Computational investigation of chromium carbene-alkyne coupling reactions

Reaction of pentacarbonyl(cyclopropylmethoxy)carbenemetal(0) complexes (where the metal is Cr, Mo, or W) with alkynes gives mainly two types of products; five-membered cyclic system as a result of [4+2+1-2] cycloaddition reaction when chromium is employed, and seven-membered cyclic system as a result of [4+2+1] cycloaddition reaction with the other two metals. The absolute selectivity of this reaction has been studied at the B3LYP/6-31G*-SDD level of theory. We found that the vinyl carbene complex plays the critical role. It favors carbene-CO coupling in the case of chromium, while it favors cyclopropyl ring opening-CO coupling in the case of the molybdenum and tungsten complexes. Intermediates formed in these two steps have different chemical reactivities which allow them to continue in two separate pathways to give the two types of products.; In the chromium reaction mentioned above, the reaction is expected to afford cyclopentadienone chromium complexes when the reaction is done in anhydrous reaction conditions however, only the metal-free products have been observed. In presence of water cyclopentadienones are reduced to cyclopentenones. The stability of the expected chromium complexes and the reduction process have been studied at the B3LYP/6-31G* level of theory by considering Cr(CO) 3(CPO) (CPO is cyclopentadienon) as the model compound. Our analysis revealed that this complex is unstable under the employed reaction conditions. The employed CpO ligand can be replaced by CO or CO and N2, two ligands that exist in the reaction medium. Regarding the reduction process, we found the water serves as source of two hydrogen atoms and coordination of water to chromium in Cr(CO)3(CPO) is an essential step.; Calculations at the B3LYP/6-31G*-SDD level of theory showed that Moore's cyclization can take place while the organic skeleton coordinates to chromium with reasonable amount of activation energy. This type of cyclization produces two radical centers. In the presence of alkene in the tether the 5-exo cyclization product is expected. However, the experimental results confirmed that the 6-endo cyclization product is also possible. Calculations provided explanation for this observation and revealed that the final product can greatly depend on the substituent bonding to the alkene. Calculations show that the radical stabilizing group (carbonyl group in our study) favors the 5-exo product.