prepared by a highly regioselective palladium-catalyzed
8
hydrostannylation; none of the regioisomeric vinyl stannanes
were ever observed (Scheme 2).
Scheme 1a
product of this kinetically controlled process is easily
separated from the minor â isomer by chromatography and
then can be utilized in strategies to control the stereochem-
istry of incipient chiral centers along the diene periphery.
As a result of our efforts to expand the synthetic applications
of these complexes, we sought to prepare a series of C4-
substituted sulfinyl dienes additionally possessing an ortho-
substituted aryl unit at C3. Molecular models suggested that
the iron diene complexes obtained from such compounds
might display restricted rotation about the aryl-C3 single
bond provided that the ortho-substituent was of a sufficient
size. Thus, four diastereomers could be produced by com-
plexation of these substrates; each facial isomer could exist
as diastereomeric atropisomers, as shown in Figure 1.
a
Reagents and conditions: (a) HS(CH
OH, 5 mol % Pd(PPh
Sn-C≡C-CH OTHP, 3 mol % Pd(Pt-
2
)
3
SH, Amberlyst-15 resin,
THF, rt. (b) H-C≡C-CH
0 °C (50 °C for 2c). (c) Bu
Bu , PhMe, rt.
2
3 4
)
, pyrrolidine,
8
3
2
3 2
)
The required 3-aryl-1-sulfinyl dienes 5a-d, 5c′, and 5d′
were prepared by our previously reported approach via Stille
coupling of an enantiopure (Z)-iodovinyl sulfoxide and the
3
corresponding vinyl stannane (Scheme 2). It is interesting
to note that we were unable to effect Stille coupling of aryl
vinyl stannanes with ortho-substituents that did not contain
a benzylic sulfur, suggesting a favorable interaction between
the palladium and sulfur atoms that accelerates transmeta-
9
1
lation. The H NMR spectra of sulfinyl dienes 5a and 5b
demonstrated significant broadening at room temperature
(presumably a result of slow, but not completely restricted,
rotation about the aryl-C3 single bond on the NMR time
scale). However, sulfinyl dienes 5c, 5c′, 5d, and 5d′ had
sharply resolved absorbances in their NMR spectra; indeed,
Figure 1. Possible diastereomers formed upon complexation of
(
1E,3Z)-1-sulfinyl-3-(o-substituted)-aryl dienes with an iron tricar-
bonyl fragment. The syn or anti notation refers to the relative
positions of the Fe(CO) unit and R ; the R or â nomenclature refers
to the location of the Fe(CO) unit relative to the sulfoxide.
5c′, 5d, and 5d′ were each obtained as atropisomeric mixtures
3
L
3
that could be separated by chromatography.
Each of the sulfinyl dienes, 5a-5d, 5c′, and 5d′, were
subjected to our usual conditions for complexation (using a
The preparation of the required 3-aryl-1-sulfinyl dienes,
each equipped with a dithiane as the bulky aryl ortho-
substituent (R ), was initiated with bromoaldehydes 2.
L
4
- to 5-fold excess of (bda)Fe(CO)
afford the sulfinyl iron diene complexes 6a-d, 6c′, and 6d′
Scheme 3). Sulfinyl diene 5a and 5b each afforded only
two of the four possible diastereomeric complexes of 6a and
b; for dienes 5c, 5c′, 5d, and 5d′, three diastereomers were
obtained. For each set of complexes, facial and axial
3
in toluene at 45 °C) to
(
Conversion to the corresponding dithianes followed by
4
Sonogashira-type coupling with propargyl alcohol proceeded
6
5
smoothly for 2a-c to produce aryl alkynes 3a-c. On the
other hand, the sterically demanding 3d was more efficiently
6
prepared from aldehyde 2d by PdP(t-Bu
3
)
2
-catalyzed Stille
(6) Clive, D. L. J.; Sannigrahi, M.; Hisaindee, S. J. Org. Chem. 2001,
7
coupling followed by one-pot dithiane formation and THP
66, 954.
(
7) Littke, A. F.; Schwarz, L.; Fu, G. C. J. Am. Chem. Soc. 2002, 124,
deprotection (Scheme 1). Vinylic stannanes 4a-d were then
6
343.
(
8) Liron, F.; Le Garrec, P.; Alami, M. Synlett 1999, 246.
(
4) Alami, M.; Ferri, F.; Linstrumelle, G. Tetrahedron Lett. 1993, 34,
403.
5) Compound 2c is available in one step from the corresponding benzyl
alcohol: Swindell, C. S.; Fan, W. J. Org. Chem. 1996, 61, 1109.
(9) (a) Itami, K.; Kamei, T.; Yoshida, J. J. Am. Chem. Soc. 2001, 123,
6
8773. (b) Crisp, G. T.; Gebauer, M. G. Tetrahedron Lett. 1995, 36, 3389.
(c) Vedejs, E.; Haight, A. R.; Moss, W. O. J. Am. Chem. Soc. 1992, 114,
6556.
(
310
Org. Lett., Vol. 5, No. 3, 2003