Structure-Based Rationale for Selectivity in the Asymmetric Allylic Alkylation of Cycloalkenyl Esters Employing the Trost ‘Standard Ligand’ (TSL): Isolation, Analysis and Alkylation of the Monomeric form of the Cationic η3-Cyclohexenyl Complex [(η3-c-C6H9)Pd(TSL)]+

Abstract

The solution-phase structures of the monomeric forms of the cationic Pd-η3-allyl and Pd-η3-cyclohexenyl complexes [Pd(R,R)-1(η3-C3H5)]+ (7+) and [Pd(R,R)-1(η3-C6H9)]+ (8+) bearing the trans-cyclohexylenediamine-based Trost ‘Standard Ligand’ (R,R)-1 have been elucidated by NMR, isotopic labeling and computation. In both complexes, (R,R)-1 is found to adopt a C1-symmetric conformation, leading to a concave shape in the 13-membered chelate in which one amide group in the chiral scaffold projects its NH unit out of the concave surface in close vicinity to one allyl terminus. The adjacent amide has a reversed orientation and projects its carbonyl group out of the concave face in the vicinity of the opposite allyl terminus. Stoichiometric and catalytic asymmetric alkylations of [8+][X−] by MCHE2 (E = ester, M = ‘escort’ counterion, X = Pd allyl counterion) show the same selectivities and trends as have been reported for in situ-generated catalysts, and a new model for the enantioselectivity has been explored computationally. Three factors are found to govern the regioselectivity (pro-S vs pro-R) of attack of nucleophiles on the η3-C6H9 ring in 8+ and thus the ee of the alkylation product: (i) a pro-R torquoselective bias is induced by steric interaction of the η3-C6H9 moiety with one phenyl ring of the ligand; (ii) pro-S delivery of the nucleophile can be facilitated by hydrogen-bonding with the concave orientated amide N−H; and (iii) pro-R delivery of the nucleophile can be facilitated by escort ion (M) binding to the concave orientated amide carbonyl. The latter two opposing interactions lead to the selectivity of the alkylation being sensitive to the identities of X− and M+. The generation of 8+ from cyclohexenyl ester substrate has also been explored computationally. The concave orientated amide N−H is able to activate the leaving group of the allylic ester by hydrogen bonding to its carbonyl group. However, this interaction is only feasible for the (S)-enantiomer of substrate, leading to the prediction of a powerful kinetic resolution (kS kR), as is found experimentally. This new model involving two regiochemically distinct (NH) and (CO) locations for nucleofuge or nucleophile binding, may prove of broad utility for the interpretation of the selectivity in asymmetric allylic alkylation reactions catalyzed by Pd complexes of (R,R)-1 and related ligands.<br/