Computational studies of ene reactions between aminoborane (FC)B[double bond, length as m-dash]N(CH) and substituted propenes: additive effects on barriers and reaction energies.

Systematic computational studies of concerted pericyclic ene-type reactions between aminoborane (FC)B[double bond, length as m-dash]N(CH), 1, and substituted propenes (R)(R)C[double bond, length as m-dash]C(R)-C(R)(R)H (R = Me, CF, F; a = axial position in transition state, e = equatorial position in transition state) show that in all cases but one the reactions are exothermic. The reactions proceed through six-membered cyclic envelope-like transition states except in the case of 1 + CH(CF). The data allow isolation of substitutional effects on barrier heights; the effects appear to be additive in all cases.

A semi-homodesmotic approach for estimating ring strain energies (RSEs) of highly substituted cyclopropanes that minimizes use of acyclic references and cancels steric interactions: RSEs for c-C3R6 that make sense.

Estimation of ring strain energies (RSEs) of substituted cyclopropanes c-C(3)H(x)R(6-x) (R = F, Cl, Me; x = 0, 2, 4) using homodesmotic reaction methods has been plagued by implausible results. Prior work suggests that this stems from poorly canceled interactions between substituents on the acyclic reference molecules. We report a semi-homodesmotic approach that minimizes use of acyclic references, focusing instead on canceling substituent interactions.

Frustrated Lewis pairs derived from N-heterocyclic carbenes and Lewis acids.

The chemistry of frustrated Lewis pairs derived from N-heterocyclic carbenes and a number of Lewis acids has been probed. The combination of 1,3-bis[2,6-(di-iso-propyl)phenyl]-1,3-imidazol-2-ylidene (IDipp) (1) with B(C6F5)3 was shown to give the classical Lewis acid-base adduct (IDipp)B(C6F5)3 (2) which was unreactive. In contrast, the combination 1,3-di-tert-butyl-1,3-imidazol-2-ylidene (3) with B(C6F5)3 proved to form a frustrated Lewis pair, and reacts with H2 to give the salt [ItBuH][HB(C6F5)3] (4) in high yield.

From classical adducts to frustrated Lewis pairs: steric effects in the interactions of pyridines and B(C6F5)3.

The pyridine adducts of B(C(6)F(5))(3), (4-tBu)C(5)H(4)NB(C(6)F(5))(3) 1, ((2-Me)C(5)H(4)N)B(C(6)F(5))(3) 2, ((2-Et)C(5)H(4)N)B(C(6)F(5))(3) 3, ((2-Ph)C(5)H(4)N)B(C(6)F(5))(3) 4, ((2-C(5)H(4)N)C(5)H(4)N)B(C(6)F(5))(3) 5, (C(9)H(7)N)B(C(6)F(5))(3) 6, and ((2-C(5)H(4)N)NH(2-C(5)H(4)N))B(C(6)F(5))(3) 7, were prepared and characterized. The B-N bond lengths in 2-7 reflect the impact of ortho-substitution, increasing significantly with sterically larger and electron-withdrawing substituents. In the case of 2-amino-6-picoline, reaction with B(C(6)F(5))(3) affords the zwitterionic species (5-Me)C(5)H(3)NH(2-NH)B(C(6)F(5))(3) 8.

Donor-Acceptor Dissociation Energies of Group 13-15 Donor-Acceptor Complexes Containing Fluorinated Substituents: Approximate Lewis Acidities of (F3C)3M vs (F5C6)3M and the Effects of Phosphine Steric Bulk.

To study donor-acceptor complexes containing fluoroalkyl and -aryl substituents on the acceptors, ONIOM methods for optimizing large complexes and determining single point energies were tested. A two-layer ONIOM optimization procedure utilizing the MPW1K model followed by single point calculations using the composite three-layer ONIOM G2R3 method proved acceptable. The optimization model predicts M-X bond distances well when compared to experiment and shows that the distances increase discontinuously with the bulk of the phosphine.