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. The method requires employing homodesmotic group equivalent reactions only for disubstituted cyclopropanes and relies solely on absolute energy calculations for more substituted rings. This provides RSEs consistent with chemical intuition regardless of the degree of substitution. We find that RSEs increase with substitution regardless of the electronic nature of R, although the increase is more dramatic when R is electron-withdrawing. The RSEs determined are consistent with QTAIM data, which show that progressive substitution always increases critical path angles. Overall, the semi-homodesmotic approach is simpler than hyperhomodesmotic reaction methods, and gives more trustworthy results.

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. In contrast, lutidine/B(C(6)F(5))(3) yields an equilibrium mixture containing both the free Lewis acid and base and the adduct (2,6-Me(2)C(5)H(3)N)B(C(6)F(5))(3) 9. This equilibrium has a DeltaH of -42(1) kJ/mol and DeltaS of -131(5) J/mol x K. Addition of H(2) shifts the equilibrium and yields [2,6-Me(2)C(5)H(3)NH][HB(C(6)F(5))(3)] 10. The corresponding reactions of 2,6-diphenylpyridine or 2-tert-butylpyridine with B(C(6)F(5))(3) showed no evidence of adduct formation and upon exposure to H(2) afforded [(2,6-Ph(2))C(5)H(3)NH][HB(C(6)F(5))(3)] 11 and [(2-tBu)C(5)H(4)NH][HB(C(6)F(5))(3)] 12, respectively. The energetics of adduct formation and the reactions with H(2) are probed computationally. Crystallographic data for compounds 1-10 are reported.

Computational studies of ene reactions between aminoborane (F3C)2B[double bond, length as m-dash]N(CH3)2 and substituted propenes: additive effects on barriers and reaction energies.

Systematic computational studies of concerted pericyclic ene-type reactions between aminoborane (F3C)2B[double bond, length as m-dash]N(CH3)2, 1, and substituted propenes (R1a)(R1e)C[double bond, length as m-dash]C(R2)-C(R3a)(R3e)H (R = Me, CF3, 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 + C3H(CF3)5. The data allow isolation of substitutional effects on barrier heights; the effects appear to be additive in all cases. Substitution at positions 1a, 1e, and 3a increases barriers, while substitution at positions 2 and 3e has variable impacts. The former observation is ascribed to steric crowding in the transition states, and is particularly prevalent for substitution at positions 1a and 1e. Substitution at position 2 lowers barriers for R = Me, F, possibly due to electronic demands, while raising them for R = CF3 because of 1,3-diaxial repulsions between boron- and carbon-bound CF3 groups. Substitution at position 3e has little impact on the barriers for R = Me, CF3, but significantly raises them for R = F. This seems to arise from charge effects on the position of the transition states.

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. In a similar fashion, addition of (3) to a series of amine-borane adducts including H3NB(C6F5)3 (5), PhH2NB(C6F5)3 (6) and PhH2NB(C6F5)3 (7) led to deprotonation and formation of imidazolium amido-borate salts [ItBuH][H2NB(C6F5)3] (8), [ItBuH][PhHNB(C6F5)3] (9) and [ItBuH][Ph2NB(C6F5)3] (10). Similar reactions of the amine-borane adducts EtNH2B(C6F5)3 (11) tBuNH2B(C6F5)3 (12) and Et2NHB(C6F5)3 (13) gave the amidoboranes EtHNB(C6F5)2 (14) tBuHNB(C6F5)2 (15) and Et2NB(C6F5)2 (16) respectively, with liberation of C6F5H and an equivalent of unreacted carbene. Mechanistically these reactions are thought to proceed through transient imidazolium salts similar to (8)-(10). In addition, the combination of carbene (3) and the cation [CPh3]+ in frustrated Lewis pair chemistry has been probed. Reaction of this combination at room temperature results in the immediate formation of [C3H2N2tBu2(C6H5)CPh2][B(C6F5)4] (17) stemming from carbene attack at a para-carbon of one of the phenyl rings of trityl. Addition of carbene to the benzyl amine adduct of [CPh3][B(C6F5)4] results in the formation of [ItBuH][B(C6F5)4] (18) and the secondary amine Ph3CNHCH2Ph (19), thus providing the first example of an all carbon-based frustrated Lewis pair. Crystallographic data for (2), (4), (9) and (13) are reported.

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. Unexpectedly, (RF)3B-XR3 and (RF)3Al-XR3 bond dissociation energies (ΔEDA) are comparable for several R substituents. For RF = CF3, both are predicted to exhibit M-X ΔEDA values in the range 55-80 kcal mol(-1), exceptionally strong for dative bond energies. For RF = C6F5, the ΔEDA values are predicted to lie in the range 30-45 kcal mol(-1). (F5C6)3BP(t-Bu)3, which does not contain a B-P bond, is predicted to display ΔEDA = 19 kcal mol(-1). The ΔEDA energies do not change smoothly as the steric bulk of the phosphine increases. However, intrinsic ΔEDA energies ΔEint show a regular increase as the donor ability of the phosphine increases, confirming that the reorganization energy of the individual moieties contributes sizably to the overall ΔEDA. The data indicate that PPh3 is approximately equivalent to PMe3 as a donor in terms of ΔEint.