Page 467 - Advanced Organic Chemistry Part B - Reactions & Synthesis

440 Ketones can also be reduced to alkenes via enol triflates. The use of Pd OAc 2
and triphenylphosphine as the catalyst and tertiary amines as the hydrogen donors is
CHAPTER 5 226
effective.
Reduction of
Carbon-Carbon Multiple CH 3 CH
Bonds, Carbonyl N Pd(O CCH ) , PPh 3 N
Groups, and Other 2 3 2 3
2
Functional Groups CO CH 3 CO CH 3
2
(C H ) N, HCO H
2 5 3
2
O SCF 3
3
Ref. 227
Reductive removal of oxygen from aromatic rings can also be achieved by
reductive cleavage of aryl diethyl phosphate esters.
O
K, NH 3
CH 3
CH 3 OP(OC H )
2 5 2
OCH 3 77%
OCH 3
Ref. 228
There are also examples in which phosphate esters of saturated alcohols are reductively
deoxygenated. 229 Mechanistic studies of the cleavage of aryl dialkyl phosphates have
indicated that the crucial C−O bond cleavage occurs after transfer of two electrons. 230

O
2e –
–
ArOP(OC H ) [ArOPO(OEt) 2 ] 2– Ar + (EtO) PO 2 –
2
2 5 2
For preparative purposes, titanium metal can be used in place of sodium or lithium in
liquid ammonia for both the vinyl phosphate 231 and aryl phosphate 232 cleavages. The
titanium metal is generated in situ from TiCl by reduction with potassium metal in
3
tetrahydrofuran.
Scheme 5.11 shows some examples of these reductive reactions. Entry 1 is
an example of conditions that have been applied to both alkyl and aryl halides.
The reaction presumably proceeds through formation of a Grignard reagent, which
then undergoes protonolysis. Entries 2 and 3 are cases of the dehalogenation of
polyhalogenated compounds by sodium in t-butanol. Entry 4 illustrates conditions that
were found useful for monodehalogenation of dibromo- and dichlorocyclopropanes.
This method is not very stereoselective. In the example given, the ratio of cis:trans
product was 1.2:1. Entries 5 to 7 are cases of dissolving-metal reduction of vinyl and
aryl phosphates.

226 W. J. Scott and J. K. Stille, J. Am. Chem. Soc., 108, 3033 (1986); L. A. Paquette, P. G. Meister,
D. Friedrich, and D. R. Sauer, J. Am. Chem. Soc., 115, 49 (1993).
227
K. I. Keverline, P. Abraham, A. H. Lewin, and F. I. Carroll, Tetrahedron Lett. 36, 3099 (1995).
228 R. A. Rossi and J. F. Bunnett, J. Org. Chem., 38, 2314 (1973).
229 R. R. Muccino and C. Djerassi, J. Am. Chem. Soc., 96, 556 (1974).
230
S. J. Shafer, W. D. Closson, J. M. F. van Dijk, O. Piepers, and H. M. Buck, J. Am. Chem. Soc., 99,
5118 (1977).
231 S. C. Welch and M. E. Walters, J. Org. Chem., 43, 2715 (1978).
232
S. C. Welch and M. E. Walters, J. Org. Chem., 43, 4797 (1978).