Diazo compounds, alkylation protonation

Two methods for converting carboxylic acids to esters fall into the mechanistic group under discussion the reaction of carboxylic acids with diazo compounds, especially diazomethane and alkylation of carboxylate anions by halides or sulfonates. The esterification of carboxylic acids with diazomethane is a very fast and clean reaction.41 The alkylating agent is the extremely reactive methyldiazonium ion, which is generated by proton transfer from the carboxylic acid to diazomethane. The collapse of the resulting ion pair with loss of nitrogen is extremely rapid. 

You met diazonium salts in Chapter 23. Arene diazonium saits are stabie compounds, but alkyl diazonium saits, whlph are formed by protonation of diazo compounds, are not. They decompose rapidiyto carbocations—this was how the carboxyiic acid got methylated at the beginning of... 

Nelsen et al. have discovered that simple alkyl diazo compounds, which are normally not dienophiles, will react in a Diels-Alder fashion if protonated (equation 67). " It was suggested that protonation facilitates the reaction thermodynamically, since hydrazines are much stronger bases than azo compounds. 

Finally, we note that deamination could in principle be achieved by generating an aliphatic diazonium ion in the presence of a hydride source this could lead to an alkane either via the diazene, or by Sn2 attack with loss of N . Protonating an aliphatic diazo compound, or treating an /V -alkyl, N -aryltriazene with protic or Lewis acid in the presence of a suitable reducing agent (NaBHsCN, NaBH(OAc)3 or RsSiH) might achieve this transformation. 

A-Phthaloyl-protected (S)-phenylalanine has been used as a ligand for rhodium in the formation of metallocarbenes from diazo compounds for C-H insertion reactions (Section D.1.2.2.3.2.). Ar-Sulfonyl-protected (S)-alanine and (S)-valine are efficient ligands for chiral Lewis acids used in the Diels-Alder reaction (Section D.1.6.1.1.1.3.). A -Sulfonyl-pro-tected (S)-phenylalanine methyl ester has been used for the enantioselective protonation of lactone enolates (Section D.2.I.). The terf-butyl ester of (S)-valine readily forms imines with carbonyl compounds which are used for the highly efficient alkylations of their azaenolates (Sections D.1.1.1.4.1D.1.5.2.4.). All these derivatives can be obtained by the standard methods described in Houben-Weyl3. 

Sulfur ylides are useful reagents in organic synthesis. The ylide is formally a zwitterion in which a carbanion is stabilized by interaction with an adjacent sulfonium centre. They are usually prepared by proton abstraction from a sulfonium salt with a suitable base or by reaction of a sulfide with an alkylating agent such as Me30+BF4 or a carbene formed, for example, by metal-catalysed or photolytic decomposition of a diazo compound (1.103). 

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. 

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