Isotope effects on the enzymatic and nonenzymatic reactions of chorismate

The important biosynthetic intermediate chorismate reacts thermally by two competitive pathways, one leading to 4-hydroxybenzoate via elimination of the enolpyruvyl side chain, and the other to prephenate by a facile Claisen rearrangement. Measurements with isotopically labeled chorismate derivatives indicate that both are concerted sigmatropic processes, controlled by the orientation of the enolpyruvyl group. In the elimination reaction of [4-2H]chorismate, roughly 60% of the label was found in pyruvate after 3 h at 60 degrees C. Moreover, a 1.846 +/- 0.057 2H isotope effect for the transferred hydrogen atom and a 1.0374 +/- 0.0005 18O isotope effect for the ether oxygen show that the transition state for this process is highly asymmetric, with hydrogen atom transfer from C4 to C9 significantly less advanced than C-O bond cleavage. In the competing Claisen rearrangement, a very large 18O isotope effect at the bond-breaking position (1.0482 +/- 0.0005) and a smaller 13C isotope effect at the bond-making position (1.0118 +/- 0.0004) were determined. Isotope effects of similar magnitude characterized the transformations catalyzed by evolutionarily unrelated chorismate mutases from Escherichia coli and Bacillus subtilis. The enzymatic reactions, like their solution counterpart, are thus concerted [3,3]-sigmatropic processes in which C-C bond formation lags behind C-O bond cleavage. However, as substantially larger 18O and smaller 13C isotope effects were observed for a mutant enzyme in which chemistry is fully rate determining, the ionic active site may favor a somewhat more polarized transition state than that seen in solution.     

Isochorismate pyruvate lyase: a pericyclic reaction mechanism?

Isochorismate pyruvate lyase (IPL) catalyzes the cleavage of isochorismate to give salicylate and pyruvate, a key step in bacterial siderophore biosynthesis. We investigated the enzyme from Pseudomonas aeruginosa using isochorismate selectively deuterated at C2 as a substrate. Monitoring the reaction by 2H NMR spectroscopy revealed that the label is quantitatively transferred from C2 to C9, producing stoichiometric amounts of [3-2H]pyruvate as product. Moreover, the deuterium kinetic isotope effect of 2.34 +/- 0.08 on kcat indicates that C-H cleavage is significantly rate limiting. Consistent with these data, hybrid density functional theory (HDFT) calculations at the Becke3LYP/DZ+(2d,p) level of theory predict a concerted but highly asynchronous pericyclic transition structure, in which carbon-oxygen bond cleavage is more advanced than hydrogen atom transfer from C2 to C9; the calculated 2H isotope effect of 2.22 at C2 is in excellent accord with the experimental value. Together, these findings indicate that IPL should be added to the small set of proteins that are known to catalyze pericyclic reactions. They also raise the possibility that enzymes, such as chorismate pyruvate lyase, salicylate synthase, 4-amino-4-deoxychorismate lyase, and anthranilate synthase, which accelerate formally similar reaction steps, may also exploit pericyclic mechanisms.     

Cyclization of electron-deficient cyclopentadienone with 2-alkenyl and 2-alkynylamines via sequential pericyclic reaction pathway.

The pericyclic reactions of 2,5-bis(methoxycarbonyl)-3,4-diphenylcyclopentadienone (1a) with both allylic and propargylic amines have been investigated. The reaction proceeded via initial formation of the 1,4 adducts followed by the ene cyclization and/or sequential pericyclic reactions depending upon the structures of the amines. The reaction of 1a with diallylamine (2a) gave the tetracyclic compound (3a). On the other hand, the reaction of 1a with 2-propynylamine (2c) gave exclusively the bicyclic compound (5c). In the reactions with the secondary 2-propynylamines (2d,e), the tetracyclic compounds (3d,e) were exclusively formed. The reactions of 1a with alpha-branched primary 1,1-dialkyl-2-propynylamines (2f,g) gave mixtures of 3- and 5-type compounds. The tetracyclic compounds 3 were formed from the intramolecular [4+2]pi cycloadditions of the [1,5]-sigmatropic rearrangement products of the 1,4 adducts of 1a and 2, followed by the [1,5]-sigmatropic rearrangement of hydrogen and dehydrogenation. The bicyclic compounds 5 were derived from the [2pi+2pi+2sigma] reaction of the 1,4 adducts of 1a and 2. The one-pot multistage sequential pericyclic reactions were discussed on the basis of the X-ray crystallographic structures and the MO calculation data.     

Mechanism of glucose oxidation by quinoprotein soluble glucose dehydrogenase: insights from molecular dynamics studies

We have generated 3 ns molecular dynamic (MD) simulations, in aqueous solution, of the bacterial soluble glucose dehydrogenase enzyme.PQQ.glucose complex and intermediates formed in PQQ reduction. In the MD structure of enzyme.PQQ.glucose complex the imidazole of His144 is hydrogen bonded to the hydroxyl hydrogen of H[bond]OC1(H) of glucose. The tightly hydrogen-bonded triad Asp163-His144-glucose (2.70 and 2.91 A) is involved in proton abstraction from glucose concerted with the hydride transfer from the C1[bond]H of glucose to the >C5[double bond]O quinone carbon of PQQ. The reaction is assisted by Arg228 hydrogen bonding to the carbonyl oxygen of >C5[double bond]O. The rearrangement of [bond](H)C5(O-)[bond]C4([double bond]O)[bond] of II to [bond]C5(OH)[double bond]C4(OH)[bond] of PQQH(2) hydroquinone is assisted by general acid protonatation of the >C4[double bond]O oxygen by protonated His144 and hydrogen bonds of Arg228 to the oxyanion O5. The continuous hydrogen bonding of the amide side chain of Asn229 to >C4[double bond]O4 oxygen and that of the O5 oxygen of the cofactor to Wat89 is observed throughout the entire reaction.     

A pericyclic cascade to the stereocontrolled synthesis of 9-cis-retinoids

A domino reaction that is pericyclic in nature is thought to be triggered upon treatment of alkenynol 10 with arylsulfenyl chlorides. The process comprises an ordered sequence of sigmatropic rearrangements: a reversible [2,3]-allyl sulfenate to allyl sulfoxide shift, followed by a [2,3]-propargyl sulfenate to allenyl sulfoxide rearrangement, and last a stereodifferentiating [1,5]-sigmatropic hydrogen migration leading to polyene 13. The occurrence of the C7 to C11 hydrogen migration has been demonstrated by labeling experiments. The double diastereoselection of the [1,5]-sigmatropic hydrogen shift to afford a single isomer of the final polyene 13 is thought to arise from a combination of the electronic effect of the sulfoxide at one terminus, and the steric effect imparted by the bulky trimethylcyclohexenyl substituent at the other terminus. The overall process thus constitutes a stereoselective synthesis of an E,Z,Z-triene fragment from an alkenynol and, in particular, a retinoid with the 7E,9Z,11Z,13E configuration on the conjugated polyenic side chain. Application of this method to the synthesis of retinoids, including labeled analogues, is straightforward.     

Addition of nitriles to alkaline earth metal complexes of 1,2-bis[(phenyl)imino]acenaphthenes

Compounds [Sr(dpp-bian)(thf)4] (2), [Ba(dpp-bian)(dme)2.5] (3) and [Mg(dtb-bian)(thf)2] (4) (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene; dtb-bian = 1,2-bis[(2,5-di-tert-butylphenyl)imino]acenaphthene) were prepared by reduction of dpp-bian and dtb-bian with an excess of metallic Mg, Sr, or Ba in THF or DME. Reactions of [Mg(dpp-bian)(thf)3], 3, and 4 with diphenylacetonitrile gave keteniminates [Mg(dpp-bianH)(NCCPh2)(thf)2] (5), [Mg(dtb-bianH)(NCCPh2)(thf)2] (6), and [Ba(dpp-bianH)(NCCPh2)(dme)2] (7), respectively. The reaction of 2 with CH3C[triple chemical bond]N in THF gave [{Sr(dpp-bianH)[N(H)C(CH3)C(H)CN](thf)}2] (8). The compounds 2, 3, 5-8 were characterized by elemental analysis, and IR and NMR spectroscopy. Molecular structures of 2, 3, 7, and 8 were determined by single-crystal X-ray diffraction. In contrast to reactions of alkali-metal reagents, magnesium amides, or yttriumalkyls with alpha-H acidic nitriles, which are accompanied by an amine or an alkane elimination, the reactions of [Mg(dpp-bian)(thf)3] (1), 2, 3, and 4 with such nitriles proceeded with formation of Mg, Sr, and Ba keteniminates and simultaneous protonation of one nitrogen atom of the bian ligand. The NMR spectroscopic data obtained for complex 5 indicated that in solution the amino hydrogen atom underwent the fast (on the NMR timescale) shuttle transfer between both nitrogen atoms of the dpp-bianH ligand.     

The mass spectra of some simple phenylhydrazides and a re-examination of the fragmentations of phenylhydrazine

The elimination of small neutral fragments from acetyl-, formyl- and ethoxycarbonyl- phenylhydrazines with formation of [C6H8N2]⁺? ions has been studied. Evidence is obtained from deuterium labelling and from metastable peak intensity ratios, to show that ketene loss from both acetylphenylhydrazines is accompanied (or preceded) by hydrogen transfer to the acylated nitrogen atom to give ions structurally analogous to the phenylhydrazine molecular ion. The decomposing [C6H8N2]⁺? ions formed from formyl- and ethoxycarbonylphenylhydrazines are also suggested to have a phenylhydrazine-like structure. In the molecular ion of phenylhydrazine interchange occurs between the two ortho hydrogen atoms and two of the three hydrazine hydrogens prior to decomposition; labelling data suggest that the N-1 hydrogen does not participate in the interchange process.     

Understanding the different activities of highly promiscuous MbtI by computational methods

Salicylate synthase from Mycobacterium tuberculosis, MbtI, is a highly promiscuous Mg(2+) dependent enzyme with up to four distinct activities detected in vitro: isochorismate synthase (IS), isochorismate pyruvate lyase (IPL), salicylate synthase (SS) and chorismate mutase (CM). In this paper, Molecular Dynamic (MD) simulations employing hybrid quantum mechanics/molecular mechanics (QM/MM) potentials have been carried out to get a detailed knowledge of the IS and the IPL activities at the molecular level. According to our simulations, the architecture of the MbtI active site allows catalyzing the two reactions: the isochorismate formation, by means of a stepwise mechanism, and the salicylate production from isochorismate, that appears to be pericyclic in nature. Findings also explain the role of the magnesium cation and the pH dependence activity experimentally observed in MbtI. Mg(2+) would be polarizing and pre-organizing the substrate and active site, as well as shifting the pK(a) values of key active site residues.     

Cyclodehydrogenation reactions to cyclopentafused polycyclic aromatic hydrocarbons

B3LYP/6-31G(d,p) electronic structure calculations are employed to elucidate the reaction mechanisms for the conversion of the alternant C(18)H(12) polycyclic aromatic hydrocarbon benzo[c]phenanthrene into the nonalternant C(18)H(10) PAHs cyclopenta[cd]pyrene and benzo[ghi]fluoranthene. Isomerization reactions such as 5/6-ring switching and hydrogen atom scrambling are analyzed. Bay region chemistry, involving the rupture of one benzene ring followed by the formation of a new five-membered ring, is also studied, together with the mechanism for the formation of an aryne. The rearrangement of the latter yields annelated cyclopentadienylidenecarbene, which is then trapped intramolecularly.     

Fascinating transformations of donor-acceptor complexes of group 13 metal (Al,Ga, In) derivatives with nitriles and isonitriles: from monomeric cyanides to rings and cages

Formation of the donor-acceptor complexes of group 13 metal derivatives with nitriles and isonitriles X(3)M-D (M = Al,Ga,In; X = H,Cl,CH(3); D = RCN, RNC; R = H,CH(3)) and their subsequent reactions have been theoretically studied at the B3LYP/pVDZ level of theory. Although complexation with MX(3) stabilizes the isocyanide due to the stronger M-C donor-acceptor bond, this stabilization (20 kJ mol(-1) at most) is not sufficient to make the isocyanide form more favorable. Relationships between the dissociation enthalpy DeltaH degrees (298)(diss), charge-transfer q(CT), donor-acceptor bond energy E(DA), and the shift of the vibrational stretching mode of the CN group upon coordination Deltaomega(CN) have been examined. For a given metal center, there is a good correlation between the energy of the donor-acceptor bond and the degree of a charge transfer. Prediction of the DeltaH degrees (298)(diss) on the basis of the shift of CN stretching mode is possible within limited series of cyanide complexes (for the fixed M,R); in contrast, complexes of the isocyanides exhibit very poor Deltaomega(CN) - DeltaH degrees (298)(diss) correlation. Subsequent X ligand transfer and RX elimination reactions yielding monomeric (including donor-acceptor stabilized) and variety of oligomeric cage and ring compounds with [MN]n, [MC]n, [MNC]n cores have been considered and corresponding to thermodynamic characteristics have been obtained for the first time. Monomeric aluminum isocyanides X(2)AlNC are more stable compared to Al-C bonded isomers; for gallium and indium situation is reversed, in qualitative agreement with Pearson's HSAB concept. Substitution of X by CN in MX(3) increases the dissociation enthalpy of the MX(2)CN-NH(3) complex compared to that for MX(3)-NH(3), irrespective of the substituent X. Mechanisms of the initial reaction of the X transfer have been studied for the case X = R = H. The process of hydrogen transfer from the metal to the carbon atom in H(3)M-CNH is thermodynamically favorable and is likely to be intramolecular. By contrast, intramolecular hydrogen transfer in H(3)M-NCH has been definitely ruled out. Head-to-tail dimeric species [H(3)M-(NC)H](2) are formed exothermically and exhibit low H.H distances, which can assist in hydrogen transfer, and are likely to be the starting point for H(2) elimination. Elimination of H(2), CH(4), and C(2)H(6) from X(3)M-(NC)R adducts is very favorable thermodynamically; by contrast, elimination of HCl and CH(3)Cl is highly unfavorable even if formation of oligomer species takes place. Thus, high-temperature generation of gas-phase rings and clusters has been predicted viable in the cases X = H,CH(3) and their presence in the reactor media should not be neglected. Moderate stability of [HMCH(2)NH](4) clusters (especially in the cases M = Ga, In) makes these species viable intermediates of gas-phase reactions. Their formation may be responsible for the carbon contamination in the course of metal organic chemical vapor deposition processes of group 13 binary nitrides.     

Fragmentation of the deprotonated ions of peptides containing cysteine, cysteine sulfinic acid, cysteine sulfonic acid, aspartic acid, and glutamic acid

We examined the fragmentation of the electrospray-produced [M-H]- and [M-2H]2- ions of a number of peptides containing two acidic amino acid residues, one being aspartic acid (Asp) or glutamic acid (Glu), and the other being cysteine sulfinic acid [C(SO2H)] or cysteine sulfonic acid [C(SO3H)], on an ion-trap mass spectrometer. We observed facile neutral losses of H2S and H2SO2 from the side chains of cysteine and C(SO2H), respectively, whereas the corresponding elimination of H2SO3 from the side chain of C(SO3H) was undetectable for most peptides that we investigated. In addition, the collisional activation of the [M-H]- ions of the C(SO2H)-containing peptides resulted in the cleavage of the amide bond on the C-terminal side of the C(SO2H) residue. Moreover, collisional activation of the [M-2H]2- ions of the above Asp-containing peptides led to the cleavage of the backbone N-Calpha bond of the Asp residue to give cn and/or its complementary [zn-H2O] ions. Similar cleavage also occurred for the singly deprotonated ions of the otherwise identical peptides with a C-terminal amide functionality, but not for the [M-H]- ions of same peptides with a free C-terminal carboxylic acid. Furthermore, ab initio calculation results for model cleavage reactions are consistent with the selective cleavage of the backbone N-Calpha bond in the Asp residue.     

Interplay of orbital symmetry and nonstatistical dynamics in the thermal rearrangements of bicyclo[n.1.0]polyenes.

CASSCF and CASPT2 calculations have been carried out on some of the thermal rearrangements of bicyclo[2.1.0]pentene (BCP), bicyclo[4.1.0]hepta-2,4-diene (BCH), bicyclo[6.1.0]nona-2,4,6-triene (BCN), and 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene (DCBCN). In addition, experiments have been conducted to determine the stereoselectivity and temperature dependence of the nondegenerate rearrangement of 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene-exo-15N. The calculations and experiments allow a consistent picture to be drawn for these reactions. The principal conclusions are as follows. (1) The ring-walk rearrangements of BCP, BCN, and DCBCN are pericyclic reactions occurring with a strong preference for inversion of configuration at the migrating carbon. However, the ring-walk rearrangement of BCH is a nonpericyclic reaction. (2) The rearrangement of DCBCN to 9,9-dicyanobicyclo[4.2.1]nona-2,4,7-triene occurs with a preferred stereochemistry corresponding to a 1,3 migration with retention. However, this reaction is not a pericyclic process; the stereoselectivity is probably of dynamic origin. (3) Cyano substituents can significantly reduce the activation energy for a reaction occurring via a singlet biradical, but they do not necessarily cause the intermediate to sit in a deeper local minimum on the potential energy surface.     

Fragmentation reactions of alkylphenylammonium ions

The fragmentation reactions of a variety of alkylphenylammonium ions, C(6)H(5)NH(3 -n)R(n)(+) (n >/= 1, R = CH(3), C(2)H(5), i-C(3)H(7), n-C(4)H(9)) were studied by energy-resolved mass spectrometry. Ionization was by fast atom bombardment (FAB) or electrospray ionization. Energy-resolved fragmentation data were obtained by low-energy collision-induced dissociation (CID) in the quadrupole cell of a hybrid sector/quadrupole instrument following FAB ionization and by cone-voltage CID in the interface region of the electrospray/quadrupole instrument. A comparison of the two methods of obtaining energy-resolved data showed that very similar results are obtained by the two methods. The fragmentation reactions of the alkylphenylammonium ions are rationalized in terms of competitive formation of an [R(+)-NC(6)H(5)H(3-n)R(n-1)] complex or a [C(6)H(5)H(3-n)R(n-1)N(+.)-(.)R] complex. The former complex fragments by internal proton transfer to yield C(6)H(5)H(3 -n)R(n -1)NH(+) and [R -H] whereas the latter complex fragments to form C(6)H(5)H(3 -n)R(n -1)N(+) and an alkyl radical. Alkane elimination, which is very prominent for tetraalkylammonium ions, most likely involves sequential elimination of an alkyl radical and either an H atom or an alkyl radical for the phenyl-substituted ammonium ions. Copyright 1999 John Wiley & Sons, Ltd.     

Reduction of chromium in ethylene polymerisation using bis(imido)chromium(VI) catalyst precursors

Hybrid density functional calculations on [Cr(NR)2C3H7(C2H4)]+ (R = H, tBu) have revealed a facile reductive elimination reaction involving beta-hydrogen transfer from the alkyl chain, suggesting that the active species in ethylene polymerisation with bis(imido)chromium(VI) precursors contains a reduced chromium atom.     

One and two hydrogen atom migrations in the retro-diels-alder fragmentation of norbornene and bicyclo [2.2.2] octene derivatives under electron impact

Norbornene derivatives with one or two carbonyl-containing substituents at positions 5 and 6 (anhydrides, esters, amides, cyclic ketones) undergo an electron impact induced rerro-Diels-Alder fragmentation accompanied by the migration of one hydrogen atom (RDA + H) giving rise to the [dienophile + H]⁺ ions. Bicyclo[2.2.2]octene derivatives substituted at C(5) and C(6) by a ring with two carbonyl groups (anhydrides, imides, diketones) undergo an RDA fragmentation accompanied by the transfer of two hydrogen atoms (RDA + 2H). Bicyclo[2.2.2]octene 5,6-diesters undergo both the RDA + H and RDA + 2H fragmentations, and the relative abundance of the resulting [dienophile + H]⁺ and [dienophile + 2H]⁺˙ ions is strongly affected by configuration.     

Ion-molecule reactions in alkyl cyanides: Identification of reactive hydrogen in precursor ions

The relative abundance of [M + H]⁺ ions in the spectra of different nitriles depends on the nature of the nitrile. A new method for the identification of ion-molecule reactions has been applied, by determining the [M + D]⁺ ion intensity with respect to the [M + H]+ ion intensity in the spectra of partially deuteriated alkyl cyanides. This intensity ratio is correlated with the hydrogen-deuterium content of the suspected primary ions. In addition not only the reacting primary ions, but also the reactive hydrogen atom in the primary ion could be indicated. There is clear evidence that the proton attached to the nitrogen atom in the H₂C?C?N⁺˙? H rearrangement ion is transferred to the nitrile molecule.     

Understanding the Mechanisms of Unusually Fast HH,CH,and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon–carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co‐workers have demonstrated extremely rapid C?C reductive elimination from cis‐[AuPPh₃(4‐F‐C₆H₄)₂Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy‐atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis‐[Au(PPh₃)₂(4‐F‐C₆H₄)₂]⁺ was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for H?H, C?H, and C?C bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp³ carbon atoms. Metal–carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis‐[AuPPh₃(H)CH₃]⁺ predict that at ?52 °C, about 82 % of the reaction occurs by hydrogen‐atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.     

Trends in the reactions of gaseous phenyl pnictogen radical cations C6H5EH2*+ (E = N, P, As)

In context of an analysis of the effect of the central atom E of gaseous radical cations of phenyl pnictogens C(6)H(5)EH(2), E = N (1), P (2), and As (3), the mass spectrometric reactions of phenyl phosphane 2 have been re-investigated by D-labeling and by using methods of tandem mass spectrometry. The 70 eV mass spectrum of 2 shows the base peak for ion [M-2H](*+) and significant peaks for ions [M-H](+), [M-(2C,3H)](+), [M-PH] (*+), and [M-(C,P,2H)](+). Metastable 2(*+) fragments exclusively by loss of H(2), and the investigation of deuterated 2-d(2) shows that excessive H/D migrations occur before fragmentation. Other significant fragment ions in the mass spectrum of 2 arise by losses of C(2)H(2,) P, or HCP from the ion [M-H](+). This mass spectrometric behavior puts the radical cation 2(*+) in between the fragmentation reactions of aniline radical cation 1(*+) (loss of H and subsequent losses of C(2)H(2,) or HCN) and phenyl arsane radical cation 3(*+) (elimination of H(2) and loss of As from ion [M-H](+)). The fragmentation mechanisms of the radical cations 1(*+) -3(*+) and of related ions were analyzed by calculations of the enthalpy of relevant species at the stationary points of the minimum enthalpy reaction pathways using the DFT hybrid functionals UBHLYP/6-311+G(2d,p)//UBHLYP/6-311+G(d). The results show that, in contrast to ionized aniline 1(*+), the reactions of the derivatives 2(*+) and 3(*+) of the heavier main group elements P and As are characterized by an easy elimination of H(2)via a reductive elimination of group C(6)H(5)-E (E = P, As) and by a special stability of bicyclic isomers of 2(*+) and 3(*+). Thus, while 1(*+) rearranges by ring expansion and formation an 7-aza-tropylium cation by loss of H., the increased stability of bicyclic intermediates in the rearrangement of 2(*+) and in particular of 3(*+) results in separate rearrangement pathways. The origin of these effects is the more extended and diffuse nature of the 3p and 4p AO of P and As.     

Intriguing roles of reactive intermediates in dissociation chemistry of N-phenylcinnamides

In mass spectrometry of protonated N-phenylcinnamides, the carbonyl oxygen is the thermodynamically most favorable protonation site and the added proton is initially localized on it. Upon collisional activation, the proton transfers from the carbonyl oxygen to the dissociative protonation site at the amide nitrogen atom or the α-carbon atom, leading to the formation of important reactive intermediates. When the amide nitrogen atom is protonated, the amide bond is facile to rupture to form ion/neutral complex 1, [RC(6)H(4)CH[double bond, length as m-dash]CHCO(+)/aniline]. Besides the dissociation of the complex, proton transfer reaction from the α-carbon atom to the nitrogen atom within the complex takes place, leading to the formation of protonated aniline. The presence of electron-withdrawing groups favored the proton transfer reaction, whereas electron-donating groups strongly favored the dissociation (aniline loss). When the proton transfers from the carbonyl oxygen to the α-carbon atom, the cleavage of the C(α)-CONHPh bond results in another ion/neutral complex 2, [PhNHCO(+)/RC(6)H(4)CH[double bond, length as m-dash]CH(2)]. However, in this case, electron-donating groups expedited the proton transfer reaction from the charged to the neutral partner to eliminate phenyl isocyanate. Besides the cleavage of the C(α)-CONHPh bond, intramolecular nucleophilic substitution (a nucleophilic attack of the nitrogen atom at the β-carbon) and stepwise proton transfer reactions (two 1,2-H shifts) also take place when the α-carbon atom is protonated, resulting in the loss of ketene and RC(6)H(5), respectively. In addition, the H/D exchanges between the external deuterium and the amide hydrogen, vinyl hydrogens and the hydrogens of the phenyl rings were discovered by D-labeling experiments. Density functional theory-based (DFT) calculations were performed to shed light on the mechanisms for these reactions.     

分子CH_3NH=B:的结构及重排反应的理论研究

采用量子化学中的从头计算方法, 在MP2/6-31G(d,p)水平上研究了不饱和硼烯CH3NH=B:的结构及重排反应机理。结果表明, CH3NH=B:的单线态结构比三线态结构稳定, 该分子的基态是单线态。分子CH3NH=B:可以发生3种不同的重排反应。本文找到了这3种重排反应的过渡态, 并详细计算了不饱和硼烯CH3NH=B:重排反应的动力学函数, 据此讨论了不饱和硼烯CH3NH=B:的稳定性问题。