without a nucleophile solely formed (no other isomer was observed by NMR) the Mo-complex 74 without transposition of the label. The structure of 74 was probed based on NMR studies by comparison with NMR studies and the X-ray structure of the protio complex 71. Nucleophilic attack of sodium malonate on the Mo complex 74 provided the S-product 75, where the D atom remained at the Z-position. On the other hand, stoichiometric reaction with mismatched R-carbamate having the D atom in the Z-position 76 without a nucleophile generated the Mo complex 80 as sole product, based on NMR studies. The structure of the complex 80 was elucidated by NMR. In 80, Mo is located on the same face as in 74 but the D atom is transposed from the Z to the E position. The transposition could be explained as follows. Initially the n-allyl Mo-complex 77 (unobserved) must form with retention. Mo complex 77 is equilibrated into the more stable Mo complex 80, where the D atom is moved 

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