Ylid ions

Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

In neutral and alkaline media, the rate of exchange at the 3 and 6 position of 4-aminopyridazine is independent of acidity but decreases markedly when the media become more acidic. This was interpreted in terms of a rate-determining removal of the 6-proton by deuteroxide ion to give the ylid (XXIV), which reacts with deuterium oxide in a fast step. A similar result for the 3 and 6 positions of py-ridazin-4-one suggests the same mechanism. For reaction at the 5 position, the rate-acidity profile indicated reaction on the free base as did that for the 5 position of pyridazin-3-one, though the appearance of a maximum in the rate at — HQ = 0.8 was anomalous and suggested incursion of a further mechanism. 

It it better to disconnect (22) directly to (24) and use a sulphur ylid for the reaction. Ketone (24) is a 1,5-dicarbonyl compound with an obvious ring-chain disconnect ion. 

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]. 

Under favourable circumstances, the initially formed /V-ylid reacts further through C-N cleavage. Thus, in the presence of a strong nucleophile, such as a phenoxide anion, the quaternary dichloromethylammonium cation forms an ion-pair with the phenoxide anion (Scheme 7.27), which decomposes to yield the alkyl aryl ether and the /V-formyl derivative of the secondary amine [22, 23]. Although no sound rationale is available, the reaction appears to be favoured by the presence of bulky groups at the 4-position of the aryl ring. In the absence of the bulky substituents, the Reimer-Tiemann reaction products are formed, either through the breakdown of the ion-pair, or by the more direct attack of dichlorocarbene upon the phenoxide anion [22,23],... 

Cis- and rranx-cyclopropane-1,2-diamines (both primary and secondary) react with a range of aldehydes, R CHO, to give pyrroles under very mild conditions. NMR has been used to identify the intermediates. The key steps involve ring expansion of the monoiminium ion (22), via an azomethine ylid (23), to yield a dihydropyrrolium ion (24). 

The reductive cleavage of sulphonium salts in aprotic solvents leads to the generation of radical and then carbanions in a further electron transfer step. Protonation of the carbanion by extraneous water leaves a hydroxide ion. Basic species formed in this way can abstract a proton from sulphonium ion to give the ylid, which is not reducible. A good example is the reduction of 9 in dimethylsulphox-ide, which consumes only one Faraday and follows the course shown [58]. 

A great deal of work has been carried out on the thianthrene radical ion(l+), which can be produced from thianthrene by a variety of one-electron oxidations. The radical cation reacts at sulfur with nucleophilic species, giving rise to 5-substituted products, oxides, ylids, and 5-R-thianthrenium salts. 

Benzilic acid rearrangement, 232 Benzoin condensation, 231 cyanide ion in, 231 thiazolium ylids in, 232... 

The physical and chemical properties of the X -phosphorins 118 and 120 are comparable to those of phosphonium ylids which are resonance-stabilized by such electron-pulling groups as carbonyl or nitrile substituents Thus they can be viewed as cyclic resonance-stabilized phosphonium ylids 118 b, c, d). As expected, they do not react with carbonyl compounds giving the Wittig olefin products. However, they do react with dilute aqueous acids to form the protonated salts. Similarly, they are attacked at the C-2 or C-4 positions by alkyl-, acyl- or diazo-nium-ions Heating with water results in hydrolytic P—C cleavage, phosphine oxide and the hydrocarbon being formed. 

Zwitterions are stable and can provide a living polymerization, however, at the smaller chain lenght (smaller distance between ions) formation of ylids was noted Q... 

Thiazolium dipolar ion (ylid) 732, 733, 733s Thin layer chromatography 103... 

Orotidine 5 -phosphate undergoes an unusual decarboxylation (Fig. 25-14, step/), which apparently is not assisted by any coenzyme or metal ion but is enhanced over the spontaneous decarboxylation rate 1017-fold. No covalent bond formation with the enzyme has been detected.268 It has been suggested that the enzyme stabilizes a dipolar ionic tautomer of the substrate. Decarboxylation to form an intermediate ylid would be assisted by the adjacent positive charge.269,270 Alternatively, a concerted mechanism may be assisted by a nearby lysine side chain.270a d Hereditary absence of this decarboxylase is one cause of orotic aciduria. Treatment with uridine is of some value.271... 

When the substituent becomes very anion-stabilising, as in 42, the ylid may not react with ketones and anions of phosphonate esters are usually preferred in the Homer-Wadsworth-Emmons (HWE) variant.11 The reagent triethyl phosphonoacetate 46 is made by combining a phosphite (EtO)3P instead of a phosphine, with ethyl bromoacetate. Displacement of bromide 44 gives a phosphonium ion that is dealkylated by bromide 45. 

Due to the presence of silicon which stabilizes the a-carbanion, strong bases such as butyllithium and sodium amide are able to give an ylid from silylmethylammonium halides. Fluoride ion-assisted desilylation of these salts represents another means to create an ylid. These ylids are prone to rearrange and the results differ upon the nature of... 

Fluoride ion-assisted desilylation has been extensively used to create an ylid from a /V-silyl methyl-quaternary ammonium salt. Its evolution to final produces) is variable and Sommelet-Hauser and Stevens rearrangement products were obtained (often as major products) in a ratio that can be shifted from one structure to another very close one, as in examples 1 and 2 dealing with //-benzyl salts.246,366 Differences in the solvents used are not significant because in the first example, HMPA does not reverse the ratio, yields and selectivity being just a bit lower, /so-toluene was proposed as an intermediate in example 1 it might also be the intermediate in example 2. Thus product partition reflects the relative ability of the C-H or the C-C bonds to be cleaved to produce aromatization with proton or a-amine carbocation migration. 

When reacted with trimethylsilylmethyl triflate, pyridines give pyridinium methyl-ides, a salt which is a good precursor for azomethine ylids under treatment with fluoride ion. Thus, indolizine derivatives can be prepared from non-stabilized pyridinium... 

With the chemical activation process, the precursor is oxidized by silver ion. An electrochemical process using a carbon-carbon pair of electrodes, has been shown to promote the formation of the ylid which is reacted with electron-poor olefins 452... 

Stepwise, but heterolytic, mechanisms have been suggested in the insertion of carbenes into oxygen-hydrogen bonds. The reactivity of water and halide ions towards dihalomethylenes parallels their reactivity in Sn2 displacements (Hine and Dowell, 1954), suggesting that an electrophilic carbene attacks water initially by way of the non-bonded electron pair on oxygen giving an ylid (equation 21). An analogous mechanism could be followed in the insertion of carbenes into the... 

However, under somewhat different conditions (aqueous acetonitrile at 85°), no evidence for the free carbonium ion could be found (Bethell et al., 1965). Moreover, the invariance of the product proportions when water is replaced by deuterium oxide, coupled with the observation of a large tritium isotope effect on the formation of diphenylmethanol, is consistent only with the ylid mechanism (equation 21) (Bethell et al., 1969). For reaction of diarylmethylenes with alcohols, substantial hydrogen-isotope effects are observed, consistent with both equations 21 and 22. 

An ylid is first formed by loss of a proton—again, you have seen this—and then chloride is lost to form the same cation that we used in the alkylation reaction. In this step there is no nucleophile available except chloride ion so that adds to the carbon atom. 

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