RZnI vs. R2Zn in Pd(0)-catalyzed cross-coupling reactions with thioimidates

Article abstract of DOI:10.1139/v03-205

Thioimidates 1-Z undergo facile cross-coupling with Pd(0)-RZnI but are inert to Pd(0)-R2Zn. This difference is due to the lower Lewis acidity of R2Zn, as opposed to lower nucleophilicity. Various sources of RZnI (R = Me, Et) and Pd(0) were evaluated for their convenience and reactivity.

Full text of DOI:10.1139/v03-205

279
RZnI vs. R2Zn in Pd(0)-catalyzed cross-coupling
reactions with thioimidates1
William P. Roberts, Indranath Ghosh, and Peter A. Jacobi
Abstract: Thioimidates 1-Z undergo facile cross-coupling with Pd(0)–RZnI but are inert to Pd(0)–R2Zn. This difference
is due to the lower Lewis acidity of R2Zn, as opposed to lower nucleophilicity. Various sources of RZnI (R = Me, Et)
and Pd(0) were evaluated for their convenience and reactivity.
Key words: thioimidates, cross-coupling, dialkylzincs, alkylzinc iodides.
Résumé : Les thioimidates (1-Z) donnent facilement des réactions de couplages croisés avec le Pd(0)–RZnI, mais ils
sont inertes vis-à-vis du Pd(0)–R2Zn. Cette différence est attribuée à l’acidité de Lewis plus faible du R2Zn plutôt que
d’un caractère nucléophile plus faible. On a évalué diverses sources de RZnI (R = Me, Et) pour leur facilité d’accès et
leur réactivité.
Mots clés : thioimidates, couplage croisé, dialkylzinc, iodures d’alkylzinc.
[Traduit par la Rédaction] Roberts et al. 284
Introduction
In ongoing studies we have evaluated a number of proce-
dures for preparing RZnI (R = Me, Et) (3). One of the most
widely employed methods involves transmetallation of Grig-
nard reagents with ZnI2 (eq. [1], R = Me, Et) (4).
Recently we described an unusual reactivity pattern of E-
and Z-thioimidates 1-E,Z, which undergo Pd(0)-catalyzed
cross-coupling with alkyl zinc iodides at markedly different
rates (Scheme 1) (1). Thioimidates 1-Z were cleanly con-
verted to imines 3-Z using Pd(0)–RZnI, while the geometric
isomers 1-E were inert (R = Me, Et). This difference was
traced to a previously unrecognized activating effect of RZnI
and other Lewis acids, which serve to polarize the thio-
imidate C—S bond by complexing with the imidate nitro-
gen. NMR studies showed that the reactive Z-isomers readily
coordinate Zn(II), most likely as η1- or η5-chelates (cf. 2-Z)
(2a, 2b). In either geometry, oxidative addition to Pd(0) is
facilitated by the increased electrophilicity of the C—S
bond. In contrast, Zn(II) chelation is not possible with 1-E,
and simple imine complexes of type 2-E are sterically
disfavored.3 In the absence of such activation, oxidative ad-
dition does not occur, even with stoichiometric Pd(0). Cross-
coupling to afford 3-E therefore fails. Interestingly, however,
addition of the smaller (and harder) Lewis acid BF3 effi-
ciently catalyzed the conversion of 1-E to 3-E when B = H.
These conditions were also effective with other systems pre-
viously regarded as unreactive (2c).
[1]
RMgBr + ZnI2 → RZnI + MgBrI
However, in our hands this reaction gave RZnI of rela-
tively low reactivity. For example, MeZnI prepared by this
route reacted sluggishly with 1-Z when E = CO2t-Bu and not
at all when E = CHO (in general, the formyl derivatives of
1-Z are slightly less reactive toward cross-coupling).4 More
satisfactory results were obtained following eq. [2] (R = Me,
Et), involving oxidative addition of RI to activated Zn (5).
[2]
RI + Zn → RZnI
Both MeZnI and EtZnI prepared in this manner were con-
sistently more reactive, regardless of the substitution pattern
of 1-Z (E = CO2t-Bu, CHO; B = H, Me, Ph). However, we
also experienced a number of drawbacks to this procedure,
which requires activation of Zn with 1,2-dibromoethane and
trimethylsilyl chloride (5b–5d). In addition, one has to guard
against competitive Wurtz coupling, and the RZnI must be
prepared fresh before every reaction. Equation [3] (R = Me,
Et) takes advantage of the Schlenk equilibrium between
Received 23 October 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 6 February 2004.
Dedicated to Professor Edward Piers, in appreciation of his seminal contributions to synthetic organic chemistry and chemical
education.
W.P. Roberts, I. Ghosh, and P.A. Jacobi.2 Department of Chemistry, 6128 Burke Laboratory, Dartmouth College, Hanover,
NH 03755, U.S.A.
1This article is part of a Special Issue dedicated to Professor Ed Piers.
2Corresponding author (e-mail: p.jacobi@dartmouth.edu).
3Detailed NOE and X-ray studies showed that the imidate nitrogen in 1-E is sterically shielded by the flanking S-Me and C-5-alkyl
groups.
4Regardless of the method of preparation, we consistently found MeZnI to be less reactive than EtZnI.
Can. J. Chem. 82: 279–284 (2004)
doi: 10.1139/V03-205
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Can. J. Chem. Vol. 82, 2004
Scheme 1.
Scheme 2.
lated cross-coupling reactions (9). Although not quite as re-
active as homogeneous PdL4, Pd–C gave yields of imines 3-
Z in the range of 70%.
Given the ready availability of Me2Zn and Et2Zn (vide su-
pra), it was of interest to examine the cross-coupling of
these species with thioimidates 1-Z. In contrast to the case
with RZnI (5c), there are only scattered reports of transition-
metal-catalyzed coupling of dialkylzincs R2Zn with thio-
organics. The most notable of these stem from the elegant
studies of Liebeskind and co-workers (10),6 who showed
that thioglycolic acids 4 are excellent substrates for coupling
with both RZnI and R2Zn (Scheme 2). However, these exam-
ples represent a special case, since strong Zn-chelation
assists both oxidative addition (o.a.) and subsequent trans-
metalation (t.m.). Under identical conditions, thiomethyl de-
rivatives 9 are essentially inert.
In the present work we observed a dramatic difference in
reactivity between RZnI and R2Zn (R = Me, Et). As noted
above, Pd(0)-catalyzed cross-coupling of thioimidates 1-Z
with RZnI occurred readily to give high yields of the corre-
sponding imines 3-Z (Scheme 1). In contrast, salt-free
Me2Zn and Et2Zn failed to react under Pd(0) catalysis7 and
gave only trace amounts of 3-Z when more reactive Ni cata-
lysts were used (10).6 The reasons for this difference were
unclear. Previously we showed that the conversion of
thioimidate 1-Z to imine 3-Z using Pd(0)–RZnI requires five
steps, involving a minimum of 3 equiv. of RZnI for complete
cross-coupling (Fig. 1) (1). Two equivalents are taken up in
complexation (steps 1 and 3), while the third is the source of
RZnI for intermolecular transmetalation (step 4) (10a). It
seemed unlikely that the rate of reductive elimination (step
5) would be greatly effected by a change from EtZnI to
Et2Zn. Therefore, we focused our attention on steps 1–4,
substituting R2Zn for RZnI.
R2Zn and ZnI2, which Abraham and Rolfe (6) showed lies
far to the right in polar solvents such as THF.
→
2RZnI
< − − −
[3]
R2Zn + ZnI2
In general, this procedure worked well, but for reproduc-
ible results the ZnI2 had to be freshly sublimed. By far the
most convenient procedure for generating RZnI of uniform
reactivity was the method of Charette and co-workers
(eq. [4], R = Me, Et) (7).
[4]
R2Zn + I2 → RZnI + RI
This method takes advantage of the nucleophilic character
of R2Zn and its ability to effect substitution on I2. Among
other advantages, displacement is carried out at –40 °C and
is complete within minutes. Furthermore, both Me2Zn and
Et2Zn are commercially available and relatively stable.
With a reliable means of preparing RZnI, we explored the
effect of various Pd catalysts on converting 1-Z to 3-Z (cf.
Scheme 1). Hayashi et al. (8) reported that dichloro[1.1′-bis-
(diphenylphosphino)ferrocene]palladium(II) [(PdCl2(dppf)]
was particularly effective in coupling n- and sec-BuZnCl
with bromobenzene. The efficacy of PdCl2(dppf) was as-
cribed to its large P-Pd-P bond angle, which presumably ac-
celerates reductive elimination. In our case, however, we
observed no significant advantage of PdCl2(dppf) over a
number of simpler homogeneous catalysts.5 Perhaps the
most convenient catalyst system tested was heterogeneous
Pd–C, which has recently been employed in a number of re-
It was tempting to attribute this failure to a difference in
nucleophilicity between RZnI and R2Zn, which could have a
significant impact on transmetallation (step 4). However,
there is some indication that dialkylzincs are at least as reac-
tive as alkylzinc halides toward simple electrophiles (4). In
one experiment we compared the relative rates of addition of
5 Including, among others, PdCl2(PPh3)4, Pd(PPh3)4, and Pd2(dba)3–tris(2-furylphosphine).
6 See ref. 5c for an additional example.
7 Me2Zn and Et2Zn were purchased neat from Strem Chemicals, Inc., and as toluene solutions from Sigma-Aldrich Chemical Co.
© 2004 NRC Canada

Roberts et al.
281
Fig. 1. Steps involved in conversion of thioimidates 1-Z to im-
ines 3-Z using RZnI–Pd(0): (1) N-complexation with Zn; (2) oxi-
dative addition; (3) S-complexation with Zn; (4) transmetalation;
(5) reductive elimination.
Scheme 3.
Scheme 4.
Et2Zn and EtZnI to benzaldehyde. With Et2Zn, GC analysis
shows that the reaction is 80% complete after 16 h at 50 °C
in THF–PhCH3 (Scheme 3; see also Experimental section)
(11).8 During this same period, EtZnI reacts to the extent of
26%. Similar results have been found for PrZnI, which is re-
ported to be unreactive at temperatures up to 60 °C (12).9 In
any event, both species are only weakly nucleophilic in the
absence of added Lewis acids or bases (4).
A more likely possibility was that dialkylzincs are less ef-
fective activators for oxidative addition (step 2) and (or)
transmetalation (step 4). Both processes require strong
complexation with Zn (1, 10a). This argument finds support
in the fact that dialkylzincs are generally poorer acceptors
than alkylzinc iodides (i.e., weaker Lewis acids) (4, 13, 14).
In both species, Zn has a low-lying, filled 3d shell, and its
coordination chemistry is influenced mainly by electrostatic
effects (14b). In alkylzinc iodides, the metal center is cova-
lently bonded to an electronegative iodine atom, which ren-
ders Zn more electron deficient (13). Consequently, there is
a strong driving force for coordination between alkylzinc io-
dides and Lewis bases (EtZnI exists as a coordination poly-
mer in the solid state) (13). In contrast, dialkylzincs are
bonded to two R groups, and Zn is less electron deficient.
This is reflected in the fact that dialkylzincs are less prone to
undergo association or complexation and always occur as
monomers (14).
cess EtZnI (17a) effects immediate loss of the N-H proton
and concomitant liberation of ethane (Scheme 4). The result-
ing Zn complex 18 readily undergoes oxidative addition to
Pd(0) to afford 19a and, ultimately, imine 20a following
transmetallation and reductive elimination. Under identical
conditions, but employing Et2Zn (21a), deprotonation and
complexation of 16 requires several hours. At the end of this
period, the resultant complex 22a was treated with Pd(0),
but little or no oxidative addition could be detected. These
experiments were carried out with 6 equiv. of 21a and
stoichiometric Pd(0), which returned only unchanged 16
NMR studies demonstrated the difference in the
complexing abilities of RZnI and R2Zn with thioimidates of
type 1-Z. With thioimidate 16, for example, addition of ex-
8 See also ref. 4, p. 109.
9 See also ref. 4.
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Can. J. Chem. Vol. 82, 2004
upon quenching. It is worth noting that Zn complexes 18
and 22a differ only in one ligand on Zn (I vs. Et). We be-
lieve it is the greater electron deficiency of Zn in 18 that
provides the driving force for oxidative addition.
tive addition and (or) transmetallation. However, these mod-
ifications were only marginally successful in our case. Inclu-
sion of either Bu4NI or Bu4NCl with Me2Zn–Pd(0) had a
modest effect in cross-coupling experiments with thio-
imidate 16, raising the yield of imine 20b from 0% to ~30%
(cf. Scheme 4). It is possible, however, that more powerful
donors might have a greater impact on accelerating these re-
actions, and this merits further investigation.
We performed a number of experiments to test this idea.
Thioimidate 16 was first combined with 2 equiv. of Et2Zn
(21a) in THF–PhCH3, and the resulting solution was al-
lowed to stand at room temperature (RT) until complexation
to give 22a was complete. The reaction was then treated
with catalytic Pd(0), and 1 equiv. of EtZnI (17a) was slowly
added in the dark.10 As expected based upon the pathway
outlined in Scheme 4, we observed no formation of imine
20a, since under these conditions oxidative addition to give
23a does not occur. However, when this sequence was re-
versed the results were markedly different. Treatment of 16
with 2 equiv. of EtZnI (17a) and catalytic Pd(0), followed by
1 equiv. of either EtZnI (17a) or Et2Zn (21a) led to forma-
tion of imine 20a at approximately the same rate. This dem-
onstrates that once oxidative addition and S-complexation
with RZnI to give 19a occurs, transmetalation and reductive
elimination are equally facile with RZnI or R2Zn. Next, we
carried out a crossover experiment using MeZnI (17b) and
Et2Zn (21a). In this case, treatment of 16 with 2 equiv. of
MeZnI – catalytic Pd(0), followed by slow addition of
Et2Zn, gave almost exclusively imine 20a (R = Et). Interest-
ingly, it was necessary to carry out this experiment with
strict exclusion of light. Roughly equal amounts of 20a and
20b were obtained if this precaution was not taken.11
Experimental section
Melting points were determined in open capillaries and
are uncorrected. 1H NMR spectra were recorded at 300, 400,
or 500 MHz and are expressed as parts per million (ppm)
downfield from tetramethylsilane. All reactions were carried
out in oven-dried glassware under an inert atmosphere of ni-
trogen or argon.
Formation of EtZnI
A flask containing 0.634 g (2.5 mmol) of iodine and
2.5 mL of THF was stirred at room temperature for 15 min
to ensure that the iodine had completely dissolved. The solu-
tion was then cooled to –40 °C, and 2.5 mL (2.5 mmol,
1 equiv.) of 1 mol L–1 Et2Zn in toluene was added in one
portion. After an additional 5 min at –40 °C, the cold bath
was removed, and the flask was allowed to warm to room
temperature. This solution was then titrated against a freshly
prepared 1 mol L–1 iodine-in-THF solution.
These experiments left unanswered the question of
whether the lower complexing ability of R2Zn also impacted
on transmetallation. That is, do 19 and 23 undergo trans-
metallation at different rates? As shown by Liebeskind and
co-workers (10a), this step requires considerable weakening
of the Pd—S bond. Some indication that this was a problem
could be inferred from our early experiments with Ni cata-
lysts and R2Zn (vide supra), which also produced little or no
cross-coupling product 3-Z. In these examples, however, oxi-
dative addition to Ni almost certainly occurred (10).6 To ex-
plore this issue further, we treated 16 with 0.9 equiv. of
EtZnI, followed by catalytic Pd(0) and 2 equiv. of Et2Zn.
Our reasoning was that there should be sufficient EtZnI to
produce complex 18, which in turn is activated toward
oxidative addition. At this point, however, there would be in-
sufficient EtZnI to further activate the Pd—S bond to trans-
metalation.12 Any such activation would have to come from
Et2Zn. Under these conditions we saw only very slow forma-
tion of imine 20a (much slower than when employing
3 equiv. of EtZnI under otherwise identical conditions).
Finally, we briefly explored the possibility of in situ acti-
vation of both Et2Zn and Me2Zn toward Pd(0)-catalyzed
cross-coupling. In this regard we were particularly drawn to
the studies of Chastrette and Amouroux (15), who showed
that the nucleophilic properties of linear dialkylzincs are
markedly increased by tetra-n-butylammonium halides
(Bu4NX), while still maintaining the strong acceptor charac-
ter of Zn (14b). Properties of this type might facilitate oxida-
5-[1-(5-Ethyl-4,4-dimethyl-3,4-dihydro-pyrrol-2-ylidene)-
ethyl]-3,4-dimethyl-1H-pyrrole-2-carboxylic acid tert-
butyl ester (20a)
A solution of 25.0 mg (0.069 mmol) of thioimidate 16 and
4.8 mg (6.9 µmol, 0.1 equiv.) of PdCl2(PPh3)2 in 1 mL of to-
luene was treated with 1.2 mL (0.414 mmol, 6 equiv.) of
0.4 mol L–1 EtZnI in THF–toluene (1:1) under argon. After
stirring for 60 min at room temperature, the mixture was
taken up in 5 mL of Et2O, stirred for 10 min, and filtered
through celite. To the filtrate was added 5 mL of saturated
(sat’d) NH4Cl, and the aqueous layer was extracted with
Et2O. The combined organic layers were washed with sat’d
NaHCO3, dried over Na2SO4, filtered, and concentrated to
dryness under reduced pressure. The residue was purified by
flash chromatography (silica gel, EtOAc:hexanes = 1:9 to
1:5) to give 16 mg (70%) of the imine 20a as an off-white
1
solid. Rf (1:4 EtOAc:hexanes) 0.54. H NMR (500 MHz,
CDCl3): 1.18 (s, 6H), 1.33 (t, J = 7.32, 3H), 1.57 (s, 9H),
2.11 (s, 2H), 2.18 (s, 3H), 2.29 (s, 3H), 2.42 (q, J = 7.32,
2H), 2.58 (s, 2H), 11.69 (br s, 1H). 13C NMR (500 MHz,
CDCl3): 189.5, 161.36, 149.10, 132.16, 127.36, 119.06,
118.38, 114.26, 79.86, 47.83, 44.44, 28.87, 26.40, 22.19,
18.47, 11.73, 11.67, 10.54. HR-MS (FAB) calcd. for
C21H32N2O2: 344.2464; found: 344.2543 ([M + H]). Anal.
calcd. for C21H32N2O2: C 73.22, H 9.36, N 8.13; found: C
73.20, H 9.44, N 8.08.
10Unless otherwise noted, the source of Pd(0) is PdCl2(PPh3)2.
11The mechanism for this alkyl group exchange is unknown at present but may be related to the facile photoinduced synthesis of organozinc
reagents recently described by Charette et al. (cf. ref. 7b).
12An alternative explanation is that oxidative addition to 18 requires activation by complexation of EtZnI at both the imidate N- and S-atoms.
However, such a species would suffer prohibitive steric strain from the adjacent geminal dimethyl groups.
© 2004 NRC Canada

Roberts et al.
283
Fig. 2. The ratio of benzaldehyde to 1-phenyl-propan-1-ol.
itoring of the reaction by TLC (1:9 EtOAc:hexanes) showed
formation of 20a after 30 min, and at no point was 20b ob-
served. Upon completion of the addition of Et2Zn, the reac-
tion was allowed to stir for an additional 3 h and then
Crossover experiment, 2 EtZnI + 1 Et2Zn
A total of 10 mg (27.6 µmol) of thioimidate 16 was dis-
solved in 0.4 mL of toluene. To this was added 110 µL
(55.2 µmol, 2 equiv.) of 0.5 mol L–1 EtZnI–THF, and the re-
action was stirred at room temperature under argon. After
stirring for 5 min, 2 mg (2.7 µmol, 0.1 equiv.) of
PdCl2(PPh3)2 was added, and the flask was purged with ar-
gon and wrapped in foil. After stirring for an additional
30 min, the reaction was checked by TLC, and no product
was observed. At this time 28 µL (27.6 µmol, 1 equiv.) of
1 mol L–1 Et2Zn in toluene was added dropwise over a pe-
riod of 30 min. Monitoring of the reaction by TLC (1:9
EtOAc:hexanes) showed formation of 20a after 30 min.
1
worked up as in the synthesis of 20a (vide supra). H NMR
confirmed that no 20b was formed. If no foil was employed
and (or) addition of Et2Zn was completed too quickly, both
20a and 20b were observed.
Crossover experiment, 0.9 EtZnI + 2Et2Zn
A solution of 10 mg (27.6 µmol) of thioimidate 16 in
0.4 mL of toluene was treated with 49 µL (25 µmol,
0.9 equiv.) of 0.5 mol L–1 EtZnI–THF, and the resultant so-
lution was stirred at room temperature under argon. After
stirring for 5 min, 2 mg (2.7 µmol, 0.1 equiv.) of
PdCl2(PPh3)2 was added, and the flask was purged with ar-
gon and wrapped in foil. After stirring an additional 30 min,
the reaction was checked by TLC, and no products were ob-
served. At this time, 55 µL (55 µmol, 2 equiv.) of 1 mol L–1
Et2Zn in toluene was added dropwise over a period of
60 min. Monitoring of the reaction by TLC (1:9 EtOAc:hex-
anes) showed formation of 20a after 50 min. The identical
procedure employing 3 equiv. of EtZnI showed immediate
formation of 20a.
Crossover experiment, 2Et2Zn + 1 EtZnI
A total of 10 mg (27.6 µmol) of thioimidate 16 was dis-
solved in 0.4 mL of toluene. To this was added 55 µL
(55.2 µmol, 2 equiv.) of 1 mol L–1 Et2Zn–THF, and the reac-
tion was stirred at room temperature under argon. After
stirring for 6 h to ensure complexation, 2 mg (2.7 µmol,
0.1 equiv.) of PdCl2(PPh3)2 was added, and the flask was
purged with argon and wrapped in foil. After an additional
30 min passed, the reaction was checked by TLC, and no
products were observed. At this time, 55 µL (27.6 µmol, 1
equiv.) of 0.5 mol L–1 EtZnI in THF was added dropwise
over 1 h. Monitoring of the reaction by TLC (1:9
EtOAc:hexanes) showed no formation of 20a.
Addition of Et2Zn to benzaldehyde
To 30 µL (3.3 mmol) benzaldehyde was added 1.2 mL of
THF under argon. This solution was then treated with
1.0 mL (9.9 mmol, 3 equiv.) of 1 mol L–1 Et2Zn–toluene,
and the flask was immersed into an oil bath maintained at
50 °C. Using a 100 µL syringe, 50 µL aliquots were taken
and quenched into 300 µL of MeOH. A 25 µL portion of
each quenched aliquot was injected onto an HP 6890 GC,
employing an HP-35 column (30 m × 0.25 mm × 0.25 µm
film thickness). The temperature profile for the GC was as
follows: 10 min at 60 °C, then an increase of 15 °C min–1 up
to 220 °C, followed by 10 min at 220 °C. The ratio of
benzaldehyde to 1-phenyl-propan-1-ol is plotted in Fig. 2 us-
ing Microsoft Excel 98®.
Crossover experiment, 2 MeZnI + 6Et2Zn
A solution of 10 mg (27.6 µmol) of thioimidate 16 in
0.4 mL of toluene was treated with 110 µL (55.2 µmol,
2 equiv.) of 0.5 mol L–1 MeZnI–THF, and the resultant solu-
tion was stirred at room temperature under argon. After stir-
ring for 5 min, 2 mg (2.7 µmol, 0.1 equiv.) of PdCl2(PPh3)2
was added, and the flask was purged with argon and
wrapped in foil. After stirring an additional 30 min, the reac-
tion was checked by TLC, and no products were observed.
−1
At this time, 10 µL (10 µmol, 0.4 equiv.) of 1 mol L Et2Zn
in toluene was added dropwise every 10 min for 2.5 h. Mon-
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Can. J. Chem. Vol. 82, 2004
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423 (2003).
Addition of EtZnI to benzaldehyde
A
solution consisting of 30 µL (3.3 mmol) of
benzaldehyde and 2.2 mL (9.9 mmol, 3 equiv.) of 1 mol L–1
EtZnI in 1:1 THF:toluene was stirred in an oil bath main-
tained at 50 °C. The reaction progress was monitored and
analyzed, as in the reaction employing Et2Zn and benzalde-
hyde (see Fig. 2).
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Acknowledgements
Financial support of this work by the National Institutes
of Health, General Medical Sciences (NIGMS) Grant No.
GM38913, is gratefully acknowledged.
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(a) P. Knochel and R.D. Singer. Chem. Rev. 93, 2117 (1993);
(b) P. Knochel and P. Jones (Editors). Organozinc reagents: A
practical approach. Oxford University Press, New York. 1999.
4. E. Erdik. Organozinc reagents in organic synthesis. CRC
Press, Boca Raton. 1996.
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Chem. Soc. 121, 4155 (1999), and references cited therein;
(b) C. Jubert and P. Knochel. J. Org. Chem. 57, 5425 (1992);
see also M.C.P. Yeh, H.G. Chen, and P. Knochel. Org. Synth.
15. M. Chastrette and R. Amouroux. Tetrahedron Lett. 59, 5165
(1970).
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