2232 J . Org. Chem., Vol. 62, No. 7, 1997
Cho et al.
Ta ble 4. Rela tive Ra te, Ha m m ett G, kH/kD, a n d
Activa tion P a r a m eter s for th e Syn a n d An ti Elim in a tion s
fr om (E)- a n d (Z)-XC6H4CLdNOC(O)C(CH3)3 P r om oted by
DBU in MeCN
Since the steric strain in 2 would undoubtedly decrease
as the Câ-H and NR-O bonds are stretched in the
transition state, the unusually large anti/syn rate ratio
for the nitrile-forming eliminations from 1 and 2 can most
reasonably be attributed to both the higher energy of the
reactant 2 and the favorable overlap between the devel-
oping p orbitals at the â-carbon and R-nitrogen atoms in
the transition state.
compd
rel rate
(E)-isomer
(Z)-isomer
20 000
1
F
2.4 ( 0.1
2.7 ( 0.3
12.5 ( 0.2
-31.0 ( 0.6
1.4 ( 0.1
kH/kD
7.8 ( 0.3
∆Hq, kcal/mol
∆Sq, eu
8.8 ( 0.1
-23.6 ( 0.4
The Hammett F value for the anti elimination is much
smaller than that for the syn elimination, indicating a
smaller extent of negative charge development at the
â-carbon in the former transition state. In contrast, the
kH/kD value is much larger for the former. Since the
smaller isotope effect for the latter has been attributed
to an extensive proton transfer past halfway (vide supra),
this result should indicate a smaller extent of proton
transfer near halfway in the former transition state. The
activation parameters listed in Table 4 are consistent
with this interpretation. The enthalpy of activation is
smaller for the anti than for the syn elimination probably
because less energy is required to cleave the Câ-H bond
and more energy is liberated by the greater extent of
triple bond formation in the transition state. In addition,
the higher energy of 2 would also decrease the ∆Hq.
Finally, since the transition state for the anti elimination
is less associated with respect to the base-proton bond,
the entropy of activation should be less negative. There-
fore, the transition state for the anti eliminations from
2 appears to be more symmetrical with a smaller degree
of proton transfer, less negative charge development at
the â-carbon, and a greater extent of triple bond forma-
tion than that for the corresponding syn eliminations.
The Hammett F value increased from 1.6 to 2.4, indicat-
ing a significant increase in the negative charge develop-
ment at the â-carbon in the transition state. On the other
hand, the kH/kD value decreased from 3.7 to 2.7 with a
stronger base. In view of the prediction that the kH/kD
increases until it reaches a maximum value and then
decreases as the extent of proton transfer increases, the
decrease in the kH/kD value may be interpreted as either
a greater or a smaller extent of proton transfer in the
transition state.18 However, the former interpretation
seems more compatible with the larger F value observed
with DBU. Therefore, the transition state for DBU-
promoted eliminations from 1 appears to have more
carbanionic-like character with increased negative charge
development at the â-carbon and a greater extent of
Câ-H bond cleavage than that for the Et3N-promoted
eliminations from the same substrate.
Tr a n sition -Sta te Differ en ces for th e Syn a n d An ti
Elim in a tion s F or m in g Nitr iles. It is generally ac-
cepted that the anti elimination is more facile than the
syn elimination and proceeds via a more symmetrical
transition state.19 Thus, the alkyne-forming anti elimi-
nation of (Z)-p-nitro-â-chlorostyrene promoted by t-BuOK-
t-BuOH proceeds at a 700-fold faster rate than the
corresponding syn elimination.1b The much faster rate
of anti elimination has been interpreted with a favorable
overlap between the developing p orbitals at Câ and CR
in the transition state. In addition, the smaller kH/kD
value for the latter was attributed to the greater extent
of Câ-H bond cleavage in the transition state. In
contrast, except for Hegarty’s report that the OH--
promoted elimination from (Z)-p-nitrobenzaldehyde O-
methyloxime proceeds 73-fold faster than that from the
(E)-isomer,5 nothing is known about the differences
between the syn and anti eliminations forming nitriles.
The data presented in Table 4 reveal that the rate and
the structure of the transition state change dramatically
with the stereochemistry of the reactants. The anti/syn
rate ratio calculated with the k2 values of 7.64 × 10-4
and 15.3 M-1 s-1 for 1a and 2a , respectively, is ap-
proximately 20 000. The ratio is much larger than 700
observed for the t-BuOK-promoted eliminations from (E)-
and (Z)-p-nitro-â-chlorostyrenes.1b
Exp er im en ta l Section
Ma ter ia ls. (E)-Benzaldehyde O-pivaloyloximes 1a -e were
available from the previous study.13 (Z)-Benzaldoximes were
synthesized by the isomerization of the corresponding (E)-
isomer by the literature procedure.20 All of the (Z)-benzalde-
hyde O-pivaloyloximes except for 2c were prepared in reason-
able yields by slowly adding (Z)-benzaldoximes (2.0 mmol) to
the solution of pivaloyl chloride (0.27 g, 2.2 mmol) in 7.0 mL
of pyridine at -40 °C. The solution was stirred for 5 min at
-40 °C and poured into 70 mL of ice-water. The products were
precipitated as needle crystals.
Synthesis of 2c was carried out by the same procedure as
above using a mixture of 20 mL of hexane-7.0 mL of pyridine
as the solvent. When the reaction was completed, the hexane
layer was separated, washed several times with cold water,
dried over anhydrous MgSO4, and then evaporated.
In most cases, the products were analytically pure and used
without further purification. The spectral and analytical data
of the compounds were consistent with the proposed struc-
tures. The yield (%), melting point (°C), NMR (CDCl3), IR
(KBr, CdO, cm-1), and combustion analysis data for the new
compounds are as follows. (Z)-C6H5CHdNOC(O)C(CH3)3
(2a ): yield 59; mp 63-65; IR 1758; NMR δ 7.85-7.82 (m, 2H),
7.80 (s, 1H), 7.53-7.45 (m, 3H), 1.34 (s, 9H). Anal. Calcd for
C12H15NO2: C, 70.22; H, 7.37; N, 6.83. Found: C, 70.18; H,
7.40; N, 6.84. (Z)-C6H5CDdNOC(O)C(CH3)3 (2b): yield 68; mp
63-65; IR 1758; NMR δ 7.86-7.83 (m, 2H), 7.54-7.46 (m, 3H),
1.34 (s, 9H). (Z)-p-MeOC6H4CHdNOC(O)C(CH3)3 (2c): yield
43; mp 33-35; IR 1752; NMR δ 7.82 (dd, 2H), 7.68 (s, 1H),
An ab initio calculation with the 6-31G basis set
reveals that the structures of both 1 and 2 are planar
and the lengths of the corresponding bonds in the two
isomers are very similar, although the bond angles are
appreciably different (Table S1). In addition, the (E)-
isomer is more stable than the (Z)-isomer by 4.094 kcal/
mol, probably due to the unfavorable steric interactions
between the phenyl and the leaving group in the latter.
6.97 (dd, 2H), 3.87 (s, 3H), 1.36 (s, 9H). Anal. Calcd for C13H17
-
NO3: C, 66.36; H, 7.28; N, 5.95. Found: C, 66.37; H, 7.25; N,
6.25. (Z)-m-BrC6H4CHdNOC(O)C(CH3)3 (2d ): yield 82; mp
61-64; IR 1757; NMR δ 8.10 (s, 1H), 7.76 (s, 1H), 7.67 (d,
(17) Wolkoff, P. J . Org. Chem. 1982, 47, 1944-1948.
(18) Smith, P. J . In Isotopes in Organic Chemistry; Buncel, E., Lee,
C. C., Eds.; Elsevier: Amsterdam, 1976; pp 239-241.
(19) Bach, R. D.; Badger, R. C.; Lang, T. J . J . Am. Chem. Soc. 1979,
101, 2845-2848. Gandler, J . R.; J encks, W. P. J . Am. Chem. Soc. 1982,
104, 1937-1951.
(20) Schoenewaldt, F. E.; Kinnel, R. B.; Paul Davis, P. J . Org. Chem.
1968, 33, 4270-4272
(21) Coetzee, J . F. Prog. Phys. Org. Chem. 1965, 4, 45-92.