The proline-catalyzed direct aldol reaction between acetone and acetaldehyde has been investigated using density functional theory. Proline catalyzes the reaction according to the enamine mechanism characteristic of the natural class I aldolase enzymes. Although it has been postulated that the rate-limiting step in the proposed mechanism is enamine and/or C-C bond formation, the initial reaction between proline and acetone is accompanied by a very large barrier which may inhibit further progression of the reaction. However, an alternative lower energy reaction pathway is utilized when the ionizing solvent DMSO is present to assist in the formation and stabilization of separated charges. The direct aldol reaction between acetone and acetaldehyde illustrates the catalytic potential of simple organic molecules in asymmetric synthesis.

Oxidative dealkylation is a unique mechanistic pathway found in the alpha-ketoglutarate-Fe(II)-dependent AlkB family of enzymes to remove the alkylation damage to DNA bases and regenerate nucleobases to their native state. The B3LYP density functional combined with a self-consistent reaction field was used to explore the triplet, quintet, and septet spin-state potential energy surfaces of the multistep catalytic mechanism of AlkB. The mechanism was found to consist of four stages. First, binding of dioxygen to iron in the active-site complex occurs concerted with electron transfer, thereby yielding a ferric-superoxido species. Second, competing initiation for the activation of oxygen to generate the high-valent iron-oxygen intermediates (ferryl-oxo Fe(IV)O and ferric-oxyl Fe(III)O(*) species) was found to occur on the quintet and septet surfaces. Then, conformational reorientation of the activated iron-oxygen ligand was found to be nearly thermoneutral with a barrier of ca. 50 kJ mol(-1). The final stage is the oxidative dealkylation of the damaged nucleobase with the rate-controlling step being the abstraction of a hydrogen atom from the damaging methyl group by the ferryl-oxo ligand. For this step, the calculated barrier of 87.4 kJ mol(-1) is in good agreement with the experimental activation energy of ca. 83 kJ mol(-1) for the enzyme-catalyzed reaction.

The series of 2-substituted diphenylphosphine pyridines 1−5 were synthesized and subsequently oxidized with silyl or aryl azides to give the series of pyridine−phosphinimine ligands 2-(Me3SiNPPh2)C5H4N (9), 2-(2,6-Me2C6H3NPPh2)C5H4N (10), 2-(2,6-i-Pr2C6H3NPPh2)C5H4N (11), 2-(2,6-Me2C6H3NPPh2)-6-MeC5H4N (12), 2-(2,6-i-Pr2C6H3NPPh2)-6-MeC5H4N (13), 2-(2,6-Me2C6H3NPPh2)-6-BnC5H4N (14), 2-(2,6-i-Pr2C6H3NPPh2)-6-BnC5H4N (15), 2-(2,6-Me2C6H3NPPh2)-6-SiMe3C5H4N (16), 2-(2,6-i-Pr2C6H3NPPh2)-6-SiMe3C5H4N (17), 2-(2,6-Me2C6H3NPPh2)-6-PhC5H4N (18), and 2-(2,6-i-Pr2C6H3NPPh2)-6-PhC5H4N (19). Attempts to oxidize the fluorinated phosphine 2-P(C6F5)2-6-PhC5H3N (6) were unsuccessful. The ligand 9 reacted with PdCl2(PhCN)2 to the give the square-planar, diamagnetic compound (L)PdCl2 (20; L = 9), while the remaining ligands were used to prepare (L)NiBr2 and (L)FeCl2 complexes 21−30 and 31−40 (L = 10-19), respectively. In these complexes the P atoms become part of the chelate backbone. In addition, the pyridine−phosphinimines 2-(Ph3PNCH2)(C5H4N) (44), 2-(Ph3PNCH2)-6-Me(C5H3N) (45), and 2-(Ph3PNCH2)-6-Ph(C5H3N) (46) were also prepared from the reaction of 2-azidomethyl−pyridines with PPh3. In a similar fashion the complexes (L)PdCl2 (47, 48; L = 44, 45), (L)NiBr2 (49−51; L = 44−46), (L)FeCl2 (52, 53), and (L)CoCl2 (54, 55; L = 44, 45) were prepared. In addition, the imidazole−phosphines 1-Me-2-(PPh2)C3H2N2 (58), 1-Me-2-(PPh2)-4,5-Ph2C3N2 (59), and 1-Me-2-(PPh2)C7H6N2 (60) were prepared and oxidized to give the imidazole−phosphinimines 1-Me-2-(2,6-Me2C6H3NPPh2)C3H2N2 (61), 1-Me-2-(2,6-i-Pr2C6H3NPPh2)C3H2N2 (62), 1-Me-2-(2,6-Me2C6H3NPPh2)-4,5-Ph2C3N2 (63), 1-Me-2-(2,6-i-Pr2C6H3NPPh2)-4,5-Ph2C3N2 (64), 1-Me-2-(2,6-Me2C6H3NPPh2)C7H6N2 (65), 1-Me-2-(2,6-i-Pr2C6H3NPPh2)C7H6N2 (66) and 1-Me-2-(2,6-i-Pr2C6H3NPPh2)-4-(t-Bu)C3HN2 (67). Subsequent complexation afforded the species (L)PdCl2 (68; L = 61), (L)NiBr2 (69−75; L = 61−67), and (L)FeCl2 (76−82; L = 61−67). Preliminary screening for activity as catalyst precursors for ethylene polymerization indicated that ethylene oligomerization may be occurring. In the case of complexes 30, 40, 74, and 82 activation with Et2AlCl(ClC6H5) at 35 °C under 300 psi of ethylene effected modest catalytic dimerization of ethylene to mainly C4 alkenes. DFT computations suggested that inclusion of P into the ligand results in diminished electrophilicity at the metal and thus a weakened ethylene−metal interaction, accounting for the modest catalytic activity. X-ray structure determinations were obtained for 2, 20, 26, 27, 35, 37, 40, 49−51, 54, 68, 79, and 82.

The effects of different basis sets and computational methods on calculated isotropic hyperfine couplings have been investigated for a set of representative small radicals (OH, H2O+, CN, HCN-, FCN-, HCCH-, CH3, CH4 +, NH2, NO2, and H2CO+). Particular emphasis has been placed on the performance of the QCISD approach, when used in combination with moderately large basis sets. It is found that the 6-311+G(2df,p) basis set generally gives good results and that the IGLO-III basis set performs nearly as well. The cc-pVXZ and aug-cc-pVXZ basis sets, on the other hand, display large and unpredictable fluctuations in hyperfine couplings even at the cc-pVQZ level. As noted previously, the reason for this erroneous behavior can be traced to the contraction of the s-shell. The error due to the unbalanced nature of the pVXZ basis sets is greatly reduced on going to the core-valence correlated aug-cc-pCVXZ sets. The calculated hyperfine coupling constants are very sensitive to changes in geometry. In turn, the geometries of radical anion systems in particular are sensitive to level of theory. The 6-311+G(2df,p) basis set has also been tested with other spin-unrestricted methods (UHF, UMP2, UQCID, and five DFT functionals), but none of these are found to perform comparably to QCISD. Inclusion of triple excitations (QCISD(T)) leads to hyperfine couplings that generally lie within 2−3 G of the QCISD results.

The ability of conventional electron correlation (MP2 and QCISD) and density functional theory (B3LYP and B3P86) methods to provide accurate and reliable optimized structures, and homolytic S-N bond dissociation energies (BDEs), for a range of S-nitrosothiols (RSNOs) has been investigated. It is found that, in general, for any given method the 6-311+G(2df,p) or larger basis set must be used to obtain reliable structures. With a suitably large basis set, the different methods generally give optimized structures in close agreement with each other. However, the B3LYP method consistently overestimates the RS-NO bond length. The trends observed are found to be due in part to the fact that the RS-NO bond does not possess considerable double-bond character as previously suggested, but rather is a long single S-N bond, with the -NO moiety possessing considerable multiple-bond character. The B3P86/6-311+G(2df,p) method consistently gives BDEs in best agreement with values obtained with higher accuracy methods, e.g., CBS-Q, while the B3LYP method increasingly underestimates BDEs with increasing RSNO size. In contrast, for all RSNOs, the QCISD method significantly underestimates BDEs by as much as 55 kJ mol -1 . Overall, the B3P86/6-311+G(2df,p) method is found to perform the best of the methods considered for obtaining optimized structures and homolytic S-N BDEs of S-nitrosothiols.

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High-level ab initio calculations and variable-temperature proton-transfer equilibrium constant measurements have been used to obtain new thermochemical data for protonated halogenomethanes (CH3XH+, X = F, C1, Br, and I) and protonated diazomethane (CH3NNf). Proton affinities of CH3X and CHzNN and methyl cation affinities of HX and NZ have been derived. The theoretical and experimental results are in good agreement with one another but in several cases are in conflict with currently accepted experimental proton and methyl cation affhities. Experimental and theoretical methyl cation affinities are presented for a variety of molecules, leading to the proposal of a new methyl cation affinity scale.

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