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Lab report for Chem 3BL at UC Berkeley.
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Experiment 21 – The Wittig Reaction
Introduction:
Wittig Reaction, the reaction of an aldehyde or ketone with a triphenyl
phosphonium ylide (the Wittig reagent) to produce an alkene and triphenylphosphine
oxide, is of great use in forming carbon-carbon bond between reactants. The Wittig
reaction was discovered in 1954 by Georg Wittig, for which he was awarded the
Nobel Prize in Chemistry in 1979. 1 The development of Wittig Reagents were
examined in a recent general review by Eisch, in which the author discussed Georg
Wittig’s discovery of the reaction along with his other significant contributions in
organic chemistry.2
In this experiment, methyl (triphenylphosphoranylidene) acetate (1) was used to
react with 2-nitrobenzaldehyde (2), with the application of heat from microwave
radiation, to produce both (Z)-methyl 3-(2-nitrophenyl) acrylate (3) and (E)-methyl 3-
(2-nitrophenyl) acrylate (4), with (E)-methyl 3-(2-nitrophenyl) acrylate as the major
product, and triphenylphosphine oxide (5) (Scheme 1).
Scheme 1. Wittig Reaction of methyl (triphenylphosphoranylidene) acetate with
2-nitrobenzaldehyde.
The mechanism of this reaction (Scheme 2) involves the nucleophilic addition of
the phorphorus ylide (1) to the carbonyl carbon in 2-nitrobenzaldehyde (2), forming a
new carbon-carbon bond and providing the intermediate (6). Addition from the
electron rich oxygen to the positively charged phosphorus could then produce either
the (E) oxaphosphetane (7) or the (Z) oxaphosphetane (8). Upon breaking of the ring,
triphenylphosphine oxide (5) and either the (Z)-methyl 3-(2-nitrophenyl) acrylate (3)
or the (E)-methyl 3-(2-nitrophenyl) acrylate (4) could be produced.
Scheme 2. Mechanism of the Wittig Reaction.
Experimental Section:
.(Z)-methyl 3-(2-nitrophenyl) acrylate (3).
.(E)-methyl 3-(2-nitrophenyl) acrylate (4).
.Triphenylphosphine oxide (5).
Solid 2-nitrobenzaldehyde (80 mg,0.50 mmol), methyl (triphenylphosphoranylidene)
acetate (175 mg, 0.52 mmol), and silica gel (104 mg) were combined in a 1-dram vial.
The evenly distributed mixture was heated with microwave radiation at power level
four for two minutes. The solid mass was mixed again and cooled to room
temperature. A chromatography column was packed with 10 mL of 50:50 ethyl
acetate: hexane solution and 0.65 g of silica gel. The reaction mixture was then added
directly to the packed column. Subsequently, column chromatography was used to
separate the products. 4 fractions of 1.5 mL of solution was collected as well as a fifth
fraction of the remaining solution. Solutions of triphenylphosphine oxide and the five
fractions were each spotted in a lane on TLC pates. The plates were developed in
50:50 hexane: ethyl acetate and visualized using UV lamp. Fractions determined to
contain only the alkene product were combined in a tared 25-mL filter flask. Yellow
particle-like crystals of the alkene product were collected by evaporating the solvent
with air and pulling a vacuum on the system. Product of 0.0990g (0.478 mmol,
90.3%) impure (Z)-methyl 3-(2-nitrophenyl) acrylate (3) and (E)-methyl 3-(2-
nitrophenyl) acrylate (4) crystalline solid were obtained, with the (E) alkene isomer
determined to be the major product, mp 54.8 – 64.2 °C (lit3 72 – 74°C). 1H NMR
(400 MHz, CDCl3) δ 3.82 (s, 3H), 6.36 (d, J = 15.8 Hz, 1H), 7.55 (d of d, J = 2.16 &
J = 1.56, 1H), 7.64 (d of d, J = 5.71 & J = 2.23, 1H), 7.66 (d, J = 6.94 Hz, 1H), 8.03
(d, J = 8.0 Hz, 1H), 8.1 (d, J = 15.9 Hz, 1H). 13C NMR (100.6 MHz, CDCl3) δ
166.2, 148.2, 140.1, 133.5, 130.5, 130.3, 129.1, 124.9, 122.8, 52.01.
References:
1) Gericke, D. 1979 Nobel Prize in chemistry for the Wittig reaction as the source of
multiple syntheses. Georg Wittig the sixtieth German Nobel Prize recipient.
Fortschritte der Medizin 1979, 97(43), 1958-1964.
2) J. J. Eisch, Organometallics, 2002, 21, 5439–5463.
3) Methyl (2E)-3-(2-nitrophenyl)acrylate
http://www.chemspider.com/Chemical-Structure.4510466.html (accessed Feb 22,
2012)