Mechanism of Cycloisomerisation of 1,6‐Heptadienes Catalysed by [( t BuCN) 2 PdCl 2 ]: 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, [( t BuCN) 2 PdCl 2 ] ( 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‐[ 2 H 6 ]‐ 1 ; 3,3,5,5‐[ 2 H 4 ]‐ 1 ; 1,7‐( Z , Z )‐[ 2 H 2 ]‐ 1 ; [1,3‐ 13 C 1 ,5,7‐ 13 C 1 ]‐ 1 and [1,3‐ 13 C 1 ,6‐ 2 H 1 ]‐ 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 ‐C 6 H 4 ‐X; X=H, OMe, Me, Cl, CF 3 ) 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, β‐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 L 2 PdCl 2 , as compared to the generation of dimethyl 3‐methylene‐4‐methyl‐cyclopentane‐1,1‐dicarboxylate ( 2 ) with pre‐catalysts of the form [(MeCN) 2 Pd(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 [( t BuCN) 2 PdCl 2 ] ( 8 ) is employed as pre‐catalyst, is proposed to proceed via a mono‐ to bidentate switch in the π‐coordination of diene 1 (η 2 to bis‐η 2 ) displacing π‐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 σ + values ( ρ + =1.3, r   2 =0.975) indicative of a strong π‐resonance contribution from the aryl ring rather than a simple σ‐inductive effect. Intermolecular modulation of regioselectivity is observed and the net effect proposed to arise through the (π→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.