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.

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.

The review summarizes our data on the synthesis of selected allylic, bicyclic, and highly strained cage boranes, investigations of their physicochemical properties, and applications of these com pounds. Procedures for the construction of boron polyhedral systems, transformations of 1 boraad amantane compounds into 1 hydroxyadamantanes and 1 azaadamantanes, and the synthesis of chiral derivatives of 1 boraadamantane and 3 borabicyclo[3.3.1]nonane are considered.

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An efficient method for methyl methacrylate radical polymerization by tri n propyl , triisopropyl , and triisobutylborane ammonia complexes, including the addition of a boron containing initiating agent into the monomer in air, was developed. An advantage of this method is that the reaction occurs at room temperature, requires no peroxide components, and leads to polymers with enhanced thermal stability.