DFT Studies on the Mechanism of the Diboration of Aldehydes Catalyzed by Copper(I) Boryl Complexes

Abstract

The detailed mechanism for the diboration of aldehydes catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition states. The results show that the catalyzed diboration occurs through aldehyde insertion into Cu-B to give a Cu-O-C(boryl) species followed by sigma-bond metathesis with a diboron reagent. It is the "electron-richness", that is, the nucleophilicity of the Cu-boryl bond, which gives rise to a small insertion barrier and determines the direction of insertion. The results of our calculations also explain the formation of the product, observed experimentally, from the stoichiometric reaction of (IPr)Cu-Bpin (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with mesitylaldehyde. In the absence of a diboron reagent, the insertion intermediate having a Cu-O-C(boryl) linkage isomerizes to the thermodynamically preferred Cu-C-O(boryl) isomer via a boryl migration to the metal-bonded oxygen through an S(E)2-like transition state. We have also studied the catalyzed diboration of 2-pyridinecarboxaldehyde, which gives the unexpected reductive coupling product 1,2-di-2-pyridyl-1,2-bis(pinacolboroxy)ethane. The insertion intermediate, which contains a coordinated pyridyl group, isomerizes easily to a 1,2-dihydropyridine form, preventing its metathesis with a diboron reagent to give the expected diboration product as observed for other aldehyde substrates.

Stereospecific Cross-Coupling of Secondary Organotrifluoroborates: Potassium 1-(Benzyloxy)alkyltrifluoroborates

“…The protocol for the previously reported diboration of aldehydes 17 and ketones 18 did not involve the addition of methanol. In the absence of methanol, intermediate 3c (Scheme 2) undergoes transmetalation with a molecule of bis(pinacolato)diboron in the rate determining step to generate the desired dibora compound 3d and regenerate the active catalyst 3a .…”

Homolytic Cleavage of a B−B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

“…Diborane compounds,R 2 B À BR 2 ,a re highly useful synthetic modules for concise synthesis of organoboronates, [1] which are versatile building blocks for the generation of new chemical bonds using the Suzuki-Miyaura crossing-coupling reactions. [3,4] On the other hand, transitionmetal-free catalytic systems for activating the BÀB bond of diborane compounds have also attracted much attention. [3,4] On the other hand, transitionmetal-free catalytic systems for activating the BÀB bond of diborane compounds have also attracted much attention.…”

Synthesis of Activated Alkenylboronates from Acetylenic Esters by CuH‐Catalyzed 1,2‐Addition/Transmetalation

“…SR in the presence of pinacolborane (HBpin) undergoes a color change from red to dark red or even brown, which suggests the possible formation of a new species, [(Ph 3 P)CuH]·HBpin, perhaps analogous to that proposed for ligated CuH in the presence of silanes. [16] Control experiments confirmed that net HB does not take place in the absence of CuH. Likewise, a mixture of alkynoate and pinacolborane did not give rise to HB to any noticeable extent, even after prolonged reaction times (several hours).…”