Origins of Regioselectivity and Alkene-Directing Effects in Nickel-Catalyzed Reductive Couplings of Alkynes and Aldehydes

Abstract

Origins of regioselectivity and alkene-directing effects in nickel-catalyzed reductive coupling reactions were investigated by density functional calculations. Regioselectivity of simple alkynes is controlled by steric effects, while conjugated enynes and diynes are predicted to have increased reactivity and very high regioselectivities, placing alkenyl or alkynyl groups distal to the forming C–C bond. The reactions of enynes and diynes involve 1,4-attack on the conjugated enyne of the Ni-carbonyl complex on the conjugated enyne or diyne.

PtI2-catalyzed cyclization of 3-acyloxy-1,5-enynes with the elimination of HOAc and a benzyl shift: synthesis of unsymmetrical m-terphenyls

“…The 2.0 kcal mol −1 difference between the activation barriers in the regioisomeric cyclopropanation and the ring expansion of TSs (TS6a and TS9a) suggests a regioselective ratio greater than 20 : 1 (m-terphenylo-terphenyl). 24 This computed result is consistent with experimental results. DFT computation for the energy surface in the proposed pathways for the PtI 2 -catalyzed cyclization of ACE b, which bears a Me group at the C1-position, was also carried out (see Scheme S2 †).…”

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

“…With respect to regioselectivity, in 2010, Houk and co-workers showed that the selectivity of simple alkenes is controlled by steric effects, 313 whereas for conjugated diynes or eneynes, the calculations (at the B3LYP/6-31G(d)(LANL2DZ) level of theory) suggested that the observed regioselectivity may originate from a mechanism involving a 1,4-attack pathway that is stabilized by a π-Ni interaction and has less distortion in the transition state (relative to other possible mechanism).…”

A General Strategy for Regiocontrol in Nickel-Catalyzed Reductive Couplings of Aldehydes and Alkynes

“…However, the use of ligand 4 (SIPr) reverses selectivity, favoring regioisomer 8 with 81:19 regioselectivity. We next examined a conjugated enyne, knowing that this substrate class is one of the most biased alkyne classes in reductive couplings 3d–f,6. Standard coupling with ligand 3b (IMes) provided highly selective coupling at the aliphatic substituted alkyne terminus to produce isomer 7 as anticipated (entry 6).…”

“…Previous results from our lab illustrated that regioselectivities may be moderately impacted by ligand structure, but the effects were too small to be broadly useful 5. A recent computational study described the minimal impact that ligand structure has on regioselectivity in aldehyde-alkyne reductive couplings with Ni(0)-phosphine catalysts and organoborane reducing agents,6 thus highlighting the complexity of designing a ligand-controlled regioselective process. Herein, we describe that carefully selected carbene ligands complexed with nickel provide a general solution to regiocontrol in silane-mediated aldehyde-alkyne reductive couplings with a broad range of alkynes.…”