Recent years have seen rapid advances in the chemistry of small molecules containing electron‐precise boron–boron bonds. This Review provides an overview of the latest methods for the controlled ...synthesis of B−B single and multiple bonds as well as the ever‐expanding range of reactivity displayed by B−B multiple bonds.
B, B bonding: The chemistry of boron–boron bond containing compounds has seen rapid advances in recent years. This Review provides a comprehensive overview of the historical background, the increasingly varied and selective methods for the synthesis of electron‐precise B–B single and multiple bonds, and their ever‐expanding range of reactivity.
The coupling of two or more molecules of dinitrogen (N
) occurs naturally under the radiative conditions present in the ionosphere and may be achieved synthetically under ultrahigh pressure or plasma ...conditions. However, the comparatively low N-N single-bond enthalpy generally renders the catenation of the strongly triple-bonded N
diatomic unfavorable and the decomposition of nitrogen chains a common reaction motif. Here, we report the surprising organoboron-mediated catenation of two N
molecules under near-ambient conditions to form a complex in which a N
chain bridges two boron centers. The reaction entails reductive coupling of two hypovalent-boron-bound N
units in a single step. Both this complex and a derivative protonated at both ends of the chain were characterized crystallographically.
Magnesium Catalysis of Imine Hydroboration Arrowsmith, Merle; Hill, Michael S.; Kociok-Köhn, Gabriele
Chemistry : a European journal,
February 18, 2013, Volume:
19, Issue:
8
Journal Article
Peer reviewed
The β‐diketiminato magnesium alkyl complex LMgnBu (L=CH{CMe(NDipp)}2, Dipp=diisopropylphenyl) is shown to be a highly effective precatalyst for the hydroboration of alkyl and aryl substituted ...aldimines and ketimines with pinacol borane (HBpin). Catalysis is proposed to occur through a sequence of MgN/BH metathesis and rate‐determining MgH/NC insertion steps, a proposal strongly supported by stoichiometric studies and kinetic analysis. The reactions are observed to proceed through the intermediacy of well‐defined magnesium amides, two examples of which have been isolated and structurally characterized. Mechanistic investigations suggest that the catalytic rate‐determining process occurs at an isolated magnesium center and requires the presence of two molecules of the imine substrate for effective turnover. This latter observation is rationalized as a requirement for the secondary substrate molecule to displace HBpin from the coordination sphere of the catalytic magnesium center.
Double‐dipping catalyst: The heteroleptic magnesium alkyl complex CH{C(Me)NDipp}2MgnBu (Dipp=2,6‐iPr2C6H3) is reported as a highly efficient precatalyst for the hydroboration of aldimines and ketamines with pinacolborane (see scheme).
Room temperature hydrogenation of an SIDep‐stabilized diboryne (SIDep=1,3‐bis(diethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene) and a cAAC‐supported diboracumulene ...(cAAC=1‐(2,6‐diisopropylphenyl)‐3,3,5,5‐tetramethylpyrrolidin‐2‐ylidene) provided the first selective route to the corresponding 1,2‐dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG=19.4 kcal mol−1) two‐step asynchronous H2 addition mechanism proceeding via a bridging hydride.
No catalyst needed: Two new carbene‐stabilized 1,2‐dihydrodiborenes have been synthesized by the direct and selective monohydrogenation of boron–boron multiple bonds under mild conditions. Theoretical calculations show a two‐step asynchronous H2 addition mechanism proceeding via a bridging hydride.
A stable three-coordinate dimethylberyllium species coordinated by the 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) ligand is readily converted to the corresponding methylhydrido ...derivative through metathetical reaction with phenylsilane. Attempts to synthesize the corresponding molecular dihydrides are, however, unsuccessful and result in ring opening of an IMes ligand through hydride transfer to the donor carbon atom and the consequent formation of a heterocyclic beryllium organoamide. In agreement with previous calculations, we suggest that this process occurs via a Schlenk-type equilibration process and formation of a four-coordinate bis-NHC beryllium dihydride. These species are not observed, however, as the steric pressure exerted by coordination of the two sterically demanding IMes ligands is sufficient to induce hydride transfer. The latter deduction is supported by the observation that a similar ring-opened product, but derived from methyl and hydride transfer, is available through the introduction of a further equivalent of IMes to the isolated beryllium methyl hydride species. In the latter case the ring-opening process is more facile, which we ascribe to the increased steric pressure achieved upon the formation of four-coordinate beryllium. In a further striking reaction under more forcing thermal conditions, the carbene carbon center of an IMes ligand is observed to be completely eliminated with selective formation of a three-coordinate diamidoberyllium species.
Deprotonation of (cAAC)BH2(CN) provided clean access to the stable boryl anion, (cAAC)BH(CN)−. Whereas the addition of soft electrophiles occurred at the nucleophilic boron center, harder silyl ...electrophiles added to the harder terminal cyano nitrogen. The resulting (cAAC)BH(CNSiPh3) species behaved like a silylium boryl nucleophile as well as a neutral silylisonitrile borylene.
Whereas the salt metathesis of soft electrophiles with the (cyano)hydroboryl anion (cAAC)B(CN)H− occurs at the nucleophilic boron center, comparatively harder silyl electrophiles add to the harder terminal cyano nitrogen. The resulting (cAAC)BH(CNSiR3) species behave as both zwitterionic silylium boryl and neutral silylisonitrile borylene, a BI compound accessed through simple deprotonation and salt metathesis. cAAC=cyclic (alkyl)(amino)carbene.
2,3‐Bis(dimethylamino)‐substituted B2N2C2 heterocycles underwent selective dimethylamino/hydride exchange with either one or two equivalents of BH3⋅SMe2 to give the corresponding cyclic monohydrido‐ ...or (cis)1,2‐dihydridodiboranes(4), respectively. Upon either heating or irradiation in solution, the latter underwent ring contraction to the corresponding five‐membered BN2C2 heterocycles, whereas irradiation of the 1,2‐dimethylaminoethene‐supported 1,2‐dihydridodiborane(4) in the presence of PEt3 gave an unprecedented unsymmetrical 1,1‐dihydrodiborane(5) phosphine adduct.
Contract my ring! Selective amino– hydride exchange at boron gave the first monohydro‐ and cis‐1,2‐dihydiboranes(4), the latter undergoing photo induced isomerization to the corresponding 1,1‐dihydrodiborane (see scheme).
Reduction of carbene‐borane adduct (cAAC)BBr2(CN) (cAAC=1‐(2,6‐diisopropylphenyl)‐3,3,5,5‐tetramethylpyrrolidin‐2‐ylidene) cleanly yielded the tetra(cyanoborylene) species (cAAC)B(CN)4 presenting a ...12‐membered (BCN)4 ring. The analysis of the Kohn–Sham molecular orbitals showed significant borylene character of the BI atoms. (cAAC)B(CN)4 was found to reduce two equivalents of AgCN per boron center to yield (cAAC)B(CN)3 and fragmented into two‐coordinate boron(I) units upon reaction with IMeMe (1,3,4,5‐tetramethylimidazol‐2‐ylidene) to yield the corresponding tricoordinate mixed cAAC‐NHC cyanoborylene. The analogous cAAC‐phosphine cyanoborylene was obtained by reduction of (cAAC)BBr2(CN) in the presence of excess phosphine.
All square: Reduction of a carbene‐borane adduct provides the tetra(cyanoborylene) species (cAAC)B(CN)4 with a 12‐membered (BCN)4 ring. This species was found to reduce two equivalents of AgCN to yield (cAAC)B(CN)3 and fragmented into two‐coordinate boron(I) units upon reaction with an NHC.
Berylliant! Interaction of a well‐defined adduct of MeBeH and an N‐heterocyclic carbene (NHC) with PhSiH3 results in complete rupture of the heterocycle, and activation of the NHC through effective ...BeH2 insertion into a CN bond of the heterocycle (see scheme; Ar=2,6‐diisopropylphenyl, IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene).
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