And phosphorus ylids

Introduction Conjugate, 1,4- or Michael addition vs direct or 1,2-addition Sulfur and phosphorus ylids Where direct (1,2-) addition is necessary Using Copper (I) to Achieve Michael Addition... 

D.J.H. Smith, Phosphazenes and phosphorus ylids in D. Barton and W.D. Olhs, Eds., Comprehensive Organic Chemistry, Pergamon, 1979. 

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also called a phosphorane) to give an alkene. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from the phosphine and an alkyl halide (10-44) ... 

Phosphorus ylids also react in a similar manner with the C=0 bonds of ketenes/ isocyanates, and certain anhydrides and imides, the N=0 of nitroso groups, and the C=N of imines, for example. 

Phosphorus ylids are quite common (see 16-47) and keto-phosphorus ylids (RCOCH=PPh3) are also known. When these compounds are heating (flash vacuum pyrolysis, FVP) to great than >500°C, alkynes are formed. Simple alkynes can... 

Reaction between phosphorus ylids and aldehydes or ketones... 

This reaction involves Michael addition of (28) to the enone followed by cycllsation (29) with displacement of DMSO, Sulphur y,llds react in these ways rather than removing oxygen, as phosphorus ylids do, because the SO bond is weaker than the PO bond and the lower valency states of sulphur more stable than those of phosphorus. 

With acyl and alkoxycarbonyl azides, 2 -triazoles are isolated, although the l.ff-isomers are detected as intermediates. The reaction is not very general it fails with the corresponding sulfonium ylids, and fairly minor changes in substituents on the phosphorus ylid result in the formation of diazoketones rather than triazoles. ... 

Complexes of nickel constitute a distinct group of homogeneous alkylalumi-nium-free catalysts for olefin polymerisation. An efficient catalyst for ethylene polymerisation is formed in the reaction of bis(cycloocta-l,5-diene)nickel(0) [Ni(Cod)2] with phosphorus-ylid and triphenylphosphine in toluene solvent [181] ... 

In their search for less complex pentaalkylphosphoranes114), Schmidbaur and coworkers could provide convincing evidence for the intermediate occurence of monocyclic pentaalkylphosphoranes of type 104 in reactions of phosphorus ylids with silacyclobutanes. Isolation of such phosphoranes 104 was, however, never possible 115). 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this intermediate. 

We could compare sulfonium ylids with the carbenoids we discussed in Chapter 40—both are nucleophilic carbon atoms carrying a leaving group, and both form three-membered rings by insertion into n bonds. Sulfonium ylids are therefore useful for making epoxides from aldehydes or ketones other ways you have met of making epoxides (Chapters 20 and 45) started with alkenes that might be made with phosphorus ylids. 

You will recall from Chapter 31 that we divided phosphorus ylids into two categories, stabilized and un stabilized , in order to explain the stereochemistry of their alkene-forming reactions. Again, there is a similarity with sulfonium ylids the same sort of division is needed—this time to explain the different regioselectivities displayed by different sulfonium ylids. Firstly, an example. 

Sulfoxides are sometimes drawn as S=0 and sometimes as S+-0 . The second representation might remind you oF the phosphorus ylids used in the Wittig reaction (Chapters j 14 and 31), which can be drawn with a P=CH2 double s, bond or as P+-CH 2-All of these representations are 5- correct—it is a matter of personal choice which you prefer. 

So-called C-linked calixsugars were prepared by Wittig reaction of the calix[4]arene tetraaldehyde 33 with galactose-6- or ribose-5-phosphorus ylids (obtained in situ from the known phosphonium iodides, Figure 5)70 followed by reduction of the double bonds and deprotection of the hydroxyl groups. Thus, the tetra- and trigalactosides 34c,d (all as P-anomers) and tetra- and tri-L-lyxosides 34 (as a mixture of a- and P-anomers) were obtained. 

In all the examples given above, the atom that donates the electron parr is oxygen and, indeed, oxygen is the most common such atom. But in another important class of compounds, called ylids, this atom is carbon. There are three main types of ylids phosphorus, nitrogen, and sulfur ylids, although... 

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