P ylide

Such ylides are unstable and react with carbonyl compounds to give both the Wittig product (p. 545) as well as AsPh3 and an epoxide. However, this very reactivity is sometimes an advantage since As ylides often react with carbonyl compounds that are unresponsive to P ylides. Substituted quaternary arsonium compounds are also a useful source of heterocyclic organoarsanes, e.g. thermolysis of 4-(1,7-dibromoheptyl)trimethylarsonium bromide to l-arsabicyclo[3.3.0]octane ... 

Recent research on aminocarbenes has led to the development of a very fruitful field. The synthesis of relevant complexes (Scheme 19) such as aminobis(yhde) carbene species (69) [147], cyclic C-amino P-ylides (70) (easily transformed into carbenes) [148] and their corresponding complexes (71) [149], and special ylides (72), which also transform very easily into carbenes by loss of pyridinium group, has been reported. Emphasis has been made on the transformation between ylides and carbenes and on the donor properties of the ylides. From the results obtained the ylides have shown a stronger a-donor behavior compared with the carbenes. 

Figure 6.33 Thiourea derivatives evaluated for catalytic efficiency in the Mannich addition of P-ylides to N-Boc-protected benzaldimine.

The Wlltig reaction uses P ylides to change O of the carbonyl group to... 

In conclusion, the dominant resonance structures for the ylides are as follows for non-stabilized ylides, A for stabilized ylides, 16b and 16c for P=C=P ylides, 22 (although further study on this class is advised to determine if the gas-phase structure may in fact be linear) and for P=C=C ylides, 28b and 28c. 

According to Section 11.1.3, P-ylides and aldehydes first react in a [2+2]-cycloaddition to form a heterocycle, which is referred to as oxaphosphetane (Figure 4.44). 

Fig. 4.44. syn-Selective eliminations from oxaphosphetanes in Wittig olefinations with unstabilized (upper row gives ris-olefin) and stabilized P-ylides (bottom row gives trans-olefin). 

A, vy/ -elimination of Ph2MeP=0 and simultaneous stereoselective alkene formation from an oxaphosphetane are shown in Figure 4.45 (note that this oxaphosphetane is not produced via a P-ylide). In Figure 4.45, this elimination is part of a an alkene inversion in which, via a four-step reaction sequence, an alkene such as cw-cyclooctene, a molecule with little strain, can be converted into its trans- and, in this case, more highly strained, isomer. 

P-Ylide PI13P—CH Alkyl PI13P—CHAryl PI13P—CH— C02R... 

All P ylides for Wittig reactions are obtained by deprotonation of phosphonium salts. Depending on whether one wants to prepare a nonstabilized, a semistabilized, or a stabilized ylide, certain bases are especially suitable (see Table 11.1 an unusual, i.e., base-free, generation of ylides is described in Side Note 11.1). In stereogenic Wittig reactions with aldehydes, P ylides exhibit characteristic stereoselectivities. These depend mainly on whether the ylide involved is nonstabilized, semistabilized, or stabilized. This can also be seen in Table 11.1. 

The [2+2]-cycloaddition between P ylides and carbonyl compounds to give oxaphosphetanes can be stereogenic. It is stereogenic when the carbanionic C atom of the ylide bears—besides the P atom—two different substituents and when this holds true for the carbonyl group, too. The most important stereogenic oxaphosphetane syntheses of this type start from monosubstituted ylides PhgP —CH —X and from substituted aldehydes R— CH=0. We will therefore study this case in Figure 11.3. 

The P ylides react with C=0 double bonds faster the more electrophilic these C=0 double bonds are. One can therefore occasionally olefinate aldehydes even in the presence of ketones. If one works salt-free, this is also accomplished with cA-selectivity (Figure 11.5). 

The C=0 double bond of esters is usually not electrophilic enough to be olefinated by P ylides. Only formic acid esters can undergo condensation with Ph3P -CH2 and then they give enol ethers of the structure H—C(=CH2)—OR. a,/3-Unsaturated esters can sometimes react with P ylides, but this then results in a cyclopropanation ... 

Table 13.4 also shows that the deprotonation of isopropanol with LiHMDS is less than half as exothermic as the deprotonations with LDA or LTMP. Hence, LiHMDS is a much weaker base than the other two amides. This is due to the ability of the SiMe3 groups of LiHMDS to stabilize the negative charge in the a-position at the N atom. The mechanism of this stabilization might be the same as in the case of the isoelectronic triphenylphosphonium center in P ylides (Figure 11.1), that is, a combination of an inductive effect and anomeric effect. Because of its relatively low basicity, LiHMDS is employed for the preparation of enolates primarily when it is important to achieve high chemoselectivity. 

P ylides condense with saturated and unsaturated carbonyl compounds to give olefins (Section 9.3). 

Condensation of P Ylides with Carbonyl Compounds Wittig Reaction... 

P-Ylide Ph3P-CH Alkyl Ph3P—CHAryl Ph3P—CH— C02R... 

---