高等有机化学 教案 ylide负碳离子

+
ex) (C6H5)3P CH CH2 + CH3Li X-
ether
+
(C6H5)3P
_ CH CH2CH3 + LiX
3. Addition of a carbene to a phosphine
ex) (C6H5)3P + CCl2
+
(C6H5)3P CCl2
Reactions of PhosphorusYlides
Formation reactions of phosphorus ylides
1. Proton abstraction from the corresponding "onium" salt by a strong base
ex) (C6H5)3P + CH3Br
+
(C6H5)3P C H3 B r-
+_ (C6H5)3P CH2
(C6H5)3P CH2
Fra Baidu bibliotek
t riphenylphosphonium m et hylide
met hylene t riphenyl phosphorane
Stabilit y and Reactivit y:
a) As t he electron-donat ing abilit y of subst it uent s bonded t o t he heteroatom increases, t st abilit y of t he ylide increases, and it s react ivity decreases.
O
b) Coupling
C4H9Li ether
phosphonium salt
+_ (C6H5)3P C H2
+ C4H10 + LiBr
H3C S H3C
O + CH3Br
+
(CH3)3S O B r-
NaH
sulfoxonium salt
C H_3 H3C +S CH2 + H2 + NaBr
O
2. Addition of a carbanion to a vinyl "onium" salt
+
(C6H5)3P CR2
i.e.)
_
O CR2
a betaine
(C6H5)3P O
CR 2 CR 2
+_ k R2C CR2 + (C6H5)3P O
Other Phosphorus Ylide Reactions
+_ a) Acylation: (C6H5)3P CH 2 + R C OR'
b) With electron-donating groups on phosphor us, phosphorus is less inclined to bond to the anionic oxygen atom and the betaine decomposes via k( reverse) instead of k.
+_ (C6H 5)3P CH 2 + C6H 5 C H
O
+
(C6 H 5 )3 P
CH 2
CH _
C6H 5 (a betaine)
O
(has bee n isolated)
W ittig re a c tio n :
A c c e p te d m
+_
+
(C6H5)3P CR2
kfor (C6H5)3P CR2
b) As t he electron-wit hdrawing abilit y of subst it uent s bonded to the carbanion carbon increases, t he stabilit y of t he ylide increases, and its react ivity decreases.
c) The isolation of a cyclic intermediate from the decomposition of betaine to products is consistent with the expected tr ansition state for the change.
Wittig re action:
+_ (C 6 H 5 ) 3 P CH 2 + R R'
C
O
+_ R R' + (C 6 H 5 ) 3 P O
C
CH 2
Facts pertinent to the mechanism of the Wittig re action:
a) Electron-withdrawing groups on the carbanion carbon and electron-donating groups bonde d to phosphorus retard the Wittig re action.
Ylides
Definition: Com pounds in which a carbanion is direct ly bonded t o a positively charged he
_+ CZ
where Z = N, P, As, Sb, S, Se
Structure and Nomenclat ure: Most ylides (except those containing nitrogen) can be repr by t he following resonance forms:
b) The following observa tions have been ma de: + __ +
(C6H 5)3P C C P(C 6H 5)3 + F3C CF 3 C
O
_+ (C6H 5)3P C C P(C 6H 5)3
O C CF 3 CF 3
(has bee n isolated)
O CR2
k re v
_ O CR2
a betaine
Rat ionale
e c h a n is m
+_
k
R2C CR2 + (C6H5)3P O
a) With electron-withdrawing groups on the carbanion carbon, the nucleophilicity of the ylide is reduced and attackon the carbonyl carbon is retarded.