Catalytic asymmetric synthesis of quaternary carbon centers.: Exploratory investigations of intramolecular Heck reactions of (E)-α,β-Unsaturated 2-haloanilides and analogues to form enantioenriched spirocyclic products

The effect of chiral diphosphine structure, method of catalyst generation, reaction solvent, and HI scavenger on the formation of enantioenriched 3,3-disubstituted 2-oxindole 5 from asymmetric Heck cyclization of 4 was studied (eq 1). Depending upon whether the HI scavenger was a silver salt or a basic tertiary amine, either enantiomer of 5 could be formed with good selectivity using the same enantiomer of BINAP. Using Pd-BINAP as catalyst, a variety of enantioenriched 3,3-disubstituted oxindoles, indolines, and dihydrobenzofurans was prepared from (E)-alpha,beta-unsaturated 2-haloaniline substrates (Table 5). With but one exception, cyclizations conducted in the presence of Ag3PO4 or 1,2,2,6,6-pentamethylpiperidine (PMP) afforded opposite enantiomers of the spirocyclic product. Which HI acceptor results in highest enantioselection is substrate dependent. These studies demonstrate, for the first time, that asymmetric Heck reactions of halide substrates can proceed with useful levels of enantioselectivity in the absence of silver or thallium salts.