J. Am. Chem. Soc. 1998, 120, 4863-4864
Evidence for a Continuous Transition between
4863
Thiaphosphetane and Betaine-Type Structures in the
Thio-Wittig Reaction
†
Carsten Puke, Gerhard Erker,* Nicola C. Aust,
,†
Ernst-Ulrich W u¨ rthwein,* and Roland Fr o¨ hlich
Organisch-Chemisches Institut der
UniVersit a¨ t M u¨ nster, Corrensstrasse 40
D-48149 M u¨ nster, Germany
ReceiVed June 23, 1997
The Wittig olefination belongs to a small group of essential
reaction types in organic synthesis for building up carbon-
containing frameworks. Its reaction mechanism has been a subject
Figure 1. Dependence of the 31P NMR chemical shift of the 4a/4′a
1
of intensive investigation. The key role of oxaphosphetane
13
intermediate (prepared from Ph
a small amount of the PPh decomposition product) with the toluene/
dichloromethane (toluene-d /dichloromethane-d ratio in volume %) ratio
at 233 K).
3 2 2
Pd CH and Ph CdS; an asterisk marks
intermediates was established unequivocally.2 However, the
involvement of betaine intermediates was controversial for a long
time.3 It now appears to be established experimentally that
betaines are not formed in the reaction between a nonstabilized
3
8
2
(
1
2
1
2
phosphorus ylide of the type Ar PdCR R (R , R ) H or alkyl)
3
Scheme 1
with ketones or aldehydes.4 We have recently found that a
reactive intermediate exhibiting a pronounced thiaphosphetane
character is formed upon treatment of Ph
3
Pd*CH
2
(1a) with
13
Ph
when necessary). In toluene-d
2
*CdS (2a) (both reagents C labeled at the marked positions
5
8
solution at -30 °C, the
2
3
1
intermediate 4′a (R ) R ) Ph, R ) R ) H) is formed. It
exhibits a 31P NMR signal at -40 ppm, i.e., in the range expected
of a thiaphosphetane structure. We have now carried out the
reaction between the ylide 1a and thiobenzophenone 2a in
dichloromethane-d
spectroscopic feature at δ +1.0 ppm (at 243 K). The chemical
behavior of the thus-formed intermediate in CD Cl is not much
different from the species obtained in toluene-d . It starts to
decompose at slightly elevated temperatures (τ1/2 ≈ 20 min at
63 K) to produce Ph PS + Ph CdCH via a 2,2-diphenylthiirane
intermediate (5a) and PPh (see Scheme 1).
In a second set of experiments, the intermediate (4a/4′a) was
generated in toluene-d , and dichloromethane-d was then added
and obtained a markedly different 31P NMR
2
2
2
8
2
3
2
2
3
toward a limiting value that is probably marked by the 31P NMR
chemical shift in pure CD Cl solvent (see above). Addition of
other polar solvents (e.g., chloroform, propylene carbonate)
8
2
2
2
31
to the solution. This had a marked effect on the P NMR
chemical shift of the intermediate. The presence of ca. 20 vol %
1
3
furnished qualitatively similar effects. By using C-labeled
solvent results in shifting the 31P NMR signal
13
of the polar CD Cl
2 2
starting materials, we monitored a gradual shift of the CH
NMR resonance adjacent to phosphorus from δ 65.1 (in toluene-
) to 51.7 ppm (in CD Cl ) and a continuous decrease of the
2
-
of 4a from δ -40 to -20 ppm (at 243 K). The consecutive
addition of another four 0.185 mL portions of CD
2
Cl
2
to the
d
8
2
2
sample resulted in a further shifting of the 31P NMR resonance
and the observation of a continuous transition (see Figure 1)
1
respective JPC coupling constant from 94 to 82 Hz. Both features
are as expected for an increased phosphonium character, although
1
the still large JPC coupling constant indicates some remaining
(1) For reviews, see: Schlosser, M. Top. Stereochem. 1970, 5, 1. Maryanoff,
6
13
distorted pentavalent geometry at phosphorus. The CPh
2
B. E.; Reitz, A. E. Chem. ReV. 1989, 89, 863. Vedejs, E.; Peterson, M. J.
Top. Stereochem. 1994, 21, 1. Vedejs, E.; Marth, C. F. In Phosphorus-31
NMR Spectral Properties in Compound Characterization and Structural
Analysis; Quin, L. D., Verkade, J. E., Eds.; VCH: New York, 1994; pp 297-
1
2
resonance (δ 50.7), the JCC (38 Hz) and JPC (1.6 Hz) coupling
constants were not affected by the change of solvent.
The 31P NMR chemical shift of the thio-Wittig intermediate
(4a/4′a) shows only a very small temperature dependence in
3
13.
(2) (a) For NMR studies, see: Vedejs, E.; Snoble, K. A. J. J. Am. Chem.
Soc. 1973, 95, 5778. Vedejs, E.; Meier, G. P.; Snoble, K. A. J. J. Am. Chem.
Soc. 1981, 103, 2823. Maryanoff, B. E.; Reitz, A. B.; Mutter, M. S.; Inners,
R. R.; Almond, H. R., Jr.; Whittle, R. R.; Olofson, R. A. J. Am. Chem. Soc.
either of the solvents (toluene-d
K); CD Cl δ +4.6 (223 K), +1.0 (243 K)). The P NMR spectra
of the intermediate were found to be static in both solvents down
8
δ -38.6 (223 K), -40.1 (243
31
2
2
1986, 108, 7664. Maryanoff, B. E.; Reitz, A. B. Phosphorus Sulfur 1986, 27,
167. Vedejs, E.; Marth, C. F. J. Am. Chem. Soc. 1988, 110, 3940. Geletneky,
7
to the lowest temperature monitored (203 K in CD
2 2
Cl ). We
C.; F o¨ rsterlin, F.-H.; Bock, W.; Berger, S. Chem. Ber. 1993, 126, 2397. (b)
For X-ray crystal structure analyses, see: Kawashima, T.; Kato, K.; Okazaki
R. J. Am. Chem. Soc. 1992, 114, 4008; Angew. Chem. 1993, 105, 941; Angew.
Chem., Int. Ed. Engl. 1993, 32, 869 and references therein. For a Review,
see: Kawashima, T.; Okazaki, R. Synlett 1996, 600.
carried out a number of control experiments. These showed, e.g.,
+
-
3 4
(6) Treatment of 4a with [Et O ][BF ] gave the phosphonium salt
31
+
-
2 2 4
CPh SEt][BF ] which exhibits a P NMR signal at δ +19.1 and
13 13 1
[Ph
3
P CH
C NMR features δ 35.6 ( CH
(7) This experiment cannot distinguish between a double-minimum situation
1
3
(
3) For a comprehensive review of this development, see: Vedejs, E.;
2 2
) and 56.1 ( CPh ), and JPC ) 62 Hz.
Peterson, M. J. In AdVances in Carbanion Chemistry: Snieckus, V., Ed.; JAI
Press: Greenwich, CT, 1996; Vol. 2, p 1-85, and references therein.
(4 h 4′) that is rapid on the NMR time scale and a one-minimum situation
characterized by a system-dependent continuous 4/4′ transition (which is
graphically indicated by the new symbol of two curved arrows in Scheme 1).
The additional evidence presented (see below) lead us to favor the latter
explanation.
(
4) Vedejs, E.; Marth, C. F. J. Am. Chem. Soc. 1990, 112, 3905.
(
5) Wilker, S.; Laurent, C.; Sarter, C.; Puke, C.; Erker, G. J. Am. Chem.
Soc. 1995, 117, 7293. See also: Okuma, K.; Ishida, T.; Morita, S.; Ohta, H.;
Inoue, T. Heteroat. Chem. 1995, 6, 265.
S0002-7863(97)02066-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/05/1998