pubs.acs.org/Organometallics Published on Web 04/07/2010 r 2010 American Chemical Society 2026 Organometallics 2010, 29, 2026–2033 DOI: 10.1021/om900881x Carbon-Oxygen Bond Forming Mechanisms in Rhenium Oxo-Alkyl Complexes Mu-Jeng Cheng, Robert J. Nielsen,* Ma˚rten Ahlquist, and William A. Goddard, III Materials and Process Simulation Center (139-74), California Institute of Technology, Pasadena, California 91125 Received October 9, 2009 Three C-X bond formation mechanisms observed in the oxidation of (HBpz 3 )ReO(R)(OTf) [HBpz 3 =hydrotris(1-pyrazolyl)borate; R=Me, Et, and iPr; OTf=OSO 2 CF 3 ] by dimethyl sulfoxide (DMSO) were investigated using quantum mechanics (M06//B3LYP DFT) combined with solvation (using the PBF Poisson-Boltzmann polarizable continuum solvent model). For R=Et we find the alkyl group is activated through R-hydrogen abstraction by external base OTf - with a free energy barrier of only 12.0 kcal/mol, leading to formation of acetaldehyde. Alternatively, ethyl migration across the MdO bond (leading to the formation of acetaldehyde and ethanol) poses a free energy barrier of 22.1 kcal/mol, and the previously proposed R-hydrogen transfer to oxo (a 2þ2 forbidden reaction) poses a barrier of 44.9 kcal/mol. The rate-determining step to formation of the final product acetaldehyde is an oxygen atom transfer from DMSO to the ethylidene, with a free energy barrier of 15.3 kcal/mol. When R=iPr, the alkyl 1,2-migration pathway becomes the more favorable pathway (both kinetically and thermodynamically), with a free energy barrier (ΔG q = 11.8 kcal/mol) lower than R-hydrogen abstraction by OTf - (ΔG q =13.5 kcal/mol). This suggests the feasibility of utilizing this type of migration to functionalize M-R to M-OR. We also considered the nucleophilic attack of water and ammonia on the Re-ethylidene R-carbon as a means of recovering two-electron-oxidized products from an alkane oxidation. Nucleophilic attack (with internal deprotonation of the nucleophile) is exothermic. However, the subsequent protonolysis of the Re-alkyl bond (to liberate an alcohol or amine) poses a barrier of 37.0 or 42.4 kcal/mol, respectively. Where comparisons are possible, calculated free energies agree very well with experimental measurements. 1. Introduction Our interest in the functionalization of alkyl groups generated through activation of alkanes by less electronega- tive metals led us to study the free energy surfaces of the carbon-heteroatom bond forming mechanisms observed by Mayer et al. 1,2 Other than reductive elimination and nucleo- philic attack (typical of later, electrophilic transition metals), there are few known radical-free, highly selective mecha- nisms for the conversion of metal alkyls M-R to metal alkoxides M-OR. 1,3-5 Four mechanisms for C-X bond formation were observed by Mayer et al. in the oxidations of (HBpz 3 )ReO(R)(OTf) [HBpz 3 = hydrotris(1-pyrazolyl)bo- rate; R = Ph, Et; OTf = OSO 2 CF 3 ]: (1) aryl 1,2-migration across a metal-oxo bond (X=O); 1 (2) nucleophilic attack on an alkylidene carbon (X = N or S); 2 (3) oxidation of an alkylidene carbon (X = O); 2 and (4) alkylidene coupling to form olefins (X=C). 2 The first three are considered in this work. The first thermal aryl 1,2-migration across a metal -oxo bond was reported by Brown and Mayer in the oxidation of (HBpz 3 )ReO(Ph)(OTf) by oxygen atom donors dimethyl sulf- oxide (DMSO) or pyridine N-oxide (pyO) (eq 1). 1 Mechanistic studies showed that the rhenium(V) species (HBpz 3 )ReO(Ph)- (OTf) is first oxidized to a more reactive rhenium(VII) dioxo intermediate, [(HBpz 3 )ReO 2 (Ph)]OTf, which is then converted to phenoxide complexes [(HBpz 3 )ReO(OPh)(py)]OTf, (HB- pz 3 )ReO(OPh)(Cl), and (HBpz 3 )ReO(O 2 C 6 H 4 ), through a 1,2- migration of the phenyl group. This migration suggests a new avenue for functionalizing alkyls to alkoxides. However, when phenyl is replaced by an ethyl group, the oxidation leads to more complicated reaction mechanisms, *Corresponding author. E-mail: smith@wag.caltech.edu. (1) Brown, S. N.; Mayer, J. M. J. Am. Chem. Soc. 1996, 118, 12119. (2) DuMez, D. D.; Mayer, J. M. J. Am. Chem. Soc. 1996, 118, 12416. (3) Conley, B. L.; Ganesh, S. K.; Gonzales, J. M.; Ess, D. H.; Nielsen, R. J.; Ziatdinov, V. R.; Oxgaard, J.; Goddard, W. A.; Periana, R. A. Angew. Chem., Int. Ed. 2008, 47, 7849. (4) Conley, B. L.; Ganesh, S. K.; Gonzales, J. M.; Tenn, W. J.; Young, K. J. H.; Oxgaard, J.; Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2006, 128, 9018. (5) Tenn, W. J.; Conley, B. L.; Hovelmann, C. H.; Ahlquist, M.; Nielsen, R. J.; Ess, D. H.; Oxgaard, J.; Bischof, S. M.; Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2009, 131, 2466.