Stereoselective coordination: a six-membered P,N-chelate tailored for asymmetric allylic alkylation.

Császár, Zs. and Farkas, Gergely and Bényei, Attila Csaba and Lendvay, György and Tóth, I. and Bakos, József (2015) Stereoselective coordination: a six-membered P,N-chelate tailored for asymmetric allylic alkylation. DALTON TRANSACTIONS, 44 (37). pp. 16352-16360. ISSN 1477-9226

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Abstract

Six-membered chelate complexes [Pd()Cl2], () and [Pd()(eta(3)-PhCHCHCHPh)]BF4, () of P,N-type ligands , ((2S,4S)-2-diphenyl-phosphino-4-isopropylamino-pentane) and , ((2S,4S)-2-diphenyl-phosphino-4-methylamino-pentane) have been prepared. The Pd-complexes have been characterized in solution by 1D and 2D NMR spectroscopy. The observed structures were confirmed by DFT calculations and in the case of also by X-ray crystallography. Unexpectedly, the coordination of the all-carbon-backbone aminophosphine resulted in not only a stereospecific locking of the donor nitrogen atom into one of the two possible configurations but also the conformation of the six-membered chelate rings containing three alkyl substituents was forced into the same single chair structure showing the axially placed isopropyl group on the coordinated N-atom. The stereodiscriminative complexation of led to the formation of a palladium catalyst with a conformationally rigid chelate having a configurationally fixed nitrogen and electronically different coordination sites due to the presence of P and N donors. The stereochemically fixed catalyst provided excellent ee's (up to 96%) and activities in asymmetric allylic alkylation reactions. In contrast, the chelate rings formed by exist in two different chair conformations, both containing axial methyl groups, but with the opposite configurations of the coordinated N-atom. Pd-complexes of provided low enantioselectivities in similar alkylations, therefore emphasizing the importance of the stereoselective coordination of N-atoms in analogous P-N chelates. The factors determining the coordination of the ligands were also studied with respect to the chelate ring conformation and the nitrogen configuration.

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