摘要:

ArCH2NH2 + Ph—CH C(CN)(C6H4NO2) PhCH NHCH2Ar CH(CN)(C6H4NO2) (1) Ph CH C CN Ar Ph CH C CN Ar + H2NR N+H2R – k1 k–1 1 1 Ph CH CH CN Ar NHR k3 [H2NR] R = CH2Ph Ar = p-NO2C6H4 388 J. CHEM. RESEARCH (S) 1997 J. Chem. Research (S) 1997 388–389 J. Chem. Research (M) 1997 2358–2377 Correlation Analysis of Reactivity in the Addition of Substituted Benzylamines to a-Cyano-4-nitrostilbene Bindu Varghese Deepa Suri Seema Kothari and Kalyan K. Banerji* Department of Chemistry J.N.V. University Jodhpur 342 005 India The addition of benzylamine to a-cyano-4-nitrostilbene involves the formation of a zwitterionic species in an equilibrium and its subsequent decomposition catalysed by a second molecule of the amine. Synthetic and mechanistic studies of the additions to activated carbon–carbon double bonds are of immense importance.However not many reports are available about the addition of neutral nucleophiles to activated double bonds.5–10 In this paper we report the addition of a number of monosubstituted benzylamines to a-cyano-4-nitrostilbene (CNS). Attempts have been made to correlate the rate and structure in this reaction. CNS was prepared by the reported method.11 The reaction was studied under pseudo-first-order conditions by keeping a large excess (Å10 or greater) of benzylamine over CNS. The solvent was acetonitrile. The reaction was followed spectrophotometrically by monitoring the decrease in [CNS] at 340 nm for ca. 80% reaction. The pseudo-first-order rate constant kobs was evaluated from the linear (r2a0.998) plots of log [CNS] vs. time. Addition of benzylamine to CNS leads to the formation of 1-benzylamino-2-cyano-1-phenyl-2-(4-nitrophenyl)ethane PhCH(PhCH2NH)CH(CN)(C6H4NO2) as ascertained by its 1H NMR spectrum.The overall reaction may be represented as eqn. (1). The reaction is first order with respect to CNS. Values of kobs increase with an increase in the concentration of benzylamine. The apparent order in [amine] as determined by a log–log plot is greater than unity (slope=1.88�0.01; r2=0.9998). It was observed that a plot of kobs/[amine] vs. [amine] is curvilinear with a negligible intercept. The absence of an intercept indicates that an uncatalysed pathway is not important in this reaction. Therefore a mechanism involving formation of the zwitterionic intermediate in the pre-equilibrium with its subsequent decompsition into the product via a proton transfer catalysed by a second molecule of the amine is proposed (Scheme 1).The addition of deuterated benzylamine to CNS exhibited a substantial primary kinetic isotope effect kH/kD=5.49 at 293 K. This confirmed the cleavage of an N·H bond in the rate-determining step and supports the proposed mechanism. Application of the steady-state treatment to this mechanism (Scheme 1) gives the rate law (3). kobs [amine] =k2= k1k3[amine] kµ1+k3[amine] (3) A plot of (k2)µ1 against [amine]µ1 is a straight line (r2=0.9990). The inverse of the intercept gives the rate constant k1 of the nucleophilic attack by amine on CNS. The kinetics of the addition of benzylamine and 27 mono-substituted benzylamines to CNS were studied. The kinetics were similar in all cases. The rate constants k1 at different temperatures were evaluated.The activation parameters were also calculated. The rate constants for addition of meta- and para-substituted benzylamines do not show any significant correlation with the pKa values of the amine or the Hammett s values. *To receive any correspondence. Scheme 1 Table 3 Rate constants k1 at different temperatures for the addition of substituted benzylamines to CNSa k1/mol dmµ3 sµ1 k1/mol dmµ3 sµ1 Subst. 293 K 303 K 313 K 323 K Subst. 293 K 303 K 313 K 323 K Hp -Me p-OMe p-F p-Cl p-NO2 p-CF3 p-CO2Me p-Br p-HNAc o-Me o-OMe o-F o-Cl 3.51 5.65 9.20 3.15 2.18 0.32 0.76 0.94 2.13 5.31 2.69 3.65 1.43 0.83 5.54 8.65 13.7 5.04 3.56 0.61 1.33 1.62 3.51 8.22 4.31 5.90 2.45 1.46 8.82 13.7 19.7 7.95 5.78 1.11 2.31 2.83 5.80 12.6 6.98 9.39 4.15 2.57 13.8 20.7 29.6 12.7 9.43 2.00 3.98 4.79 9.28 19.6 10.9 14.5 6.80 4.36 o-Br o-NO2 o-CF3 o-CO2Me o-NHAc m-Me m-OMe m-F m-Cl m-I m-NO2 m-CF3 m-CO2Me m-NH2 0.72 0.20 0.25 0.51 1.58 4.61 3.56 1.33 1.22 1.40 0.31 0.78 1.01 6.89 1.27 0.38 0.47 0.91 2.66 6.69 5.42 2.23 2.07 2.36 0.61 1.43 1.79 9.76 2.25 0.73 0.91 1.69 4.49 10.9 8.83 3.89 3.61 4.03 1.17 2.53 3.12 15.2 3.85 1.37 1.67 2.95 7.33 17.0 14.0 6.50 6.03 6.75 2.06 4.31 5.22 23.3 aAbbreviated version of the corresponding table contained in the full text.J. CHEM. RESEARCH (S) 1997 389 Similarly the rate constants for the ortho- compounds did not correlate well with s0 values.21 The correlation of the rate constants for meta and para compounds in terms of Taft’s dual substituent parameter equation22 also was poor. The rate constants k1 were therefore analysed in terms of Charton’s23 LDR eqn.(8) log k1=Ls1+Dsd+Rse+h (8) Here s is a localized (field and/or inductive) effect parameter sd is the intrinsic delocalized (resonance) electrical effect parameter when active-site electronic demand is minimal and se represents the sensitivity of the substituent to change in electronic demand by the active site. The latter two substituent parameters are related by eqn. (9) sD=nse+sd (9) where n represents the electronic demand of the reaction site which is given by n=R/D and sD represents the delocalized electrical parameter of the diparametric LD equation. For ortho-substituted compounds it is necessary to account for the possibility of steric effects. The LDR equation is therefore modified to LDRS eqn. (10) where V is the well known Charton’s steric parameter based on Van der Waals radii.24 log k1=Ls1+Dsd+Rse+SV+h (10) The rates of addition of the ortho- meta- and para-substituted benzylamines showed excellent correlations with LDR/ LDRS equations with all the four regression coefficients L D R and S being negative.The negative values of L D and R indicate an electron-deficient reaction centre in the transition state of the reaction. The positive value of n adds a negative increment to sd [eqn. (9)] increasing the donor effect of the substituent where sd is negative and decreasing the acceptor effect where sd is positive. The substituent is therefore better able to stabilize a cationic reaction site. This also supports the presence of an electron-deficient centre in the transition state of the rate-determining step.The magnitude of n points to a relatively small electronic demand of the reaction centre. The negative value of S indicates that the reaction is subject to steric hindrance by the ortho substituent. This may be due to steric hindrance of the ortho substituent to the approach of amines to CNS. Comparison of the L and D values for the substituted benzylamines showed that the addition of para-substituted benzylamines is more susceptible to delocalization rather than localized effects. However the addition of ortho- and meta-substituted compounds exhibits a greater dependence on the field effect. Thanks are due to the University Grants Commission (India) for financial support. Techniques used 1H NMR spectrophotometry correlation analysis References 25 Figure 1 Table 1 Rate constants for the addition of benzylamine to CNS at 293 K Table 2 Rate constants k1 at different temperatures and activation parameters for the addition of substituted benzylamines to CNS Table 3 Correlation analysis of the rates of addition of meta- and para-substituted benzylamines to CNS with Taft’s dual substituentparameters at 298 K Table 4 Correlation analysis of the rates of addition of substituted benzylamines to CNS with LDR/LDRS equations at different temperatures Received 24th December 1996; Accepted 25th July 1997 Paper E/7/08621G References cited in this synopsis 5 C.F. Bernasconi R. A. Rentfrow and P. R. Tia J. Am. Chem. Soc. 1986 108 4541. 6 C. F. Bernasconi and M. Panda J. Org. Chem. 1987 52 3042. 7 R. Kada V. Knoppova J. Kovac and I. Malenakova Collect. Czech. Chem. Commun. 1984 49 2496. 8 A. F. Popov I. F. Perepichka and L. I. Kostenko J. Chem. Soc. Perkin Trans. 2 1989 395. 9 C. F. Bernasconi d R. B. Killion J. Org. Chem. 1989 54 2878. 10 A. Shunmugasundaram L. Thanuligam and R. Murugesan Indian J. Chem. Sect. A 1991 30 609. 11 A. Schonne E. Braye and A. Bruylants Bull. Soc. Chim. Belg. 1953 62 155. 21 M. T. Tribble and J. G. Traynham J. Am. Chem. Soc. 1969 91 379. 22 S. Dayal S. Ehrenson and R. W. Taft J. Am. Chem. Soc. 1972 94 9113. 23 M. Charton and B. Charton Bull. Soc. Chim. Fr. 1988 199 and references cited therein. 24 M. Charton J. Org. Chem. 1975 40 407.

ISSN:0308-2342    

DOI:10.1039/a608621g

出版商:RSC    

年代:1997

数据来源: RSC