N-heterocyclic carbene boryl radicals: a new class of boron-centered radical

Synthesis and reactions of N-heterocyclic carbene boranes

Boranes are widely used Lewis acids and N-heterocyclic carbenes (NHCs) are popular Lewis bases, so it is remarkable how little was known about their derived complexes until recently. NHC-boranes are typically readily accessible and many are so stable that they can be treated like organic compounds rather than complexes. They do not exhibit "borane chemistry", but instead are proving to have a rich chemistry of their own as reactants, as reagents, as initiators, and as catalysts. They have significant potential for use in organic synthesis and in polymer chemistry. They can be used to easily make unusual complexes with a broad spectrum of functional groups not usually seen in organoboron chemistry. Many of their reactions occur through new classes of reactive intermediates including borenium cations, boryl radicals, and even boryl anions. This Review provides comprehensive coverage of the synthesis, characterization, and reactions of NHC-boranes.

New boryl radicals derived from N-heteroaryl boranes: generation and reactivity

Recently, boryl radicals have been the subject of revived interest. These structures were generated by hydrogen-abstraction reactions from the corresponding boranes (i.e., from amine or phosphine boranes). However, the classical issue remains their high B--H bond-dissociation energy (BDE), thereby preventing a very efficient hydrogen-abstraction process. In the present paper, new N-heteroaryl boranes that exhibiting low B--H BDE are presented; excellent hydrogen-transfer properties have been found. Both the generation and the reactivity of the associated boryl radicals have been investigated through their direct observation in laser flash photolysis. The boryl radical interactions with double bonds, oxygen, oxidizing agent, and alkyl halides have been studied. Some selected applications of N-heteroaryl boryl radicals as new polymerization-initiating structures are proposed to evidence their high intrinsic reactivity.

Substitution reactions at tetracoordinate boron: synthesis of N-heterocyclic carbene boranes with boron-heteroatom bonds

Boryl halide, carboxylate and sulfonate complexes of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (dipp-Imd-BH(2)X, X = halide or sulfonate) have been prepared from the parent borane dipp-Imd-BH(3) by (1) substitution reactions with R-X (X = halide or sulfonate), (2) reactions with electrophiles (like I(2) or NIS), or (3) acid/base reactions with HX (provided that HX has a pK(a) of about 2 or less). Dipp-Imd-BH(2)I is most conveniently prepared by reaction with diiodine while dipp-Imd-BH(2)OTf is best prepared by reaction with triflic acid. These and other less reactive complexes behave as electrophiles and can be substituted by a wide range of heteroatom nucleophiles including halides, thiolates and other sulfur-based nucleophiles, isocyanate, azide, nitrite, and cyanide. The resulting products are remarkably stable, and many have been characterized by X-ray crystallography. Several are members of very rare classes of functionalized boron compounds (boron azide, nitro compound, nitrous ester, etc.).

Stable singlet carbenes as mimics for transition metal centers

This perspective summarizes recent results, which demonstrate that stable carbenes can activate small molecules (CO, H(2), NH(3) and P(4)) and stabilize highly reactive intermediates (main group elements in the zero oxidation state and paramagnetic species). These two tasks were previously exclusive for transition metal complexes.

Hydrogen atom donors: recent developments

This review highlights recent developments in the field of hydrogen atom transfer (HAT) reagents that circumvent the disadvantages of classical group 14 reagents, such as Bu₃SnH. Special emphasis is laid on the lowering of bond dissociation energies (BDEs) of molecules that could, as yet, not be used as HAT reagents and on the use of organometallic HAT reagents.

Radical deoxygenation of xanthates and related functional groups with new minimalist N-heterocyclic carbene boranes

Minimalist N-heterocyclic carbene boranes 1,3-dimethylimidazol-2-ylideneborane and 2,4-dimethyl-1,2,4-triazol-3-ylideneborane are readily available and have low molecular weights. They exhibit superior performance to first-generation NHC-boranes, providing improved yields in reductions of xanthates and related functional groups.

CAAC Boranes. Synthesis and characterization of cyclic (alkyl) (amino) carbene borane complexes from BF₃ and BH₃

In situ formation of two cyclic (alkyl) (amino) carbenes (CAACs) followed by addition of BF₃•Et₂O provided the first two examples of CAAC–BF₃ complexes: 1-(2,6-diisopropylphenyl)-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene trifluoroborane, and 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene trifluoroborane. These CAAC–BF₃ complexes are robust compounds that are stable to ambient laboratory conditions and silica gel chromatography. They were characterized by spectroscopy and X-ray crystallography. In contrast, a CAAC complex with borane (BH₃) was readily formed in situ according to ¹H and ¹¹B NMR analysis, but did not survive the workup conditions. These results set the stage for further studies of the chemistry of CAAC boranes.

Radical stability and its role in synthesis and catalysis

The thermodynamic stability of carbon-centered radicals may be defined in quantitative terms using the hydrogen transfer reaction shown in . The stability values obtained in this way for substituted systems may be understood as the stabilizing or destabilizing influence of substituents on the neighboring radical center. This approach can be easily adapted to oxygen- or sulfur-centered radicals as expressed in eqn (b). [Formula: see text] The stability values obtained in this way do not only serve as a quantitative basis for the discussion of substituent effects, but also allow for quantitative estimates of reaction energies for hydrogen transfer reactions. These occur as key steps in a multitude of synthetically useful radical-chain processes in apolar solution, in enzyme-mediated non-chain processes in biological systems, and in the oxidative degradation of a variety of biomolecules. The review will highlight the usefulness of radical stability values for the rationalization of successful (and not so successful) synthetic radical reactions as well as the potential design of new radical reactions.