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Nature of the Interactions between Polar β‐Substituents and Palladium in η 3 ‐Allylpalladium Complexes—A Combined Experimental and Theoretical Study
Author(s) -
Szabó Kálmán J.
Publication year - 1997
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19970030415
Subject(s) - substituent , chemistry , palladium , nucleophile , ring (chemistry) , moiety , allylic rearrangement , ligand (biochemistry) , chemoselectivity , stereochemistry , medicinal chemistry , catalysis , organic chemistry , biochemistry , receptor
Deuteromethanolysis of six β‐methoxy‐substituted η 3 ‐allylpalladium complexes ( 1–5 ) was studied under the same reaction conditions. The reaction rates depend on the ring size of the cyclic complexes, on the σ‐donor/π‐acceptor nature of the ancillary ligand, on the configuration of the allylic moiety, and on the position of the β‐substituent with respect to the palladium atom. Replacement of the methoxy group proceeds about 1000 times faster in the trans ‐β‐substituted cycloheptylallyl palladium complex 2 than in the cyclooctyl analogue 3 ; this indicates that the C–O bond strength is a function of the ring and substituent conformations. A theoretical analysis of the structure and stability of slightly simplified model compounds 6–10 was performed with density functional theory at the B 3 PW91 level in order to elucidate the relationship between the rate of deuteromethanolysis and the electronic interactions between the β‐methoxy substituent and the palladium atom. It was concluded that the strength of these β‐substituent effects critically depends on the relative position of the Pd–C3 and C4–O bonds. In cyclohexyl and cycloheptylallyl complexes 6 and 7 , the trans ‐β‐methoxy substituent adopts a conformation that is particularly favoured by the β‐substituent effect. Since the η 3 ‐allylpalladium complexes studied and their derivatives are key intermediates of important palladium‐catalysed transformations, the implications of the β‐substituent effect for the regio‐ and chemoselectivity of nucleophilic attack are also discussed.