Mechanism of halide exchange in reactions of CpRu(PPh₃)₂Cl with haloalkanes

Abstract

Kinetic measurements of the reaction between CpRu(PPh3)2Cl (1a) and 1-bromobutane reveal a nearly first order dependance on the concentration of haloalkane and a negative entropy of activation, ∆S†<0. The rate...

Kinetics and mechanism of halide exchange in reactions of CpRu(PPh3)2Cl with alkyl halides: evidence for radical pairs

“…There is also no spectroscopic evidence for ATRA of acetyl bromide to styrene using either 1 or 2 and AIBN as catalysts. These observations run counter to evidence for a radical mechanism between 1 and CH 3 COBr based on Kohn-Sham density functional theory (DFT) and the effect of radical traps or initiators on t 1/2 for halide exchange in benzene-d 6 . 11 The observation of ATRA reactions for CBr 4 , CHBr 3 , CBr 3 CO 2 Et and CHBr 2 CO 2 Et with styrene suggests a role for radical pairs as shown in Fig.…”

“…We have argued that the absence of ATRA reactivity of styrene with n C 4 H 9 Br and the absence of any significant effect of radical initiators supports a mechanism where either oxidative addition or s-bond metathesis is more likely than a mechanism involving radical pairs. 6 The mechanism of halide exchange is, therefore, substrate dependent suggesting that the success of ruthenium catalyzed ATRA reactions are similarly affected by the choice of haloalkane.…”

“…The ATRA reactions are all accompanied by halide exchange. 31 P NMR spectroscopy of reaction mixtures containing 1 and 10 equivalents of CBr 4 in fluorobenzene/10% v/v d 6 -benzene reveal complete conversion to 3 after 12.5 hours at 35 1C (Fig. S6, ESI †).…”

“…The choice of solvent is based on the solubility of 1 and 3 in C 6 H 5 F and to allow for comparison with data from our earlier studies using bromobutane. Benzene-d 6 is added to the solutions to provide a lock signal for the spectrometer. The chemical shifts for the product, CpRu(PPh 3 ) 2 Br (3, 38.1 ppm) is well resolved from the resonance for CpRu(PPh 3 ) 2 Cl, (1, 39.3 ppm).…”

“…Reexamination of the data for the reaction between 1 and bromobutane suggests a similar rate law for halide exchange. 6 Efforts to study the effect of added PPh 3 on the reaction rate were stymied by precipitation of white solids when PPh 3 is added to solutions of PPh 3 and CBr 4 , CHBr 2 CO 2 Et or CBr 3 CO 2 Et in fluorobenzene/10% C 6 D 6 in the presence or absence of 1. The formation of phosphonium ions,[PPh 3 R] + [Br] À , means that all that is accomplished is to remove RBr from solution.…”