Other Ylides

Phosphoniutn ylides can be generated by treatment of diazoacetates and triphe-nylphosphine or triethyl phosphite with catalytic amounts of RuCl2(PPh3)2 [ 1343] or ReOCl3(PPh3)2 [1344]. If these reactions are conducted in the presence of aldehydes, carbonyl olefination takes place in high yields. 

Electrophilic carbene complexes can also react with organic halides to yield halonium ylides. Reaction of acceptor-substituted carbene complexes with allyl 

One of the most efficient procedures for the synthesis of cyclopropanes is the reaction of alkenes with electrophilic carbene complexes. In this process up to three stereogenic centers can be generated in one step. Cyclopropanes are a key structural element encountered in many natural products with interesting biological activity. Further, by virtue of the ability of cyclopropanes to undergo ring-opening reactions these compounds can be valuable synthetic intermediates. 

A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [1015], 1,3-dienes [1002], vinyl iodides [1347,1348], arenes [1349], fullerenes [1350], heteroare-nes, enol ethers or esters [1351-1354], ketene acetals, and A-alkoxycarbonyl-[1355,1356] or A-silyl enamines [1357], Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,1015], 

Alkynes can be converted into cyclopropenes by inter- [587,1022,1052,1060-1062] or intramolecular [1070] cyclopropanation with electrophilic carbene complexes, Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result from rearrangement or other transformations of the cyclopropenes initially formed (cf. Section 4,1,3), 

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To examine potentiality of other ylides and their metal complex containing Sb, As, P, Bi, and Se as new novel initiator in polymer synthesis via living radical polymerization. 

Ylides in which the heteroatom is N, P, As, S, or Se are well known. Other ylides containing Sb, Bi, O, Te, I, or Br are also known, but they are rarely used as ligands since they are very unstable, and they will not be treated here. The synthesis of the ylides is achieved through several preparative methods, most of which have been comprehensively reviewed [2-11]. The most relevant of these requires two steps, and involves the reaction of a halide with an EZ nucleophile (NR3, PR3, ASR3, SR2, etc.) and subsequent dehydrohalogenation of the onium salt (method a) as represented in Scheme 2 [2-6]. This process has been reported in a wide variety of experimental conditions, using virtually all kinds of solvents and bases (provided that they are compatible). The desilylation of some a-SiMe3 onium salts (method b)... 

As an example of a o-arabino phosphonium salt, XVIa was prepared. Simple Wittig reactions with the ylide derived from XVIa did not proceed as smoothly as with the other ylides, and for that reason salt XVIb, which proved to be much better behaved (results not shown), was also prepared. 

Ylides of other elements have been used much less commonly than sulfur ylides in cyclopropanations. Rather, other ylides are better known for their uses in other types of reactions, the best example being the use of phosphonium ylides in the Wittig reaction with carbonyl compounds to give alkenes. Nonetheless, some cases of cyclopropanations have been reported with phosphonium ylides and the related arsenic derivatives. Examples are given in Table 9. 

It is possible to remove a proton from the methyl group of a trialkylmethylammonium halide with strong bases. Thereby, a betaine (see Section 4.7.4) is produced with the structure R3N+—CH2. A betaine in which the positive and the negative charges are located on adjacent atoms as in R3N+—CH2 is called an ylide. The yl part of the name ylide refers to the covalent bond in the substructure N+—CH2. The ide part indicates that it also contains an ionic bond. When one wants to distinguish the ylide R3N+—CH2 from other ylides, it is called an ammonium ylide or an N ylide. 

If there are electron-withdrawing substituents conjugated with the ylidic carbon atom, further dipolar structures such as 4 and 5 may make major contributions to the overall structure. Delocalization of the charge in this way frequently leads to the ylides being isolable. Such ylides are commonly described as stable ylides in this context stable is, in effect, a synonym for isolable. Many ylides are not, however, isolable, because of their high reactivity, in particular their very ready hydrolysis. In this article such ylides are called reactive ylides. Some other ylides, notably benzylylides, have reactivity intermediate between those ylides which are obviously stable or obviously reactive. They are described as semi-stabilized ylides. 

One model for bonding in a diazo compound would be die ylide (89 equation 37)." Unlike many other ylides, diazoalkanes are stable to air and water. With acid, however, protonation can lead to the highly reactive salt (90), the functional equivalent of the corresponding carbocation. As the substituents on the diazo group are made increasingly electron withdrawing, the ylide becomes less basic, and thus more stable to acid. Reaction of a diazo compound with a transition metal can also often be understood as proceeding via initial donation of electron density by (89) to a coordinatively unsaturated metal center. 

Other ylides which have been reported include (46), which undergoes... 

In addition to sulfur ylides, examples are known of cyclopropanations using other ylides containing group V or group VI elements. Among these, triphenylphosphonium ylides have most frequently been studied. Some examples are collected in Table 19. The products obtained in entries 1-3 and 10 of Table 19 are suitable precursors for chrysanthemic acid. An enantioselec-tive synthesis of cyclopropanes was achieved by using electron-deficient alkenes substituted with an optically pure oxazolidine or acetonide function. When the alkene is stabilized by a ketone or nitro function instead of an ester function, the yield dropped considerably. 

The oldest syntheses of chrysanthemates are those starting from 2,5-dimethyl-2,4-hexadiene (238). There have been more papers on the use of rhodium or antimony to catalyze the addition of diazoacetate and chiral copper complexes to create asymmetry during the addition (see Vol. 4, p. 482, Refs. 219-222). The problem with this route is to avoid the use of diazo compounds. An old synthesis of Corey and Jautelat used the ylide addition of a sulfurane to a suitable precursor (in this case a C3 unit was added to methyl 5-methyl-2,4-hexadienoate, 239), and a recent paper gives details about the addition of ethyl dimethylsulfuranylideneacetate to 2,5-dimethyl-4-hexen-3-one (240). This led exclusively to the tran -isomer 241, from which ethyl trans-chrysanthemate (185, R = Et) was made. Other ylide additions are mentioned below. 

Ylide complexes [PdCl2(PPhMe2XYl)] (Y1 = Ph3AsCH2COPh, or Mej SCHCOPh) and similar Pt compounds (see p. 76) have been described. Other ylide... 

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