Base-promoted elimination reactions of ( E )- and ( Z )-arylaldehyde O-benzoyloximes. Effects of stereochemistry,  -aryl group, and base-solvent on the nitrile-forming transition states

This account summarizes the results of mechanism studies on the nitrile-forming eliminations from ( E )- and ( Z )-arylaldehyde O -benzoyloximes promoted by 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) in MeCN, R 1 R 2 NH in MeCN, and R 1 R 2 NH/R 1 R 2 NH 2+ in 70mol% MeCN(aq). The rate of anti -elimination was 36,000-fold faster than that of syn -elimination. The change of the  - aryl group from Ph to thienyl to furyl shifted the E2 transition state slightly towards product-like. When ( E )-2,4-dinitrobenzaldehyde O -benzoyloxime was employed as the reactant and R 1 R 2 NH/R 1 R 2 NH 2+ in 70mol% MeCN(aq) was used as the base-solvent, the (E1cb) irr was observed.


Introduction
2][3][4][5][6][7] The results of the structure-reactivity relationships studies have led to a qualitative understanding of the relationship between the reactant structure and the transition state structure.One of the most important achievements from these studies is the development of the More-O'Ferrall-Jencks energy diagram, which has become a useful tool for the interpretation of not only elimination reactions, but also substitution reactions and the reactions of carbonyl compounds, and found its way in advanced organic chemistry textbooks. 8,9In contrast, not much attention has been focused on the elimination reactions in which the -bond is formed between carbon and a heteroatom, such as nitrogen, oxygen, or sulfur.
The imine-forming elimination reactions were thoroughly investigated by the groups of Hoffman, Bartsch, and subsequently by Cho and an account summarizing the results was jointly published by them. 11Two decades ago, Cho group has initiated a systematic investigation on the nitrile-forming eliminations reactions.There are several differences between imine-and nitrileforming eliminations.First, the energy difference between C-N and C=N bonds is significantly larger than that between C=N and C≡N bonds.Second, the carbon in the imine-forming elimination is sp 3 hybridized, whereas in the nitrile-forming elimination it is sp 2 hybridized.This is expected to stabilize the developing negative charge in the nitrile-forming transition state.Third, both the CH bond and developing partial multiple bonds are parallel to the orbitals of the aryl group in the imine-forming transition state, whereas they are orthogonal to each other in the nitrile-forming transition state.Hence, the aryl group is expected to stabilize the developing negative charge more in the imine-than in the nitrile-forming transition state. 12The combined effects would predict that the imine-forming eliminations would favor E1-like or E2 central transition state, whereas the nitrile-forming eliminations would favor E1cb-like transition state or an E1cb mechanism.4][25] This series of compounds provided unusual opportunities to study the effects of changing the stereochemistry, aryl group, leaving group, and base-solvent on the nitrile-forming transition states.The results reveal a wide variation of the transition state structures ranging from E2-central to E1cb mechanism.

Base-promoted nitrile-forming eliminations from (E)-and (Z)benzaldehyde O-benzoyloximes
An excellent understanding of electronic effects and the effects of changes in reactant structure and reaction conditions on the structure of the E2 transition state for alkene-and alkyne-forming anti-eliminations has evolved. 1,4,8,26Similarly, the complementary syn eliminations have also been studied. 1,4,26The results of these studies reveal that the anti-elimination is more facile and proceeds through the transition state with less carbanionic character and smaller extents of CH bond cleavage than the corresponding syn-elimination reactions.In contrast, little was known about the differences between the syn-and anti-eliminations forming carbon-nitrogen triple bonds.
To provide an insight into the transition state differences between the syn-and anti-eliminations forming nitriles, we have investigated the reactions of (E)-and (Z)-benzaldehyde Obenzoyloximes with 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) in MeCN (Scheme 1). 23We have determined the kH/kD, Hammett , and Brönsted  and 1g values for both reactions.The structure-reactivity parameters permitted a mapping of the transition state for these reactions on the More-O'Ferrall-Jencks reaction coordinate energy diagram.These results provided a complete picture of the transition state differences for the syn-and anti-eliminations forming nitriles.

Mechanism of eliminations from 1 and 2 promoted by DBU in MeCN
Results of product studies and kinetic investigations revealed that the reactions of 1 and 2 with DBU in MeCN proceeded by the E2 mechanism. 23Since the reactions produced only elimination products and exhibited second-order kinetics, all but bimolecular pathways could be ruled out.In addition, an E1cb mechanism could be negated by the substantial values of kH/kD and |lg|. 4,8,26

Transition state differences for the Syn-and Anti-eliminations forming nitriles
The rate of anti-elimination reaction from 2 promoted by DBU in MeCN proceeded at approximately 36,000-fold faster rate than that of the syn-elimination from 1 (Table 1).An ab initio calculation with the 6-31G basis set revealed that 2 was less stable than the (E)-isomer by 3.734 kcal/mol due to the unfavorable steric interactions between the phenyl and the leaving group that distorted the structure.Hence, the much faster rate of anti-elimination was attributed to the steric strain in the (Z)-isomer and the greater extent of partial triple bond formation in the anti-transition state.
The structures of the transition states were assessed by comparing the Hammett , kH/kD, and |lg| values.The Hammett  value for the anti-elimination was much smaller than that for the syn-elimination, indicating a smaller extent of negative charge developed at the -carbon in the transition state.In addition, the kH/kD value was much larger for the former.Although this result could be interpreted with either a greater or a smaller extent of proton transfer in the transition state, 28 the latter interpretation was more compatible with the smaller  value observed for the anti-eliminations.Further, the |lg| value was smaller for the anti-than for the syn-elimination, indicating a smaller degree of leaving group bond cleavage in the anti transition state.These results indicated that the transition state for the anti-eliminations from 2 appeared to be more symmetrical with a smaller degree of proton transfer, less negative charge development at the carbon, and a smaller extent of leaving group bond cleavage than that for the corresponding syn eliminations from 1.The activation parameters were also consistent with this interpretation.The enthalpy of activation (H  ) was smaller for 2 probably because of the smaller extents of the C-H and N-OC(O)Ar bond cleavages and greater degree of partial triple bond formation in the transition state.The higher reactant energy of 2 should also decrease the H  (vide supra).Further, since the transition state for the anti-elimination was less associated with respect to the base-proton bond, the entropy of activation (S  ) should be less negative.

Mapping of the transition state
Changes in the structure-reactivity parameters provided additional evidences for the above conclusions.These changes can usually be described on the energy surface of More-O´Ferrall-Jencks diagram. 28An energy surface for the elimination reactions of DBU-promoted eliminations from 1 and 2 is shown in Figure 1.Table 2 shows that the kH/kD values for 1 increased slightly as the leaving group was made less basic.Since the smaller isotope effect for the former had been attributed to an extensive proton transfer past halfway (vide supra), this result indicated a gradual decrease in the extent of proton transfer in the transition state.The result could be described by a positive pxy interaction coefficient, pxy = ∂β/∂pKlg > 0, which provided additional support for the concerted E2 mechanism.On the More-O'Ferrall-Jencks energy diagram in Figure 1, a change to better leaving group would raise the energy of the top edge of the diagram.The transition state with greater extent of proton transfer than Nα-OC(O)Ar bond cleavage would then move slightly toward the right as depicted by a shift from A to B on the energy diagram, resulting in a small increase in kH/kD (vide supra). 28On the other hand, the kH/kD values for 2 decreased with the same variation of the leaving group.This could also be explained with a decrease in the extent of proton transfer by assuming less than half proton transfer in the former transition state, a positive pxy interaction coefficient, pxy = ∂β/∂pKlg > 0, and a shift of the transition state from A´ to B´ in Figure 1. 26,28,29 The kH/kD value decreased for 1 but increased for 2, respectively, as the electronwithdrawing ability of the β-aryl substituent was increased (Table 2).Since the extent of proton transfer for the two reactions was assumed to be on different sides of the midpoint, these results indicated an increase in the extent of proton transfer in both transition states (vide supra).This effect could be described by a positive pxy' interaction coefficient, pxy' = ∂β/∂σ > 0, and the reaction coordinate that has large components of proton transfer and Nα-OC(O)Ar bond cleavage. 26,29These changes in the kH/kD values could be described on the More-O'Ferrall-Jencks diagram (Figure 1). 28An electron-withdrawing β-aryl substituent will lower the energy of the carbanion intermediate in the upper left corner of the diagram.The transition state will then move slightly toward the upper left corner, with more proton and larger or smaller kH/kD , as depicted by the shifts from A to C and A´ to C´ for 1 and 2, respectively, on the energy diagram.
As shown in Table 3, there was a progressive decrease in the Hammett ρ values with a better leaving.This result could be described by a negative pyy' interaction coefficient, pyy' = -∂ρ/∂pKlg = -∂βlg/∂σ < 0. 26,29 The decrease in the |βlg| values with a stronger electron-withdrawing β-aryl substituent (Table 4) provided additional evidence for this effect, i.e., pyy' = -∂βlg/∂σ < 0. The negative pyy' coefficients observed in these reactions were consistent with an E2 mechanism and the reaction coordinates that have large components of proton transfer and Nα-OC(O)Ar bond cleavage so that a better leaving group would shift the transition state from A to B and A´ to B´ for 1 and 2, respectively, to decrease the extent of negative charge development and the ρ values (Figure 1).Moreover, an electron-withdrawing substituent would shift the transition state from A to C for 1 in the direction of decreased Nα-OC(O)Ar bond cleavage and smaller |βlg| values. 28The nearly identical |βlg| values for 2 may be attributed to the relative insensitivity of anti-transition state to the variation of the β-aryl substituent.All of these results were consistent with a slightly E1cb-like transition state for 1 and a more symmetrical transition state for 2, respectively, as depicted in Figure 1.For both reactions, the structures of the transition states changes only slightly with the variation of the β-aryl substituent and the leaving group.
In summary, the nitrile-forming anti-eliminations from 2 proceeded at a 36,000 fold faster rate than that of 1 via more symmetrical transition states with a smaller degree of proton transfer, less negative charge development at the -carbon, and a smaller extent of leaving group bond cleavage.The extent of proton transfer and negative charge density at the -carbon decreased with a better leaving group, and the extent of the leaving group departure decreased as the electron-withdrawing ability of the -aryl substituent was increased.Noteworthy was the relative insensitivity of both transition states to the structural variations in the reactants.

Elimination reactions of (Z)-thiophene-and (Z)-furan-2-carbaldehyde Obenzoyloximes
It is well known that the aromatic resonance energies of thiophene and furan are much smaller than that of benzene.Since significant negative charge is developed at the -carbon in the nitrileforming transition state, it is conceivable that the heterocyclic rings could provide different stabilization of the respective transition states. 30To assess this possibility, we have studied the elimination reactions of (Z)-thiophene-and (Z)-furan-2-carbaldehyde O-benzoyloximes promoted by DBU in MeCN (Scheme 2). 12We have determined the kH/kD, Hammett ρ, and βlg values and compared the results with those for 2 under the same conditions.

Mechanism of elimination from 3 and 4 promoted by DBU in MeCN
The reactions of (Z)-thiophene-and (Z)-furan-2-carbaldehyde O-benzoyloximes 3 and 4 with DBU in MeCN produced elimination products quantitatively, exhibited second-order kinetics with substantial values of kH/kD and |lg|, indicating a common E2 mechanism for both reactions. 4,26The conclusion was supported by the interaction coefficients.The kH/kD values for 3 and 4 decreased slightly as the leaving groups were made less basic (Table 5).Since smaller  values were observed for the better leaving groups, this result could be interpreted by a decrease in the extent of proton transfer in the transition state (Table 6).This could be described by a positive pxy interaction coefficient, pxy = /pKlg, that describes the interaction between the base catalyst and the leaving group. 26,29On the More-O'Ferrall-Jencks energy diagram in Figure 2, a change to a better leaving group would raise the energy of the top edge of the diagram.The transition state on the vertical reaction coordinate would then move slightly toward the right as depicted by a shift from A to B on the energy diagram, resulting in a small increase in kH/kD (vide supra). 8,28The positive pxy coefficients provided additional support for the concerted E2 mechanism. 26,29or both reactions, the kH/kD value decreased as the electron-withdrawing ability of the aryl substituents were increased (Table 5).This was interpreted with an increase in the extent of proton transfer in the transition state because smaller |lg| value was determined with a stronger electron-withdrawing -aryl substituent (Table 7).This effect could be described by a positive pxy' interaction coefficient, pxy' = / > 0, which describes the interaction between the base catalyst and the -aryl substituent. 26,29Positive pxy' coefficients were in good agreement with an E2 mechanism and the reaction coordinate that has large components of proton transfer and N-OC(O)Ar bond cleavage. 26,29These changes in the kH/kD values could be described on the More-O'Ferrall-Jencks energy diagram (Figure 2).  a Substituents at 4-and 5-positions for phenyl and heterocyclic compounds, respectively, except otherwise noted.b X = 5-CH3.
Table 6 shows that there was a progressive decrease in the Hammett  value as the leaving group is made less basic.This result could be described by a negative pyy' interaction coefficients, pyy' = -/pKlg = -lg/ < 0, which describes the interaction between the leaving group and the -aryl substituent. 26,29The decrease in the |lg| values with a stronger electron-withdrawing aryl substituent provided additional evidence for this effect, i.e., pyy' = -lg/ < 0 (Table 7).The negative pyy' coefficients observed in these reactions were consistent with an E2 mechanism and the reaction coordinates that have large components of proton transfer and N-OC(O)Ar bond cleavage, so that a better leaving group would shift the transition state from A to B in Figure 2 to decrease the extent of negative charge development and the  values.In addition, an electron-withdrawing substituent would shift the transition state from A to C for both 3 and 4 in Figure 2 in the direction of decreased N-OC(O)Ar bond cleavage and smaller |lg| values. 8,28All of these results were very similar to those for closely-related eliminations from 2.

Effect of -aryl group on the nitrile-forming transition state
Table 6 shows that the relative rate and transition state parameters for DBU-promoted eliminations from 2, 3, and 4 were similar.This revealed that the structures of the transition states were similar for all of the elimination reactions, despite the large difference in the aromatic resonance energy of the -aryl group.Similar values of H  and S  provided additional evidence for this conclusion.However, a closer examination of the data revealed that there was small, but clear, trend in the transition state parameters.The kH/kD, Hammett , and |lg| values increased gradually as the -substituent was changed in the order phenyl < thienyl < furyl.The transition state appeared to change slightly toward product-like with a larger degree of proton transfer, more negative charge development at the -carbon, and a greater extent of the leaving group departure in the same order.The trend could not be explained with the aromatic resonance energy of the -aryl group.An ab initio calculation had revealed that all of the three reactants are nearly planar.Hence, if the planarity was retained in the transition state, the  orbitals of the aryl groups should be nearly orthogonal to the developing negative charge at the -carbon and ARKAT USA, Inc.
the partial triple bond character in the transition state.This would predict that the aromatic resonance energy of the -substituent should have little influence on the transition state structures.On the other hand, the result could readily be attributed to the increased inductive effect of the -substituent.Since the 5-thienyl and 5-furyl substituents were closer to the -carbon than the phenyl substituents, the electronic effect of the former would be more efficiently transmitted to the reaction site. 31Hence the negative charge density at the -carbon and the partial triple bond character in the former transition states would be better stabilized by the electronwithdrawing substituents, which would in turn change the transition state structures more product-like.
In summary, the nitrile-forming anti-eliminations from 3 and 4 proceeded by the E2 mechanism via symmetrical transition states.The structures of the transition states changed slightly towards more product-like with larger degree of proton transfer, more negative charge development at the -carbon, and greater extent of the leaving group departure as the substituent is changed from phenyl to thienyl to furyl.Noteworthy was the negligible influence of the aromatic resonance energy the -substituent upon the nitrile-forming transition state.

Elimination reactions of (E)-2,4-dinitrobenzaldehyde O-benzoyloximes
Previous studies have revealed that the nitrile-forming eliminations from the E and Z isomers of benzaldehyde O-pivaloyloximes with DBU in MeCN proceeded by the same E2 mechanism with similar transition state structures, despite the 36,000-fold difference in rate. 21,23The results were somewhat surprising since the latter had syn stereochemistry, a poor leaving group, and a sp 2hybridized -carbon atom.It is well established that the transition state for the syn-elimination has more carbanion character than that for anti-elimination because of the poor overlap between the developing p-orbitals. 4,26,32,33Moreover, carboxylates are poor leaving groups, which should also increase the negative charge density at the -carbon in the transition state. 26Furthermore, an sp 2 -hybridized -carbon atom should stabilize the negative charge density more than an sp 3 hybridized carbon atom. 26,33All of these factors favor an E1cb mechanism or E1cb-like transition state.However, such a transition state had never been observed in the nitrile-forming reactions until recently.
To determine whether a change to the E1cb mechanism could be realized by introducing a more electron-withdrawing -aryl substituent, we have studied the reactions of (E)-2,4dinitrobenzaldehyde O-benzoyloxime promoted by R1R2NH in MeCN and R1R2NH/R1R2NH2 + in 70mol% MeCN(aq) (Scheme 3). 24This substrate was the most strongly activated one studied so far in the (E)-benzaldehyde O-benzoyloxime series.

Mechanism of elimination from 5 promoted by R2NH in MeCN
The reactions of 5 with R1R2NH in MeCN produced only elimination products, exhibited second-order kinetics and substantial values of β and |βlg|, thereby indicating an E2 mechanism. 4,26This conclusion was supported by the interaction coefficients.Table 9 shows that the β value for 5 decreased slightly as the leaving group was made less basic.The result could be described by a positive pxy interaction coefficient, pxy = ∂β/∂pKlg = ∂βlg/∂pKBH, that describes the interaction between the base catalyst and the leaving group. 26,29he observed increase in the |βlg| values with a less basic catalyst was another manifestation of this effect, i.e., pxy = ∂βlg/∂pKBH > 0 (Table 10).On the More-O'Ferrall-Jencks energy diagram shown in Figure 3, a change to a better leaving group would raise the energy of the top edge of the diagram.The transition state on the vertical reaction coordinate would then move slightly toward the right as depicted by a shift from C to E on the energy diagram, resulting in a decrease in β (vide supra). 8,28Similarly, a stronger base would raise the energy of the right side of the energy diagram and shift the transition state from C to D to decrease the extent of N-OC(O)Ar bond cleavage.The positive pxy coefficients were inconsistent with an E1cb mechanism for which pxy = 0 was expected, but provided additional support for the concerted E2 mechanism. 26,29All of these results were very similar to those for closely related eliminations from 1 and 2. 23

Effect of -aryl group on the nitrile-forming transition state
Table 11 shows that the rate of elimination from 5a is only 10-fold faster than that from the unsubstituted (E)-benzaldehyde O-benzoyloxime.Although this result may in part be attributed to the weaker basicity of i-Pr2NH than DBU, the difference was surprisingly small considering the large difference in the electron-withdrawing ability of the -aryl substituent. 21Comparison of the transition-state parameters revealed that the structure of the transition states for these two reactions were also similar.It was previously reported that the DBU-promoted elimination from 1 proceed via a slightly E1cb-like transition state, in which the Cβ-H bond cleavage has progressed to a greater extent than the N-OAr bond rupture. 21,23Although a direct comparison between the kH/kD and β values was not possible, β = 0.47 for elimination from 5 appeared to indicate a smaller extent of proton transfer than the kH/kD = 3.3 value for the former since the latter was attributed to more than half proton transfer. 21,23In addition, the smaller |βlg| value for the former could also be explained with a lesser degree of N-OAr bond cleavage.These results indicated that the transition state for elimination from 5 was slightly more reactant-like with lesser extents of Cβ-H and N-OAr bond cleavage than that for 1.However, it should be noted that the difference was remarkably small considering the large difference in the β-aryl substituent.The small difference in the transition-state structures may be attributed to the geometry of the reactant structure.It has been well established that the benzaldoxime esters have planar structures. 12,21,23Hence, if the planarity was retained in the transition state, the  orbitals of the βaryl groups should be nearly orthogonal to the developing negative charge at the β-carbon in the transition state. 12This would predict that the electronic effect of the β-substituent should be transmitted to the reaction site only through an inductive effect.Further, the negative charge density developed on the β-carbon would be less sensitive to the substituent effect because it should be more stable than that on a sp 3 hybridized carbon atom.It appeared that the nitrileforming transition states were intrinsically insensitive to the reactant structure variations because of the lack of resonance stabilization of charge density on the β-carbon and the increased carbanion stabilizing ability of the sp 2 -hybridized β-carbon atom.The mechanism of elimination for 5 promoted by R1R2NH/R1R2NH2 + in 70 mol % MeCN(aq) have been elucidated by the results of kinetic investigations and product studies. 25Because the reactions produced only elimination products and exhibited second-order kinetics, all but bimolecular elimination pathways can be negated.The (E1cb)ip and internal return mechanisms were ruled out by the observed general base catalysis with the Brönsted β values ranging from 0.28 to 0.32 because these mechanisms would exhibit either a specific base catalysis or Brönsted β values near unity.The distinction between the two mechanisms had been made by the interaction coefficients.Table 12 shows that the β values for 5a remained almost the same within experimental error regardless of the base strength.The result could be described by a negligible pxy interaction coefficient, pxy = ∂β/∂pKlg  0, which described the interaction between the base catalyst and the leaving group. 26,28,29,36,37The similar |βlg| values for all bases was another manifestation of this effect, pxy = ∂βlg/∂pKBH  0 (Table 13).On the More-O'Ferrall-Jencks energy diagram shown in Figure 3, a change to a better leaving group would raise the energy of the top edge of the diagram.The transition state on the horizontal coordinate would remain at nearly the same position because there is no diagonal character.This would predict negligible change in |βlg|. 28imilarly, a change to a stronger base would raise the energy of the right side of the diagram.The transition state on the horizontal coordinate would then move toward the right as depicted by a shift from A to B, resulting in little change in |βlg|. 28The negligible pxy coefficients were inconsistent with the E2 mechanism for which pxy > 0 is expected (Figure 3) but provided a strong evidence for the (E1cb)irr mechanism. 26,28,29,36,37

Effect of base-solvent
Table 14 shows that the rate of elimination from 1a decreased slightly as the base-solvent was changed from i-Bu2NH in MeCN to i-Bu2NH/i-Bu2NH2 + in 70 mol% MeCN(aq), presumably due to the decreased basicity in more protic solvent. 38The extent of Cβ-H and Nα-OC(O)Ar bond cleavage decreased remarkably as revealed by the large decrease in the Brönsted β and |βlg| values by the same variation of the base-solvent system (Table 14).The change in the transitionstate structure with the base-solvent variation could be attributed to a solvent effect.If the partial negative charge developed at the β-carbon in the transition state was better stabilized by solvation in more protic 70 mol % MeCN(aq), the transition state should be less sensitive to the base strength.This would predict a smaller β value, as observed.Moreover, the negative charge should be transferred from the β-carbon toward the α-nitrogen to form partial triple bond and to break the Nα-OC(O)Ar bond.If a smaller amount of negative charge was transferred from the βcarbon, the extent of Nα-OC(O)Ar bond cleavage should be smaller too.The most interesting result from this study was the change of the reaction mechanism from E2 to (E1cb)irr by the basesolvent variation as revealed by the interaction coefficients (pxy > 0  pxy ~ 0) (Table 14).The change in the elimination reaction mechanism had been realized by the combined effects of strongly electron-withdrawing β-aryl substituents, intrinsically carbanion stabilizing sp 2hybridized β-carbon, syn-stereochemistry and enhanced anion-solvating ability of more protic solvent.-0.41 ± 0.01 -0.28 ± 0.02 pxy > 0 0 a Reference 24.b R1R2NH = i-Bu2NH.c R1R2NH = Bz(i-Pr)NH.

Conclusions
We have studied the nitrile-forming elimination reactions from a series of (E)-and (Z)arylaldehyde O-benzoyloxime derivatives promoted by DBU in MeCN, R1R2NH in MeCN and R1R2NH/R1R2NH2 + in 70mol% MeCN(aq).The rate of anti-elimination from 2a was 36,000-fold faster than that of syn-elimination from 1a due to the steric strain in 2a and the greater extent of partial triple bond formation in the anti-transition state.The change of the -aryl group from Ph 2 to thienyl 3 to furyl 4 shifted the transition state structures slightly towards product-like without altering the reaction rate.A further change in the -aryl group from phenyl 1 to 2,4dinitrophenyl 5 resulted in little change in the transition state structure except for the 10-fold increase in the reaction rate.The change of reaction mechanism from E2 to (E1cb)irr was realized only when most activated 5 was employed as the reactant and R1R2NH/R1R2NH2 + in 70mol% MeCN(aq) was used as the base-solvent.To change the reaction mechanism from E2 to E1cb, the combined effects of strongly electron-withdrawing β-aryl group, intrinsically carbanion stabilizing sp 2 -hybridized β-carbon, syn-stereochemistry, and enhanced anion-solvating ability are necessary.

Figure 2 .
Figure 2. Reaction coordinate diagram for nitrile-forming eliminations.The effects of the change to a better leaving group and a stronger electron-withdrawing -aryl substituent are shown by the shift of the transition state from A to B and A to C, respectively.

Table 6 .aTable 7 .a
Hammett  values for eliminations from (Z)-ArCL=NOC(O)C6H4Y promoted by DBU in MeCN at 25.Calculated with the data for X = H and 5-NO2.b Calculated with the data for X = 5-Me Values of lg for eliminations from (Z)-ArCL=NOC(O)C6H4Y promoted by DBU in MeCN at 25.Substituents at 4-and 5-positions for phenyl and heterocyclic compounds, respectively, except otherwise noted.b X = 5-CH3.

Figure 3 .
Figure 3. Reaction coordinate diagram for nitrile-forming eliminations from (E)-2,4dinitrobenzaldehyde O-benzoyloximes.(top) The effect of the change to a stronger base on the horizontal reaction coordinate is shown by the shift of the transition state from A to B. The effect of the change to a better leaving group is omitted because there will be little shift.(center) The effects of the change to a stronger base and a better leaving group on the diagonal reaction coordinate are shown by the shifts from C to D and C to E, respectively.These effects can be described by pxy > 0.

Table 2 .
Primary isotope effect values for eliminations from (E)-and (Z)-XC6H4CL=NOC(O)C6H4Y a promoted by DBU in MeCN at 25.0 C kH

Table 3 .
Hammett a Calculated with the rate data for 2cb' and 2db'.

Table 4 .
Values of βlg for eliminations from (E)-and (Z)-XC6H4CL=NOC(O)C6H4Y a promoted by DBU in MeCN at 25.0 C

Table 5 .
Primary isotope effect values for eliminations from (Z)-XArCL=NOC(O)C6H4Y promoted by DBU in MeCN at 25.0 C

Table 11 .
Relative rate, Brönsted β, and βlg values for eliminations from (E)- 8herefore, the (Elcb)R mechanism was ruled out by the linear dependence of the kobs values on the base concentration.On the other hand, the β = 0.28-0.32 and |βlg| = 0.28-0.32 were consistent with an E2 mechanism with limited cleavage of the Cβ-H and Nα-OC(O)Ar bonds in the transition state, and a mechanism in which k1 is the rate limiting [(E1cb)irr], for which a small or negligible leaving group effect is expected.8 2626Hence, the most likely mechanism for this bimolecular process was either E2 or E1cb.If the reaction proceeded via a carbanion intermediate, the rate equation could be expressed as kobs = k1k2'[B]/(k-1[BH + ] + k2') (Scheme 4).26The (E1cb)R mechanism required that the first step must be reversible, i.e., k-1[BH + ] >> k2', and the rate expression could be simplified to kobs = k1k2´[B]/(k-1[BH + ]). Thisould predict that the kobs should remain constant regardless of the buffer concentration because [B]/[BH + ] = 1.0 was maintained throughout the reaction.ARKAT USA, Inc.