Thomas Kupfer

Structural Evidence for Antiaromaticity in Free Boroles

Unsaturated boron-containing heterocycles such as borirenes (I), boroles (II), and borepins (III) have attracted fundamental interest owing to their electronic structure. The interaction of the empty pz orbital at boron in these systems with the unsaturated carbon backbone might result in a stabilization (I and III) or destabilization (II) of the entire p system, depending on the number of available p electrons. In particular, boroles (II) are of interest because of their close relationship to the cyclopentadienyl cation (IV), which

Base‐Stabilized Diborenes: Selective Generation and η2 Side‐on Coordination to Silver(I)

(B)olefin complexes: Reductive coupling of designed monoborane precursors (see scheme; Dur=2,3,5,6-tetramethylphenyl) gives convenient access to N-heterocyclic carbene stabilized diborenes. The presence of B-B multiple bonds in the dark red diborenes is shown experimentally and theoretically. Reaction with AgCl afforded a Ag(I) species with an unprecedented, olefin-like η(2) coordination mode.

Generation of a carbene-stabilized bora-borylene and its insertion into a C-H bond.

A novel NHC adduct of a dihalodiborane(4), 1, is reduced by KC(8) with formation of the five-membered boracycle 2. The reaction most likely proceeds via C-H insertion of an intermediate NHC-stabilized free bora-borylene species.

Boron Radical Cations from the Facile Oxidation of Electron‐Rich Diborenes

The realization of a phosphine-stabilized diborene, Et3P⋅(Mes)B=B(Mes)⋅PEt3 (4), by KC8 reduction of Et3P⋅B2Mes2Br2 in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC-stabilized diborene IMe⋅(Dur)B=B(Dur)⋅IMe (1). Importantly, both species feature unusual electron-rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the BB double bond system. Thus, the stronger σ-donor strength and larger electronegativity of the NHC ligand results in an energetically higher-lying HOMO, making 1 a stronger neutral reductant as 4 (1: E(1/2)=-1.55 V; 4: -1.05 V), and a smaller HOMO-LUMO gap of 1 accompanied by a noticeable red-shift of its lowest-energy absorption band with respect to 4. Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron-centered radical cations (5 and 6).

Borylene-Based Direct Functionalization of Organic Substrates: Synthesis, Characterization, and Photophysical Properties of Novel π-Conjugated Borirenes

Room temperature photolysis of aminoborylene complexes, [(CO)(5)M=B=N(SiMe(3))(2)] (1: M = Cr, 2: Mo) in the presence of a series of alkynes and diynes, 1,2-bis(4-methoxyphenyl)ethyne, 1,2-bis(4-(trifluoromethyl)phenyl)ethyne, 1,4-diphenylbuta-1,3-diyne, 1,4-bis(4-methoxyphenyl)buta-1,3-diyne, 1,4-bis(trimethylsilylethynyl)benzene and 2,5-bis(4-N,N-dimethylaminophenylethynyl)thiophene led to the isolation of novel mono and bis-bis-(trimethylsilyl)aminoborirenes in high yields, that is [(RC=CR)(mu-BN(SiMe(3))(2)], (3: R = C(6)H(4)-4-OMe and 4: R = C(6)H(4)-4-CF(3)); [{(mu-BN(SiMe(3))(2)(RC=C-)}(2)], (5: R = C(6)H(5) and 6: R = C(6)H(4)-4-OMe); [1,4-bis-{(mu-BN(SiMe(3))(2)(SiMe(3)C=C)}benzene], 7 and [2,5-bis-{(mu-BN(SiMe(3))(2) ((C(6)H(4)NMe(2))C=C)}-thiophene], 8. All borirenes were isolated as light yellow, air and moisture sensitive solids. The new borirenes have been characterized in solution by (1)H, (11)B, (13)C NMR spectroscopy and elemental analysis and the structural types were unequivocally established by crystallographic analysis of compounds 6 and 7. DFT calculations were performed to evaluate the extent of pi-conjugation between the electrons of the carbon backbone and the empty p(z) orbital of the boron atom, and TD-DFT calculations were carried out to examine the nature of the electronic transitions.

Boron as a Powerful Reductant: Synthesis of a Stable Boron‐Centered Radical‐Anion Radical‐Cation Pair

Despite the fundamental importance of radical-anion radical-cation pairs in single-electron transfer (SET) reactions, such species are still very rare and transient in nature. Since diborenes have highly electron-rich BB double bonds, which makes them strong neutral reductants, we envisaged a possible realization of a boron-centered radical-anion radical-cation pair by SET from a diborene to a borole species, which are known to form stable radical anions upon one-electron reduction. However, since the reduction potentials of all know diborenes (E1/2 =-1.05/-1.55 V) were not sufficiently negative to reduce MesBC4 Ph4 (E1/2 =-1.69 V), a suitable diborene, IiPr⋅(iPr)BB(iPr)⋅IiPr, was tailor-made to comply with these requirements. With a halfwave potential of E1/2 =-1.95 V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4 Ph4 by SET to afford a stable boron-centered radical-anion radical-cation pair.

The Pentaphenylborole–2,6-Lutidine Adduct: A System with Unusual Thermochromic and Photochromic Properties†

Organoboranes have attracted great attention owing to their interesting photophysical properties, which enable promising applications as optoelectronics and colorimetric chemosensors. 2] Three-coordinate boron has a low-lying empty p orbital, which bestows a strong Lewis acidity to the resulting substances and promotes electron transport through conjugated systems. The electron affinity of trivalent boron leads to the formation of various stable adducts of triaryl/alkyl boranes and Lewis bases. Remarkably, some of the borane– pyridine adducts have interesting nonlinear optical properties. Recently, Wang and co-workers have demonstrated that irradiation of adducts such as A leads to the formation of B, which contains a boracyclopropane ring with the B N linkage remaining intact (Scheme 1). Analogous rearrangement reactions have been found during photolysis of tetraphenylborates, where the generation of free radical species has also been observed (Scheme 1). Detailed investigations regarding the photochemistry of Lewis base adducts of boracycles are however scarce, even though theoretical and experimental studies have shown that the incorporation of boron into a cyclic p system implicates a substantial impact on the electronic properties of the molecule. Herein, we present the structure and the thermochromic behavior of Lewis base adducts of pentaphenylborole (PPB) with pyridine bases, and in particular an unusual photoinduced transformation to a borataalkene. To this end, the Lewis base adducts of pentaphenylborole with 4-picoline (1a) and 2,6-lutidine (1b) were prepared in a straightforward manner (Scheme 2). Interestingly, these two reactions substantially differ in their photophysical behavior. Treating a CD2Cl2 solution of PPB with 4-picoline resulted in an immediate color change from blue to yellow, and an upfield shift of the B NMR signal from d = 65.4 ppm to d = 3.5 ppm was observed in the B NMR spectrum, thus suggesting the formation of 1a. However, upon addition of 2,6-lutidine to PPB, the color of the solution remained deep blue under the same conditions. Surprisingly, the B NMR signal of 1b detected at room temperature (d = 21.0 ppm) is at much lower field than that of 1a. Cooling a toluene solution of 1b from room temperature to 40 8C resulted in a color change from deep blue to light yellow. The UV/Vis spectrum of 1 b at ambient temperature features a broad absorption band at 578 nm (Supporting Information, Figure S5) that decreased in intensity upon cooling. At 40 8C, the B NMR signal is shifted to d = 3.3 ppm, which clearly confirms the formation of a fourScheme 1. Light-induced rearrangements of four-coordinate boron compounds.

Chemical Reduction and Dimerization of 1‐Chloro‐2,3,4,5‐tetraphenylborole

As neutral isoelectronic analogues of the elusive cyclopentadienyl cation, boroles have been of interest for their prospective applications as strong Lewis acids, chromophores, and electron acceptors. Recently our group discovered a π-nucleophilic boryl anion based on the borole system. In an effort to extend borole chemistry, we now report the molecular structure of 1-chloro-2,3,4,5-tetraphenylborole (1) and its corresponding borole dianion resulting from the two-electron reduction of 1 with KC(8). The thermally induced dimerization of 1 yields an unprecedented boracyclohexadiene/borolene spiro-bicyclic compound and the resulting dimer was fully characterized including a single-crystal X-ray analysis.

Synthesis and reactivity studies of iminoboryl complexes.

A range of new iminoborylcomplexes of the type [L(n)M-B[triple bond]N-R], which are isoelectronic with sigma-alkynyl complexes [L(n)M-C[triple bond]C-R], was obtained by systematically varying the metal M, the coligands L, and the nitrogen bound substituent R. Selected examples include, for example, trans-[(Cy3P)2(Br)Pt(B[triple bond]N iBu)], which is characterized by a sterically less demanding N-R group or the unprecedented rhodium species cis,mer-[(Br)2(Me3P)3Rh(B[triple bond]NSiMe3)]. All compounds were fully characterized in solution by multinuclear NMR spectroscopy and, where appropriate, in the solid state by X-ray crystallography. Subsequent reactivity studies revealed that particularly the combination of smaller N-R groups with Pt-B linkages of increased stability opens up opportunities for novel reactivity patterns of this class of compounds. Within the scope of these study, we inter alia succeeded in synthesizing the unusual bridged boryl species 1,4-trans-[{(Cy3P)2(Br)Pt(B{NH iBu}NH)}2C6H4] and a complex bearing both an acetylide ligand and an iminoboryl ligand, respectively.

NHC-stabilized 1-hydro-1H-borole and its nondegenerate sigmatropic isomers.

Electrophilic quenching of a carbene-stabilized π-boryl anion with the Brønsted acid Et(3)NHCl provides a convenient synthetic route to the parent NHC-stabilized 1-hydroborole, which isomerizes to the corresponding 3-hydroborole via two successive nondegenerate [1,5]-sigmatropic hydrogen migrations. Molecular structures of both isomers have been determined by X-ray crystallography.