Rhodium–Carbon Bond Energies in Tp′Rh(CNneopentyl)(CH₂X)H: Quantifying Stabilization Effects in M–C Bonds

Abstract

A series of substituted methyl derivatives of the type Tp'Rh(CNneopentyl)(CH2X)H (CH2X = CH2C(═O)CH3, CH2C≡CCH3, CH2O-t-Bu, CH2CF3, CH2F, CHF2) was synthesized either by photolysis of Tp'Rh(CNneopentyl)(PhNCNneopentyl) in neat CH3X or by exchange with the labile hydrocarbon in Tp'Rh(CNneopentyl)(n-pentyl)H or Tp'Rh(CNneopentyl)(CH3)H. Only a single product was observed in each case. Clean reductive elimination was observed for all compounds in C6D6. Structures of these complexes and their corresponding chlorinated derivatives have been characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. Relative Rh-C bond energies are calculated using previously established kinetic techniques, and two separate linear correlations are observed versus known C-H bond strengths, one for the parent hydrocarbons, and one for the substituted hydrocarbons. Both correlations have slopes of 1.4, and are separated vertically by 7.5 kcal mol(-1) (-CH2X above -CxHy). In addition, it is now clear that preferences for linear vs branched olefin insertion products in substituted derivatives can be predicted on the basis of the strengths of the β-C-H bonds. The DFT calculations of the metal-carbon bond strengths in these Rh-CH2X derivatives with α-substitution show a trend that is in good agreement with the experimental results.

β-H Abstraction/1,3-CH Bond Addition as a Mechanism for the Activation of CH Bonds at Early Transition Metal Centers

“…The reaction of 1 with furan has been investigated in detail. The disappearance of 1 was monitored (with adamantane as an internal standard) by 1 H NMR spectroscopy in C 6 Overall, the kinetic results are consistent with a ratedetermining intramolecular loss of cyclopropane from 1 by β-H abstraction that generates a reactive unsaturated intermediate A, which then activates rapidly and selectively an α-CH bond of furan to give 3 in a 1,3-stereospecific manner (Scheme 6). As mentioned above, this mechanistic scheme for CH bond formation/activation has rarely been observed when an alkene intermediate is involved.…”

The Effects of Ancillary Ligands on Metal–Carbon Bond Strengths as Determined by C–H Activation

“…In each case, exclusive activation of the methyl C-H bond was observed, giving products of the type Tp 0 Rh(CNR)(CH 2 X)H (X ¼ m-xylyl, 2-propenyl, OMe, O t Bu, CN, Cl, F, CF 3 , CCMe, or C(¼O)Me, Eq. 5) [14]. Difluoromethane also underwent clean oxidative addition of the C-H bond, but trifluoromethane proved unreactive, perhaps due to steric hindrance from the fluorines.…”

“…The vertical separation of the lines at D C-H ¼ 100 kcal mol À1 is 7.5 kcal mol À1 . Reproduced with permission of the ACS from Jiao et al [14] The )]. Using 5, many hydrocarbon and substituted methyl products could be prepared (Scheme 3) [18].…”

“…The second trend seen is in the methyl-substituted derivatives Rh-CH 2 X. Again, a parallel line is observed with a slope of 1.71 (8), which compares to the slope seen with L ¼ CNneopentyl of 1.40 (14). Terminal C-H bond strengths in italics for alkynes were calculated using DFT; B3LYP/6-31g** a Energies are in kcal mol…”

“…Reproduced with permission of the ACS from Jiao et al [14] 3 . n-hexyl, p-MeOC 6 H 4 , CF 3 , Ph, p-CF 3 C 6 H 4 ).…”