An ab Initio MO Study on the Transformation of Acetylene to Vinylidene in the Coordination Sphere of Rhodium(I). The Intra- and Intermolecular Proton Transfer Mechanism

Abstract

The transformation of a rhodium(I) η 2 -alkyne model complex RhCl(PH 3 ) 2 (HCtCH) (A) into the vinylidene form RhCl(PH 3 ) 2 (CdCH 2 ) (E) has been examined by ab initio theoretical calculations using MP2 level geometry optimizations and localized molecular orbital (LMO) analysis. The vinylidene form E has been found to be 7.8 kcal/mol more stable than A. The previously found intraligand 1,2-hydrogen shift mechanism in the Ru(II)-coordinated alkyne-vinylidene isomerization is not relevant for the present Rh system. The reaction proceeds via the oxidative addition product RhCl(PH 3 ) 2 (H)(CtCH) (C), followed by a bimolecular hydrogen shift from the metal to the terminal carbon of a second molecule rather than by intramolecular 1,3-hydrogen transfer. The LMO analysis of the transition state of the unimolecular 1,3-hydrogen shift indicates that the hydrogen moves as a proton while it interacts with the three centers simultaneously, i.e., Rh, CR, and C in the transition state. The hydrogen was analyzed to migrate also as a proton in the bimolecular mechanism. The barrier of the bimolecular pathway has been further calculated for a more realistic system with substituted phosphines, RhCl(P i Pr 3 ) 2 (H)(CtCH), using the integrated MO + MM (MP2:MM3) method. It was concluded that in the real system with substituents on both the phosphines and the alkyne, RhCl(P i Pr 3 ) 2 (HCtCR), the bimolecular hydrogen shift is still favored by ca. 15 kcal/mol in free energy of activation; unimolecular 1,3-H migration should become important in special cases like solid state isomerizations.

Mono‐ and Dinuclear Rhodium and Iridium Complexes with Chiral Phospholanes as Ligands

“…[20,21] With regard to the course of the conversion of 25 to 26, we note that owing to recent ab inito MO calculations both a mononuclear as well as a bimolecular mechanism seems possible. [22] In contrast to the related ruthenium system, [23] the alkynyl(hydrido)rhodium() species lies in a relative energy minimum, which is in agreement with the results presented in Scheme 6.…”

Metal Vinylidenes as Catalytic Species in Organic Reactions

“…In the absence of an available hydrogen atom for the [1,5] shift, the reaction can follow a different course. This is seen in the following example, in which enediyne 245 was converted to silacycle 246 in moderate yield (Scheme 93).…”

Synthesis, Molecular Structure, and CC Coupling Reactions of Carbeneruthenium(II) Complexes with C5H5Ru(CRR′) and C5Me5Ru(CRR′) as Molecular Units

“…[26] As an alternative to path a, the alkyne compound A could be protonated to give D, which, via E and the allene(hydrido)-metal intermediate F, could be converted to 19 (Scheme 3 path b). Precedence for the metal-assisted conversion of alkynes (not only propyne) to allenes stems from previous studies by Richards et al [27] on six-coordinate rhenium(i) compounds, as well as from our own work on square-planar and halfsandwich-type rhodium(i) and iridium(i) complexes, respectively.…”