Scheme 4. A Plausible Mechanism for the Inversion of
Table 2. Effect of Solvent on the Reaction of 1ca
Configuration
of the boronic ester and participates anti to the entering
electrophile.9,10 In the ensuing step, the same group departs
concertedly with the leaving electrofuge, the boronate group,
in an anti fashion after bond rotation. That is, the inversion
selectivity can be explained by anti-addition of the internal
oxy group and iodine(III) followed by anti-elimination of
the oxy group and boronic ester. A similar anti-addition-
anti-elimination mechanism has been proposed for the
substitution reaction of alkenylboronic ester with bromine
in the presence of base.11,12
vol % of Et2O
1c/(E)-2c/(Z)-2cb
(E)-2c/(Z)-2cc
0
10
50
54:15:31 (32:68)
87:12:1 (90:10)
86:14:0 (>98:2)
29:71
91:9
>98:2
a The reaction of 1 (0.10 mmol) with PhI(OAc)2 (0.14 mmol) was carried
out in the presence of BF3‚OEt2 (0.16 mmol) in CH2Cl2/Et2O solvent (2
mL) at 0 °C for 30 min. b Product distribution of the reaction mixture
1
determined by H NMR before workup. The values in parentheses are the
E/Z ratio of 2c. c Ratio of the isolated products.
(E)-Iodonium salt 2c was selectively formed with retention
of configuration during the reaction of (E)-boronate 1c in
the ether-dichloromethane solution, and the stereoselectivity
was affected by the content of the ether (Table 2 and entries
4, 5 in Table 1). The ether might be impeding the neighboring
group participation process to drive vinylic substitution to
the normal retention mode.
The usual substitution reaction of alkenylborates to form
alkenyliodonium salt proceeds with retention of the olefin
geometry, and this is rationalized by the addition-elimination
mechanism illustrated in Scheme 3.1k Electrophilic attack of
In summary, some oxy-functionalized alkenyliodonium
salts have been prepared from the corresponding alkenyl-
boronic esters with complete inversion of configuration. The
selectivity of substitution is controlled by the reaction
conditions used. In particular, both (E)- and (Z)-2c were
selectively obtained from E substrate 1c depending on the
precise conditions and reagent employed. The unexpected
and divergent E/Z selectivity is rationalized by participation
of the internal oxy group.
Scheme 3. Retention of Configuration via
Addition-Elimination Mechanism
Supporting Information Available: General procedures
and characterization of all new compounds. This material is
OL0601850
the iodine(III) reagent to the carbon-carbon double bond
of vinylboronic ester must initiate the exchange of boronic
ester for the iodonium group. Electrophilic attack is followed
by the elimination of the electrofuge, where the cleaving
bond is parallel to the vacant orbital on the positive carbon.
Favorable rotation around the C1-C2 bond generally occurs
as illustrated in Scheme 3 to result in the selective formation
of the E product with retention of configuration due to the
lower rotational barrier.
The opposite selectivity we observed in the reaction of
oxy-functionalized alkenylboronic esters may be accounted
for as illustrated in Scheme 4. The internal oxy group can
interact with the positive charge developed at the â-position
(9) (a) Participation of carbonyl oxygen has been discussed for the vinylic
substitution of silane for iodine.9b The reactions of both (E)- and (Z)-3-
acyloxyprop-1-enyl(trimethyl)silane with N-iodosuccinimide gave prefer-
entially (Z)-iodide in contrast to retention of configuration for the simple
alkenylsilane substrates. The convergent stereoselectivity can also be
explained by the thermodynamic stability of the cationic intermediate. (b)
Stamos, D. P.; Taylar, A. G.; Kishi, Y. Tetrahedron Lett. 1996, 37, 8647-
8650.
(10) (a) Participation of internal chloride has been reported in the
preparation of vinyl-λ3-bromane (vinylbromonium salt).10b The participation
does not affect the E/Z selectivity, but results in internal chloride shift. (b)
Ochiai, M.; Nishi, Y.; Mori, T.; Tada, N.; Suefuji, T.; Frohn, H. J. J. Am.
Chem. Soc. 2005, 127, 10460-10461.
(11) (a) Brown H. C.; Hamaoka, T.; Ravindran, N. J. Am. Chem. Soc.
1973, 95, 6456-6457. (b) Brown H. C.; Subrahmanyam, C.; Hamaoka,
T.; Ravindran, N.; Bowman, D. H.; Misumi, S.; Unni, M. K.; Somayaji,
V.; Bhat, N. G. J. Org. Chem. 1989, 54, 6068-6075. (c) Brown H. C.;
Hamaoka, T.; Ravindran, N.; Subrahmanyam, C.; Somayaji, V.; Bhat, N.
G. J. Org. Chem. 1989, 54, 6075-6079.
(12) For reviews of reactions of vinyl boranes, see: (a) Hall, D. G. In
Boronic Acids; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2005; pp 1-99.
(b) Negishi, E.-I.; Idacavage, M. J. Org. React. 1985, 33, 1-246.
(8) 5-Benzoyloxypent-1-yne was detected in the crude mixture obtained
from the reaction of 1c in the presence of ether, but the content was less
than 5%. Thus, (E)-2c is still a major product under these conditions even
if unstable (Z)-2c is decomposed to the alkyne during the reaction.
Org. Lett., Vol. 8, No. 7, 2006
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