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J. Am. Chem. Soc. 1996, 118, 1549-1550
1549
Scheme 1. Alkylation of Spirooxyphosphorane 1
Reactivity of Carbon Anions r to Pentacoordinated
Phosphorus: Spirooxyphosphoranyl C-Anions as
Valuable Intermediates in Olefination Chemistry
Mihaela L. Bojin, Salim Barkallah, and
Slayton A. Evans, Jr.,*
William Rand Kenan, Jr. Laboratories of Chemistry
UniVersity of North Carolina at Chapel Hill
Chapel Hill, North Carolina 27599-3290
Table 1. Summary of Alkylation Reactions of Phosphorane 1
ReceiVed June 26, 1995
Oxyphosphoranes, e.g., pentaoxyphosphoranes, are well-
documented as useful models designed to mimic the structural
and electronic features of phosphorous intermediates in phos-
phate ester hydrolysis.1 The synthesis and unique geometrical
characteristics of many spirooxyphosphoranes have been de-
scribed previously,2 and these novel spatial features are envi-
sioned as structural platforms for developing new stereoselective
methods.3
The spirooxyphosphoranyl substructure (e.g., 1) was selected
because the acyloxy groups are highly electron withdrawing and
exhibit a higher apicophilicity (preference for the axial orienta-
tion in the trigonal bipyramidal arrangement of substituents
attached to phosphorus)4 than the ethereal P-O-C oxygens.5
Consequently, the anticipated high barrier attending the stereo-
mutation process, through Berry6 or turnstile7 mechanisms,
should ensure a strong, if not exclusive, preference for the
spirooxyphosphoranyl conformational substructure represented
in derivatives of 1.
Scheme 2. The Olefination Reaction
nances resulted from the E and Z stereochemistry of the 2-buten-
1-yl fragment in 2d.
We anticipated that the acidity of the diastereotopic methylene
hydrogens in 2 would be influenced by the inductive effects of
the ethereal ring oxygens (through the “spiro-linked” phosphorus
atom), as well as that of the acyloxy groups. Once formed, the
R-“carbanion” might benefit from stabilization caused by
possible polarization functions,8a d-functions as valence
participants,8b and/or negative hyperconjugation effects.9
The objectives of our investigations were to (a) establish the
propensity for the diastereotopic methylene hydrogens in
spirooxyphosphorane 2 to exhibit different and measurable
kinetic acidities,10 (b) define the utility of the conjugate base
of phosphorane 2 (Pv-CHLi-EWG) for initiating selective
alkylations, and (c) demonstrate the feasibility of a new
stereoselective olefination procedure through the condensation
of the Pv-CHLi-EWG species with benzaldehyde. In this
report, we describe our findings on the developments of a new
olefination procedure.
Spirooxyphosphoranes 2a-e were obtained by the deproto-
nation of phosphorane 1 with triethylamine, followed by
alkylation as described by Munoz et al.11 (Scheme 1).
A view of the data (Table 1) indicated that the relative ease
of substitution and yields of these reactions appeared to be
sensitive to the increased steric crowding attending the displace-
ment of the halide ion. The 31P NMR resonances were in accord
with the expectations for a substituted pentacoordinate trigonal
bipyramidal phosphorane.12 In addition, two 31P NMR reso-
Viewing the R-phosphoranyl organolithiums derived from the
C-spirooxyphosphoranes 2a-e as possible candidates for Hor-
ner-Wadsworth-Emmons-type olefinations,13 we examined the
lithiation of (methyloxy)carbonyl spirooxyphosphorane 2e using
lithium hexamethyldisilazane (LiHMDS), followed by treatment
with benzaldehyde at -78 °C. Under these conditions, a
mixture of (E)- and (Z)-methyl cinnamates 3 was obtained in
81% yield (Scheme 2).
The E olefinic stereochemistry was assigned to (E)-3 by
comparison of its 1H NMR spectral data with previously reported
data,14 while the stereochemistry of the (Z)-3 was ascertained
by correlation of its 1H NMR and MS spectral data with those
of (E)-3.
A more detailed and systematic 31P NMR study provided
insights into a probable mechanism for the formation of
(10) (a) While the acidity (and reactivity of the conjugate base) of the
methylene hydrogens R to the pentacoordinate phosphorus atom in 2 have
not been explored previously, R-lithiation of 4,4-bis(trifluoromethyl)-1,2λ5-
phosphetanes 11a,b have been reported.
(b) Kawashima, T.; Kato, K.; Okazaki, R. Angew. Chem., Int. Ed. Engl.
1993, 32, 869. (c) Kawashima, T.; Kato, K.; Okazaki, R. J. Am. Chem.
Soc. 1992, 114, 4008. (d) Kawashima, T.; Iwama, N.; Okazaki, R. Ibid.
1992 114, 7598.
(1) Swamy, K. C. K.; Burton, S. D.; Holmes, J. M.; Day, R. O.; Holmes,
R. R. Phosphorus, Sulfur Silicon Relat. Elem. 1990, 53, 437-455.
(2) Koenig, M.; Munoz, A.; Garrigues, B. Wolf, R. Phosphorus Sulfur
Relat. Elem. 1979, 6, 435.
(3) Unpublished results with Powers, T. A., Bojin, M. L., and McKithen,
M., University of North Carolina, Chapel Hill, NC.
(11) (a) Lamande, L.; Munoz, A. Phosphorus Sulfur Relat. Elem. 1987,
32, 1. (b) Lamande, L.; Munoz, A. Tetrahedron 1990, 46, 3527-3534.
(12) (a) Munoz, A.; Lamande, L. Phosphorus, Sulfur Silicon Relat. Elem.
1990, 49/50, 373-376. (b) Munoz, A.; Lamande, L. Phosphorus, Sulfur
Silicon Relat. Elem. 1992, 70, 263-272. (c) Compounds 2a-e were
characterized by employing 1H, 13C, and 31P NMR. For example, 2c
exhibited the following spectroscopic data: 31P NMR (CDCl3) δ -28.23
ppm; 1H NMR δ 1.10 (s, C-CH3, 6H), 1.55 (s, C-CH3, 6H), 2.28 (s, aryl-
(4) (a) Holmes, R. R. In Pentacoordinated Phosphorus, Vol. 1, Structure
and Spectroscopy; ACS Monograph 175; American Chemical Society:
Washington, DC, 1980. (b) Well, M.; Albers, W.; Fischer, A.; Jones, P.
G.; Schmutzler, R. Chem. Ber. 1992, 125, 801-808.
2
2
(5) For an alternative view, e.g., equatoriphilicity and stabilization of
the equatorial plane of the phosphorane as the major determinant for
conformational preference, see: Wasade, H.; Hirao, K. J. Am. Chem. Soc.
1992, 114, 16.
Me, 3H), 2.38 (s, aryl-Me), 3.40 (dd, P-C-H, JP2H ) 17.9 Hz, JHH )
2
12.8 Hz, 1H), 3.65 (dd, P-C-H, JPH ) 21.9 Hz, JHH ) 12.8 Hz, 1H),
and 6.95-7.15 ppm (m, 3H); 13C NMR (CDCl3) δ 19.07 (s, aryl-Me), 20.73
(s, aryl-Me), 23.86 (d, P-O-C-CH3, 3JPC ) 7.40 Hz, 2C), 26.07 (s, P-O-
1
(6) Berry, R. S. J. Chem. Phys. 1960, 32, 933.
C-CH3, 2C), 38.64 (d, P-CH2 JPC ) 165.26 Hz), 81.01 (d, P-O-C,
(7) Ramirez, F.; Ugi, I. AdV. Phys. Org. Chem. 1971, 9, 256.
(8) (a) Bestman, H. J.; Kos, A. J.; Witzgall, K.; Schleyer, P. v. R. Chem.
Ber. 1986, 1331. (b) Magnusson, E. J. Am. Chem. Soc. 1990, 112, 7940.
(9) Reed A. E.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 1434.
2JPC ) 6.41 Hz, 2C), 127-136 (m, aryl, 6C), and 172.16 ppm (d, P-O-
2
C(O), JPC ) 8.55 Hz).
(13) (a) Maryanoff, B. E.; Reitz, A. B. Chem. ReV. 1989, 89, 863-927.
(b) Vedejs, E.; Peterson, M. J. Top. Stereochem. 1994, 21, 1.
0002-7863/96/1518-1549$12.00/0 © 1996 American Chemical Society