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Base‐Promoted Selective Activation of Benzylic Carbon‐Hydrogen Bonds of Toluenes with Rhodium(III) Porphyrin Chloride: Synthetic Scopes and Mechanism
Author(s) -
Choi Kwong Shing,
Chiu Peng Fai,
Chan Chung Sum,
Chan Kin Shing
Publication year - 2013
Publication title -
journal of the chinese chemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.329
H-Index - 45
eISSN - 2192-6549
pISSN - 0009-4536
DOI - 10.1002/jccs.201200635
Subject(s) - chemistry , rhodium , porphyrin , medicinal chemistry , nucleophile , dimer , aryl , cationic polymerization , photochemistry , catalysis , free base , polymer chemistry , organic chemistry , alkyl , salt (chemistry)
Toluenes underwent base‐promoted selective benzylic carbon‐hydrogen bond activation (CHA) with rhodium porphyrin chlorides (Rh(por)Cl). In the absence of nucleophilic base, both aryl and benzylic rhodium porphyrins were formed. In the presence of nucleophilic base, the selectivity, rates and functional group compatibilities of benzylic activation reactions were enhanced. K 2 CO 3 was found to be the optimal base to achieve the best yields. Ortho‐, meta‐ and para‐substituted toluenes in the presence of K 2 CO 3 yielded the corresponding rhodium porphyrin benzyls in high yields both in solvent‐free conditions and in benzene solvent. Mechanistically, in the absence of nucleophilic base, a cationic rhodium(III) porphyrin species together with some rhodium(II) porphyrin are the most likely intermediates to account both the aryl and benzylic CHA. In the presence of a base, Rh(por)OH is generated by ligand substitution with Rh(por)Cl and rapidly undergoes reduction to give rhodium(II) porphyin dimer and H 2 O 2 . The key rhodium porphyrin intermediates for benzylic CHA were found to be rhodium(II) porphyrin dimer and hydrides as observed by 1 H NMR spectroscopy, which underwent parallel benzylic CHA reactions with the rhodium(II) porphyrin dimer being the more reactive intermediate.