How the bicyclo[4.1.0] substrate isomerizes into 4,5-dihydrobenzo[b]furan: the contribution from W(CO)5 and NEt3

The cycloisomerization of a bicyclo[4.1.0] substrate into 4,5-dihydrobenzo[b]furan was investigated by using density functional theory (DFT). Comparative studies on four models (model I: with W(CO)5 and NEt3; model II: without NEt3; model III: without W(CO)5; model IV: without W(CO)5 and NEt3) indicate that this reaction is the most likely to proceed under model I to give the product. The ring closure process is greatly associated with the H1 and H2 transfer processes, because in the H1 transfer process, the carbene C3 atom is mainly stabilized by W(CO)5, and in the H2 atom transfer process the C3 atom is mainly stabilized by the O1 atom. The rearrangement of 12 to give 14 is the rate-determining step of this reaction with a free energy barrier of 31.0 kcal/mol. The presence of W(CO)5 can not only promote the H1 transfer and the ring closure (1→6-[W]) but can also be slightly favorable for the isomerization of 6-[W] into 11-[W](6-[W]→11-[W]). NEt3 mainly has an effect in the 6-[W]→11-[W] stage, in which it mainly plays proton-transfer bridge and proton-adsorption roles.