Central versus Terminal Attack in Nucleophilic Addition to (π-Allyl)palladium Complexes. Ligand Effects and Mechanism

Abstract

Nucleophilic addition to (η3-2-chloropropenyl)palladium complexes 1 with stabilized carbanions such as dialkyl malonates was studied. These complexes are used as probes to determine whether nucleophilic attack occurs at the central or terminal carbon of the π-allyl group. Attack at the central carbon leads to substitution of chloride via a palladacyclobutane intermediate. The regiochemistry of the reaction (central versus terminal attack) is controlled by proper choice of ligands. Thus, σ-donor ligands direct the attack of the nucleophile to the central carbon (C-2) of the allyl group whereas π-acceptor ligands direct the attack to the terminal carbons (C-1 or C-3). It was found that there is a correlation between the relative rate of central versus terminal attack and the 13C NMR shifts of the allyl group. The shift difference between the central and terminal carbons, Cc − Ct, can be used to predict the site of attack. Ab initio calculations were performed on (π-allyl)palladium complexes as well as on the postulated palladacyclobutane. The calculations support the experimental results, and for the π-allyl complexes with the σ-donor ligands the LUMO from the calculations is the symmetrical orbital with a large coefficient at the central carbon.

(2-Fluoroallyl)palladium complexes as intermediates in Pd-catalyzed Tsuji-Trost 2-fluoroallylations: Synthesis and reactivity

“…All these results are in a good agreement with previously studied reactivity of (2-chloroallyl)palladium complexes [16], as well as with ligand screening in catalytic version of this reaction in ref. [10].…”

(η3-Phenylallyl)(phosphanyloxazoline)palladium Complexes: X-Ray Crystallographic Studies, NMR Investigations, and Ab Initio/DFT Calculations (Phosphanyloxazoline)palladium Complexes, Part II. Part I see: ref. 5.

“…[9]) and excellent results have been achieved on the aspects of isomer distributions [5,23] and substitution reactions (transition-state structure, [24] regioselectivity, and [25] electronic substituent effects [26] ). Complex 1, displaying as four different isomers in solution and in addition providing X-ray crystal data, was the most interesting target for quantum-chemical calculations.…”

Synthesis of Cyclopropanes by Intramolecular Attack of N-Nucleophiles on the Central Carbon of (π-Allyl)palladium Complexes

“…[5] The existence of an intermediate palladocyclobutane was subsequently confirmed by isolation and X-ray structure determination [6] and additional experimental [7] and theoretical [8,9] studies concerning central versus terminal attack have been reported. [5] The existence of an intermediate palladocyclobutane was subsequently confirmed by isolation and X-ray structure determination [6] and additional experimental [7] and theoretical [8,9] studies concerning central versus terminal attack have been reported.…”