Page 186 - Advanced Organic Chemistry Part B - Reactions & Synthesis

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158 NMR spectroscopic studies ( H C, and 31 P) are consistent with the dipolar ylide
structure and suggest only a minor contribution from the ylene structure. 234 Theoretical
CHAPTER 2 235
calculations support this view. The phosphonium ylides react with carbonyl
Reactions of Carbon compounds to give olefins and the phosphine oxide.
Nucleophiles with
Carbonyl Compounds
+ –
R P CR 2 + R′ C O R C CR′ 2 + R 3 P O
2
3
2
There are related reactions involving phosphonate esters or phosphines oxides. These
reactions differ from the Wittig reaction in that they involve carbanions formed by
deprotonation. In the case of the phosphonate esters, a second EWG substituent is
usually present.
O O
base R C O
2
(R′O) PCH -EWG (R′O) PCH-EWG R C CH-EWG
2
2
2
2
-
2.4.1.1. Olefination Reactions Involving Phosphonium Ylides. The synthetic potential
of phosphonium ylides was developed initially by G. Wittig and his associates at
the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde
or ketone introduces a carbon-carbon double bond in place of the carbonyl bond.
The mechanism originally proposed involves an addition of the nucleophilic ylide
carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by
elimination of a phosphine oxide. The elimination is presumed to occur after formation
of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes
direct formation of the oxaphosphetane by a cycloaddition reaction. 236 There have
been several computational studies that find the oxaphosphetane structure to be an
intermediate. 237 Oxaphosphetane intermediates have been observed by NMR studies
at low temperature. 238 Betaine intermediates have been observed only under special
conditions that retard the cyclization and elimination steps. 239

(betaine intermediate)
+
Ar 3 P CR 2
– ′
O CR
+ – 2
Ar P CR 2 + R′ 2 C O Ar P O + R C CR′ 2
3
2
3
Ar 3 P CR 2
O CR′ 2
(oxaphosphetane intermediate)
234
H. Schmidbaur, W. Bucher, and D. Schentzow, Chem. Ber., 106, 1251 (1973).
235 A. Streitwieser, Jr., A. Rajca, R. S. McDowell, and R. Glaser, J. Am. Chem. Soc., 109, 4184 (1987);
S. M. Bachrach, J. Org. Chem., 57, 4367 (1992); D. G. Gilheany, Chem. Rev., 94, 1339 (1994).
236 E. Vedejs and K. A. J. Snoble, J. Am. Chem. Soc., 95, 5778 (1973); E. Vedejs and C. F. Marth, J. Am.
Chem. Soc., 112, 3905 (1990).
237
R. Holler and H. Lischka, J. Am. Chem. Soc., 102, 4632 (1980); F. Volatron and O. Eisenstein, J. Am.
Chem. Soc., 106, 6117 (1984); F. Mari, P. M. Lahti, and W. E. McEwen, J. Am. Chem. Soc., 114,
813 (1992); A. A. Restrepocossio, C. A. Gonzalez, and F. Mari, J. Phys. Chem. A, 102, 6993 (1998);
H. Yamataka and S. Nagase, J. Am. Chem. Soc., 120, 7530 (1998).
238 E. Vedejs, G. P. Meier, and K. A. J. Snoble, J. Am. Chem. Soc., 103, 2823 (1981); B. E. Maryanoff,
A. B. Reitz, M. S. Mutter, R. R. Inners, H. R. Almond, Jr., R. R. Whittle, and R. A. Olofson, J. Am.
Chem. Soc., 108, 7684 (1986).
239
R. A. Neumann and S. Berger, Eur. J. Org. Chem., 1085 (1998).