Sciencemadness Discussion Board

Wittig reaction with phosphorous ylides

whupharm_xu - 25-8-2005 at 00:58

PhCH2P(Ph3)Br+PhCOPh------PhCHPh2
procedeure:
the reactants were dissolved in DMSO(distilled), then NaH was added, stirring. the reaction proceeded at r.t. for about 24h, but TLC indicated they didn't react.
Generally, phosphorus ylides react easily with aldehyde(ie. PhCHO), ccording to the theory of witting reaction, this reaction may proceed, however, i heven't found references about the reaction.
Maybe the condition was not right in my reaction, anybody can give some suggestion?
thank you !

E.b.C:Title

[Edited on 26-8-2005 by chemoleo]

Eclectic - 25-8-2005 at 03:58

Maybe too much steric hinderance?

Sergei_Eisenstein - 25-8-2005 at 04:25

You must be referring to the Wittig reaction?

Also, the reaction of Ph3P=CHPh with Ph2CO will yield Ph2C=CHPh and not PhCHPh2

unionised - 25-8-2005 at 12:23

It's a while since I studied this sort of thing but won't NaH deprotonate DMSO? I have a vague recollection of dimsyl sodium reacting with aldehydes (though it might have been alcohols) as an oxidant giving the sulphide.

Also, would NaH act as a hydride donor reducing agent for the aldehyde?
Would it be better to add the NaH to the phosphorus compound in an inert solvent, then add the aldehyde?

I presume the DMSO was freshly distilled, otherwise you just made the world's most expensive NaOH.

whupharm_xu - 25-8-2005 at 18:56

the product should be Ph2C=CHPh and not PhCHPh2.I made a neglect,sorry!
In this reaction,the solvent DMSO was freshly distilled,NaH was used as a strong base to depronate from PhCH2P(Ph3) Br. the resulting PhCH=PPh3 attacks the C=O of PhCOPh as a nucleophilic reagent to yield Ph2C=CHPh. is it due to steric hinderance,or the charge of C=O was not positive enough because of conjugation of two Ph groups?

Sandmeyer - 28-8-2005 at 07:40

Quote:
i heven't found references about the reaction.


Wittig himself did this reaction in diethylether, unfortunately I can't get the following paper:

Wittig; Haag; Chem. Ber.; 88; 1955; 1654, 1663.


Is your purpose to solely prepare triphenylethene? If the Wittig don't work, then you might consider changing protocol to Grignard (esp since the fragments are almost identical), in your case the regioselectivity of C=C formation is not a matter - hence I see no reason of using Wittig:

http://www.orgsyn.org/orgsyn/prep.asp?prep=cv2p0606

Benzyl bromide might give better yield than chloride. Benzyl bromide might easily be made from toluene in 3h with NBS and trace benzoylperoxide as initiator:

Journal; Donxia, Li; Dexian, Wu; Yaozhong, Li; Huaming, Zhao; Tetrahedron; 42; 15; 1986; 4161-4168.

Yield 75% from paper.

[Edited on 28-8-2005 by Sandmeyer]

thegameguy5 - 6-9-2005 at 17:36

You have to make sure that you generate your Ylide before you throw in your ketone.

NaH is a good non nucleophilic base so don't worry about H- reducing your ketone but an excess of base normally removes protons alpha to your CO group (but here it is not a problem here).

I think your solvent is not appropriate. Your DMSO must be wet or it is also possible that the base (NaH) deprotonates the DMSO......like in the second step of a Swern oxidation reaction.

I use THF/LDA at -10 0 C under N2. It works well unless there is too much steric, which can also be a problem in your case.
Good luck

BromicAcid - 6-9-2005 at 18:24

Indeed, sodium hydride will deprotonate DMSO, from Technical Bulletin Reaction Solvent Dimethyl Sulfoxide (Gaylord Chemical Corporation)
Quote:
The activating influence of the sulfinyl group on on of the a-hydrogens is considerably less then that of a carbonyl group but sufficent to give a pKa of 35.1 for DMSO. Consequently, strong bases such as sodium hydride or sodium amide react with DMSO to produce solutions of sodium methylsulfinyl carbanion (dimsyl ion) which have proved to be synthetically useful.

..........

There are numerous applications of the dimsyl ion in the isomerization of alkynes and the formation of phosphorus ylides in preparing Wittig reagents

acetate - 7-9-2005 at 04:50

... very useful metod of Wittig reaction is reaction under PTC system ... MÄRKL, G. MERZ, A. Synthesis 295 (1973) and Yves Le Bigota, Michel Delmas* and Antoine Gaset Tetrahedron Volume 42, Issue 1 , 1986, Pages 339-350