A DFT Study of the Effect of the Ligands in the Reductive Elimination from Palladium Bis(allyl) Complexes

Abstract

The effect of some selected ligands (L = empty, PMe3, ethylene, maleic anhydride) in the reductive elimination of the palladium complexes cis-[Pd(η1-allyl)(η1-allyl)(PMe3)L] and Pd(η1-allyl)(η3-allyl)L to form hexa-1,5-diene was computationally studied using DFT methods. Among the various possible coupling processes (C1sp3 −C1′sp3 , C3sp2 −C3′sp2 , and C1sp3 −C3′sp2 ), C3−C3′ bond formation is the most favored in all cases, as reported before for cis-[Pd(η1-allyl)(η1-allyl)(PH3)2]. Interestingly, the activation energy for this coupling changes with the L ligand: empty (4.6 kcal/mol) < MA (5.8 kcal/mol) < CH2CH2 (12.5 kcal/mol) < PMe3 (17.3 kcal/mol). Therefore, tricoordinated Pd(η1-allyl)(η1-allyl)L complexes undergo reductive elimination at higher rates than the tetracoordinated counterparts. The order L = empty < L = MA is inverse to that found for carbon ligands (alkyl, aryl, alkenyl) that couple via direct C1−C1′ reductive elimination; the order L = empty < L = MA is also followed by the allyl groups when the disfavored C1−C1′ bond formation is considered. Structural analysis reveals that the C3−C3′ reductive elimination of cis-[Pd(η1-allyl)2(PMe3)] is particularly favored by the small distortion of the original T-shaped geometry in the transition state, which preserves the hyperconjugative dσ(C1−Pd)→π*(C2C3)-type interaction between the metal and the allyl substituents. The activation energies for the elimination of allyl groups are intermediate between those of alkenes/arenes and alkyls when palladium complexes with similar composition cis-[Pd(R)(R)(PMe3)L] are compared. Although the effect is more moderate than in other carbon substituents, π-acceptor ligands (MA) and to a lesser extent olefins (exogenous or the same substrates/products) are efficient additives in this coupling, an electronic effect that is conveyed to the distant C3 and C3′ atoms by the entire interacting system. Consistent with this proposal, the transition state for the C3sp2 −C3′sp2 reductive elimination in cis-[Pd(η1-allyl)(η1-allyl)(PMe3)L] shares both structural and electronic features with a pericyclic (homo)Cope rearrangement.

Branched/linear selectivity in palladium-catalyzed allyl-allyl cross-couplings: the role of ligands

“…As a result, we considered potential pathways for bond-formation from this complex (Figure 2). In line with the studies by Espinet 5c , barriers for diene formation from this three-coordinate system were found to be lower overall than those found for the bis-ligated system (Figure 1). From GS C , 3,3′-reductive elimination to give the branched compound 3 ( TS D , Figure 2) was found to have a barrier of only 8.4 kcal/mol.…”

“…2 Recently, our group has disclosed a number of methods that employ chiral, small-bite-angle bidentate phosphine ligands to reverse the regioselectivity of these couplings to give branched 1,5-dienes of type C with excellent regio- and enantioselectivity (eq 2). 3 Recent mechanistic and computational studies 4 have supported previous proposals 3 that the reaction is operative through an inner-sphere 3,3′-reductive elimination 5 , in which coupling at carbons 3 and 3′ through the conformation as shown in B leads to the branched isomer. These studies are in line with computational studies on the preferred reductive elimination pathways for unsubstituted allylic systems by Eschavarren 5a and Espinet 5c , which also found the 3,3′-reductive elimination to be preferred over direct coupling at the 1 and 1′ carbons.…”

“…8 These studies considered possible pathways leading to linear and branched isomers of phenyl-substituted 1,5-diene products of type 3 and 4 (Table 1), and began from the bis-ligated, bis(η 1 -allyl) complex GS A (Figure 1). Based on previous studies, 5a,c the two lowest energy routes for bond formation from GS A should be through coupling at the either 3 and 3′ ( TS A ) or 1 and 1′ carbons ( TS B ). The 3,3′-pathway to give the branched isomer 3 ( TS A ) was calculated to have an activation free energy barrier of 14.6 kcal/mol.…”

Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: An Overview of Commonly Employed DFT Methods and Mechanistic Insights

“…Although these studies utilized more classical methods that did not account well for dispersion, the results were in line with experiments. 152 The absolute barriers may likely change if dispersion is considered, but this frequently does not change the qualitative reactivity picture.…”

Synthesis of axially chiral biaryl compounds by asymmetric catalytic reactions with transition metals

“…They can be described by three successive main steps: oxidative addition [56,57,58,59], transmetalation [60,61,62,63] and reductive elimination [64,65,66] (the catalytic cycle involving the neutral intermediate is represented in Scheme 3.2). The starting point of both cycles is a Pd(0) species, suggesting that when a palladium(II) precatalyst is used, its reduction should occur first, as the oxidative addition of an aryl halide to a Pd(II) compound is not favored.…”