Ylides reactions involving

Another arsonium ylide reaction involves a notable transylidation reaction between a phosphorus and an arsenic ylide (Scheme 6). A useful arsenic ylide which provides a hydroxymethyl epoxide has been reported (equation IS) note the use of biphasic reaction conditions for ylide generation. 

The sample solution to Problem 17 16(a) showed the prepara tion of 3 methyl 3 heptene by a Wittig reaction involving the ylide shown Write equations showing the formation of this ylide beginning with 2 bromobutane... 

Deacylations are known. C-Acyl groups in 1,3,4-thiadiazoles are cleaved by sodium ethoxide in ethanol (68AHC(9)165). Imidazole-2-carbaldehyde behaves similarly, yielding imidazole and ethyl formate this reaction involves an ylide intermediate. 3-Acylisoxazoles (405) are attacked by nucleophiles in a reaction which involves ring opening (79AHC(25)147). 

Although the first metal-catalyzed asymmetric 1,3-dipolar cycloaddition reaction involved azomethine ylides, there has not been any significant activity in this area since then. The reactions that were described implied one of more equivalents of the chiral catalyst, and further development into a catalytic version has not been reported. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

The insertion of a carbene into a Z-H bond, where Z=C, Si, is generally referred to as an insertion reaction, whereas those occurring from Z=0,N are based on ylide chemistry [75]. These processes are unique to carbene chemistry and are facilitated by dirhodium(II) catalysts in preference to all others [1, 3,4]. The mechanism of this reaction involves simultaneous Z-H bond breaking, Z-car-bene C and carbene C-H bond formation, and the dissociation of the rhodium catalyst from the original carbene center [1]. 

Wittig reactions are versatile and useful for preparing alkenes, under mild conditions, where the position of the double bond is known unambiguously. The reaction involves the facile formation of a phosphonium salt from an alkyl halide and a phosphine. In the presence of base this loses HX to form an ylide (Scheme 1.15). This highly polar ylide reacts with a carbonyl compound to give an alkene and a stoichiometric amount of a phosphine oxide, usually triphenylphosphine oxide. 

A mechanism for this reaction involving nucleophilic attack of the ylide on the cyanide group and formation of the P=N linkage via a four-centred intermediate was formulated. The structure of this phosphazene was confirmed by its synthesis from the vinyl azide, Ph(N3)C=CHPh, and triphenylphosphine. Phosphoranes stabilized by electron-withdrawing... 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

In 1995, Boyd and co-workers <95TL7971 > covalently linked a porphyrin to fullerene Cgo through a 1,3-dipolar cycloaddition reaction involving the porphyrinic azomethine ylide 28 (Scheme 8). The ylide was generated in situ from befa-formyl-meso-tetraphenylporphyrin 27 and A -methylglycine, and provided the porphyrin-C6o diad 29 in good yield. 

Our first entry to the use of porphyrins as dipolarophiles in 1,3-DC reactions involved the reaction of porphyrins with azomethinic ylides, generated in situ from a-amino acids and aldehydes, to yield chlorins and isobacteriochlorins (bisadducts) <99CC1767, 05JOC2306>. 

The process shown in Scheme 3.16 is rather interesting. It should be noted that in most cases this reaction is very stereoselective with respect to the arrangement of the substituents at C-4 and C-5 atoms. In light of recent data on the possible isomerization of nitrocyclopropanes (13) to form five-membered cyclic nitronates (5) (for more details, see Section 3.2.2.1.2), low chemoselectivity of many reactions involving sulfur ylides does not seem to be so fatal. 

Since the Wittig reaction involves ionic intermediates it is ideally suited to the ionic liquid environment. Wittig reactions using stabilized ylides have been... 

The formation of the phosphoranyl alkyne 19, in the spontaneous room temperature decomposition of the phosphinodiazoketone lm, has been explained in terms of an intramolecular Wittig-type reaction involving the phosphorus-vinyl-ylide form of the phosphinocarbene 2m.39... 

Diazoalkanes are the carbene complex precursors most commonly used for the catalytic cyclopropanation of alkenes. Reactions involving this type of ylide will be discussed in this section. 

The outcome of reactions involving carbonyl ylides is not always as easy to predict, as the examples in Table 4.20. Depending on the basicity of the intermediates, proton migrations might occur and unexpected results can be obtained. Two examples of such peculiar conversions and the proposed mechanisms are sketched in Figure 4.16. 

In reactions involving sulfonium ylides fewer side-reactions than with oxonium ylides are usually observed. This is probably because of the stabilization of the former by dtt-pK interaction this is not possible with oxonium ylides. 

These reactions involve the intermediacy of a bis-ylide which undergoes a Wittig-type reaction with aldehydes or a Michael 1,4-addition to the enone. 

Cycloaddition reactions involving thermal/photochemical/catalytic decomposition of iodonium ylides are applicable to oxazole derivatives... 

The thermal S-dealkylation of benzothiazine ylides was shown in Scheme 11 and the dealkylation of a sulfonium salt in Scheme 37. All other reported reactions involve N-substituents. 

This unusual reaction involves the reductive dimerization of protected iodohy-drin (44) that produced a symmetrically substituted carbonyl ylide (48). The mechanism proposed for this interesting process involved initial reduction of the... 

Historically, the first reactions involving thiocarbonyl ylides involve the preparation of thiiranes and 1,3-dithiolanes from diazomethane and thiocarbonyl compounds reported early in the last century by Staudinger and co-workers (12,13). Similar reactions have been smdied by Schonberg and co-workers (14—16) during the 1960s, but neither was the reaction mechanism understood nor have thiocarbonyl ylides been recognized as key intermediates. [For some remarks to this subject see (8) and (10) in (17).]... 

The reaction of the sterically crowded thiocarbonyl ylide 69 with highly electron-deficient alkenes such as 2,3-dicyano fumarate and maleate, tetracya-noethene, a-cyano cinnamates, and l,2-bis(trifluoromethyl)ethene-l,2-dicarboni-trile occurred in a nonstereospecific manner (27,89,96,97,136-138). The formation of a mixture of cis/trans tetrahydrothiophenes of type 82 is the result of a stepwise reaction involving zwitterionic intermediates of type 81 (Scheme 5.29). Ylide 69 fulfills the fundamental requirements for a two-step reaction with electron-deficient alkenes. This species corresponds to an electron-rich 1,3-dipole that also contains a bulky substituent at one terminus (89). 

Saito et al. <1995S87> described a new method for the synthesis of heterocycle-fused[c]thiophenes via reaction of aryl heteroaryl thioketones with the carbene precursors. Heteroaromatic thioketones A react with carbenoids generated from bis(arylsulfonyl)diazomethanes or phenyliodonium bis(phenylsulfonyl)methylides to give heterocycle-fused[f]thiophenes B. The reaction involves the ring closure of the intermediary thiocarbonyl ylides, followed by restorative aromatization via the elimination of a sulfenic acid (Equation 11). 

Furans (133) are obtained from acyltriafulvenes [(acylmethylene)cyclopropenes] (131) and azomethine-imines, -ylides or oxides (132) (75TL3919). The reaction involves the transfer of the group X from the azomethine dipole to the triafulvene and may proceed as shown in Scheme 28. I... 

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