_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Organic derivatives of Group V
Butyllithium is an unsuitable base on account of its sensitivity to air and
moisture. In the process the phosphonium salt precursor (formed from the
alcohol, Ph,P and hydrogen halide), together with aldehyde in methanol, are
treated with methanolic sodium methoxide at ~ 30 C. After reaction the solution
is neutralized with dilute sulphuric acid, the Vitamin A ester extracted into a
hydrocarbon solvent and isolated after removal of solvent by distillation.
~ PPh 3 °~0COCH 3
-Ph3PO
Generally the Wittig reaction yields a mixture of cis and trans geometrica!
isomers of an alkene. The stereochemical considerations are discussed elsewhere.
(b) ARSENIC YLIDES. Arsenic ylides can be prepared by deprotonation of
quaternary arsonium salts.
+ ~~ ' -
R,As + R'CH 2 X-----+ [R,AsCH 2 R']X ----> R,As=CHR'( +-->R,As -CHR')
(R = alkyl. aryl: R' = H. alkyl. aryl. acyl. C0 2 R, CN)
Potentiometric titrations indicate that they are much stronger bases than the
corresponding phosphorus ylides. On the basis of J
derived from 13 C n.m.r. spectra it is concluded that the carbon atom in Ph,P=
( 1 'C- 1 H) coupling constants
CH 2 is in a trigonal planar environment (sp 2 C). whereas in Ph,As + CH 2 it is
probablypyramidal(sp' C;J = l36.7Hzcomparedwith 142 HzinPh,AsCH 1 r).
There therefore seems tobe weaker ( 2p- 3d)n bonding in the arsenic compound,
which means that the As-C hond is more polar in the sense PhJ\s-CH 1 . This
shows up in the enhanced reactivity. most noticeably of 'stabilized' arsenic ylides.
compared with their phosphorus analogues. While the ylides PhE=CHR' (E = P.
R' = C0 2 Me, CN or COPh) do not react with ketones readily if at ali, the arsenic
ylides give yields ofbetween 50-80% ofthe expected alkenes. With non-stabilized
\ "
139