Reductions, Reductive Alkylations, and Intramolecular Cyclizations of Acyl Silanes with Samarium Diiodide or Tributyltin Hydride

A series of acyl silanes including aliphatic-, aromatic-, and bis-acyl silanes, as well as the acyl silanes bearing other substituents such as a bromine atom and alkenyl, succinimide, and carbonyl groups, were prepared, and their reactions with samarium diiodide or tributylstannane were studied. The reactions of acyl silanes occurred in various manners such as reductions, reductive alkylations, intramolecular radical cyclizations, pinacol couplings, aldol reactions, and Tishchenko reactions, depending on the nature of substrates and reaction conditions. Acyl silanes were generally reduced to give the corresponding alpha-silyl alcohols without transfer of silyl groups. Intramolecular radical cyclizations of 5-hexenoyl silanes and 1-silyl-1,5-pentanedione were realized to give alpha-silyl cyclopentanols and 1,2-cyclopentanediol derivatives, respectively. On treatment with samarium diiodide in tetrahydrofuran, 1-(trimethylsilyl)-1,6-hexanedione underwent a pinacol coupling reaction in the presence of t-BuOH, whereas it underwent a Tishchenko reaction in the presence of MeOH. The Tishchenko reaction of 1-silyl-1,5-pentanedione gave a delta-silyl-delta-lactone. On treating with samarium diiodide, 1-(trimethylsilyl)-1,5-hexanedione and 1,5-bis(trimethylsilyl)-1,6-hexanedione, underwent, respectively, intramolecular aldol reactions.     

Time-resolved gas-phase kinetic, quantum chemical, and RRKM studies of reactions of silylene with alcohols

Time-resolved kinetic studies of silylene, SiH(2), generated by laser flash photolysis of 1-silacyclopent-3-ene and phenylsilane, have been carried out to obtain rate constants for its bimolecular reactions with methanol, ethanol, 1-propanol, 1-butanol, and 2-methyl-1-butanol. The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF(6) bath gas, at room temperature. In the study with methanol several buffer gases were used. All five reactions showed pressure dependences characteristic of third body assisted association reactions. The rate constant pressure dependences were modeled using RRKM theory, based on E(0) values of the association complexes obtained by ab initio calculation (G3 level). Transition state models were adjusted to fit experimental fall-off curves and extrapolated to obtain k(∞) values in the range (1.9-4.5) × 10(-10) cm(3) molecule(-1) s(-1). These numbers, corresponding to the true bimolecular rate constants, indicate efficiencies of between 16% and 67% of the collision rates for these reactions. In the reaction of SiH(2) + MeOH there is a small kinetic component to the rate which is second order in MeOH (at low total pressures). This suggests an additional catalyzed reaction pathway, which is supported by the ab initio calculations. These calculations have been used to define specific MeOH-for-H(2)O substitution effects on this catalytic pathway. Where possible our experimental and theoretical results are compared with those of previous studies.     

Atmospheric chemistry of dimethyl phosphonate, dimethyl methylphosphonate, and dimethyl ethylphosphonate

Rate constants for the reactions of OH radicals and NO3 radicals with dimethyl phosphonate [DMHP, (CH3O)2P(O)H], dimethyl methylphosphonate [DMMP, (CH3O)2P(O)CH3], and dimethyl ethylphosphonate [DMEP, (CH3O)2P(O)C2H5] have been measured at 296 +/- 2 K and atmospheric pressure using relative rate methods. The rate constants obtained for the OH radical reactions (in units of 10(-12) cm3 molecule(-1) s(-1)) were as follows: DMHP, 4.83 +/- 0.25; DMMP, 10.4 +/- 0.6; and DMEP, 17.0 +/- 1.0, with a deuterium isotope effect of k(OH + DMMP)/k(OH + DMMP-d9) = 4.8 +/- 1.2. The rate constants obtained for the NO3 radical reactions (in units of 10(-16) cm3 molecule(-1) s(-1)) were as follows: DMHP, < 1.4; DMMP, 2.0 +/- 1.0; and DMEP, 3.4 +/- 1.4. Upper limits to the rate constants for the O3 reactions of < 8 x 10(-20) cm3 molecule(-1) s(-1) for DMHP and < 6 x 10(-20) cm3 molecule(-1) s(-1) for DMMP and DMEP were determined. Products of the reactions of OH radicals with DMHP, DMMP, and DMEP were investigated in situ using atmospheric pressure ionization mass spectrometry (API-MS) and, for the DMMP and DMEP reactions, Fourier transform infrared (FT-IR) spectroscopy. API-MS analyses showed the formation of products of molecular weight 96 and 126, attributed to CH3OP(O)(H)OH and (CH3O)2P(O)OH, respectively, from DMHP; of molecular weight 110, attributed to CH3OP(O)(CH3)OH, from DMMP; and of molecular weight 124 and 126, attributed to CH3OP(O)(C2H5)OH and (CH3O)2P(O)OH, respectively, from DMEP. FT-IR analyses showed formation (values given are % molar yields) of the following: from DMMP, CO, 54 +/- 6; CO2, 5 +/- 1 in dry air; HCHO, 3.9 +/- 0.7; HC(O)OH, < 1.4 in dry air; RONO2, approximately 4; and formate ester, approximately 8; and from DMEP, CO, 50 +/- 7; CO2, 11 +/- 4; CH3CHO, 18 +/- 8; HCHO, < 7; HC(O)OH, < 6; RONO2, < or = 5; and formate ester, 5.0 +/- 1.5. Possible reaction mechanisms are discussed.     

类锗烯H2C=GeLiCl与RH(R=F,OH, NH2)的插入反应

采用DFT B3LYP和QCISD方法研究了不饱和类锗烯H2C=GeLiCl与RH(R=F, OH, NH2)的插入反应. 在B3LYP/6-311+G(d,p)水平上优化了反应势能面上的驻点构型. 结果表明, H2C=GeLiCl与HF、H2O 或NH3发生插入反应的机理相同. QCISD/6-311++G(d,p)//B3LYP/6-311+G(d,p)计算的三个反应的势垒分别为173.53、194.48和209.05 kJ·mol-1, 反应热分别为60.18、72.93和75.34 kJ·mol-1. 相同条件下发生插入反应时, 反应活性顺序都是H—F>H—OH>H—NH2.     

Thermochemical kinetics of hydrogen-atom transfers between methyl, methane, ethynyl, ethyne, and hydrogen

Saddle point properties of three symmetric and one asymmetric hydrogen transfer and the energy of reaction of the asymmetric reactions are investigated in the present work. These reactions were calculated by various density functionals, many of which were developed in recent years, by coupled cluster theory, and by multicoefficient correlation methods based on wave function theory. Instead of comparing calculated results to "semi-experimental" values, we compared them to very accurate theoretical values (e.g., to values obtained by the Weizmann-1 method). Coupled cluster theory and the multicoefficient correlation methods MC-QCISD/3 and MCQCISD-MPW are very accurate for these reactions with mean unsigned errors below 0.94 kcal/mol. Diagnostics for multireference character add additional reliability to these results. The newly developed hybrid density functional M06-2X shows very good performance for these reactions with a mean unsigned error of only 0.77 kcal/mol; the BHandHLYP, MPW1K, and BB1K density functionals, can also predict these reactions well with mean unsigned errors less than 1.42 kcal/mol.     

Intramolecular homolytic translocation chemistry: an ab initio study of 1,n-silyl, germyl, and stannyl group transfer and related ring-closure reactions

Ab initio calculations using 6-311G**, cc-pVDZ, aug-cc-pVDZ, and a (valence) double-zeta pseudopotential (DZP) basis set, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional methods (B3LYP) predict that 1,n-homolytic transfers (n = 1-5) of silyl, germyl, and stannyl groups from group IV heteroatoms to carbon radicals can proceed via a frontside attack mechanism. At the B3LYP/DZP level of theory, energy barriers (DeltaE++) of 101.2, 98.8, 58.9, and 63.4 kJ/mol are calculated for the 1,2-, 1,3-, 1,4-, and 1,5-translocation reactions, respectively, of SiH3 between silicon atoms. Similar results are obtained for reactions involving germanium and tin with energy barriers (DeltaE++) of 85.9-113.1, 84.4-109.0, 41.7-73.3, and 48.5-78.2 kJ/mol for the 1,2-, 1,3-, 1,4-, and 1,5-translocation reactions, respectively. This study also predicts that four- and five-membered ring-closure reactions can be competitive with the 1,4- and 1,5-translocation reactions. These results suggest that while 1,2- and 1,3-translocation four-membered ring-formation reactions are unlikely to be synthetically viable, 1,4- and 1,5-transfers and five-membered ring-formation have synthetic possibilities.     

三重态类硅烯HB＝SiLiF的结构及其与R—H(R=F,OH,NH_2)的插入反应

用密度泛函理论(DFT)和二次组态相互作用(QCISD)方法研究了三重态类硅烯HB=SiLiF的结构及其与RH(R=F,OH,NH2)的插入反应.计算结果表明,类硅烯HB=SiLiF有三种平衡构型,其中四元环构型能量最低,是其存在的主要构型.HB=SiLiF与HF,H2O和NH3发生插入反应的机理相同.QCISD/6-311++G(d,p)//B3LYP/6-311+G(d,p)计算的三个反应的势垒分别为124.85,140.67和148.16kJ·mol-1,反应热分别为-2.22,20.08和23.22kJ·mol-1.相同条件下发生插入反应时,反应活性都是H—FH—OHH—NH2.     

Atmospheric chemistry of diethyl methylphosphonate, diethyl ethylphosphonate, and triethyl phosphate

Rate constants for the reactions of OH radicals and NO(3) radicals with diethyl methylphosphonate [DEMP, (C(2)H(5)O)(2)P(O)CH(3)], diethyl ethylphosphonate [DEEP, (C(2)H(5)O)(2)P(O)C(2)H(5)], and triethyl phosphate [TEP, (C(2)H(5)O)(3)PO] have been measured at 296 +/- 2 K and atmospheric pressure of air using relative rate methods. The rate constants obtained for the OH radical reactions (in units of 10(-11) cm(3) molecule(-1) s(-1)) were as follows: DEMP, 5.78 +/- 0.24; DEEP, 6.45 +/- 0.27; and TEP, 5.44 +/- 0.20. The rate constants obtained for the NO(3) radical reactions (in units of 10(-16) cm(3) molecule(-1) s(-1)) were the following: DEMP, 3.7 +/- 1.1; DEEP, 3.4 +/- 1.4; and TEP, 2.4 +/- 1.4. For the reactions of O(3) with DEMP, DEEP, and TEP, an upper limit to the rate constant of <6 x 10(-20) cm(3) molecule(-1) s(-1) was determined for each compound. Products of the reactions of OH radicals with DEMP, DEEP, and TEP were investigated using in situ atmospheric pressure ionization mass spectrometry (API-MS) and, for the TEP reaction, gas chromatography with flame ionization detection (GC-FID) and in situ Fourier transform infrared (FT-IR) spectroscopy. The API-MS analyses show that the reactions are analogous, with formation of one major product from each reaction: C(2)H(5)OP(O)(OH)CH(3) from DEMP, C(2)H(5)OP(O)(OH)C(2)H(5) from DEEP, and (C(2)H(5)O)(2)P(O)OH from TEP. The FT-IR and GC-FID analyses showed that the major products (and their molar yields) from the TEP reaction are (C(2)H(5)O)(2)P(O)OH (65-82%, initial), CO(2) (80 +/- 10%), and HCHO (55 +/- 5%), together with lesser yields of CH(3)CHO (11 +/- 2%), CO (11 +/- 3%), CH(3)C(O)OONO(2) (8%), organic nitrates (7%), and acetates (4%). The probable reaction mechanisms are discussed.     

Kinetics of sulfur oxide, sulfur fluoride, and sulfur oxyfluoride anions with atomic species at 298 and 500 K

The rate constants and product-ion branching ratios for the reactions of sulfur dioxide (SO2-), sulfur fluoride (SFn-), and sulfur oxyfluoride anions (SOxFy-) with H, H2, N, N2, NO, and O have been measured in a selected-ion flow tube (SIFT). H atoms were generated through a microwave discharge on a H2/He mixture, whereas O atoms were created via N atoms titrated with NO, where the N had been created by a microwave discharge on N2. None of the ions reacted with H2, N2 or NO; thus, the rate constants are <1 x 10(-12) cm3 s-1. SOxFy- ions react with H by only fluorine-atom abstraction to form HF at 298 and 500 K. Successive F-atom removal does not occur at either temperature, and the rate constants show no temperature dependence over this limited range. SO2- and F- undergo associative detachment with H to form a neutral molecule and an electron. Theoretical calculations of the structures and energetics of HSO2- isomers were performed and showed that structural differences between the ionic and neutral HSO2 species can account for at least part of the reactivity limitations in the SO2- + H reaction. All of the SOxFy- ions react with O; however, only SO2- reacts with both N and O. SOxFy- reactions with N (SO2- excluded) have a rate constant limit of <1 x 10(-11) cm3 s-1. The rate constants for the SOxFy- reactions with H and O are < or =25% of the collision rate constant, as seen previously in the reactions of these ions with O3, consistent with a kinetic bottleneck limiting the reactivity. The only exceptions are the reactions of SO2- with N and O, which are much more efficient. Three pathways were observed with O atoms: F-atom exchange in the reactant ion, F- exchange in the reactant ion, and charge transfer to the O atom. No associative detachment was observed in the N- and O-atom reactions.     

Enthalpies of formation, bond dissociation energies, and molecular structures of the n-aldehydes (acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal) and their radicals

Aldehydes are important intermediates and products in a variety of combustion and gas-phase oxidation processes, such as in low-temperature combustion, in the atmosphere, and in interstellar media. Despite their importance, the enthalpies of formation and bond dissociation energies (BDEs) for the aldehydes are not accurately known. We have determined enthalpies of formation for acetaldehyde, propanal, and butanal from thermodynamic cycles, using experimentally measured reaction and formation enthalpies. All enthalpy values used for reference molecules and reactions were first verified to be accurate to within around 1 kcal mol-1 using high-level ab initio calculations. Enthalpies of formation were found to be -39.72 +/- 0.16 kcal mol-1 for acetaldehyde, -45.18 +/- 1.1 kcal mol-1 for propanal, and -49.27 +/- 0.16 kcal mol-1 for butanal. Enthalpies of formation for these three aldehydes, as well as for pentanal, hexanal, and heptanal, were calculated using the G3, G3B3, and CBS-APNO theoretical methods, in conjunction with bond-isodesmic work reactions. On the basis of the results of our thermodynamic cycles, theoretical calculations using isodesmic work reactions, and existing experimental measurements, we suggest that the best available formation enthalpies for the aldehydes acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal are -39.72, -45.18, -50.0, -54.61, -59.37, and -64.2 kcal mol-1, respectively. Our calculations also identify that the literature enthalpy of formation of crotonaldehyde is in error by as much as 1 kcal mol-1, and we suggest a value of -25.1 kcal mol-1, which we calculate using isodesmic work reactions. Bond energies for each of the bonds in the aldehydes up to pentanal were calculated at the CBS-APNO level. Analysis of the BDEs reveals the R-CH(2)CH=O to be the weakest bond in all aldehydes larger than acetaldehyde, due to formation of the resonantly stabilized vinoxy radical (vinyloxy radical/formyl methyl radical). It is proposed that the vinoxy radical as well as the more commonly considered formyl and acetyl radicals are important products of aldehyde combustion and oxidation, and the reaction pathways of the vinoxy, formyl, and acetyl radicals are discussed. Group additivity values for the carbon-oxygen-hydrogen groups common to the aldehydes are also determined. Internal rotor profiles and electrostatic potential surfaces are used to study the dipole induced dipole-dipole interaction in the synperiplanar conformation of propanal. It is proposed that the loss of this dipole-dipole interaction in RC(.-)HCH(2)CH=O radicals causes a ca. 1-2 kcal mol-1 decrease in the aldehyde C-H and C-C bond energies corresponding to RC(.-)HCH(2)CH=O radical formation.     

Theoretical study of the gas-phase thermolysis reaction of alkyl (ethyl,isopropyl, and tert-butyl) N,N-dimethylcarbamates and N,N-diethylcarbamates

Theoretical studies on the thermolysis in the gas phase of alkyl N,N-dialkylcarbamates were carried out using ab initio theoretical methods, at the MP2/6-31G(d), MP2/6-31++G(d,p) and MP2/6-311++G(2d,p)//MP2/6-31G(d) levels. The reactions have two steps: the first one corresponds to the formation of an alkene and a neutral dialkylcarbamic acid intermediate via a six-membered cyclic transition state; the second one is the decarboxylation of this intermediate via a four-membered cyclic transition state, leading to carbon dioxide and the corresponding dialkylamine. The progress of the reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states have character intermediate between reactants and products, and the calculated synchronicities show that the reactions are slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming ones, indicating a bond deficiency in the transition states. The rate constants calculated for all the reactions agree very well with the available experimental data.From the Proceedings of the 28th Congreso de Químicos Teóricos de Expresión Latina (QUITEL 2002)     

Chiral hetero Diels-Alder products by enantioselective and diastereoselective zirconium catalysis. Scope,limitation, mechanism,and application to the concise synthesis of (+)-Prelactone C and (+)-9-deoxygoniopypyrone

Catalytic asymmetric hetero Diels-Alder (HDA) reactions using a chiral zirconium complex have been developed. The reactions of aldehydes with Danishefsky's dienes proceeded smoothly to afford the corresponding pyranone derivatives in high yields with high diastereo- and enantioselectivities in the presence of a chiral zirconium complex, which was prepared from zirconium tert-butoxide, (R)-3,3'-diiodobinaphthol or its derivative, a primary alcohol, and a small amount of water. It is noted that 2,3-trans-pyranone derivatives were obtained with remarkably high diastereo- and enantioselectivities in the reaction with 4-methyl Danishefsky's diene. This is the first example of catalytic asymmetric trans-selective hetero Diels-Alder reactions of aldehydes. Furthermore, asymmetric HDA reactions with 4-benzyloxy Danishefsky's dienes were conducted to afford 2,3-cis-pyranone derivatives in high selectivities. Isolation of an intermediate of this asymmetric hetero Diels-Alder reaction indicated that the reaction proceeded in a stepwise cycloaddition pathway. Finally, these catalytic, asymmetric hetero Diels-Alder reactions were successfully applied to concise syntheses of biologically important natural pyranone derivatives, (+)-Prelactone C and (+)-9-deoxygoniopypyrone.     

Kinetics and mechanisms of the tropospheric reactions of menthol, borneol, fenchol, camphor, and fenchone with hydroxyl radicals (OH) and chlorine atoms (Cl)

Relative kinetic techniques have been used to measure the rate coefficients for the reactions of oxygenated terpenes (menthol, borneol, fenchol, camphor, and fenchone) and cyclohexanol with hydroxyl radicals (OH) and chlorine atoms (Cl) at 298 ± 2 K and atmospheric pressure. The rate coefficients obtained for the reactions of the title compounds with OH are the following (in units of 10(-11) cm(3) molecule(-1) s(-1)): (1.48 ± 0.31), (2.65 ± 0.32), (2.49 ± 0.30), (0.38 ± 0.08), (0.39 ± 0.09) for menthol, borneol, fenchol, camphor, and fenchone, respectively. For the corresponding reactions with Cl atoms the rate coefficients are as follows (in units of 10(-10) cm(3) molecule(-1) s(-1)): (3.21 ± 0.26), (3.40 ± 0.28), (2.72 ± 0.13), (2.93 ± 0.17), (1.59 ± 0.10), and (1.86 ± 0.29) for cyclohexanol, menthol, borneol, fenchol, camphor, and fenchone, respectively. The reported error is twice the standard deviation. Product studies of the reactions were performed using multipass in situ FTIR (Fourier transform infrared spectroscopy) and solid-phase microextraction (SPME) with analysis by GC-MS (gas chromatography-mass spectrometry). A detailed mechanism is proposed to justify the observed reaction products.     

Kinetic study of the phthalimide N-oxyl radical in acetic acid. Hydrogen abstraction from substituted toluenes, benzaldehydes, and benzyl alcohols

The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO* is 1.36 x 10(3) L mol(-1) cm(-1) at lambda(max) 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 +/- 0.1 L mol(-1) s(-1) at 25 degrees C. The reactions of PINO(*) with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with sigma+ (rho = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with sigma (rho = -0.91). The kinetic isotope effect was also studied and significantly large values of k(H)/k(D) were obtained: 25.0 (p-xylene), 27.1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25 degrees C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d(10), the difference of the activation energies, E(a)(D) - E(a)(H), was 12.6 +/- 0.8 kJ mol(-1) and the ratio of preexponential factors, A(H)/A(D), was 0.17 +/- 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.     

类硅烯H2C＝SiLiBr与RH (R＝F, OH, NH2)的插入反应

采用DFT B3LYP和QCISD方法研究了类硅烯H2C＝SiLiBr与RH (R＝F, OH, NH2)的插入反应. 在B3LYP/6- 311＋G(d,p)水平上优化了反应势能面上的驻点构型. 结果表明, H2C＝SiLiBr与HF, H2O或NH3发生插入反应的机理相同. QCISD/6-311＋＋G(d,p)//B3LYP/6-311＋G(d,p)计算的三个反应的势垒分别为148.62, 164.42和165.07 kJ&#8226;mol－1, 反应热分别为－69.63, －43.02和－28.27 kJ&#8226;mol－1. 相同条件下发生插入反应时, 反应活性都是H—F＞H—OH＞H—NH2.     

Syntheses, crystal and solution structures, ligand-exchange, and ligand-coupling reactions of mixed pentaarylantimony compounds

All possible combinations of mixed pentaarylantimony compounds bearing p-methylphenyl and p-trifluoromethylphenyl groups were synthesized; ArnTol5-nSb (n=0-5: Ar=p-CF3C6H4, Tol=p-CH3C6H4): Tol5Sb (1), ArTol4Sb (2), Ar2Tol3Sb (3), Ar3Tol2Sb (4), Ar4TolSb (5), and Ar5Sb (6). Compounds 2-5 are the first well-characterized examples of mixed acyclic pentaarylantimony species. The structures of 2-6 were determined by X-ray crystallography to feature trigonal-bipyramidal (TBP) geometry with the more electronegative p-trifluoromethylphenyl substituents selectively occupying the apical positions. Consideration of the chemical shifts of the ipso carbons of the aryl and tolyl groups suggested that the solution structures of 1-6 were also TBP, although their pseudorotation could not be frozen even at -80 degrees C. Ligand-exchange reactions (LERs) took place between 1 and 6 at approximately 60 degrees C in [D6]benzene and all six species 1-6 were found in the equilibrium mixture. The relative stabilities of 1-6 were determined quantitatively by comparison of the observed molar ratios of 1-6 in equilibrium with calculated statistical molar ratios, and Ar2Tol3Sb (3) was found to be the most stable. The ligand-coupling reactions (LCRs) of 2-5 in solution were greatly accelerated by adding Cu(acac)2 or Li+TFPB- (TFPB: [3,5-(CF3)2 C6H3]4 B), whereby the rate becomes comparable to the LER. The use of flash vacuum thermolysis (FVT) allowed the LCR to occur with very little ligand-exchange; the exception ArTol4Sb had very fast ligand-exchange. The selectivities of the LCRs were calculated from the yield of the biaryls synthesized by using FVT. These results were highly consistent with reactions catalyzed in solution, in which bitolyl was not obtained at all. The experimental results suggested that the LCR of pentaarylantimony compounds proceeds in the manner of apical-apical coupling.     

Reactivity and selectivity of the N-acetyl-Glu-P-1, N-acetyl-Glu-P-2, N-acetyl-MeIQx,and N-acetyl-IQx nitrenium ions: comparison to carbocyclic N-arylnitrenium ions

The model ultimate carcinogens 1a-d, related to the metabolites of the food-derived carcinogenic heterocyclic amines Glu-P-1, Glu-P-2, MeIQx, and IQx, spontaneously decompose in neutral aqueous solution to generate the heterocyclic nitrenium ions, 2a-d. The less reactive esters 1a and 1b also undergo acid-catalyzed ester hydrolysis to generate the corresponding hydroxamic acids at pH <2, while the more reactive 2c and 2d are prone to rearrangement in nonaqueous solvents. The reactions of the nitrenium ions with AcO(-), HPO(4)(2-), N(3)(-), and 2'-deoxyguanosine (d-G) were characterized in aqueous solution by using a combination of competitive trapping methods and product isolation and identification. The reactions with N(3)(-) and d-G generally follow patterns previously established for carbocyclic nitrenium ions, but the reactions with AcO(-) and HPO(4)(2-) are unusual. Similar reactions have previously only been reported for heterocyclic 1-alkyl-2-imidazolium ions. The N(3)(-)/solvent selectivities of these ions (5.1 x 10(6) M(-1) for 2a, 2.3 x 10(6) M(-1) for 2b, 1.2 x 10(5) M(-1) for 2c, and 5.2 x 10(4) M(-1) for 2d) are comparable to those of highly selective carbocyclic nitrenium ions. If k(az) for these ions is diffusion limited at ca. 5 x 10(9) M(-1) s(-1) the aqueous solution lifetimes of these ions range from 10 micros (2d) to 1 ms (2a). These ions are also highly selective for trapping by d-G, but comparisons to other nitrenium ions show that they are 10- to 50-fold less selective for trapping by d-G than they would be if both the N(3)(-) and d-G reactions were diffusion limited. This is not a consequence of their heterocyclic structures. Several carbocyclic ions show similar behavior. The relatively inefficient trapping of 2c and 2d by d-G may account for the observation of the unusual minor N-2 d-G adduct that is isolated for both of these nitrenium ions, but has not previously been observed for the reactions of other nitrenium ions with monomeric d-G.     

Gas-phase synthesis of the homo and hetero organocuprate anions [MeCuMe]-, [EtCuEt]-, and [MeCuR]-

The homocuprates [MeCuMe]- and [EtCuEt]- were generated in the gas phase by double decarboxylation of the copper carboxylate centers [MeCO2CuO2CMe]- and [EtCO2CuO2CEt]-, respectively. The same strategy was explored for generating the heterocuprates [MeCuR]- from [MeCO2CuO2CR]- (R = Et, Pr, iPr, tBu, allyl, benzyl, Ph). The formation of these organocuprates was examined by multistage mass spectrometry experiments, including collision-induced dissociation and ion-molecule reactions, and theoretically by density functional theory. A number of side reactions were observed to be in competition with the second stage of decarboxylation, including loss of the anionic carboxylate ligand and loss of neutral alkene via beta-hydride transfer elimination. Interpretation of decarboxylation of the heterocarboxylates [MeCO2CuO2CR]- was more complex because of the possibility of decarboxylation occurring at either of the two different carboxylate ligands and giving rise to the possible isomers [MeCuO2CR]- or [MeCO2CuR]-. Ion-molecule reactions of the products of initial decarboxylation with allyl iodide resulted in C-C coupling to produce the ionic products [ICuO2CR]- or [MeCO2CuI]-, which provided insights into the relative population of the isomers, and indicated that the site of decarboxylation was dependent on R. For example, [MeCO2CuO2CtBu]- underwent decarboxylation at MeCO2- to give [MeCuO2CtBu]-, while [MeCO2CuO2CCH2Ph]- underwent decarboxylation at PhCH2CO2- to give [MeCO2CuCH2Ph]-. Each of the heterocuprates [MeCuR]- (R = Et, Pr, iPr, allyl, benzyl, Ph) could be generated by the double decarboxylation strategy. However, when R = tBu, intermediate [MeCuO2CtBu]- only underwent loss of tBuCO2-, a consequence of the steric bulk of tBu disfavoring decarboxylation and stabilizing the competing channel of carboxylate anion loss. Detailed DFT calculations were carried out on the potential energy surfaces for the first and second decarboxylation reactions of all homo- and heterocuprates, as well as possible competing reactions. These reveal that in all cases the first decarboxylation reaction is favored over loss of the carboxylate ligand. In contrast, other reactions such as carboxylate ligand loss and beta-hydride transfer become more competitive with the second decarboxylation reaction.     

Recognition of cyclic, acyclic, exocyclic, and spiro acetals via structurally diagnostic ion/molecule reactions with the (CH3)2N-C(+)=O acylium ion

Reactions of the model acylium ion (CH3)2N-C(+)=O with acyclic, exocyclic, and spiro acetals of the general formula R(1)O-CR(3)R(4)-OR(2) were systematically evaluated via pentaquadrupole mass spectrometry. Characteristic intrinsic reactivities were observed for each of these classes of acetals. The two most common reactions observed were hydride and alkoxy anion [R(1)O(-) and R(2)O(-)] abstraction. Other specific reactions were also observed: (a) a secondary polar [4(+) + 2] cycloaddition for acetals bearing alpha,beta-unsaturated R(3) or R(4) substituents and (b) OH(-) abstraction for exocyclic and spiro acetals. These structurally diagnostic reactions, in conjunction with others observed previously for cyclic acetals, are shown to reveal the class of the acetal molecule and its ring type and substituents and to permit their recognition and distinction from other classes of isomeric molecules.     

Highly anti-selective asymmetric aldol reactions using chiral zirconium catalysts. Improvement of activities, structure of the novel zirconium complexes, and effect of a small amount of water for the preparation of the catalysts

Catalytic asymmetric aldol reactions of silyl enol ethers with aldehydes (Mukaiyama aldol reactions) have been performed using novel chiral zirconium catalysts. The reactions proceeded in high yields under mild conditions, and anti-adducts were obtained in high diastereo- and enantioselectivities. The catalysts were first prepared from zirconium(IV) tert-butoxide (Zr(O(t)Bu)(4)), (R)-3,3'-diiodo-1,1'-binaphthalene-2,2'-diol ((R)-3,3'-I(2)BINOL), a primary alcohol, and a small amount of water. It was revealed that the primary alcohol played an important role in completing the catalytic cycle and that a small amount of water was essential for obtaining high selectivities. Moreover, activities of the chiral zirconium catalysts were enhanced by using new ligands, (R)-3,3'-I(2)-6,6'-X(2)BINOL (X = Br, I, C(2)F(5)), and it has been shown that even aldol reactions of less reactive substrates proceeded smoothly using the novel zirconium catalysts. Finally, NMR studies of these catalysts were performed, which suggested that the catalyst would form a dimeric structure and that the water affected the catalyst formation.