Halides ylides

Phosphorus ylides are prepared from alkyl halides by a two step sequence The first step is a nucleophilic substitution of the 8 2 type by triphenylphosphme on an alkyl halide to give an alkyltriphenylphosphonium salt... 

The alkyllnphenylphosphomum sail producls are ionic and cryslalhze m high yield from Ihe nonpolar solvenls m which Ihey are prepared After isolalion Ihe alkyllriphe nylphosphonmm halide is converted lo Ihe desired ylide by deprolonalion wilh a slrong base... 

Arsonium Ylides. Arsonium ylides were first prepared by reaction between an arsonium halide and phenyUithium. Thus methyitriphenyiarsonium iodide [1499-33-8], C H gAsI, and phenyUithium give tripbenylarsonium metbylide [19365-61-8], C H yAs ... 

N-Alkylations, especially of oxo-di- and tetra-hydro derivatives, e.g. (28)->(29), have been carried out readily using a variety of reagents such as (usual) alkyl halide/alkali, alkyl sulfate/alkali, alkyl halide, tosylate or sulfate/NaH, trialkyloxonium fluoroborate and other Meerwein-type reagents, alcohols/DCCI, diazoalkanes, alkyl carbonates, oxalates or malon-ates, oxosulfonium ylides, DMF dimethyl acetal, and triethyl orthoformate/AcjO. Also used have been alkyl halide/lithium diisopropylamide and in one case benzyl chloride on the thallium derivative. In neutral conditions 8-alkylation is observed and preparation of some 8-nucleosides has also been reported (78JOC828, 77JOC997, 72JOC3975, 72JOC3980). 

The alkyltriphenylphosphonium salt products are ionic and crystallize in high yield from the nonpolar- solvents in which they are prepared. After isolation, the alkyltriphenylphosphonium halide is converted to the desired ylide by deprotonation with a strong base ... 

The ylide Ph3P=CH2 can readily be made by deprotonating a quaternary phosphonium halide with n-butyllithium and many such ylides are now known ... 

Since cbiral sulfur ylides racemize rapidly, they are generally prepared in situ from chiral sulfides and halides. The first example of asymmetric epoxidation was reported in 1989, using camphor-derived chiral sulfonium ylides with moderate yields and ee (< 41%) Since then, much effort has been made in tbe asymmetric epoxidation using sucb a strategy without a significant breakthrough. In one example, the reaction between benzaldehyde and benzyl bromide in the presence of one equivalent of camphor-derived sulfide 47 furnished epoxide 48 in high diastereoselectivity (trans cis = 96 4) with moderate enantioselectivity in the case of the trans isomer (56% ee). ... 

Phosphorus ylides like 1 can be prepared by various routes. The most common route is the reaction of triphenylphosphine 5 with an alkyl halide 6 to give a triphenylphosphonium salt 7, and treatment of that salt with a base to give the corresponding ylide 1 ... 

On treatment with a strong base such as sodium hydride or sodium amide, dimethyl sulfoxide yields a proton to form the methylsulfinyl carbanion (dimsyl ion), a strongly basic reagent. Reaction of dimsyl ion with triphenylalkylphosphonium halides provides a convenient route to ylides (see Chapter 11, Section III), and with triphenylmethane the reagent affords a high concentration of triphenylmethyl carbanion. Of immediate interest, however, is the nucleophilic reaction of dimsyl ion with aldehydes, ketones, and particularly esters (//). The reaction of dimsyl ion with nonenolizable ketones and... 

Triphenylphosphine reacts with alkyl halides to form alkyltriphenylphosphonium salts. Upon reaction with strong bases, the salts release a proton to form an ylide (alkylidenetriphenylphosphorane), which is capable of reacting with aldehydes or ketones providing an unambiguous route to olefins. Since there are virtually no... 

An aldehyde or ketone reacts with a phosphorus ylide to yield an alkene in which the oxygen atom of the carbonyl reactant is replaced by the =0 2 of the ylide. Preparation of the phosphorus ylide itself usually involves reaction of a primary alkyl halide with triphenylphosphine, so the ylide is typically primary, RCH = P Ph)3-This means that the disubstituted alkene carbon in the product comes from the carbonyl reactant, while the monosubstituted alkene carbon comes from the ylicle. 

Metzner and co-workers reported a one-pot epoxidation reaction in which a chiral sulfide, an allyl halide, and an aromatic aldehyde were allowed to react to give a trons-vinylepoxide (Scheme 9.16c) [77]. This is an efficient approach, as the sulfonium salt is formed in situ and deprotonated to afford the corresponding ylide, and then reacts with the aldehyde. The sulfide was still required in stoichiometric amounts, however, as the catalytic process was too slow for synthetic purposes. The yields were good and the transxis ratios were high when Ri H, but the enantioselectivities were lower than with the sulfur ylides discussed above. 

Quinoxaline (see Section 2.1.3) and many of its derivatives may be converted into A-alkylquimoxalinium or even A,A -dialkylquinoxalinediium salts by treatment with alkyl halides or the like. Occasionally, when the molecule bears a suitable acidic grouping, it may be possible to deprive the quaternary salt of its gegenion by treatment with a base to form an ylide (in which a carbon atom of the molecule bears the negative charge) or other zwitterionic entity, such as a quinoxaliniumolate. 

Usually, C-mercury substituted phosphorus ylides are monomers and in order to stabilize these complexes the presence of a second substituent on the carbon is necessary to balance the electron-donating effect of the metal. However a dimeric complex 85 has been obtained by the reaction of mercuric halides HgX2... 

The investigations included donor adducts with PO [93], 34,PS [+) [94], 35, and P(NMes ) +) [95], 36. While 34 was only formulated as an intermediate species, the other donor-acceptor complexes, 35 and 36, were characterized by X-ray investigations. To complete this series it may also be compared with bis(ylide)-substituted phosphonium halides [96], 37. For these cases the donors refer to... 

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. 

Salt-free ylides have been prepared from phosphonium chlorides and bromides by treatment with sodamide in refluxing THF. The sodium halide precipitates and is removed by filtration. Allylidene- and benzylidene-trimethylphosphoranes have been obtained as low melting distillable solids from the phosphonium chlorides and butyl-lithium in ether. The allylidenephosphorane on standing at room temperature slowly decomposed to give methylenetrimethylphosphorane. 

B. Reactions.—(/) Halides. Whereas ylides are alkylated in the normal way on treatment with a-bromo- or a-iodo-esters, quite different reactions occur with a-fluoro- and a-chloro-acetates. When salt-free ylides were refluxed in benzene with ethyl fluoroacetate or trifluoroacetate normal Wittig olefin synthesis took place with the carbonyls of the ester groups to give vinyl ethers, e.g. (14). On the other hand, methyl chloroacetate with... 

The reaction of ylides with phosphorus(iii) halides has been extended to the ylides (Me2N) Me3 P CH2, = 1, 2, or 3. Alkylation of the resulting stabilized ylides (20) with methyl iodide took place on the tervalent phosphorus, e.g. 

Trzcinskabancroft, B., Knachel, H., Dudis, D., Delord, T.J. and Marler, D.O. (1985) Experimental And Theoretical-Studies Of Dinudear Gold(I) And Gold(II) Phosphorus Ylide Complexes - Oxidative Addition, Halide Exchange, And Structural-Properties Including The Crystal And Molecular-Structures Of [Au (CH2)2PPh2]2 And [Au(CH2)2PPh2]2(CH3) Bri. Journal of the American Chemical Society, 107(24), 6908-6915. 

Phosphonium ylides are usually prepared by deprotonation of phosphonium salts. The phosphonium salts that are used most often are alkyltriphenylphosphonium halides, which can be prepared by the reaction of triphenylphosphine and an alkyl halide. The alkyl halide must be reactive toward Sw2 displacement. 

Alkyltriphenylphosphonium halides are only weakly acidic, and a strong base must be used for deprotonation. Possibilities include organolithium reagents, the anion of dimethyl sulfoxide, and amide ion or substituted amide anions, such as LDA or NaHMDS. The ylides are not normally isolated, so the reaction is carried out either with the carbonyl compound present or with it added immediately after ylide formation. Ylides with nonpolar substituents, e.g., R = H, alkyl, aryl, are quite reactive toward both ketones and aldehydes. Ylides having an a-EWG substituent, such as alkoxycarbonyl or acyl, are less reactive and are called stabilized ylides. 

Other heteropolynuclear gold(II) complex that can be obtained by replacement of halide groups in bis(ylide)gold(II) species by other anionic nucleophilic metal complex is the tin derivative [Au2 Sn[N(p-Tol)SiMe2]3SiMe 2 M-(CH2)2PPh2 2] (350).2041... 

Quaternization of benzo[/][l,7]naphthyridine with halogenoacyl halides occurs at N-7, and the quaternary salts may then be deprotonated to give the ylides. These ylides can then react with a variety of dienophiles to give the corresponding pyrrolonaphthyridines (Scheme 72) <1996PJC1324, 1999AJC149>. 

We have also found that ultrasound will promote the liberation of hydrogen from phenylacetylene to give the nucleophile phenylacet-ylide which can be efficiently quenched with an alkyl halide(19) ... 

A mechanistic picture which reconciles the experimental results is given in Scheme 24. It is assumed that both the heteroatom and the double bond of the allyl halide compete for an electrophilic metal carbene. Heteroatom attack yields a metalated ylide 129, which may go on to ylide 131 by demetalation and/or to allylmetal complex 130. Symmetry-allowed [2,3] rearrangement of 131 accounts for product 132, and metal elimination from 130 gives rise to products 132 and 133, corresponding to [2,3] and [1,2] rearrangement, respectively, as well as haloacetate (if R3 = CHc ). 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

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