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The identification of a suitable base is often an important step when optimizing a Wittig reaction. Because phosphonium ylides are seldom isolated, the byproduct(s) generated upon deprotonation essentially plays the role of an additive in a Wittig reaction. As a result, the choice of base has a
156:), or sodium hydride (NaH) are also commonly used. For stabilized Wittig reagents bearing conjugated electron-withdrawing groups, even relatively weak bases like aqueous sodium hydroxide or potassium carbonate can be employed.
176:
Electron-withdrawing groups (EWGs) enhance the ease of deprotonation of phosphonium salts. This behavior is illustrated by the finding that deprotonation of triphenylcarbethoxymethylphosphonium requires only
226:
Wittig reagents are prepared by deprotonation of alkyl phosphonium salts, and this reaction can be reversed. The methodology can be useful in the preparation of unusual Wittig reagents.
85:. Quaternization of triphenylphosphine with secondary halides is typically inefficient. For this reason, Wittig reagents are rarely used to prepare tetrasubstituted alkenes.
185:
is somewhat air-stable. It is however less reactive than ylides lacking EWGs. For example they usually fail to react with ketones, necessitating the use of the
521:; Basil, J. D. (1982). "Oxidative Addition of Methyl Iodide to a Dinuclear gold(I) Complex. The X-Ray Crystal Structure of Bis-iodomethyldigold(II)(Au-Au), Au
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as an alternative. Such stabilized ylides usually give rise to an E-alkene product when they react, rather than the more usual Z-alkene.
614:
57:. They are typically generated and used in situ. THF is a typical solvent. Some are sufficiently stable to be sold commercially.
357:
416:
Crystallographic characterization of methylenetriphenylphosphorane shows that the phosphorus atom is tetrahedral. The PCH
367:
298:
Although ylides are "electron-rich", they are susceptible to deprotonation of alkyl substituents. Treatment of Me
25:
168:
strong influence on the efficiency and, when applicable, the stereochemical outcome of the Wittig reaction.
555:
Schmidbaur, H. (1983). "Phosphorus Ylides in the
Coordination Sphere of Transition Metals: An Inventory".
211:
582:
Bart, J. C. J. (1969). "Structure of the Non-Stabilized
Phosphonium Ylid Methylenetriphenylphosphorane".
41:
469:
448:
Wittig reaction in
Organic Syntheses, Coll. Vol. 5, p. 361 (1973); Vol. 45, p. 33 (1965). (
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Because they typically hydrolyze and oxidize readily, Wittig reagents are prepared using
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distance is 1.661 Å, which is much shorter than the other P-C distances (1.823 Å).
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Appel, Rolf; Morbach, Wolfgang (1977). "(Chloromethylene)triphenylphosphorane".
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The alkylphosphonium salt is deprotonated with a strong base such as
33:
441:, Coll. Vol. 10, p. 703 (2004); Vol. 75, p. 153 (1998). (
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Li. Having carbanion-like properties, lithiated ylides function as
191:
378:
Wittig reagents are usually described as a combination of two
36:. They are used to convert ketones and aldehydes to alkenes:
81:. Wittig reagents are usually derived from a primary
134:
BuLi), other strong bases like sodium and potassium
557:Angewandte Chemie International Edition in English
493:Angewandte Chemie International Edition in English
413:form, which is the more familiar representation.
8:
246:−X, produces substituted phosphonium salts:
89:Bases for deprotonation of phosphonium salts
65:Wittig reagents are usually prepared from a
470:"(Carbethoxymethylene)triphenylphosphorane"
148:(LiHMDS, NaHMDS, KHDMS, where HDMS = N(SiMe
158:
461:
363:Triphenylcarbethoxymethylenephosphorane
183:triphenylcarbethoxymethylenephosphorane
348:(Chloromethylene)triphenylphosphorane
144:BuOK), lithium, sodium and potassium
7:
353:Methoxymethylenetriphenylphosphorane
69:, which is in turn prepared by the
409:form and the latter is called the
14:
584:Journal of the Chemical Society B
242:with a primary alkyl halide R−CH
187:Horner–Wadsworth–Emmons reaction
40:
196:A "stabilized" Wittig reagent.
1:
358:Methylenetriphenylphosphorane
306:with butyl lithium affords Me
210:Wittig reagents are used for
163:Br, typical phosphonium salt.
61:Formation of phosphonium salt
368:Hexaphenylcarbodiphosphorane
282:in the usual way to give Ph
32:P=CHR', where R is usually
631:
615:Organophosphorus compounds
26:organophosphorus compounds
420:centre is planar and the
405:The former is called the
569:10.1002/anie.198309071
505:10.1002/anie.197701801
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164:
212:olefination reactions
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596:10.1039/J29690000350
380:resonance structures
146:hexamethyldisilazide
542:10.1021/om00066a021
437:Wittig reaction in
278:These salts can be
172:Substituent effects
55:air-free techniques
334:Li is a potential
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75:triphenylphosphine
439:Organic Syntheses
18:organic chemistry
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131:-butyllithium (
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116:P=CHR + LiX + C
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22:Wittig reagents
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83:alkyl halide
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590:: 350–365.
411:phosphorane
222:Protonation
214:, i.e. the
206:Olefination
138:-butoxide (
49:Preparation
457:References
230:Alkylation
528:(CH3)I".
374:Structure
322:. Thus Me
201:Reactions
609:Category
342:Examples
127:Besides
77:with an
476:27 June
450:Article
443:Article
320:ligands
286:P=CH−CH
141:BuONa,
112:Li → Ph
262:X → Ph
34:phenyl
407:ylide
258:+ RCH
104:X + C
588:1969
478:2019
398:P=CR
394:↔ Ph
326:P(CH
310:P(CH
290:R.
254:P=CH
238:P=CH
24:are
592:doi
565:doi
538:doi
501:doi
422:P=C
390:PCR
302:PCH
274:R X
266:PCH
73:of
16:In
611::
586:.
561:22
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532:.
497:16
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386:Ph
382::
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270:CH
250:Ph
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122:10
100::
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540::
534:1
526:2
523:2
507:.
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424:H
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244:2
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236:3
154:2
152:)
150:3
136:t
129:n
120:H
118:4
114:3
110:9
108:H
106:4
96:n
30:3
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