Mechanism of cycloisomerisation of 1,6-heptadienes catalysed by [(tBuCN)2PdCl2]: Remarkable influence of exogenous and endogenous 1,6- and 1,5-diene ligands

The mechanism of the highly regioselective cycloisomerisation of dimethyl hept-1,6-dienyl-4,4-dicarboxylate (1) by a neutral pre-catalyst, [(tBuCN)₂PdCl₂] (8), to generate dimethyl 3,4-dimethylcyclopent-2-ene-1,1-dicarboxylate (3) has been investigated by isotopic labelling (reactions involving single and mixed samples of 1,1,2,6,7,7-[²H₆]-1; 3,3,5,5-[²H₄]-1; 1,7-(Z,Z)-[²H₂]-1; [1,3-¹³C₁,5,7-¹³C₁]-1 and [1,3-¹³C₁,6-²H₁]-1) and by study of the reactions of dimethyl 1-aryl-hept-1,6-dienyl-4,4-dicarboxylates (9?a–e, where aryl is p-C₆H₄-X; X=H, OMe, Me, Cl, CF₃) and dimethyl hept-1,5-dienyl-4,4-dicarboxylate (14), a 1,5-diene isomer of 1. The mechanism proposed involves the generation of a monochloro-bearing palladium hydride which undergoes a simple hydropalladation, carbopalladation, Pd/H dyotropy, ß-H elimination sequence to generate 3. A key point that emerges is that chelation of the 1,6-diene 1 at various stages in the mechanism plays an important role in determining the regioselectivity of the reaction. The selective generation of 3 with pre-catalysts of the form L₂PdCl₂, as compared to the generation of dimethyl 3-methylene-4-methyl-cyclopentane-1,1-dicarboxylate (2) with pre-catalysts of the form [(MeCN)₂Pd(allyl)]OTf (5) is ascribed to the absence of chloride ion in the latter, which makes an additional coordination site available throughout turnover. Liberation of the product 3 when [(tBuCN)₂PdCl₂] (8) is employed as pre-catalyst, is proposed to proceed via a mono- to bidentate switch in the p-coordination of diene 1 (?² to bis-?²) displacing p-coordinated 3 from Pd. When 1-aryl-1,6-dienes 9 are employed as substrates, the electron-donor property of the aryl group is found to influence the regioselectivity of cyclisation. Electron-withdrawing groups favour dimethyl 3-arylmethyl-4-methylcyclopent-2-ene-1,1-dicarboxylates (10), whilst electron-donating aryl groups favour 3-arylidene-4-methyl-cyclopentane-1,1-dicarboxylates (11). The regioselectivity (10/11) correlates with the Hammett s⁺ values (?⁺=1.3, r?²=0.975) indicative of a strong p-resonance contribution from the aryl ring rather than a simple s-inductive effect. Intermolecular modulation of regioselectivity is observed and the net effect proposed to arise through the (p?d) donation ability of the vinyl arene in the diene displacing product (10/11) via a mono- to bidentate switch in coordination. The isomerisation process increasingly sequesters Pd as turnover proceeds leading to a powerful inhibition mechanism and ultimately a limitation in turnover number to about 80.