Ylids bonding

In almost all compounds that have pn-dn bonds, the central atom is connected to four atoms or three atoms and an unshared pair and the bonding is approximately tetrahedral. The pn-dn bond, therefore, does not greatly change the geometry of the molecule in contrast to the normal tc bond, which changes an atom from tetrahedral to trigonal. Calculations show that nonstabilized phosphonium ylids have nonplanar ylidic carbon geometries, whereas stabilized ylids have planar ylidic carbons. ... 

Double-bond compounds that undergo the Michael reaction (15-21) can be converted to cyclopropane derivatives with sulfur ylids. Among the most common of these is dimethyloxosulfonium methylid (109), which is widely used to... 

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. 

The double bond in (32) is trans since the Wittig used a stabilised ylid and an aldehyde. 

Diene alcohol (38) is the pheromone of the codling moth, tlie creature responsible for the grubs in apples, and of the various possible disconnections (a) is best as it gives most simplification and a stabilised ylid (39) which will produce the requi red trans double bond. Allylic bromide (40) and aldehyde-ester (41) are available,... 

Disconnection (a2) leads to the industrial synthesis as the half aldehyde, hall ester (46) of fumarie acid (100% tpans) is available and the Wittig reaction with unstabilised ylid (45) gives 85% cis geometry in the new double bond. 

Ansuey If we disconnect the c-is double bond first (50a), we must use unstahilised ylid (51) and aldehyde (52) which can be made from stabilised ylid (53) and available half aldehyde, half ester (54). 

Example Mali do (15, R=Pr-n) was used by Butenandt in his original synthesis of bombykol Cp iSf ) (16). Wittig disconnection of the tvans double bond requires cie allylic ylid (17), It is easier to use acetylenic ylid (18) and half aldehyde (19) in this step because el availability,... 

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. 

Both Wittigs use stabilised ylids and so will give ipans double bonds as required. Protection of... 

Diazepines (150) can be prepared by ultraviolet photolysis of iV-iminopyridinium ylids (151) but N—N bond homolysis in the ylid substrates presents a problem in regard to the synthesis of N-l substituted... 

Other 1,3-dipolar reagents show the same mode of reactivity towards cyclopropenones. Thus, the Munchnones 412 serving as potential azomethine ylides259-261 or the nitrile ylids 41 3262 effect expansion of the three-membered ring to the 4-pyridone systems 411/414 as a result of (2 + 3) cycloaddition to the C /C2 bond. 

These reactions are covered in other chapters of Volume 11 (Chapters 11.06 and 11.07). This part deals only with examples which are in connection with other sections of this chapter. Additions of metallocarbenoids to unsaturated partners have been extensively studied. Most of the initial studies have involved the transition metal-catalyzed decomposition of cr-carbonyl diazo compounds.163,164 Three main reaction modes of metallocarbenoids derived from a-carbonyl diazo precursor are (i) addition to an unsaturated C-C bond (olefin or alkyne), (ii) C-H insertion, and (iii) formation of an ylid (carbonyl or onium).1 5 These reactions have been applied to the total synthesis of natural... 

Compared with primary and secondary amines, tertiary amines are virtually unreac-tive towards carbenes and it has been demonstrated that they behave as phase-transfer catalysts for the generation of dichlorocarbene from chloroform. For example, tri-n-butylamine and its hydrochloride salt have the same catalytic effect as tetra-n-butylammonium chloride in the generation of dichlorocarbene and its subsequent insertion into the C=C bond of cyclohexene [20]. However, tertiary amines are generally insufficiently basic to deprotonate chloroform and the presence of sodium hydroxide is normally required. The initial reaction of the tertiary amine with chloroform, therefore, appears to be the formation of the A -ylid. This species does not partition between the two phases and cannot be responsible for the insertion reaction of the carbene in the C=C bond. Instead, it has been proposed that it acts as a lipophilic base for the deprotonation of chloroform (Scheme 7.26) to form a dichloromethylammonium ion-pair, which transfers into the organic phase where it decomposes to produce the carbene [21]. 

In oxidative decarboxyiation of pyruvate to acetyi-CoA, the enzyme-bound disulfide-containing coenzyme lipoic acid is also involved. The electron-rich enamine intermediate, instead of accepting a proton, is used to attack a sulfur in the lipoic acid moiety. This leads to fission of the S-S bond, and thereby effectively reduces the lipoic acid fragment. Regeneration of the TPP ylid via the reverse aldol-type... 

Cleavage reactions are best carried out in aqueous solution. In aprotic solvents, electrogenerated bases lead to the conversion of onium salts to the ylids which are not reducible [49]. The sequence of reactions shown in Scheme 5.2 shows that the bond cleavage process for phosphonium salts proceeds with retention of configuration around the phosphorus atom [50]. Retention of configuration at arsenic is also observed [51]. This electrochemical process is a route to asymmetric trisub-stituted phosphorus and arsenic centres. 

Polarographic half-wave potentials ( %), in the pH independent region, for the cleavage of bonds adjacent to a carbonyl function pH valu at the onset of ylid formation. 

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