Formations of Mixed β,β-Dihaloenals from Halogenated Secondary Alkynols

Article abstract of DOI:10.1016/0040-4039(93)88022-B

Iodine and Koser's reagent have been used in stoichiometric amounts to convert 3-bromo-1-phenylpropynol to (Z)-3-bromo-3-iodo-2-phenylpropenal in high yield.The latter is a major component of reaction mixtures of 3-iodo-1-phenylpropynol and bromine with K

Full text of DOI:10.1016/0040-4039(93)88022-B

Letters. Vol. 33. No. SO. pp.
Britain
in
Formations of Mixed
from Halogenated Secondary Alkynols
Pakorn Bovonsombat and Edward
Department of Chemistry, New York University
New York, New York
Abstract. Iodine and Koser’s reagent have been used in stoichiometric amounts to convert
phenylpropynol to
reaction mixtures of
in high yield. The latter is a major component of
and bromine with Koser’s reagent.
Recently we reported the conversions of secondary alkynols
iodosuccinimide and catalytic quantities of Lewis acids such as
to a-iodoenones
with
Koser’s reagent). Bromoenones were formed in an analogous manner with N-bromosuccinimide (NBS) and
catalytic silver
c = c
‘ C - R
Catalytic acid
H ’
H
a, R =
a, R =
b, R =
X = or Br
X = or Br
This “Halo-Meyer-Schuster” reaction stands in contrast with the conversions of tertiary alkynols to
With stoichiometric or greater amounts of iodine and
acid
were converted to
c =
Y
I
Y =
Y =
Y
I:
Y
Br
7705

7706
The conversion of
and HTIB of
halogens by transition metal-catalyzed coupling techniques.
to
was lately modified to give mixed
whose stereospecific formations render them excellent templates for exchanges of the
A similar conversion to mixed
(such as the conversion with
would expand the utility of such methodologies, but the starting materials would be secondary alkynols, which
seem to prefer the “Halo-Meyer-Schuster” pathways.
We now report such is not the case if the
secondary alkynols are the starting materials. The
mechanistic guideline for this direction would be that halonium attack at the a-position required of a “Halo-Meyer-
Schuster” process would lead to vinyl cation destabilized by halogen at the P-position. A more stable cation
would be formed by attack at the P-position and would be the terminus of a group shift from the alcohol-bearing
carbon. In particular,
phenylpropenal by means of iodine and
has been converted to
in stoichiometric quantities.
= c - x
C - C _
c =
X
I
a, X = Br, Y = I; b, X = I, Y= Br
X = Br; b, X = 1
When
(1 mmol) was treated with iodine mmol) and
(1 mmol) in acetonitrile
at room
temperature overnight, mixed
formed in 96% yield.” When the quantities of iodine and HTIB
were reduced to one-half molar, the yield was 7 1%. The uses of
and catalytic quantities of HTIB with
of p-toluenesulfonic acid with NIS in
but the selectivity to was 71%. The results with
in
acetonitrile or methanol were ineffective. The catalytic
acetonitrile, however, led to the conversion (65%) of
different iodinating systems reflect the electron withdrawing effect of the bromine on the alkyne and the
subsequent deactivation of any intermediate vinyl cations.
The spectral data for the clear oil
1600 (m), 1270 (s), 1090 (s) cm-t;
was follows,
(neat) 3030 (m), 2850 (m), 2730 (w), 1690 ( s ) ,
7.45 (m, 9.86 (s,
192.66 (Cl); mass
(3, M+-I), (1 M+-I-CO), (11,
(CDC13) 7.16 (m,
80.57
128.49, 128.76, 128.82, 131.19, 135.07, 147.60, 152.14
spectrum, m/e (rel. intensity)
127
(6,
C,H.

7707
An assignment of stereochemistry was based on a conversion of
followed by an oxidation with pyridinium chlorochromate (PCC).
to
by means of a
reaction
1)
2)
In order to rule out an isomerization in the oxidation step in the conversion of to
the haloenal was reduced at
low temperature with to obtain alcohol which was converted back to with
c:’
The intermediates and were not isolated but were characterized by
m/e (tel. int.)
m/e int.)
(M-I,
(M-I,
(1,
43
102 (100). The products of the PCC oxidation of and had the same
NMR and
IR as authentic samples.
Further evidence for the assigned stereochemistry
that exchanged the vinyl iodine atom with a hydrogen atom.
stemmed from a reaction with
and
H’
c = c
‘ B r
Compound was isolated by silica gel chromatography:
NMR
7.30 (m,
7.45 (m,
7.70
(s,
9.69
NMR
128.66, 129.41, 129.45, 130.28, 130.51, 132.51, 134.30, 134.35,
1328 (s), 1172 (m), 1064 1020 (m), 809 (m), 772 (m),
IR (nujol) 1706
1603 (m), 1495
716
The appearance of the vinyl singlet at 7.70 ppm is particularly significant. It indictaes that the proton
according to the studies of Piers on a variety of cyclic and acyclic In general, for
is syn to the

7708
haloenones protons syn to
values between 6.28 and 6.96 ppm.
The starting material was prepared from I-phenylpropynol and NBS in acetone stirred over catalytic
amounts of silver nitrate.8 This general procedure was used to prepare the iodinated analogue
had
between 7.35 and 7.70 ppm, whereas those that were anti had
from
phenylpropynol and
Compound
as well.
was treated with NBS
catalytic amounts of
or
in
or methanol to
no avail. The addition of
ond
in stoichiometric quantities in
gave a mixture of six products.
With the use of half molar amounts of bromine and
artifactual iodobenzene were present. They were identified by
and None of the anticipated
with
three major components other than the
to be the following:
was detected.
This finding is consonant with the results of the mixed dihaloenone system wherein the principal shifted
product of the iodinated was the Z-isomer rather than the (E)-isomer. Nonetheless, so long as the
isomer is formed in high yield stereospecifically, it would serve as a valuable synthetic template for vinyl halide
exchange processes. The order of exchange allows flexibility in the selection of synthetic targets.9
Acknowledgement: We are grateful for financial assistance from New York University Technology Transfer
Fund.
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and solvent evapora-
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(Received in USA 5 August 1992; accepted 17 September 1992)