Formation of ylides

A number of ylides have been isolated in crystalline state. However, some of the ylides are transient intermediates in some of the reactions. 

The two most important methods for the preparation of ammonium ylides such as 3.44 involve either alkylation of an appropriate amine and subsequent deprotonation by a base or capture of tertiary amines by carbenes. 

Sulfur ylides can also be prepared by the reaction of a carbene with a sulfide. 

The reaction of arylchlorocarbenes, generated from arylchlorodiazirines, with trimethyle-nesulfide gives a sulfur ylide as an intermediate . 

The sulfide reacts with a diazo compound in the presence of catalytic amounts of CUSO4 to give a 50 50 mixture of sulfur ylides.  

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More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

Muller et al. have also examined the enantioselectivity and the stereochemical course of copper-catalyzed intramolecular CH insertions of phenyl-iodonium ylides [34]. The decomposition of diazo compounds in the presence of transition metals leads to typical reactions for metal-carbenoid intermediates, such as cyclopropanations, insertions into X - H bonds, and formation of ylides with heteroatoms that have available lone pairs. Since diazo compounds are potentially explosive, toxic, and carcinogenic, the number of industrial applications is limited. Phenyliodonium ylides are potential substitutes for diazo compounds in metal-carbenoid reactions. Their photochemical, thermal, and transition-metal-catalyzed decompositions exhibit some similarities to those of diazo compounds. 

For the formation of ylides from triphenylphosphine with dimethyl acetylene dicarboxylate and with halonitroalkenes see Chapter 1, Section 2. 

Rhodium catalysis have been used for formation of ylides by intramolecular reactions. 

This reaction closely resembles the formation of ylide complex 8 from the stable rhenium methylene 7 (24) ... 

Heating compounds 59 (R1, R2 = Aik, Ar, R3 = Ar, OAlk, SAlk, etc., R4 = OAlk, SAlk, and others) in benzene or toluene led to the formation of ylides that upon decomposing gave carbenes (see Section 4.06.6.2, CHEC-II(1996)). Further fates of the carbenes depend on their structures and co-reagents added. Refluxing compound 61 ( = 3) in benzene at 90 °C gave product 62 in 77% yield for compound 61 ( = 2), the yield of product 62 dropped to 33% <1996JA4214>. 

Other interesting reactions of la are the formation of ylides with acetone, pyridine, and acetonitrile (Scheme 6).25 These ylides are intensely colored with absorption maxima in Freon-113 at 500, 560, and 540 nm, respectively, and thus easily detected spectroscopically. Interestingly, transient species with absorption maxima around 620 nm and lifetimes in the order of ms are found in all three... 

Electrophilic carbene complexes can react with amines, alcohols or thiols to yield the products of a formal X-H bond insertion (X N, O, S). Unlike the insertion of carbene complexes into aliphatic C-H bonds, insertion into X-H bonds can proceed via intermediate formation of ylides (Figure 4.7). 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

Treatment of cycloadduct 287, obtained from a dibenzo thiazinylium intermediate and a diene, with base affords dibenzo[d,/]pyrrolo[2,l-lr][l,3]thia-zepine 290 in 58% yield. The proposed mechanism involves formation of ylide intermediate 288 which undergoes intramolecular rearrangement into dihydro derivative 289 and spontaneous oxidative aromatization (Scheme 62 (1999TL95)). 

Wenkert and Khatuya (51) examined the competition between direct insertion of a carbene into furan (via cyclopropanation) and ylide formation with reactive side-chain functionality such as esters, aldehydes, and acetals. They demonstrated the ease of formation of aldehyde derived carbonyl ylides (Scheme 4.30) as opposed to reaction with the electron-rich olefin of the furan. Treatment of 3-furfural (136) with ethyl diazoacetate (EDA) and rhodium acetate led to formation of ylide 137, followed by trapping with a second molecule of furfural to give the acetal 138 as an equal mixture of isomers at the acetal hydrogen position. 

For a monograph, see Johnson Ylid Chemistry, Academic Press New York. 1966. For reviews, sec Morris. Surv. Prog. Chem. 1983, 10, 189-257 Hudson Chem. Br. 1971, 7, 287-294 Lowe Chem. Ind. (London) 1970, 1070-1079. For a review on the formation of ylides from the reaction of carbenes and carbenoids with heteroatom lone pairs, see Padwa Hornbuckle Chem. Rev. 1991, 91, 263-309. 

The formation of ylides is observed in cases where the probase reduction at the electrode is reversible in cyclic voltammetric recordings, as this provides a control to monitor the... 

The electrochemical formation of ylides described in reactions 1 and 2 has been utilized for producing alkenes by reaction with aldehydes through reaction 3. The potential of the cathode is controlled at or near the cathodic reduction peak potential of the probase. The yields of the alkenes are given in Table 3. 

In the presence of base the methylol 203 (R = H, OMe) reacted with acetic anhydride or triethyl orthoformate, giving esters 204a or alcohols 204b, respectively. The formation of ylide 205 was demonstrated under neutral conditions.163... 

Ethylene and dimethylamine would result successively from the formation of ylide by deprotonation, an intramolecular carbanionic attack, and finally reprotonation. These mechanisms in which the methyl derivatives have been used are different from those proposed for montmorillonite where the ethylammonium cations were mainly implied. The origin of these differences may be partially the reactivity of the hydrogen as well as the nature of the surface acid sites. These considerations prompted us to repeat our previous experiments (1) for ethylammonium-exchanged Y zeolites. 

Tetracyanoethylene oxide (TCNEO) not only oxidizes sulfides to sulfoxides but also reduces sulfoxides to sulfides with generation of two molecules of carbonyl cyanide (17). The reduction mechanism involves a zwitterion intermediate (15) that produces sulfide and two molecules of (17) by simultaneous cleavage of the C—C and O—S bonds. A mechanism (Scheme 14) that involves a zwitterion (16) as a common intermediate is proposed for the formation of ylide and sulfoxide.311... 

Metallocarbenes derived from diazoacetates and the appropriate transition metal are generally thought to be electrophilic in nature that is, they are susceptible to nucleophilic addition. This reactivity manifold has been exploited with the formation of ylide species on reaction with a number of nucleophiles providing intermediates that can undergo either rearrangements (e.g., 41 > 42, Scheme 8.8)25 or cycloadditions (e.g., 43 > 44, Scheme 8.8)26 depending on the type of ylide formed... 

For the rate-limiting ylide complex formation no isotope effects are expected for epoxide carbons. The observed 13C KIEs (Figure 20) rules out that formation of ylide is kinetically important, because effects of Cl and C2 atoms are different than unity. The normal isotope effect of Cl corresponds to the C-O bond breakage in the rate-limiting step. The inverse isotope effect of C2 suggests that the C2-0 bond became stranger in transition state than in substrate. 

At 100 °C pentamethylarsorane (/) is quantitatively decomposed to trimethyl-arsine, methane and a little ethylene, thus supporting the assumption of intermediate formation of ylide 148 ethane is only formed in traces131). With water and acids tetramethylarsonium salts are produced m) whereas with alcohols, hydroxylamines and oximes covalent pentacoordinate arsoranes 149 are obtained in which one methyl group has been replaced by the respective electronegative group Y 132,133). 

The presence of the triphenylphosphonium substituent makes 351 (Scheme 2.122) an active dienophile. Resulting cycloadducts such as 352 are immediately recognized as precursors for the formation of ylides. Products with an exocyclic double bond such as 353 are easily synthesized via a tandem sequence of Diels-Alder and Wittig reactions. The formation of adduct 353 could have been achieved via the Diels-Alder reaction with the allene CH2 = C = CHR, but this direct route is generally unapplicable owing to the low activity of allenes as dienophiles. 

Carbene reactions provide a versatile approach to the synthesis of five-membered nitrogen-containing rings. Of particular importance here are intramolecular insertion of a carbene into C — H and N — H bonds, addition onto multiple carbon-carbon bonds, intermediate formation of ylides as a result of carbene addition onto the heteroatom followed by rearrangement, cycloaddition, and cyclization. 

The Rh(II)-catalyzed reaction of pyridone 96 with DMAD was also found to give cycloadducts derived from an intermediate azomethine ylide. The initial reaction involves generation of the expected carbonyl ylide by intramolecular cyclization of the keto carbenoid onto the oxygen atom of the amide group. A subsequent proton shift generates the thermodynamically more stable azomethine ylide, which is trapped by DMAD. This is an example of subsequent formation of ylides of two types, a phenomenon termed a dipole cascade (93JOC1144). 

Despite the pioneering work of Ando and Doyle, few synthetic applications of oxonium ylidic rearrangements have been reported. However, three examples of synthetic relevance appeared recently (Schemes 60 to 62). When a-allyloxyacetic esters are reacted with trimethylsilyl triflate and a base, the transposed material (252), resulting from a 3,2-sigmatropic rearrangement of the transient ylide (251), could be isolated in good yields. ° The same product ratio was also obtained upon treatment of the ketene acetal (253) with trimethylsilyl triflate. This second variant involves the direct formation of ylide (251) followed by its transformation into (252 Scheme 60). 

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