On the Reactivity of a NHC Nickel Bis-Boryl Complex: Reductive Elimination and Formation of Mono-Boryl Complexes.

The synthesis of the first terminal mono-boryl complexes of nickel, which are not stabilized by a pincer ligand, is reported. The reaction of the nickel bis-boryl complex cis-[Ni(i Pr2 ImMe )2 (Bcat)2 ] 1 (cat=1,2-O2 C6 H4 ) with the small donor ligand PMe3 led to a complete ligand exchange at nickel with reductive elimination of B2 cat2 and formation of the bis-NHC adduct [B2 cat2 ⋅ (i Pr2 ImMe )2 ] 3 and [Ni(PMe3 )4 ] 2 as the metal-containing species. Electrophilic attack of MeI on complex 1 or ligand dismutation of 1 with trans-[Ni(i Pr2 ImMe )2 Br2 ] led to loss of only one boryl ligand of 1 and afforded the nickel mono-boryl complexes trans-[Ni(i Pr2 ImMe )2 (Bcat)Br] 4 a and trans-[Ni(i Pr2 ImMe )2 (Bcat)I] 4 b.

NHC induced radical formation via homolytic cleavage of B-B bonds and its role in organic reactions.

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings via the homolytic cleavage of the B-B bond. We report herein that bis-NHC adducts of the type (NHC)2·B2(OR)4, which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC-BR2˙, exemplified by Me2ImMe·Bneop˙ 1a (Me2ImMe = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B-B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed. However, based on these findings, a protocol was developed using Me2ImMe·B2pin23 for the transition metal- and additive-free boryl transfer to substituted aryl iodides and bromides giving aryl boronate esters in good yields. Analysis of the side products and further studies concerning the reaction mechanism revealed that radicals are likely involved. An aryl radical was trapped by TEMPO, an EPR resonance, which was suggestive of a boron-based radical, was detected in situ, and running the reaction in styrene led to the formation of polystyrene. The isolation of a boronium cation side product, [(Me2ImMe)2·Bpin]+I-7, demonstrated the fate of the second boryl moiety of B2pin2. Interestingly, Me2ImMe NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me2ImMe·B2pin23 to aryl iodides and bromides is proposed based on these experimental observations.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes.

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

FBpin and its adducts and their role in catalytic borylations.

We present herein investigations concerning the reactivity of B2pin2 and FBpin (pin = pinacolato) with respect to fluoride ions and NHCs (NHC = N-Heterocyclic Carbene) in order to understand better the role of added fluoride in our recently reported Ni-catalyzed C-F borylation process, to confirm the nature of the NHC-adducts of FBpin, and to explain the formation of the species detected as side products of the defluoroborylation reaction. We report the calculated gas phase fluoride ion affinities (FIA) of relevant boron species and demonstrate that the presence of one equivalent of added F- in solution will bind and activate B2pin2 (FIA = +247 kJ mol-1), but it will also bind even more effectively to FBpin (FIA = +287 kJ mol-1) formed in the reaction. We prepared the NHC-adducts of FBpin by two different methods and isolated and characterized several examples of the type FBpin·NHC (NHC = iPr2Im, 1; Me2Im, 2; MeiPrIm, 3; nPr2Im, 4; Mes2Im 5). The salts [pinBF2][NMe4] 6, as well as the significantly more soluble analogue [pinBF2][NnBu4] 7, were also isolated and characterized, providing confirmation of the presence of the pinBF2 anion in various stoichiometric and catalytic borylation reactions.

Copper-catalysed borylation of aryl chlorides.

We report herein the first Cu-catalysed borylation of a wide range of aryl chlorides with different electronic and steric properties using a readily prepared NHC-stabilised Cu catalyst and KOtBu as the base with B2pin2 (pin = pinacolato) as the boron reagent. The aryl chlorides are converted into their corresponding arylboronic esters in good yields. The new procedure shows broad functional group tolerance, and B2neop2 (neop = neopentyl glycolato) can also be applied as the boron reagent.

Stoichiometric and Catalytic Aryl-Cl Activation and Borylation using NHC-stabilized Nickel(0) Complexes.

NHC-nickel (NHC=N-heterocyclic carbene) complexes are efficient catalysts for the C-Cl bond borylation of aryl chlorides using NaOAc as a base and B2 pin2 (pin=pinacolato) as the boron source. The catalysts [Ni2 (ICy)4 (µ-(η2 :η2 )-COD)] (1, ICy=1,3-dicyclohexylimidazolin-2-ylidene; COD=1,5-cyclooctadiene), [Ni(ICy)2 (η2 -C2 H4 )] (2), and [Ni(ICy)2 (η2 -COE)] (3, COE=cyclooctene) compare well with other nickel catalysts reported previously for aryl-chloride borylation with the advantage that no further ligands had to be added to the reaction. Borylation also proceeded with B2 neop2 (neop=neopentylglycolato) as the boron source. Stoichiometric oxidative addition of different aryl chlorides to complex 1 was highly selective affording trans-[Ni(ICy)2 (Cl)(Ar)] (Ar=4-(F3 C)C6 H4 , 11; 4-(MeO)C6 H4 , 12; C6 H5 , 13; 3,5-F2 C6 H3 , 14).

Facile insertion of a cyclic alkyl(amino) carbene carbon into the B-B bond of diboron(4) reagents.

We report herein the room temperature insertion of the carbene carbon atom of the cyclic (alkyl)(amino) carbene cAACMe into the B-B single bonds of the diboron(4) compounds B2pin2, B2cat2, B2neop2, and B2eg2 (pin = pinacolato, cat = catecholato, neop = neopentylglycolato, eg = ethyleneglycolato).