1,2-Azaborolyls, Isoelectronic Analogues of the Ubiquitous Cyclopentadienyl Ligand: Synthesis of B-Heteroatom-Substituted 1,2-Azaborolyl Complexes and an Assessment of Their Electronic Features

Abstract

The substituent on boron allows the electronic nature of the heterocycle of B‐heteroatom‐substituted (R=H, O, S, N, P, and F) 1,2‐azaborolyl complexes to be modulated. Given that 1,2‐azaborolyl is isoelectronic with cyclopentadienyl (see scheme), one of the most widely used ligands in organometallic chemistry, it seems likely that the diverse array of compounds synthesized in this study will stimulate the development of applications of η5‐(1,2‐azaborolyl) ligands in metal‐catalyzed processes.

Site-Selective and Stereoselective trans-Hydroboration of 1,3-Enynes Catalyzed by 1,4-Azaborine-Based Phosphine–Pd Complex

“…BN/CC isosterism, i.e., the replacement of a CC bond unit with the isoelectronic and isosteric BN bond unit, has recently emerged as a viable strategy to increase the structural diversity of organic molecules . While early applications have appeared in the area of materials science and biomedical research, efforts taking advantage of expanded chemical space provided by BN/CC isosterism in ligand-supported catalysis for organic synthesis have lagged behind. − This is surprising in view of tremendous opportunities for achieving new reactivity and selectivity in catalytic transformations that the electronic tuning through BN/CC isosterism could potentially provide. In a recent example, we disclosed that the 1,4-azaborine-based pyridine ligand A (Scheme ) exhibits a κ 2 N -η 2 -BC coordination with group 10 transition metals and that the phosphine derivative B in conjunction with Pd could uniquely catalyze the hydroboration of a terminal enyne in a trans -selective fashion .…”

Half‐Sandwich Metal‐Catalyzed Alkyne [2+2+2] Cycloadditions and the Slippage Span Model

“…The most studied organometallic half‐sandwich compounds for the [2+2+2] cycloadditions of alkynes are those containing the ubiquitous cyclopentadienyl anion (Cp, Scheme A) and the indenyl anion (Ind, Scheme B). Recently, the diheteroaromatic rings 1,2‐azaborolyl (Ab, Scheme C) and 3a,7a‐azaborindenyl (Abi, Scheme D) were also tested in silico in Rh I half‐sandwich catalysts . The idea was to design catalysts for the [2+2+2] cycloaddition of alkynes with enhanced metal slippage promoted by the low symmetry of these anions, which are isoelectronic to the parent hydrocarbons Cp and Ind, respectively.…”

1,3,2‐Diazaborolyl Anions – From Laboratory Curiosities to Versatile Reagents in Synthesis

“…In view of a host of cyclopentadienyl complexes, the quest for BN analogues such as π‐1,2‐azaborolyl metal compounds was intriguing and was eventually brought to success in the group of G. Schmid in the late 1970s and 1980s with a large variety of transition metal and main group systems featuring 1,2‐azaborolyl ligands (Scheme ) . Later contributions to this field were added by A. Ashe III, G. Fu, and Z. Hou …”