Scheme 4
a These hydrolytic experiments were made from the corresponding unlabeled dibromoalkenes.
at the competitive migration of a secondary or a primary
alkyl versus an allyl or 2-butenyl group (Scheme 4).
First, labeled, metalated cyclohexylacetylene was prepared
from cyclohexanecarboxaldehyde and 13CBr4 as above and
submitted to allylzincation. The monochlorination of the
bimetallic species 11 (R1 ) c-C6H11, R2 ) H) gave the
carbenoid 12 which upon warming delivered the two alkynes
14a and 14b in a 12/1 ratio (Table 1, entry 2). However,
this ratio is not related to the geometry of the intermediate
12, as the hydrolysis of the corresponding unlabeled com-
pound led to a mixture of chlorovinyl derivatives 13 (E/Z )
1/2).
13C NMR studies on unlabeled 14 allowed us to discrimi-
nate C(4) and C(5) via a long-range H-C COSY experiment
and thus determine the identity of 14a and 14b. The major
enyne corresponds to a migration of the allyl moiety, but
shows that this happens whatever its initial position, cis or
trans to chlorine. Thus, the total stereoselectivity observed
from 8 to 10 is related to the presence of the allylic alkoxy
moiety.
The competition between a cyclohexyl and a 2-butenyl
group was then considered (Table 1, entry 3). Lithiated
cyclohexylacetylene leads to the corresponding chlorovinyl
educts 15 (E/Z ) 2/1), whereas the Fritsch-Buttenberg-
Wiechell rearrangement performed on the labeled 1-alkyne
shows a predominance of the 2-butenyl migration of 6/1
(lower than allyl migration). Finally, the competition between
a primary n-octyl and a 2-butenyl group (Table 1, entry 4)
showed that from a E/Z ) 2/1 ratio in 17 the migration of
the n-octyl group prevails by 2/1: the primary n-octyl group,
although predominantly cis to Cl in 17 (as determined on
the unlabeled product), migrates better than the secondary
cyclohexyl in 15. It must be noted that no allylic rearrange-
ment of the 2-butenyl moiety is observed as no linear enyne
(tetradec-2-en-5-yne) is detected. Thus, the migrating aptitude
of the 2-butenyl group is related more to its substitution order
than to its allylic character.
The migrating aptitude of the groups can be classified as
follows: alkoxymethine > n-octyl >(?) allyl > 2-butenyl
> cyclohexyl. Unfortunately, the experiment corresponding
to the symbol (?) above cannot be made, because with n-alkyl
acetylides (RCH2CtCΗ) the allylmetalation does not stop,
as is documented,9 at the desired vinyl bismetallic but instead
goes on to the trismetallic derivative RCH2(allyl)2CCM3. This
relation between substitution and migration seems to be in
good agreement with the migration order observed in the
“Fritsch-Buttenberg-Wiechell type” rearrangement of true
10
alkylidene carbenes as reported recently.
In summary, the Fritsch-Buttenberg-Wiechell rearrange-
ment in the aliphatic series for zinc carbenoids depends on
the migrating aptitude of the groups, whatever the config-
uration of the carbenoid. Only in the case of allylic ether
(8) was the selective migration of the alkoxymethine moiety
trans to chlorine observed.
Acknowledgment. We thank the Japan Society for the
Promotion of Science, who made possible a 3 month stay
for S.Y. in University Pierre et Marie Curie, in the frame of
a JSPS-CNRS collaboration.
Supporting Information Available: General experimen-
tal procedures, including spectroscopic and physical data,
for the preparation of 13C-labeled dibromoalkenes and
compounds 10, 14, 16, and 18 via the tandem alkynyllithium
formation-allylzincation-Fritsch-Buttenberg-Wiechell rear-
rangement. This material is available free of charge via the
OL991117Z
(9) Frangin, Y.; Gaudemar, M. Bull. Soc. Chim. Fr. 1976, 1173.
Gaudemar, M. C. R. Acad. Sci. Paris Ser. C 1971, 273, 1669. Frangin, Y.;
Gaudemar, M. C. R. Acad. Sci. Paris Ser. C 1974, 278, 885.
(10) Graf von der Schulenburg, W.; Hopf, H.; Walsh, R. Angew. Chem.,
Int. Ed. 1999, 38, 1128-1130.
Org. Lett., Vol. 2, No. 4, 2000
421