Electron-induced ionization of undeuterated and deuterated benzoic acid isopropyl esters and nicotinic acid isopropyl esters: Some implications for the mechanism of the McLafferty rearrangement.

Electron ionization mass spectra, ionization, and appearance energies and bond energies (as dissociation energies) are reported for benzoic acid-1-methyl-ethyl ester (BAIPE), benzoic acid-1-deutero-1-methyl-ethyl ester (BAIPED1), benzoic acid-2,2,2-trideutero-1-trideuteromethyl-ethyl ester (BAIPED6) as well as nicotinic acid-1-methyl-ethyl ester (NAIPE), nicotinic acid-1-deutero-1-methyl-ethyl ester (NAIPED1), and nicotinic acid-2,2,2-trideutero-1-trideuteromethyl-ethyl ester (NAIPED6). Ionization energies of 9.39 eV for BAIPE, 9.40 eV for BAIPED1, 9.26 eV for BAIPED6 as well as 9.70 eV for NAIPE, 9.79 eV for NAIPED1, and 9.65 eV for NAIPED6 were determined. A gas-phase formation enthalpy of ΔHf0 = (-4.10 ± 0.1) eV for BAIPE is calculated as well as ΔHf0 = (-3.35 ± 0.1) eV for NAIPE. Molecular ions show two main fragmentation pathways. The first is a classical McLafferty rearrangement, characterized by the transfer of one γ-hydrogen atom from the isopropyl ester chain leading to the ions of the corresponding acid and neutral propene. The second is the double hydrogen transfer from the ester chain leading to the formation of the protonated acid and a C3H5√ allyl radical. For BAIPE, both hydrogen atoms originate from the methyl groups of the aliphatic chain with a probability of ≥98%, whereas the C-1-hydrogen is transferred with a probability of ≤2%. For NAIPE, both hydrogen atoms originate from the methyl groups of the aliphatic chain with a probability of 90%. Experimental proton affinities of PA = (8.75 ± 0.2) eV for benzoic acid and PA = (8.43 ± 0.2) eV for nicotinic acid are derived. For the protonation of the carbonyl group, B3LYP DFT calculations yielded PA = 8.66 eV for benzoic acid and PA = 8.41 eV for nicotinic acid. The overall fragmentation mechanism is explained with the initial formation of a 1,5-distonic ion by transfer of the first hydrogen. For the transfer of the second hydrogen, an intermediate ion/neutral complex is formulated.

Fe or Fe-NO catalysis? A quantum chemical investigation of the [Fe(CO)3(NO)](-)-catalyzed Cloke-Wilson rearrangement.

A quantum chemical investigation of the Bu4N[Fe(CO)3(NO)]-catalyzed Cloke-Wilson rearrangement of vinyl cyclopropanes is reported. It was found that allylic C-C bond activation can proceed through a SN2' or SN2-type mechanism. The application of the recently reported intrinsic bond orbital (IBO) method for all structures indicated that one Fe-N π bond is directly involved. Further analysis showed that during the reaction oxidation occurs at the NO ligand exclusively.

The electronic ground state of [Fe(CO)3 (NO)](-) : a spectroscopic and theoretical study.

During the past 10 years iron-catalyzed reactions have become established in the field of organic synthesis. For example, the complex anion [Fe(CO)3 (NO)](-) , which was originally described by Hogsed and Hieber, shows catalytic activity in various organic reactions. This anion is commonly regarded as being isoelectronic with [Fe(CO)4 ](2-) , which, however, shows poor catalytic activity. The spectroscopic and quantum chemical investigations presented herein reveal that the complex ferrate [Fe(CO)3 (NO)](-) cannot be regarded as a Fe(-II) species, but rather is predominantly a Fe(0) species, in which the metal is covalently bonded to NO(-) by two π-bonds. A metal-N σ-bond is not observed.

Carbonylation of alkyl halides with [Fe(CO)3(NO)]-: in silico identification of a common intermediate.

The mechanism of carbonylation of alkyl halides using [Fe(CO)3(NO)](-) has been studied using density functional theory (DFT). Our results suggest an SN2 mechanism for the alkylation event followed by a well-defined oxidative addition and alkyl migration. An experimentally elusive common intermediate [Fe(CO)2NO(Ac)] has been identified in two isomers and reacted in silico with a number of ligands (CO, PH3 and PPh3) to give the corresponding iron acyl complexes. Pathways between the two isomers, including direct involvement of a solvent molecule (THF) or iodide, have been elucidated.

Gold catalysis: phenol synthesis in the presence of functional groups.

The effect of different substituents, such as bromo, chloromethyl, hydroxymethyl, formyl, acetyl, carboxy, and acylated hydroxymethyl and ammonium groups, on the furan ring of substrates in gold-catalyzed phenol synthesis has been investigated. The furan ring was also replaced by different heterocycles, such as pyrroles, thiophenes, oxazoles, and a 2,4-dimethoxyphenyl group; gold catalysis then delivered no phenols, but occasionally other products were obtained. [Ru(3)(CO)(12)] also catalyzed the conversion of 1 at a low rate, [Os(3)(CO)(12)] failed as a catalyst, and with [Co(2)(CO)(8)] the alkyne complex 19 can be obtained, it does not lead to any phenol but reacts with norbornene to give the product of a Pauson-Khand reaction. Efforts to prepare vinylidene complexes of 1 provided the only evidence for these species; in the presence of a phosphane ligand with ruthenium an interesting deoxygenation to 22 was observed. The phenol 2 c was converted to the allyl ether, a building block for para-Claisen rearrangements, and to the aryl triflate, a building block for cross-coupling reactions.

Inversion of stereoselectivity by applying mutants of the hydroxynitrile lyase from Manihot esculenta.

The influence of Trp128-substituted mutants of the hydroxynitrile lyase from Manihot esculenta (MeHNL) on the stereoselectivity of MeHNL-catalyzed HCN additions to aldehydes with stereogenic centers, which yield the corresponding cyanohydrins, is described. In rac-2-phenylpropionaldehyde (rac-1) reactions, wild-type (wtMeHNL) and all MeHNL Trp128 mutants are highly (S)-selective toward the (R) enantiomer of rac-1; this results exclusively in (2S,3R)-cyanohydrin ((2S,3R)-2) with > or =96 % de. The (S) enantiomer of rac-1, however, only reacts (S)-selectively with wtMeHNL to give (2S,3S)-2 with 80 % de, whereas with Trp128 mutants, (R) selectivity increases with decreasing size of the amino acids exchanged. The MeHNL W128A mutant is exclusively (R)-selective, resulting in (2R,3S)-2 with 86 % de. The reaction behavior of rac-phenylbutyraldehyde (rac-5) is comparable with rac-1, which also inverts the stereoselectivity from (S) to (R) when the enzyme is exchanged from wtMeHNL to the W128A mutant. Stereogenic centers not adjacent to the aldehyde group, as in 7 and 9, do not influence the stereoselectivity of MeHNL catalysis, and (S) selectivity is observed in all cases. Stereoselectivity and inversion of stereoselectivity of MeHNL Trp128 mutant-catalyzed cyanohydrin formation can be explained and rationalized with crystal-structure-based molecular modeling.

Crystal structure of hydroxynitrile lyase from Sorghum bicolor in complex with the inhibitor benzoic acid: a novel cyanogenic enzyme.

The crystal structure of the hydroxynitrile lyase from Sorghum bicolor (SbHNL) in complex with the inhibitor benzoic acid has been determined at 2.3 A resolution and refined to a crystallographic R-factor of 16.5%. The SbHNL sequence places the enzyme in the alpha/beta hydrolase family where the active site nucleophile is predicted to be organized in a characteristic pentapeptide motif which is part of the active site strand-turn-helix motif. In SbHNL, however, a unique two-amino acid deletion is next to the putative active site Ser158, removing thereby the putative oxyanion hole-forming Tyr residue. The presented X-ray structure shows that the overall folding pattern of SbHNL is similar to that of the closely related wheat serine carboxypeptidase (CPD-WII); however, the deletion in SbHNL is forcing the putative active site residues away from the expected hydrolase binding site toward a small hydrophobic cleft, which also contains the inhibitor benzoic acid, defining thereby a completely different SbHNL active site architecture where the traditional view of a classic triad is not given any more. Rather, we propose a mechanism involving general base catalysis by the carboxy-terminal Trp270 carboxyl group and proton transfer toward the leaving nitrile group by an active site water molecule. The unexpected interactions of the inhibitor with the new SbHNL active site also reveal the structural basis for the enzyme's limited substrate specificity. The implications of this structure on the evolution of catalysis in the hydroxynitrile lyase superfamily are discussed.

Structure determinants of substrate specificity of hydroxynitrile lyase from Manihot esculenta.

Tryptophan 128 of hydroxynitrile lyase of Manihot esculenta (MeHNL) covers a significant part of a hydrophobic channel that gives access to the active site of the enzyme. This residue was therefore substituted in the mutant MeHNL-W128A by alanine to study its importance for the substrate specificity of the enzyme. Wild-type MeHNL and MeHNL-W128A showed comparable activity on the natural substrate acetone cyanohydrin (53 and 40 U/mg, respectively). However, the specific activities of MeHNL-W128A for the unnatural substrates mandelonitrile and 4-hydroxymandelonitrile are increased 9-fold and approximately 450-fold, respectively, compared with the wild-type MeHNL. The crystal structure of the MeHNL-W128A substrate-free form at 2.1 A resolution indicates that the W128A substitution has significantly enlarged the active-site channel entrance, and thereby explains the observed changes in substrate specificity for bulky substrates. Surprisingly, the MeHNL-W128A--4-hydroxybenzaldehyde complex structure at 2.1 A resolution shows the presence of two hydroxybenzaldehyde molecules in a sandwich type arrangement in the active site with an additional hydrogen bridge to the reacting center.