In continuation of our work on exploring the synergistic coordination mode of DDGs in Rh(Ⅲ)-catalyzed C-H functionalization [
43-
45], therefore, a set of density functional theory (DFT) studies were first carried out by selecting the rhodacycle
INT-0 as the starting point with TFE being the solvent. Initially, the coordination of propargyl carbonate with rhodium metal (
INT-1) followed by regioselective insertion of the alkyne into C-Rh bond
via the transition state
TS-1 (∆
G≠ = 21.4 kcal/mol) led to the formation of
INT-3, which facilely converted into a more stable intermediate
INT-4 bearing distinct coordination affinity between -OBoc and the rhodium metal center (
Fig. 1). Thereafter, the
β-
O elimination from
INT-4 proceeded
via TS-3 with an energy barrier of 7.6 kcal/mol, giving the allene intermediate
INT-5 with an exothermic process (path Ⅰ). Different possible pathways for the following [3 + 2] cycloaddition were then calculated. In path Ⅰ
a, a metal-free annulation from
INT-6c proceeded through
TS-4c with an energy barrier of 24.6 kcal/mol (from
INT-5 to
TS-4c), affording the final tetracyclic indenopyrazolopyrazolone
PC with an overall exothermicity by 33.8 kcal/mol. As a comparison, the Rh(Ⅲ)-catalyzed [3 + 2] cycloaddition
via TS-4 (∆
G≠ = -1.8 kcal/mol) and
TS-5 (∆
G≠ = -9.8 kcal/mol) led to the formation of
PC with a relatively lower energy barrier of 13.7 kcal/mol (path Ⅰ
b, from
INT-5 to
TS-4), implying the rhodium metal might be involved in the annulation process. On the other hand, removal of the -OBoc moiety from
INT-3 resulted in the coordination of the allene double bond with rhodium metal (
INT-6b), followed by the annulation through
TS-4b with an energy barrier of 24.5 kcal/mol (path Ⅰ
c). Obviously, path Ⅰ
b involving the Rh(Ⅲ)-catalyzed stepwise C-C/C-N bond formations was more reasonable based on the reaction energy profile, and -OBoc played a key role in tuning the regioselectivity and reactivity by the coordination interaction. An alternative Rh(Ⅲ)-catalyzed stepwise C-N/C-C formation process involving 8-membered rhodacycle species was ruled out due to the high energy barriers (Fig. S1 in Supporting information).