Nucleophilic Substitution of Methyl β-Chloro-(3-bromo-2,4,6-trimethyl)cinnamate

Full text of DOI:10.1021/jo970157i

5
634
J . Org. Chem. 1997, 62, 5634-5637
Nu cleop h ilic Su bstitu tion of Meth yl
â-Ch lor o-(3-br om o-2,4,6-tr im eth yl)cin n a m a te
study, the corresponding esters seem to be suitable
substrates. Such an approach was applied by Cabaret,
5
Welvart, and co-workers in a related system.
Two practical problems that may complicate the study
are the possibility of a substitution via elimination-
addition and the possibility of a racemization of the
precursor due to a low rotational barrier. Both problems
are reduced with the R-Me compound ArC(Cl)dC(Me)-
Michal Beit Yannai and Zvi Rappoport*
Department of Organic Chemistry, The Hebrew University,
J erusalem 91904, Israel
Received J anuary 28, 1997
CO
2
3 6
Me (Ar ) 3-Br-2,4,6-Me C H) since the barrier in the
acids is relatively high and there is no vinylic hydrogen
We are looking for probes for delineating the mecha-
nism of bimolecular nucleophilic vinylic substitution
proceeding by an initial nucleophilic attack on the double
available for elimination.4
a
In a study of this ester, we found that dechlorocar-
bomethoxylation reaction with a loss of both the Cl and
1
bond (the “addition-elimination” route). With mildly
2
the CO Me groups led to an acetylene and not to a
activated systems the stronger evidence for a multistep
route via an intermediate carbanion (eq 1) (i.e., A being
an intermediate and not a transition state) is the element
6
substitution product. This was apparently due to the
excessive steric hindrance to approach to the vinylic
carbon.
effect (the relative rates k when X ) F, Cl, Br, e.g., kBr
Cl), which is not always available.
/
2
k
Consequently, in the present work, we prepared the
R-H analog. It is known that racemization of the acid
takes place in n-BuOH with a half-life of 206 min at
reflux. Moreover, the system is a priori prone to R,â-
k
9
-X-
RC(X)dCYY′ + Nu-
8 RC(X)(Nu)-YY′
8
A
4
elimination. However, according to Adams, the isomer
RC(Nu)dCYY′ (1)
studied is the E isomer where the hydrogen and chlorine
are cis to one another so that E2 elimination may be
difficult.
The study of this system therefore imposes several
practical problems. First, the substitution should be
conducted at room temperature in order to avoid thermal
racemization. Second, the requirements for the involve-
ment of the elimination-addition route should also be
investigated.
-
X ) leaving group; Nu ) nucleophile; Y, Y′ )
activating groups
â-Halocinnamates belong to this class of compounds.
Methyl â-chlorocinnamates (ArC(Cl))CHCO Me, Ar )
Ph, p-ClC , p-Tol) undergo elimination with MeO /
2
-
6
H
4
MeOH,3 but with PhS , substitution takes place, giving
a-c
-
3
d
the retained substitution product. This and the forma-
tion of the isomeric E/Z substitution products in the
-
+
Resu lts a n d Discu ssion
addition of PhS /H to the corresponding methyl propi-
olates (ArCtCCO Me) exclude an elimination-addition
route, except when Ar ) p-O NC when the substitu-
2
Cr ysta l Str u ctu r e of E-1. The crystal structure of
E-1 was determined for three reasons. (a) The configu-
ration of the acid precursor to E-1 was given by Adams
as E on the basis of chemical evidence and analogy with
o,o′-substituted biphenyls, and a corroboration for this
was sought. (b) The structures of the analogous methyl
â-chlorocinnamates were determined by Youssef et al.
on the basis of UV and NMR spectra of the acids and
esters. By using additivity calculations of H NMR
chemical shifts, they assigned a Z-configuration to the
2
6 4
H
tion gives E/Z substitution products mixture.
Adams and co-workers have prepared and resolved to
enantiomers two pairs of â-chlorocinnamic acids: the
R-Me4 and R-H with 3-bromo-2,4,6-trimethyl substit-
uents; i.e., the aryl group is 3-bromomesityl. The optical
activity results from hindered rotation around the
Ar-Cdbond due to the steric interaction of the o-Me
substituents with the double-bond substituents, which
lead to atropisomerism. We reasoned that if a carbanion
a
4b
3
1
2
esters studied whose dCH and CO Me signals were
â
A with a finite lifetime is formed by attack on C giving
calculated to be at a lower field than for the E isomers.
a carbanion, the locked geometry may be relaxed, leading
to a racemized product, whereas if the reaction is
concerted (i.e., A is a transition state) this will not
happen. Since the activation by a carboxyl is low and
the negative charge on a carboxylate may complicate the
7
Similar additivity calculations for the vinylic hydrogen
of E-1 and Z-1, using the increment value for Ar for our
tetrasubstituted phenyl group, gave δ values of 6.18 and
.49 ppm, respectively. Since the observed value was δ
.48 ppm, this probe suggest that our ester is Z-1. This
6
6
is in contrast with Adams’ assignment of configuration
of the acid if we assume that the configuration is retained
(
1) (a) Rappoport, Z. Adv. Phys. Org. Chem. 1969, 7, 1. (b) Modena,
G. Acc. Chem. Res. 1971, 4, 73. (c) Rappoport, Z. Acc. Chem. Res. 1981,
4, 7. (d) Shainyan, B. Russ. Chem. Rev. 1986, 55, 511. (e) Rappoport,
Z. Acc. Chem. Res. 1992, 25, 474.
2) (a) Bunnett, J . F.; Garbisch, E. W., J r.; Pruitt, H. M. J . Am.
6
1
on esterification. We have previously shown that with
+
2 2
the R-Me acid esterification by CH N or with MeOH/H
(
gives the same (retained) geometrical isomer. Assuming
Chem. Soc. 1957, 79, 385. For element effect in vinylic systems, see
ref 1 and: (b) Rappoport, Z.; Topol, A. J . Chem. Soc., Perkin Trans. 2
975, 863. (c) Rappoport, Z.; Rav-Acha, C. Tetrahedron Lett. 1984, 25,
17. (d) Avramovitch, B.; Weyerstahl, P.; Rappoport, Z. J . Am. Chem.
that this applies also for the R-H acid, this raises a
1
1
Soc. 1987, 109, 6687. (e) Hoz, S.; Basch, H.; Wolk, J . L.; Rappoport,
Z.; Goldberg, M. J . Org. Chem. 1991, 56, 5424.
(5) Cabaret, D.; Maigret, N.; Welvart, Z.; Onong, K. N. V.; Gaud-
emer, A. J . Am. Chem. Soc. 1984, 106, 2870.
(6) Beit-Yannai, M.; Rappoport, Z. J . Org. Chem. 1996, 61, 3553.
(7) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991;
p 215.
(
3) (a) Youssef, A.-H. A.; Abdel-Reheim, A. G. Ind. J . Chem. Sect. B
1
976, 14B, 101. (b) Youssef, A.-H. A.; Abdel-Maksoud, H. M.; Youssef,
A.-H. A. J . Org. Chem. 1975, 40, 3227. (c) Youssef, A.-H. A.; Sharaf,
S. M.; El-Sadany, S. K.; Hamid, E. A. Ind. J . Chem. 1982, 21B, 359.
(
d) J . Org. Chem. 1981, 46, 3813.
4) (a) Adams, R.; Miller, A. W. J . Am. Chem. Soc. 1940, 62, 53. (b)
Adams, R.; Anderson, A. W.; Miller, M. W. J . Am. Chem. Soc. 1941,
3, 1589.
(8) The authors have deposited the atomic coordinates for E-1 with
the Cambridge Crystallographic Data Centre. The coordinates can be
obtained, on request, from the Director, Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK.
(
6
S0022-3263(97)00157-6 CCC: $14.00 © 1997 American Chemical Society

Notes
J . Org. Chem., Vol. 62, No. 16, 1997 5635
Ta ble 1. Bon d Len gth s a n d An gles in E-1
length, Å angle
1.31(1) C2-C1-C12
bond
deg
122(1)
117.0(9)
114.0(8)
129(1)
117(1)-124(1)
122(1)
127(1)
110(1)
120(1)-123(1)
176.45
89.36
170.10
C1-C2
C1-C12
C2-Cl
1.46(1)
Cl-C2-C1
Cl-C2-C3
C1-C2-C3
CCC(Ar)
1.74(1)
1.50(1)
1.51(1)
C2-C3
3
C(Ar)-C(Me)
C-C(Ar)
C7-Br
1.35(1)-1.42(1)
O1-C12-O2
O1-C12-C1
O2-C12-C1
CC(Ar)C(Me)
1.91(1)
O1-C12
O2-C12
O2-C13
1.20(1)
1.35(1)
1.46(1)
_
a
H-C1-C12 Cl-C2-C3
_
a
Cl-C2-C3 Ar
_
C1-C12-O1-O2 H-C1-C12
a
Dihedral angle.
question about the reliability of the additivity calcula-
tions, which could be solved by X-ray diffraction. (c) In
view of the atropisomerism, the dihedral angle Ar-CdC
and the source of the hindered rotation were of interest.
The crystal structure of the ester was determined on
a single crystal. The ORTEP drawing is shown in Figure
1
, and bond lengths, bond angles, and several dihedral
angles are given in Table 1.
The important conclusion is that the configuration of
the cinnamic ester is E so that the conclusion from the
chemical shift additivity calculations is unreliable. This
may be due to the fact that the calculation uses a single
value for the aryl group regardless of its substituents and,
especially regardless of the Ar-CdC dihedral angle.
Consequently, we do not know if our conclusion also
applies to Youssef’s compounds, but Adams’ assignment
is apparently correct.
Another interesting feature is the Ar-CdC torsional
angle. The Ar and the >CdC< moieties are orthogonal
to one another. From the ORTEP drawing, this seems
to be due to the presence of both the bulky geminal (Cl)
F igu r e 1.
GC-MS. The other possible acetylenic product, i.e.,
methyl 3-bromo-2,4,6-trimethylphenylpropiolate, the HCl-
elimination product, was not observed.
A ca. 2.3 ratio of the two isomers of 3 was obtained at
the end of several different experiments and also during
the followup of one of these reactions. In the H NMR
spectrum (CDCl ) the major isomer displays the higher
MeO signal (δ 3.50) but the lower dCH signal (δ 5.80),
and vicinal cis (CO
is also slightly twisted from the plane of the double bond
which itself is not planar and has a torsion angle of ca.
.5°).
Su bstitu tion of E-1 w ith MeS . The substitution of
2
Me) substituents. The ester group
(
3
1
-
3
-
E-1 with MeS gave three products (eq 2). Two of them
compared with the other isomer (δ 3.76, 5.74 ppm). The
6
7
Cl
H
additivity calculations for the δ(dCH) give much larger
differences between E-3 and Z-3 than the experimental
values (calculated: E, 5.75; Z, 6.24 ppm). In Youssef’s
C
C
CO2Me
MeS–
C
C
H +
3b
systems, both δMeO and δdCH are at a lower field for the
MeCN
Z isomers. This is consistent with our previous results
Br
Br
on 10 p-O
2
NC
6
H
4
2
C(X))C(CO Me)CN systems (including
E-1
2
X ) p-TolS) that the δMeO is always at a lower field for
9
the Z isomer, and in applying this observation to our
MeS
H
system, we assign the more abundant isomer as E-3.
Analogy with the X-ray structure of E-1 suggests that
the distance between the mesityl o-Me protons and the
dCH proton will be similar in both E-3 and Z-3, and
hence, NOE measurements will give inconclusive geo-
metrical assignments. We therefore used the three-bond
H-C coupling constant between the vinylic proton and
C1 of the mesityl ring, which is known to be much larger
when both groups are trans rather than cis to one
another. The value of 8.20 Hz obtained for the more
abundant isomer was larger than that (4.89 Hz) for the
less abundant isomer, thus corroborating the assignment
of the former as E-3.
C
C
Br
CO2Me
+
(2)
H
Br
C
C
CO2Me
MeS
E-3
Z-3
were isomeric esters that, according to the spectral data
and the microanalysis of one of them, are the substitution
products. The very similar mass spectra of the isomers
and their close NMR spectra, as well as mechanistic
considerations (see below) strongly suggest that they are
the E,Z isomers of 3. Also formed, in various amounts
under different conditions, is the arylacetylene 2, which,
when formed in small amounts, was only detected by
(9) Avramovitch, B.; Rappoport, Z. J . Am. Chem. Soc. 1988, 110,
911.

5
636 J . Org. Chem., Vol. 62, No. 16, 1997
Notes
Vinylic substitution via the addition-elimination route
nal aim of studying the resolved system as a probe for
of moderately activated systems such as esters proceed
exclusively with retention of configuration.1 Conse-
quently, the formation of both isomers indicates the
intervention of another mechanism, either operating
the detailed mechanism of the Ad
in our system.
N
-E route cannot apply
We note that we failed in this goal also when we
attempted previously to substitute the R-Me analog of
E-1. In this case, another alternative route was the
major process, i.e., nucleophilic attack on the ester group.
exclusively or accompanying the Ad -E route. The most
N
likely mechanism is an elimination-addition reaction.
This route requires a proper geometry of the precursor,
the presence of a base, and to account for the formation
of the two isomeric products, a nonstereospecific addition
We ascribe the formation of acetylene 2 in our case to a
-
similar process. MeS attacks the carbonyl of the CO
2
-
Me group and the intermediate (as depicted in eq 4) loses
-
+
-
of the MeS and H to the intermediate acetylene.
The work of Youssef et al.3d indicates that the addition
of a thio nucleophile to methyl cinnamates gives both (E)-
and (Z)-methyl â-thiocinnamate. The same may apply
2
MeSCO Me and Cl to give 2. A concerted dechlorocar-
-
in our system. While MeS is an excellent carbon
nucleophile, it can also serve as a base toward E-1.
Figure 1 shows that, whereas the approach of the
nucleophile to C is hindered, the R-hydrogen is much
â
more exposed so that preferential attack on it is not
unreasonable.
However, the geometry of E-1 with the cis disposition
of H and Cl precludes anti-elimination of HCl in our
system. If elimination takes place it should then proceed
via the E1cB route, which is not unlikely due to the
boxymethylation process can also take place. The mecha-
nistic possibilities were described previously and litera-
ture references were given, and the new information is
that the extent of this process is smaller than with the
R-Me analog of E-1. Again, this is easily rationalized by
the fact that the R-H is the most exposed reaction site in
E-1 for nucleophilic attack.
6
2
activation of the R-CO Me group. The formation of the
substitution products is then as displayed in eq 3.
Exp er im en ta l Section
(
E)-Met h yl â-Ch lor o-(3-b r om o-2,4,6-t r im et h ylp h en yl)-
cin n a m a t e (E -1). (a) To a warm solution of (E)-â-chloro-(3-
bromo-2,4,6-trimethylphenyl)cinnamic acid prepared according
to Adams4 (4.35 g, 14.1 mmol; νmax (Nujol) 1694 cm ) in dry
toluene (30 mL) was added thionyl chloride (3.9 mL, 53.4 mmol)
dropwise, and the mixture was refluxed for 105 min. The
unreacted thionyl chloride was evaporated, and the IR spectrum
-1
-
1
An essential feature of this mechanism is the incor-
poration of deuterium in E-3 and Z-3 when the reaction
is conducted in a medium containing a deuterium source.
Two other features are the probability of detecting 4 in
the reaction medium and the exchange of the R-H of E-1
in a medium containing deuterium source. We observed
no acetylene 4 and found no incorporation of deuterium
of the remainder (νmax (Nujol) 1789.5 cm ) showed the presence
of the acyl chloride.
(
b) The crude remainder of the reaction mixture above was
dissolved in MeOH (35 mL) containing pyridine (0.3 mL), and
the mixture was refluxed for 2 h. The IR spectrum then showed
disappearance of the acyl halide and formation of an ester (νmax
-
1
(neat) 1744 cm ). Evaporation of the solvent left 4.6 g of the
crude product. Chromatography on silica, using 95:5 petroleum
ether:ether eluent, gave methyl â-chloro-(3-bromo-2,4,6-trim-
into the recovered E-1 in 9:1 CD
3
CN-D
2
O. These
1
-
ethylphenyl)cinnamate as a yellow oil (3.4 g, 76%): H NMR
features could be easily explained if addition of MeS /
(CDCl
3
) δ 2.21, 2.38, 2.40 (3 × 3H, 3s, Me), 3.45 (3H, s, OMe),
+
H
to the acetylene is sufficiently rapid, thus avoiding
6
.48 (1H, s, dCH), 6.99 (1H, s, ArH); MS (EI, 70 eV) (rel
its accumulation, and if the elimination to 4 is an
irreversible E1cB. The alternative that E-1 isomerizes
to Z-1 before elimination takes place was excluded by a
control experiment.
abundance, assignment) m/z 318, 316 (36, 27, M), 287, 285 (20,
16, M - OMe), 286, 284 (25, 19, M - MeOH), 283, 281 (18, 18,
M - Cl), 223, 221 (56, 56, M - H - CO
2
Me - Cl), 202 (17, M -
3 6
Me), 143 (B, Me C -
HCtC ), 128 (66, Me
Cl - Br), 179, 177 (29, 81, M - Br - H - CO
2
+
+
+
HCdCH ), 142 (35, Me
3
C
6
2 6
C HCdCH ),
When the reaction was conducted in 9:1 CD
3
2
CN-D O,
1
1
15 (53). Anal. Calcd for C13 14BrClO : C, 49.16; H, 4.44; Cl,
H
2
GC-MS of both substitution products indicated an
extensive (ca. 50%) deuterium incorporation in each of
1.16; Br, 25.15. Found: C, 49.08; H, 4.57; Cl, 11.30; Br, 25.39.
X-ray diffraction of a sample crystallized from 95:5 MeCN-
2
D O showed that the compound is the E isomer.
them. Control experiments with E-3 in 9:1 CD
3
CN-D
2
O
showed no deuterium incorporation. Hence, the incor-
poration is part of the substitution and not a postsub-
stitution incorporation. In a fully deuteriated medium,
a complete incorporation could indicate an exclusive
elimination-addition (E-A) route. However, since the
medium contains HCl formed in the elimination and
some adventitious water, and since contribution from the
Crystallographic data: C13
H14BrClO ; space group: Pbca; a )
2
3
1
6.828(3) Å, b ) 10.113(2) Å, c ) 16.668(3) Å, V ) 2836.6(8) Å ,
-
3
-1
Z ) 8, Fcalcd ) 1.49 g cm , µ (MoK
reflections 3683, no. of reflections with I g 2σ
) 0.084.
Rea ction of E-1 w ith MeS Na . A solution of E-1 (1.0 g,
R
) ) 30.46 cm , no. of unique
I
) 1223, R ) 0.086,
R
w
-
+
3.14 mmol) and MeSNa (0.55 g, 7.9 mmol) in MeCN (15 mL)
was stirred at room temperature under argon for 3 days (when
TLC showed that the reaction is complete). The solvent was
evaporated, the organic phase was extracted with CH
filtered, and the solvent was evaporated. The remainder was
chromatographed over silica using 3:2 petroleum ether:CH Cl
eluent. The first fraction contained 350 mg (35%) of E-3: mp
90-91 °C; H NMR (CDCl ) δ 2.38, 2.32, 2.31, 2.16 (4 × 3H, 4s,
Ad -E route should give only E-3 without incorporation,
N
2 2
Cl and
the only conclusion is that the reaction proceed to a
significant extent via the E-A route. This is one of the
2
2
many examples of the intervention of a non-Ad -E route
N
1
0
1
in vinylic substitution.
Reaction with NCS , p-TolS , or Et
3
-
-
3
N did not give the
expected substitution product. Consequently, the origi-
(10) Rappoport, Z. Recl. Trav. Chim. Pays-Bas 1985, 104, 309.

Notes
J . Org. Chem., Vol. 62, No. 16, 1997 5637
3
ArMe + SMe), 3.55 (3H, s, OMe), 5.81 (1H, s, dCH), 6.96 (1H,
substitution products were ca. 50% deuteriated according to MS.
When ester E-1 was kept for 120 h in 95:5 MeCN:D O at room
2
s, ArH); MS (rel abundance, assignment) 330, 328 (74, 72, M),
3
2
15, 313 (91, 91, M - Me), 283, 281 (95, 100, M - SMe), 273,
71 (12, 12, M - MeOH - Me), 255, 253 (15, 15, M - CO Me -
HCdC ), 202 (41, M - Br -
temperature neither deuteriation in the recovered precursor nor
isomerization was observed.
A Sea r ch for Deu ter ia tion of th e Su bstitu tion P r od u ct.
A solution containing a pure sample (10 mg, 0.03 mmol) of E-3
2
+
H - Me), 223, 221 (32, 33, BrC
SMe), 143 (67, Me
6
+
Me
3
+
3
C
6
HCdCH ), 128 (62, Me
2 6
C HCdCH ), 115
(47). Anal. Calcd for C14
H
17BrO
2
S: C, 51.07; H, 5.20; S, 9.74;
3 2
and NaSMe (5 mg, 0.07 mmol) in 10:1 CD CN:D O (0.5 mL) was
Br, 24.27. Found: C, 51.30; H, 5.40; S, 9.44; Br, 23.88.
kept for 144 h at rt. No deuterium incorporation was detected
1
The second fraction contained a mixture of E-3 and Z-3; 2
by H NMR spectrum.
1
-
+
was not detectable. From this, the H NMR (CDCl
3
) of the
Rea ction of E-1 w ith p-TolS Na . E-1 (0.2 g, 0.63 mmol)
and p-TolSNa (0.46 g, 3.15 mmol) in MeCN (10 mL) were stirred
under argon at room temperature for 93 h, and the solids were
filtered. The solvent was evaporated. According to GC-MS
analysis no substitution product was formed, but di-p-tolyl
disulfide and two unidentified products were formed: (a) with
second isomer is as follows: δ 2.39, 2.35, 2.17, 1.69 (4 × 3H, 4s,
3
ArMe + MeS), 2.75 (3H, s, OMe), 5.73 (1H, s, dCH), 6.97 (1H,
s, ArH). The mass spectrum is very similar to that of the first
isomer.
During the reaction samples were withdrawn, the solvent was
evaporated, and the distribution of the E/Z-3 isomers was
determined. The E-3/Z-3 ratios were 1.8, 2.3, 2.0, and 2.3 after
+
+
M
172, containing TolS and Cl fragments, and (b) with M 260,
containing TolS but no Cl or Br.
1
.5, 2.5, 25.5, and 72 h, respectively. The reaction was complete
Rea ction of E-1 w ith Na SCN. When a solution containing
E-1 (150 mg, 0.47 mmol) and NaSCN (0.19 g, 2.4 mmol) in MeCN
(10 mL) was stirred for 5 days at room temperature no reaction
took place according to TLC and NMR.
Attem p ted Isom er iza tion of E-1. When a solution contain-
3
ing E-1 (0.15 g, 0.47 mmol) and Et N (0.2 mL, 1.43 mmol) in
MeCN (10 mL) was stirred under argon at room temperature
for 18 h, no reaction took place. On addition of a 1 mL (7.2
3
mmol) of Et N, neither substitution nor isomerization took place
after 72 h.
The assignment of the more abundant isomer as E-3 was
based on its higher three-bond (Mes-C1/vinylic H) coupling
constant compared with that of the other isomer (see text). The
coupling constants were measured using a long-range inverse
H-C correlation with gradient selection. The relevant row of
the 2D spectrum was extracted and phase adjusted to yield the
antiphase doublet. The coupling constant was determined by
deconvolusion of this spectrum.
during an additional 3 days.
For an experiment conducted for 140 h, GC-MS showed the
formation of three compounds: E-3, Z-3, and traces (1%) of (3-
bromo-2,4,6-trimethylphenyl)acetylene (2): MS (rel abundance,
assignment) 224, 222 (100, 96, M), 143 (73, M - Br), 128 (96, M
Attem p ted Isom er iza tion of th e Su bstitu tion P r od u ct.
E-3 (10 mg, 0.03 mmol) was dissolved in CD
which MeSNa (5 mg, 0.07 mmol) was added. Followup by
3
CN (0.5 mL) to
1
H
NMR did not show any formation of Z-3 after 3 days at room
temperature.
-
Br - Me), 115 (38). The compound was not isolated.
Su bstitu tion in 95:5 MeCN:H
O. When the reaction of E-1
0.25 g, 0.79 mmol) with MeSNa (0.5 g, 7.08 mmol) in 95:5
O (10 mL) was conducted for 96 h at rt and worked
up as above the products were 22% of 2 and 37%, and 41% of
E-3 and Z-3, respectively.
2
Ack n ow led gm en t. This work was supported by the
U.S.A.-Israel Binational Science Foundation (BSF),
J erusalem, Israel, to which we are indebted. We thank
Dr. Roy Hoffman for his help in measurement of the
three-bond C-H coupling constant.
(
MeCN-H
2
2
Su bstitu tion in 95:5 MeCN-D O. The reaction was identi-
cal with that described above. After 3 days at rt the products
were almost exclusively 1:1 E-3/Z-3 (80%) and ca. 1% of 2. The
J O970157I