From phosphorus ylids

Notice that sulphur ylids behave quite differently from phosphorus ylids, which would of course do the Wittig reaction (ftames 41-43). 

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) ... 

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. 

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. 

If we view the Wittig reaction from an alkyl halide starting material (alkyl halide phosphonium salt phosphorus ylid alkene), the halogen-bearing carbon of an alkyl halide must contain at least one hydrogen as in 59 (for deprotonation at the phosphonium salt stage). 

The Wittig reaction has also been carried out with phosphorus ylids other than phosphoranes, the most important being prepared from phosphonates, such as... 

A useful alternative to phosphorus ylids are the titanium reagents, such as, 71, prepared from dicyclopentadienyltitanium dichloride and trimethylaluminum. Treatment of a carbonyl compound with the titanium cyclopentadienide complex 71 Tebbe s reagent) in toluene-THF containing a small amount of pyridine " leads to the alkene. Dimethyltitanocene (Me2TiCp2), called the Petasis reagent, is a convenient and highly useful alternative to The mechanism of Petasis... 

Phosphorus yhds are quite common (see 16-44) and keto-phosphorus ylids [RCOCH=PPh3] are also known. When these compounds are heating (flash vacuum pyrolysis, FVP) to > 500°C, alkynes are formed. Simple alkynes can be formed as well as keto-alkynes and en-ynes. Rearrangement from ylids derived from tertiary amines an a-diazo ketones is also known. ... 

Phosphorus ylids are prepared by treatment of phosphonium salts with bases such as phenyllithium, butyllithium. dimsyl anion, or potassium f-butoxide. They are not isolated but used directly after preparation because they are sensitive to oxygen and moisture. The requisite phosphonium salts are available in great variety from the reaction of triphenylphosphine and primary or secondary alkyl halides, usually bromides [24]. 

Flash vacuum pyrolysis of a-benzotriazolyl-jS-oxo phosphorus ylids (472) results in 3-acetyl-1,2,4-benzotriazine (474) and o-cyanoacetophenone (475), products apparently derived from rearrange-... 

This same disconnection of a carbon atom is also helpful for epoxides (11) without carbonyl substituents. The reagent should be a nucleophilic carbene equivalent and a sulphur ylid (12) is the answer.These can be made from the sulphide (13) by a similar process to phosphorus ylid synthesis (Chapter 15), though the reactions of the two ylids with carbonyl compounds are significantly different (there is a third type of reaction with the ylid CH2N2 in Chapter 31). 

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