Regioselectivity in addition of atomic fluorine to alkenes

Irradiation of N₂F₄(NF₂) with propene and with 1,1-difluoroethene in the gas phase atroom temperature leads to the addition of F and NF₂ across the double bond. The ratioof terminal to internal addition of F to propene was found to be 1.4:1 after correctingfor the decomposition of vibrationally excited intermediates. In the case of 1,1-difluoroethene,the corrected ratio of F addition is 5:1 with addition at the methylene sitepredominating. The difference in selectivity of F addition to propene and to 1,1-difluoroethenecorrelates with spin density difference in the π-π* triplet state of the alkeneand with the difference in the HOMO coefficients of the substrate alkene, i.e. the patternof selectivity is that predicted by the State Correlation Diagram model of Shaik andCanadell.     

Regioselectivity in intramolecular addition of allylsilanes to conjugated dienones

The intramolecular addition of allylsilanes to conjugated dienones shows a divergence of regioselectivity as a function of reagent and substrate structure. The creation of 5-5, 6-5, 5-7, and 6-7 bicyclic ring systems is reported.     

Regioselectivity and equilibrium thermodynamics for addition of Rh-OH to olefins in water

Rhodium(III) tetra(p-sulfonato phenyl) porphyrin ((TSPP)Rh) aquo and hydroxo complexes react with a series of olefins in water to form beta-hydroxyalkyl complexes. Addition reactions of (TSPP)Rh-OH to unactivated terminal alkenes invariably occur with both kinetic and thermodynamic preferences to place rhodium on the terminal carbon to form (TSPP)Rh-CH(2)CH(OH)R complexes. Acrylic and styrenic olefins initially react to place rhodium on the terminal carbon to form Rh-CH(2)CH(OH)X as the kinetically preferred isomer but subsequently proceed to an equilibrium distribution of regioisomers where Rh-CH(CH(2)OH)X is the predominant thermodynamic product. Equilibrium constants for reactions of the diaquo rhodium(III) compound ([(TSPP)Rh(III)(H(2)O)(2)](-3)) in water with a series of terminal olefins that form beta-hydroxyalkyl complexes were directly evaluated and used in deriving thermodynamic values for addition of the Rh-OH unit to olefins. The DeltaG degrees for reactions of the Rh-OH unit with olefins in water is approximately 3 kcal mol(-1) less favorable than the comparable Rh-H reactions in water. Comparisons of the regioisomers and thermodynamics for addition reactions of olefins with Rh-H and Rh-OH units in water are presented and discussed.     

Regioselectivity in the palladium-catalyzed addition of carbon nucleophiles to carbocyclic derivatives.

The regioselectivity of Pd-catalyzed malonate additions and arylations to cycloalkenyl esters can be predicted by completing a stereochemical analysis of the Pd-pi-allyl complex. The Pd-catalyzed malonate additions which have the greatest degree of regioselectivity are in which substituents have a steric influence in blocking the incoming nucleophile. Cyclopentenyl substrates displayed lower regioselectivity than the cyclohexyl counterparts presumably due to increased planarity of the system. Arylations using tin and hypervalent silicon reagents were compared.     

自卤仿产生二卤卡宾的单电子转移过程

研究了从卤仿和氢氧化钠生成二卤卡宾的反应,根据化学和物理研究证明,是一单电子转移(SET)的反应机理.在类卡宾、三卤碳阴离子和二卤卡宾间设有平衡存在,卤素交换是单电子转移过程中形成的三卤甲基和二卤甲基游离基所引起的,它优先于卡宾对C=C的加成而发生,反应的初始电子给予体是CX3主要电子接受体是过量存在的卤仿.     

Regioselectivity and reactivity in the addition of trichloromethyl radicals to amides of unsaturated carboxylic acids

The AM1 method was used to analyze the factors that correlate with regioselectivity in the addition of radicals to 1,2-disubstituted unsaturated compounds. The rate constants of the addition of^.CCl₃ radicals to RCH=CHC(O)X (R = Ph, Me; X = N-pyrrolidyl) were determined by ESR. The analysis of the spin density distribution in mono- and 1,2-disubstituted alkenes and the experimental values for the rate constants of the addition of^.CCl₃ radicals to these alkenes allowed the authors to conclude that the efficiency of the addition of^.CC1₃ to unsaturated compounds depends only on steric effects.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 452–455, March, 1995.     

Regioselectivity in the addition of vinylmagnesium bromide to heteroarylic ketones: C- versus O-alkylation

The reactivity of heteroarylic ketones toward vinylmagnesium bromide (2) and the regiochemistry of the addition were investigated. The reactivity drastically increases when the carbonyl is conjugated with at least one aza group and the regiochemistry of the addition of the vinyl Grignard reagent depends on the carbonyl compound: in the series of di(heteroazolyl) ketones the O-alkylation product was observed as unique with di(1,3-benzothiazol-2-yl) ketone, and in different relative ratios with respect to the classic C-alkylation product with di(1,3-thiazol-2-yl) ketone, (1,3-benzothiazol-2-yl) (1,3-thiazol-2-yl) ketone, and di(1,3-benzoxazol-2-yl) ketone, whereas di(N-methylbenzimidazol-2-yl) ketone gave the exclusive formation of the carbinol. This behavior can be explained by the intervention of the delocalization power of the heterocyclic ring and this was confirmed by the results obtained from the reaction between vinylmagnesium bromide and a series of mixed (1,3-benzothiazol-2-yl) (para-substituted phenyl) ketones, that showed a relative O-/C-alkylation ratio dependent on the nature and on the electronic effect of the substituent on the phenyl ring. The results are in agreement with the existence of intermediate species bearing a negative charge on the benzylic carbonyl carbon atom, and make the O-alkylation reaction between vinyl Grignard reagents and carbonyl compounds no longer a rare case, since it was observed with a number of heterocyclic carbonyl compounds, such as (1,3-benzothiazol-2-yl) aryl ketones and di(heteroaryl) ketones of the pentatomic 1,3-heteroazolic series.     

混合多卤甲烷与金属镁反应产生溴氯卡宾及其反应机理的研究

应用CHBr3/CHCl3或CHBr3/CCl4和金属镁在一般的Grignard反应条件下与烯烃反应, 制备偕溴氯环丙烷衍生物。该反应原料易得, 反应条件温和, 产率良好, 后处理方便, 是制备偕溴氯环丙烷衍生物的一种简便的新方法。文中研究了上述反应的单电子转移和自由基链循环的复杂反应机理。     

Synthesis of dihalocarbene derivatives of estafiatin guaianolide

Adducts of dihalocarbenes generated from chloroform and bromoform were prepared by reaction with estafiatin guaianolide. Their structures were established by XSA. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 453–455, September–October, 2007.     

Regioselectivity in the addition of alkanethiols to vinylacetylene in superbasic media of alkali-metal hydroxides in dimethyl sulfoxide

Conclusions The reaction of vinylacetylene with alkanethiols is largely directed toward the formation of alkyl 1,3-butadien-l-yl sulfide with increasing basicity of the system consisting of an alkali-metal hydroxide and DMSO. A reduction in the basicity facilitates the formation of alkyl 2,3-butadien-l-yl sulfide.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2800–2803, December, 1985.     

Kinetics and mechanism of ozone addition to olefins and dienes

The mechanism of the initial stage of the ozonolysis of a series of olefins and trans-1,3-butadoiene has been investigated by the B3LYP density functional theory (DFT) method, B2PLYP double hybrid method based on DFT and the MP2 approximation, and CCSD coupled cluster method. Two possible butadiene and olefin ozonolysis mechanisms are considered: concerted 1,3-cycloaddition, which yields a primary ozonide (Criegee mechanism) and stepwise ozone addition via a biradical transition state (DeMore mechanism). The geometries of the initial and transition states and the energies of the elementary steps of the reaction have been determined. The geometric structures of the stationary states determining the rate constant of the reaction have been completely optimized using the above methods and the aug-cc-pVDZ basis set. The rate constants for both reaction pathways have been calculated. For butadiene, the contribution from nonconcerted addition can reaches 25%. According to the MRMP2 method, the overall rate constant (which includes both reaction pathways) is 1778 L mol^–1 s^–1; according to B2PLYP, 1640 L mol^–1 s^–1; according to CCSD, 1424 L mol^–1 s^–1 (aug-cc-pVDZ basis set). These results are in good agreement with experimental data (k = 3 × 10³ L mol^–1 s^–1 and with earlier calculations. The data calculated for olefins are also in agreement with experimental data.     

Molecular mechanism of hydrogen bromide addition to olefins

Experimental results on the products and the kinetics of the hydrogen bromide addition to alkenes-1 in non-polar solvents and on complex formation in hydrogen bromide-olefin-solvent systems are reviewed. Theoretical calculations concerning hydrogen bromide-olefin complexes are used along with the experimental evidence to suggest a molecular mechanism of addition involving intermediate cyclic structures.     

Regioselectivity of S-silylketene S,N acetals and titanium - (ate) - enethiolates addition to unsaturated ketones

(Z) S-Silylketene S,N acetals react with α-enones to afford exclusive 1-2 addition. Titanium enethiolates and titanium - ate - complexe enethiolates undergo 1-2, 1-4 addition.     

Kinetics and mechanism study of aniline addition to ethyl propiolate

The kinetics of the addition reaction of aniline to ethyl propiolate in dimethylsulfoxide (DMSO) as solvent was studied. Initial rate method was used to determine the order of the reaction with respect to the reactants, and pseudo‐first‐order method was used to calculate the rate constant. This reaction was monitored by UV–Vis spectrophotometer at 399 nm by the variable time method. On the basis of the experimental results, the Arrhenius equation for this reaction was obtained as log k = 6.07 ‐ (12.96/2.303 RT). The activation parameters, E_a, ΔH^#, ΔG^#, and ΔS^# at 300 K were 12.96, 13.55, 23.31 kcal mol^?1 and ?32.76 cal mol^?1 K^?1, respectively. The results revealed a first‐order reaction with respect to both aniline and ethyl propiolate. In addition, based on the experimental results and using also density functional theory (DFT) at B3LYP/6‐31G* level, a mechanism for this reaction was proposed. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 144–151, 2006     

Enantioselective addition of secondary phosphines to methacrylonitrile: catalysis and mechanism

A highly enantioselective intermolecular hydrophosphination reaction is described. The (Pigiphos)nickel(II)-catalyzed reaction of secondary phosphines and methacrylonitrile gives chiral 2-cyanopropylphosphines in good yield and high enantiomeric excess (ee's up to 94%; (R)-(S)-Pigiphos = bis[(R)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}cyclohexylphos phine). We propose a mechanism involving coordination of methacrylonitrile to the dicationic nickel catalyst followed by 1,4-addition of the phosphine, and then, rate-determining proton transfer. This mechanism is supported by (a) the experimentally determined rate law (rate = k'[Ni][methacrylonitrile][t-Bu(2)PH]), (b) a large primary deuterium isotope effect k(H)/k(D) = 4.6(1) for the addition of t-Bu(2)PH(D) at 28.3 degrees C in toluene-d(8), (c) the isolation and characterization of the species [Ni(kappa(3)-Pigiphos)(kappaN-methacrylonitrile)](2+), and (d) DFT calculations of model compounds.     

Kinetics and mechanism of base catalysed ethyl cyanoformate addition to aldehydes

The mechanism by which tertiary amines catalyse the formation of cyanohydrin carbonates from aldehydes and alkyl cyanoformates is investigated by means of a kinetic study. The reaction rate shows a second order dependence on amine concentration unless the amine is sterically hindered, when the rate has a first order dependence on amine concentration. The catalytic activity of the amine correlated with its pK_aH. On the basis of these results, a mechanism is proposed in which the amine acts as a base to activate a water molecule, which reacts with the ethyl cyanoformate generating cyanide in situ.     

Kinetics and mechanism of the addition of benzylamines to benzylidenediethylmalonates in acetonitrile

Kinetic studies of the benzylamine additions to benzylidenediethylmalonates (BDM: YC(6)H(4)CH[double bond]C(COOEt)(2)) in acetonitrile at 20.0 degrees C are reported. The rates in acetonitrile are consistent with that expected from the through-conjugative electron-accepting power of the activating groups, (COOEt)(2). The sign and magnitude of the cross-interaction constant, rho(XY) = -0.45, are in general agreement with those for the single-step amine additions to activated olefins. The kinetic isotope effects (k(H)/k(D) > 1.0) measured with deuterated benzylamines (XC(6)H(4)CH(2)ND(2)) increase with a stronger electron-acceptor substituent in benzylamines (partial differential sigma(X) > 0) and a stronger electron donor in the substrate (partial differential sigma(Y) < 0). These trends are the same as those found for benzylidene-1,3-indandiones but are exactly opposite to those for other activated olefin series, e.g., beta-nitrostyrene. It has been shown that the former series are thermodynamically controlled, whereas the latter are intrinsically controlled with a relatively strong transition state imbalance. The activation parameters, Delta H(++) and Delta S(++), also support our proposed transition state involving concurrent C(alpha)-N and C(beta)-H bond formation with a four-membered cyclic structure.     

Kinetics and mechanism of the addition of benzylamines to benzylidenemalononitriles in acetonitrile

Nucleophilic addition reactions of benzylamines (BA) to benzylidenemalononitrile (BMN) have been studied in acetonitrile at 15.0 degrees C. The rate is first-order with respect to both BA and BMN and no base catalysis is observed. The rate decreases as the electron-withdrawing power of the substituent (Y) in the substrate increases (rho Y < 0). This is in contrast to the similar reactions of beta-nitrostyrenes (NS) with BAs in acetonitrile and the addition reactions of NS and BMN in aqueous solution (rho Y > 0). This sign change of rho Y is considered to result from the strong electron-withdrawing power of the (CN)2 group in BMN, which leads to polarization of C alpha delta+C delta- (CN)2 in the transition state. The mechanism of amine addition to BMN in acetonitrile is radically different from that in water. The reaction is predicted to proceed concertedly in a single step with a hydrogen-bonded, four-center cyclic transition state.     

Regioselectivity in the ligand-assisted addition of vinylmagnesium bromide: an experimental and theoretical study on the gamma-alkoxycyclobutenone model

Compounds 2 (M = Mg) obtained in the mono addition of vinylmagnesium bromide to squarates are attractive structural models to determine the influence of complexation between magnesium(II) and the alkoxide group on the regioselectivity of the 1,2- versus 1,4-addition of organomagnesium by complex induced proximity effects (CIPE). The 1,4-addition is observed almost exclusively in the case of vinylmagnesium in THF solution with formation of hydroxyketones type 5, which are always side or minor products in the known reaction of alkenyllithium derivatives. A comparative study on the reactivity of alkenyllithium and magnesiun derivatives is reported. The high regioselectivitity observed in the 1,4-addition of vinylmagnesium bromide is fully understood by computational studies of compounds 2 (M = Mg) at the DFT level with the density functional B3LYP.