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J. Org. Chem. 1996, 61, 5656-5658
Mechanism of Elimination from (1-Anilino-1-cyanoethyl)benzene Promoted by Sodium Methoxide in Methanol Bong Rae Cho,*,1a Yoon Chul Oh,1a Sang Hae Lee,1a and Young Ja Park1b Department of Chemistry, Korea University, 1-Anamdong, Seoul 136-701, Korea, and Department of Chemistry, Sook Myung Women’s University, Seoul 140-742, Korea Received February 7, 1996
Introduction Extensive studies on the imine-forming elimination reactions have revealed that the reactions are much more facile than the corresponding alkene-forming eliminations and proceed via E2-central or E1-like transition states.2 For MeONa-promoted eliminations from ArCH2N(OSO2Ar)R, a competing E2 and (E1)ip mechanism was also reported.3 All of these studies utilized 1, in which the leaving group is attached to the nitrogen. Accordingly, many of the characteristics of the imineforming eliminations have been attributed to the weaker strength of the N-X bond and greater CdN bond energy than those involved in the corresponding alkene-forming eliminations. In contrast, little is known about the imine-forming eliminations from 2, in which the leaving group is attached to the carbon. Since C-X and N-H bonds are stronger than N-X and C-H bonds, respectively,4,5 eliminations from 1 are expected to be more exothermic than those from 2. On the other hand, the N-H bond is more acidic than the C-H bond.6,7 Hence, if the acidity of the β-proton is more important, the latter should react at a faster rate. However, the relative importance of these two opposing factors on the imine-forming eliminations has not been investigated in detail.
It has been reported that the pyrolytic elimination of (1-anilino-1-cyanoethyl)benzene proceeds at 210 °C to afford N-phenylacetophenonimine in high yield.8 The result is somewhat surprising considering the facility of the imine-forming elimination reactions. In order to add to the meager store of information available concerning imine-forming eliminations from 2, we have investigated (1) (a) Korea University. (b) Sook Myung Women's University. (2) Hoffman, R. V.; Bartsch, R. A.; Cho, B. R. Acc. Chem. Res. 1989, 22, 211-217. (3) Cho, B. R.; Pyun, S. Y. J. Am. Chem. Soc. 1991, 113, 39203924. (4) Bond energies for C-H, C-Cl, CH3-OH, N-H, N-Cl, and CH3N-OH are 85, 80, 80, 62, 90, and 50, respectively,5 indicating that the N-H and C-X bonds are stronger than the C-H and N-X bonds. (5) Dean, J. A. Handbook of Organic Chemistry; McGraw-Hill: New York, 1987; pp 3-13-3-37. (6) The pKa values of HCl, HCN, C6H5NH2, and C6H5CH3 in DMSO are 1.8, 12.9, 30.6, and 43, respectively.7 Although the corresponding values determined in MeOH are not available in the literature, the relative acidities may be assumed to be nearly the same in both solvents. (7) Bordwell, F. G. Acc. Chem. Res. 1986, 21, 456-463. (8) Dew, E. N.; Richie, P. D. Chem. Ind. 1969, 51-52.
S0022-3263(96)00254-X CCC: $12.00
Figure 1. ORTEP representation of the (1-anilino-1-cyanoethyl)benzene structure.
the reactions between (1-anilino-1-cyanoethyl)benzenes 3 with MeONa in MeOH (eq 1). We have determined
the X-ray structure of the reactant and measured the reaction rates. The mechanism of the reactions is assessed by the use of the transition state parameters. The results of these studies are reported here. Results and Discussion (1-Anilino-1-cyanoethyl)benzenes 3 were synthesized by reacting aniline, acetophenone, and NaCN by the literature method.8 The X-ray structure of 3a reveals that the two phenyl groups are nearly orthogonal to each other, and the dihedral angle between the N-H and the C-CN bonds is 105.2° (Figure 1), which is close to 90°, which is the most unfavorable angle for elimination reactions.9 Therefore, the high temperature required for the pyrolytic elimination from 3a may in part be ascribed to the unfavorable dihedral angle. Reaction of 3a with MeONa in MeOH produced only N-phenylacetophenonimine. Rates of eliminations from 3 were followed by monitoring the increase in the absorption at 243-244 nm. Rate constants for MeONapromoted eliminations from 3 are listed in Table 1. The second-order rate constant k2 ) 2.81 × 10-4 M-1 s-1 at 40.0 °C for MeONa-promoted elimination from 3a is approximately 130-fold smaller than k2 ) 3.65 × 10-2 M-1 s-1 for MeONa-promoted eliminations from N-chloro-Nmethylbenzylamine at 39.0 °C.10 Although a direct comparison of these rate constants may not be very meaningful because the two reactions proceed by different mechanisms (vide infra), the slower rate of 3a can (9) Bartsch, R. A.; Zavada, J. Chem. Rev. 1980, 80, 453-494. (10) Bartsch, R. A.; Cho, B. R. J. Am. Chem. Soc. 1979, 101, 35873591.
© 1996 American Chemical Society
Notes
J. Org. Chem., Vol. 61, No. 16, 1996 5657
Table 1. Second-Order Rate Constants for Eliminations from XC6H4NHC(CN)(CH3)C6H5a Promoted by MeONa in MeOH X
T, °C
[MeONa]
104k2, M-1 s-3 b,c
H H Hd H H m-Cl m-Cl
40.0 40.0 40.0 50.0 60.0 40.0 40.0
1.00 0.510 0.640 1.00 1.00 1.00 0.760
2.91 ( 0.01 2.75 ( 0.01 2.50 ( 0.02 6.28 ( 0.05 14.9 ( 0.1 8.13 ( 0.01 7.66 ( 0.01
a [Substrate] ) (2-3) × 10-5 M. b k ) k c 2 obs/[MeONa]. Average and standard deviation for two or more kinetic runs. d The basesolvent was MeONa-MeOD.
Table 2. Transition State Parameters for Eliminations from ArCH2N(Cl)CH3 and ArNHC(CN)(CH3)Ph Promoted by MeONa in MeOH rel rate kH/kD F ∆Hq, kcal/mol ∆Sq, eu
ArCH2N(Cl)CH3a
ArNHC(CN)(CH3)Ph
130 6.0 ( 0.1 1.52 ( 0.06 16.6 ( 0.1 -12.1 ( 0.3
1
