The Chemistry of Perfluoroaryl Boranes

Intramolecular pyridine-based frustrated Lewis-pairs

Besides a series of pyridine-based Lewis pairs with B(C₆F₅)₂ groups with varying electronic and steric situations and strong intramolecular B–N bonds, 2-[(CH₂)₄B(C₆F₅)₂]-6-tBu-pyridine was found to behave as a frustrated Lewis pair.

Electron transport and nonlinear optical properties of substituted aryldimesityl boranes: a DFT study

A comprehensive theoretical study was carried out on a series of aryldimesityl borane (DMB) derivatives using Density Functional theory. Optimized geometries and electronic parameters like electron affinity, reorganization energy, frontiers molecular contours, polarizability and hyperpolarizability have been calculated by employing B3PW91/6-311++G (d, p) level of theory. Our results show that the Hammett function and geometrical parameters correlates well with the reorganization energies and hyperpolarizability for the series of DMB derivatives studied in this work. The orbital energy study reveals that the electron releasing substituents increase the LUMO energies and electron withdrawing substituents decrease the LUMO energies, reflecting the electron transport character of aryldimesityl borane derivatives. From frontier molecular orbitals diagram it is evident that mesityl rings act as the donor, while the phenylene and Boron atom appear as acceptors in these systems. The calculated hyperpolarizability of secondary amine derivative of DMB is 40 times higher than DMB (1). The electronic excitation contributions to the hyperpolarizability studied by using TDDFT calculation shows that hyperpolarizability correlates well with dipole moment in ground and excited state and excitation energy in terms of the two-level model. Thus the results of these calculations can be helpful in designing the DMB derivatives for efficient electron transport and nonlinear optical material by appropriate substitution with electron releasing or withdrawing substituents on phenyl ring of DMB system.

Boron and other triel Lewis acid centers: from hypovalency to hypervalency

MP2/aug-cc-pVTZ calculations were performed on triel trifluorides, ZF3 (Z=B, Al, Ga, In, Tl), and their complexes with N2 and HCN species, which are acting as Lewis bases. Interaction of the ZF3 trigonal structure with a single HCN or N2 ligand may lead to a tetrahedral structure with the tetravalent triel atom, whereas interaction of ZF3 with two ligands results in a trigonal bipyramidal structure with a pentacoordinate Z atom. Consequently, the Z atom in ZF3 compounds suffers from electron deficiency (hypovalency), it obeys the octet rule in ZF3-NCH and ZF3-N2 moieties, and it may be considered as a hypervalent center in complexes of ZF3 with two ligands. Much weaker interactions are observed for the boron complexes than for the other triel systems. This is because of the lower acidity of the B center relative to that of the other triel centers, and it may be the result of a stronger backbonding effect for BF3 than for the other triel trifluorides. Interactions in aluminum complexes are characterized by meaningful electrostatic contributions, whereas for gallium complexes, the most important electron charge shift is observed as a result of complexation. Analysis of the geometry of the triel complexes and of the Z⋅⋅⋅N interactions (triel bonds) in these complexes is based on ab initio calculations as well as on the quantum theory of atoms in molecules and the natural bond orbitals method.

Applications of pentafluorophenyl boron reagents in the synthesis of heterocyclic and aromatic compounds

Recently, main group reagents have attracted a lot of attention in bond-forming reactions in organic synthesis. This article highlights the use of pentafluorophenyl substituted boron reagents in their reactions with C=C and C≡C π-bonds for the synthesis of heterocyclic and aromatic compounds. These cyclisation reactions fall into four general classes although there is some overlap between classes and often combinations of these different types of reactivity are observed in the formation of the final heterocyclic product: (i) 1,2- (and 1,4-) additions of nucleophile and Lewis-acidic boron centre, (ii) 1,1-carboboration, (iii) carbocation rearrangements and (iv) cycloaddition chemistry. In addition, the prospect of using such boron reagents catalytically in the synthesis of aromatic compounds such as oxazoles and dibenzopentalene derivatives is emphasised.

Activation of alkynes with B(C₆F₅)₃--boron allylation reagents derived from propargyl esters

Novel allyl boron compounds are readily synthesized via rearrangement reactions between Lewis acidic B(C6F5)3 and propargyl esters. These reactions proceed through an initial cyclization followed by ring-opening and concurrent C6F5-group migration. In the absence of disubstitution adjacent to the ester oxygen atom, an allyl boron migration rearrangement leads to formal 1,3-carboboration products. These allyl boron compounds act as allylation reagents with aldehydes introducing both a C3-allyl fragment and a C6F5-unit as a single anti-diastereomer. In these reactions, B(C6F5)3 activates the alkynes, prompting the rearrangement processes and enabling installations of C6F5 and R-groups.

B(C6F5)3 promoted cyclisation of internal propargyl esters: structural characterisation of 1,3-dioxolium compounds

Internal propargyl esters react with B(C₆F₅)₃ to give zwitterionic dioxolium heterocycles; the structures and mechanism of formation are considered in the light of DFT calculations.

Synthesis and structure of amido- and imido(pentafluorophenyl)borane zirconocene and hafnocene complexes: N-H and B-H activation

Treatment of Me(2)S·B(C(6)F(5))(n) H(3-n) (n=1 or 2) with ammonia yields the corresponding adducts. H(3)N·B(C(6)F(5))H(2) dimerises in the solid state through N-H···H-B dihydrogen interactions. The adducts can be deprotonated to give lithium amidoboranes Li[NH(2)B(C(6)F(5))(n)H(3-n)]. Reaction of the n=2 reagent with [Cp(2)ZrCl(2)] leads to disubstitution, but [Cp(2)Zr{NH(2)B(C(6)F(5))(2)H}(2)] is in equilibrium with the product of β-hydride elimination [Cp(2)Zr(H){NH(2)B(C(6)F(5))(2)H}], which proves to be the major isolated solid. The analogous reaction with [Cp(2)HfCl(2)] gives a mixture of [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)] and the N-H activation product [Cp(2)Hf{NHB(C(6)F(5 )(2)H}]. [Cp(2)Zr{NH(2)B(C(6)F(5))(2)H}(2)]·PhMe and [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)]·4(thf) exhibit β-B-agostic chelate bonding of one of the two amidoborane ligands in the solid state. The agostic hydride is invariably coordinated to the outside of the metallocene wedge. Exceptionally, [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)]⋅PhMe has a structure in which the two amidoborane ligands adopt an intermediate coordination mode, in which neither is definitively agostic. [Cp(2)Hf{NHB(C(6)F(5))(2)H}] has a formally dianionic imidoborane ligand chelating through an agostic interaction, but the bond-length distribution suggests a contribution from a zwitterionic amidoborane resonance structure. Treatment of the zwitterions [Cp(2)MMe(μ-Me)B(C(6)F(5))(3)] (M=Zr, Hf) with Li[NH(2)B(C(6)F(5))(n)H(3-n)] (n=2) results in [Cp(2) MMe{NH(2)B(C(6)F(5))(2)H}] complexes, for which the spectroscopic data, particularly (1)J(B,H), again suggest β-B-agostic interactions. The reactions proceed similarly for the structurally encumbered [Cp''(2)ZrMe(μ-Me)B(C(6)F(5))(3)] precursor (Cp''=1,3-C(5)H(3)(SiMe(3))(2) , n=1 or 2) to give [Cp''(2)ZrMe{NH(2)B(C(6)F(5))(n)H(3-n)}], both of which have been structurally characterised and show chelating, agostic amidoborane coordination. In contrast, the analogous hafnium chemistry leads to the recovery of [Cp''(2)HfMe(2)] and the formation of Li[HB(C(6)F(5))(3)] through hydride abstraction.

A highly Lewis acidic triarylborane bearing peripheral o-carborane cages

A triarylborane (2) bearing three o-carborane cages at peripheral positions on the aryl groups was prepared and its crystal structure was determined from X-ray diffraction study. Treatment of 2 with KF in the presence of 18-crown-6 led to the potassium salt, [2F](-). A UV-vis titration experiment carried out in THF/H(2)O (9/1 v/v) showed that 2 binds fluoride ions with a binding constant (K) of 4.8 × 10(4) M(-1), which is an order-of-magnitude greater than K for the mono-carborane substituted triarylborane. The enhanced fluoride ion affinity of 2 indicates an apparent additive effect of multiple carborane substitutions on the Lewis acidity enhancement of the triarylborane. The highly Lewis acidic nature of 2 was further utilized in evaluating the fluoride ion affinity of tris(pentafluorophenyl)borane (B(C(6)F(5))(3)). A fluoride exchange reaction between [2F](-) and B(C(6)F(5))(3) resulted in 15 times higher fluorophilicity for B(C(6)F(5))(3) than for 2. The lower Lewis acidity of 2 compared with B(C(6)F(5))(3) was confirmed from its greater cathodic reduction potential.

Cage-shaped borate esters with tris(2-oxyphenyl)methane or -silane system frameworks bearing multiple tuning factors: geometric and substituent effects on their Lewis acid properties

Boron complexes that contain new tridentate ligands, tris(o-oxyaryl)methanes and -silanes, were prepared. These complexes had a cage-shaped structure around a boron center and showed higher Lewis acidity and catalytic activity than open-shaped boron compounds. The cage-shaped ligands determined the properties of the borates by altering the geometry and were consistently bound to the metal center by chelation. The synthesized compounds were L⋅B(OC(6)H(4))(3)CH, L⋅B(OC(6)H(4))(3)SiMe, and its derivatives (L=THF or pyridine as an external ligand). Theoretical calculations suggested that the cage-shaped borates had a large dihedral angle (C(ipso)-O-B-O) compared with open-shaped borates. The geometric effect due to the dihedral angle means that compared with open-shaped, the cage-shaped borates have a greater Lewis acidity. The introduction of electron-withdrawing groups on the aryl moieties in the cage-shaped framework increased the Lewis acidity. Substitution of a bridgehead Si for a bridgehead C decreased the Lewis acidity of the boron complexes because the large silicon atom reduces the dihedral angle of C(ipso)-O-B-O. The ligand-exchange rates of the para-fluoro-substituted compound B(OC(6)H(3)F)(3)CH and the ortho-phenyl-substituted compound B(OC(6)H(3)Ph)(3)CH were less than that of the unsubstituted borate B(OC(6)H(4))(3)CH. The ligand-exchange rate of B(OC(6)H(4))(3)SiMe was much faster than that of B(OC(6)H(4))(3)CH. A hetero Diels-Alder reaction and Mukaiyama-type aldol reactions were more effectively catalyzed by cage-shaped borates than by the open-shaped borate B(OPh)(3) or by the strong Lewis acid BF(3) ⋅OEt(2). The cage-shaped borates with the bulky substituents at the ortho-positions selectively catalyzed the reaction with less sterically hindered substrates, while the unsubstituted borate showed no selectivity.