Mechanism of Cycloisomerisation of 1,6-Heptadienes Catalysed by [(tBuCN)2PdCl2]: Remarkable Influence of Exogenous and Endogenous 1,6- and 1,5-Diene Ligands

Bray, Katharine L., Lloyd-Jones, Guy C., Munoz-Herranz, Maria, Slatford, Paul A., Tan, Emily H. P., Tyler-Mahon, Amanda R. and Worthington, Paul A. (2006) Mechanism of Cycloisomerisation of 1,6-Heptadienes Catalysed by [(tBuCN)2PdCl2]: Remarkable Influence of Exogenous and Endogenous 1,6- and 1,5-Diene Ligands. Chemistry - A European Journal, 12 (34). pp. 8650-8663. ISSN 0947-6539

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Abstract

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.

Item Type: Article
Faculty \ School: Faculty of Science > School of Chemistry
Related URLs:
Depositing User: Rachel Smith
Date Deposited: 14 Feb 2012 16:24
Last Modified: 21 Apr 2020 19:40
URI: https://ueaeprints.uea.ac.uk/id/eprint/37109
DOI: 10.1002/chem.200600924

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