M. E. Gurskii

Determination of the activation parameters of permanent allylic rearrangement in allyl- and methallylboranes and in 1,1-bis(dipropylborylmethyl)ethylene by dynamic NMR spectroscopy

Abstract The activation parameters were determined for the permanent allylic rearrangement (PAR) in five organoboranes of allylic type by means of complete line shape analysis for the dynamic NMR spectra. The introduction of the second boron atom into the allylborane molecule leads to a significant decrease in the activation barrier of the PAR.

Regioselective β‐Hydride Transfer in Reactions of Ate Complexes of Boron Bicyclic and Cage Compounds. Synthesis of Methylenecyclohexane Derivatives.

Abstract The reactions of bicyclic and cage boron containing ate complexes with AcCl have been studied, the key stage of which involves β-bridgehead hydride abstraction. The ate complexes of 7-substituted 3-methyl-3-borabicyclo[3.3.1]non-6-ene were converted to the corresponding 5-methylene-3-alkylcyclohex-2(3)-en-1-ylmethyl(dialkyl)boranes 14 and 15 . A synthetic application of the reaction is illustrated by conversion of compounds 14 and 15 to 3,5-dimethylene-1-R-cyclohexenes 16 , particularly to 3,5-dimethylene-1-isopropenylcyclohexene ( 16c ) The β-hydride transfer in ate complexes of 2-alkyl-1-boraadamantane and of 4-alkyl-3-borahomoadamantane occurs regioselectively, at the unsubstituted bridgehead, to give, respectively, 2-alkyl-7-methylene-3-borabicyclo[3.3.1]nonanes and 8-methylene-3- borabicyclo[4.3.1]decanes.

Organoboranes-atmospheric oxygen systems as unconventional initiators of radical polymerization

This paper provides the first generalization and analysis of our results and published data about a new type of pseudoliving process: radical polymerization mediated by boroxyl radicals generated through the oxidation of organoboranes by atmospheric oxygen. The kinetics and mechanism of polymerization of methyl methacrylate and vinyl acetate in the presence of ammonia-organoboranes and boracyclanes are studied. Methods of practical use of polymerization initiated by organoborane-oxygen systems for macromolecular design are discussed.

An Unusual Mechanism of Polymerization of MMA Initiated by Ammonia—Triisobutyl Borane and Atmospheric Oxygen

The kinetics of polymerization of methyl methacrylate in vacuum initiated by ammonia-triisobutyl borane (iso-Bu3B · NH3) oxidized in air is studied. It is shown that the rate of reaction shows the first order with respect to the monomer concentration and a variable order with respect to the initiator concentration; the process is characterized by a low activation energy. It is demonstrated that polymerization proceeds according to a two-step mechanism. The mechanism of polymerization initiation and reinitiation is investigated via ESR spectroscopy, and it is found that one primary radical generated during initiation can form up to 200 substantial chains during the subsequent chain-transfer process.

Construction of nitrogen bicyclic and cage compounds with the use of allylic organoboranes

It is shown that reactions of triallylborane with pyrrole, pyridines, isoquinolines, lactams, 1-pyrroline, and acetylenes offer versatile methodology for the construction of various bicyclic and polycyclic nitrogen compounds, some of which are skeletally related to important classes of alkaloids. Optically active 3-borabicyclo[3.3.1]non-6-enes are useful precursors for synthesis of chiral 3-aza- and 3-thiabicyclo[3.3.1]non-6-enes, as well as derived chiral cyclohexenoid systems. The convenient methodology for the transformation of 1-boraadamantanes into 1-azaadamantanes is also discussed.

Structure and Bonding Nature of the Strained Lewis Acid 3-Methyl-1-boraadamantane: A Case Study Employing a New Data-Analysis Procedure in Gas Electron Diffraction

Base-free 3-methyl-1-boraadamantane was synthesized by starting from its known THF adduct, transforming it to a butylate-complex with n-butyllithium, cleaving the cage with acetyl chloride to give 3-n-butyl-5-methyl-7-methylene-3-borabicyclo[3.3.1]nonane and closing the cage again by reacting the latter with dicyclohexylborane. The identity of 3-methyl-1-boraadamantane was proven by (1) H, (11) B and (13) C NMR spectroscopy and elemental analysis. The experimental equilibrium structure of the free 3-methyl-1-boraadamantane molecules has been determined at 100 °C by using gas-phase electron diffraction. For this structure determination, an improved method for data analysis has been introduced and tested: the structural refinement versus gas-phase electron diffraction data (in terms of Cartesian coordinates) with a set of quantum-chemically derived regularization constraints for the complete structure under optimization of a regularization constant, which maximizes the contribution of experimental data while retaining a stable refinement. The detailed analysis of parameter errors shows that the new approach allows obtaining more reliable results. The most important structural parameters are: r(e) (B-C)(av) =1.556(5) Å, angle(e) (C-B-C)(av) =116.5(2)°. The configuration of the boron atom is pyramidal with ∑ angle (C-B-C)=349.4(4)°. The nature of bonding was analyzed further by applying the natural bond orbital (NBO) and atoms in molecules (AIM) approaches. The experimentally observed shortening of the B-C bonds and elongation of the adjacent C-C bonds can be explained by the σ(C-C)→p(B) hyperconjugation model. Both NBO and AIM analyses predict that the B-C bonds are significantly bent in the direction out of the cage.

1-Pyridine- and 1-Quinuclidine-1-boraadamantane as Models for Derivatives of 1-Borabicyclo[2.2.2]octane. Experimental and Theoretical Evaluation of the B−N Fragment as a Polar Isosteric Substitution for the C−C Group in Liquid Crystal Compounds†

The suitability of 1-borabicyclo[2.2.2]octane (1) as a structural element for liquid crystals was evaluated using computational methods and experimental studies of two complexes of its close analogue 1-boraadamantane (2). The molecular and crystal structures for 1-pyridine-1-boraadamantane [2-P, C(14)H(20)BN, P2(1)/m, a = 8.4404(13) A, b = 6.8469(10) A, c = 10.5269(16) A, beta = 104.712(3) degrees, Z = 2], 1-quinuclidine-1-boraadamantane [2-Q, C(16)H(28)BN, P2(1)/n, a = 6.6529(3) A, b = 10.6665(6) A, c = 19.3817(10) A, beta = 94.689(3) degrees, Z = 4], and 1-pyridine-trimethylborane [3-P, C(8)H(14)BN, C(cma), a = 6.9875(10) A, b = 15.011(2) A, c = 16.556(2) A, Z = 8] were determined by X-ray crystallography and compared with the results of DFT and MP2 calculations. Gas-phase thermodynamic stabilities of complexes 1-P, 1-Q, 2-P, and 2-Q were estimated using a correlation between theoretical (MP2/6-31+G(d)//MP2/6-31G(d) with B3LYP/6-31G(p) thermodynamic corrections) and experimental data for complexes of BMe(3) (3) with amines lacking N-H bonds. The analysis showed the generally higher thermodynamic stability for the quinuclidine (Q) complexes compared to that of the pyridine (P) analogues in the gas phase and an overall order of stability of 1 > 2 > 3. This order is paralleled by high ring strain energy of 1 (SE = 27 kcal/mol) as compared to that of 1-boraadamantane (2, SE = 16.5 kcal/mol). The chemical stability of 2-P and 2-Q, with respect to hydrolytic and oxidative reagents, is high for the pyridine derivative and satisfactory for the quinuclidine complex at ambient temperature, which implies sufficiently high stability of 1-borabicyclo[2.2.2]octane complexes for materials applications. Molecular dipole moments of 6.2 +/- 0.1 and 6.0 +/- 0.15 D were measured for 2-Q and 2-P, respectively.

Design of bicyclic and cage boron compounds based on allylboration of acetylenes with allyldichloroboranes.

Allylboration of acetylenes with allyldichloroboranes has been proposed as a first step of allylboron-acetylene condensation and a way to design condensation products from stage to stage. The chemistry has been applied to the synthesis of isomeric 3-borabicyclo[3.3.1]non-6-enes transformed into 3-methyl-1-boraadamantane and [5-D]-3-methyl-1-boraadamantane derivatives.

COMPLEXES OF 1-BORAADAMANTANE AND ITS DERIVATIVES WITH 1-AZAADAMANTANES : SYNTHESIS AND MOLECULAR STRUCTURE

The reactions of 1-boraadamantane and 2-methyl-, 2-ethyl-, 2,2-dimethyl-, and 3,5-dimethyl-substituted 1-boraadamantanes with their 1-azaadamantane analogs afforded a series of 1:1 adducts, which are stable to atmospheric air and moisture.IH,13C, and31B NMR spectra as well as mass spectra of the compounds synthesized were investigated. Only the adduct of 2,2-dimethyl-1-boraadamantane with 2,2-dimethyl-1-azaadamantane readily dissociates into the initial components due to steric hindrances that prevent strong B→N coordination. The structure and geometric parameters of the 1-boraadamantane complex with 3,5-dimethyl 1-azaadamantane were established by X-ray diffraction analysis.

Conformational behavior of 3-borabicyclo[3.3.1]nonanes. Intramolecular p–σ and p–π interactions in 3,7-endo-disubstituted 3-borabicyclo[3.3.1]nonanes

Abstract Intramolecular interactions between the unoccupied p-orbital of the boron atom and the σ- (Me) or π-orbital (Ph) of the 7-endo-substituent lead to an unusual stability of the chair–chair conformation in 3-borabicyclo[3.3.1]nonanes. X-ray analysis of 3,7α-dimethyl-3-borabicyclo[3.3.1]nonane (7) and 3-methyl-7α-phenyl-3-borabicyclo[3.3.1]nonane (8), and single point ab initio calculation (B3LYP/6-31G*) of 8 have confirmed the existence of this specific attractive interaction.