Directed C–H Alkenylation of Aryl Imines with Alkenyl Phosphates Promoted by a Cobalt–N‐Heterocyclic Carbene Catalyst

Abstract

We report herein an ortho-C-H alkenylation reactiono fa ryl imines with alkenyl phosphates promoted by ac obalt-N-heterocyclic carbene (NHC)c atalytic system. While commercially available bulky NHC ligands exhibited only modest catalytic activity,e laborationo ft he nitrogen substituents and backbone of the NHC enabled the desired transformation to proceed in high yield at am ild temper-ature.T he newC o-NHC system proveda pplicable to av ariety of aryl iminesa nd alkenyl phosphates to afford ortho-alkenylated aryl imines,w hich serve as precursors to benzofulvene derivatives.

Cobalt-catalyzed directed C–H alkenylation of pivalophenone N–H imine with alkenyl phosphates

“…By contrast, imines bearing m -methoxy, m -fluoro, or a 3,4-methylenedioxy group underwent exclusive alkenylation at the proximity of the functional group to afford the products 3ia – ka in good yields. As was also observed in previously reported cobalt-catalyzed ortho C–H functionalization reactions [ 22 – 23 28 – 29 ], this regioselectivity may be ascribed to the role of the oxygen or fluorine atom as a secondary directing group to have an electrostatic interaction with the cobalt center during the C–H activation. For compound 3ja , an increased acidity of the ortho position of the fluorine atom could have also contributed to the observed regioselectivity [ 31 ].…”

“…No significant improvement was observed using the saturated analogues of IMes·HCl and IPr·HCl ( Table 1 , entries 5 and 6) or the 2,6-diethylphenyl analogue (IEt·HCl, Table 1 , entry 7). Furthermore, the NHC precursor featuring a cyclohexane backbone and 2,6-diethylphenyl groups ( L1 ·HBr), which proved to be the optimal ligand for the C–H arylation of pivalophenone N–H imine as well as for the C–H alkenylation of N-arylimine ( Scheme 1 , b) [ 28 – 29 ], was not particularly effective for the present reaction ( Table 1 , entry 8). To our delight, we observed a remarkable improvement in the reaction efficiency using the benzofused analogue of IPr·HCl ( L2 ·HCl), affording 3aa in 88% yield without any trace of a dialkenylation product ( Table 1 , entry 9).…”

“…Note also that the present reaction could employ the relatively inexpensive diethyl phosphate, whereas, in the N-arylimine-directed alkenylation, the use of diisopropyl phosphate was necessary to achieve higher and more reproducible yields (cf. Scheme 1 ) [ 28 ].…”

“…The same catalytic system also promoted the alkenylation using alkenyl carbamates, carbonates, and phosphates. More recently, we have achieved an N-arylimine-directed arene C–H alkenylation reaction with alkenyl phosphates using a different cobalt–NHC catalyst ( Scheme 1 ) [ 28 ]. Meanwhile, we have also demonstrated that pivaloyl N–H imine serves as a powerful directing group for cobalt-catalyzed arene C–H functionalization reactions such as the hydroarylation of alkenes and alkylation/arylation using organic halides [ 29 – 30 ].…”

C–H Functionalization Strategies in the Naphthalene Series: Site Selections and Functional Diversity

“…[80][81][82][83][84][85][86][87] Cobalt-promoted monoalkenylation of naphthyl imines using alkenyl phosphates has been described recently by Yoshikai. 88 The procedure is well adapted for the installation of cyclic alkenes (Scheme 22). It has been shown that elaboration of NHC ligands and fine-tuning of the nitrogen substituents as well as the backbone of the NHC was beneficial to the alkenylation process compared with more classical and commercially available NHC ligands.…”