title: First principles (DFT) characterization of Rh(I) /dppp-catalyzed C-H activation by tandem 1,2-addition/1,4-Rh shift reactions of norbornene to phenylboronic acid.
abstract: The C-H activation in the tandem, "merry-go-round", [(dppp)Rh]-catalyzed (dppp=1,3-bis(diphenylphosphino)propane), four-fold addition of norborene to PhB(OH)2 has been postulated to occur by a C(alkyl)H oxidative addition to square-pyramidal Rh(III) -H species, which in turn undergoes a C(aryl)-H reductive elimination. Our DFT calculations confirm the Rh(I) /Rh(III) mechanism. At the IEFPCM(toluene, 373.15 K)/PBE0/DGDZVP level of theory, the oxidative addition barrier was calculated to be 12.9 kcal mol(-1) , and that of reductive elimination was 5.0 kcal mol(-1) . The observed selectivity of the reaction correlates well with the relative energy barriers of the cycle steps. The higher barrier (20.9 kcal mol(-1) ) for norbornyl-Rh protonation ensures that the reaction is steered towards the 1,4-shift (total barrier of 16.3 kcal mol(-1) ), acting as an equilibration shuttle. The carborhodation (13.2 kcal mol(-1) ) proceeds through a lower barrier than the protonation (16.7 kcal mol(-1) ) of the rearranged aryl-Rh species in the absence of o- or m-substituents, ensuring multiple carborhodations take place. However, for 2,5-dimethylphenyl, which was used as a model substrate, the barrier for carborhodation is increased to 19.4 kcal mol(-1) , explaining the observed termination of the reaction at 1,2,3,4-tetra(exo-norborn-2-yl)benzene. Finally, calculations with (Z)-2-butene gave a carborhodation barrier of 20.2 kcal mol(-1) , suggesting that carborhodation of non-strained, open-chain substrates would be disfavored relative to protonation.
authors.affiliation: School of Chemistry and Chemical Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 230009, Anhui (P.R. of China); School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore); Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 (Singapore). ekantchev@hfut.edu.cn.
authors.forename: Eric Assen B
authors.initials: EA
authors.lastname: Kantchev
authors.forename: Surya R
authors.initials: SR
authors.lastname: Pangestu
authors.forename: Feng
authors.initials: F
authors.lastname: Zhou
authors.forename: Michael B
authors.initials: MB
authors.lastname: Sullivan
authors.forename: Hai-Bin
authors.initials: HB
authors.lastname: Su
keywords: 1,4-shift; CH activation; density functional theory calculations; norbornene; rhodium