Nitrile ylides acidity

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

Reaction of iV-[(benzotriazol-l-yl)methyl]amide 707 with PCI5 gives chloroimine 708, which upon treatment with Bu OK is converted to nitrile ylide 709. Benzyl esters of ot,(3-unsaturated acids used as dipolarophiles trap species 709 to generate pyrroles 712 (Scheme 110) <2002JHC759>. When no trapping agent is added, the N-2 atom of benzotriazole act as a nucleophile, and tricyclic system 711 is formed <2001TL9109>. Addition of benzyl bromide... 

Phosphoryl substituted nitrile ylides have also been generated via the imidoyl chloride-base route using precursors 118 (R=Et, c-CeHn, f-Bu) prepared by the addition of an acid chloride to diethyl isocyanomethylphosphonate (120) (65). Treatment of the imidoyl chloride with triethylamine at —10 to 0 °C in the presence of dipolarophiles gave adducts in yields of up to 55% (e.g., 119 and 121) in ratios 1 4, 1 3 for R=Et and c = CgHn, respectively. 

It was found that the rate of the protonation step depended on both the acidity of the alcohol and the nature of the substituent on the nitrile ylide. After protonation, the azaallyl cation 370 might have been expected to react rapidly with the aUcoxy anion but, instead, it was found that it decayed according to pseudo-first-order kinetics via reaction with an alcohol molecule at a rate which depended on the alcohol pKs- The overall mechanism proposed (212) is shown in the scheme. 

Stoichiometric amount of methyl aluminum p-binaphthoxide as the Lewis acid. This reaction has recently been extended to a catalytic enantioselective version using an enantiopure methylaluminum p-binaphthoxide (Scheme 11.59) (210,211). Although the actual reacting species were not assigned, 5-alkoxyoxazoles behave as nitrile ylide 1,3-dipole equivalents in Lewis acid catalyzed reactions with aldehydes. 

In neutral medium the deuterium solvent isotope effect, (A h2o/ D2o) in the hydrolysis of diaryldiacyloxyspirosulphuranes483, 404-407 has been found to be 1.66. In acidic medium the ratio of catalytic rate constants has been found to be 0.56. Heterocondensed imidazoles 408, are produced in the reaction of N-benzylamides, 409, of nitrogen heterocyclic carboxylic acids with phosphorus pentachloride. Deuterium labelling experiments have been carried out to understand the mechanism of this reaction involving a nitrile ylide species484. 

Photo-triggered ring opening of 2//-azirines is a well-known reaction to produce pyrrolines [8, 71]. Padwa and co-workers showed that photoirradiation of azirines with a mercury arc lamp (450 W) equipped with Vycor filter generated the reactive nitrile ylide intermediate (72), which can be stabilized by the phenyl substituents. The nitrile ylide (72) then reacts with the electron-deficient olefins (73) such as acrylate and acrylonitrile in a cycloaddition reaction to form A -pyrrolines (74) (Scheme 10) [8]. Steenken and co-workers studied reaction kinetics of azirines with dipolarophiles as well as nucleophiles such as alcohols [72]. They showed that the reaction rate depends on the azirine substituents, the nucleophilicity of the reactant and the acidity of the alcohol. 

A number of other methods have also been used to generate nitrile ylides. Access to this 1,3-dipole has been realized by (a) treatment of imidoyl halides (5) with base, (b) thermal or photochemical elimination of phosphoric acid ester from 4,5-dihydro-1,3,5-oxazaphospholes (6), (c) 1,3-dipolar cycloreversion of 1-azetidines (7), (d)loss of carbon dioxide from 3-oxazolin-5-ones (g)44-47 treatment of boron-containing isonitriles (9) with base ... 

Esters of carboxylic acids which are activated by electron-withdrawing groups in the acyl or alkyl portion of the molecule also react with photochemically generated nitrile ylides to produce 5-alkoxy-3-oxazolines in high yield. The addition to the carbonyl group occurs with the same regioselectivity as observed with aldehydes and ketones. Esters which are not sufficiently activated, such as methyl acetate or benzoate, do not undergo cycloaddition. Schmid and coworkers have reported that the ester carbonyl... 

By a mechanism reminiscent of that above, 4,5-dihydro-l,3,5-oxaza-phospholes (82) undergo a thermal, as well as photochemical, cycloelimination of phosphoric acid ester giving nitrile ylides (83), which participate in cycloadditions. ... 

The cyclizations of conjugated nitrile ylides forming substituted oxazoles and thiazoles were computed up to the MP4/6-311-I-G level [OOJOC47]. Relative to 23, oxazole-4-carboxylic acid 24 is stabilized by about —38.1 kcal/mol (Scheme 18). 

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