Ylid, nitrogen

When the nitrogen atom is substituted by a nitrophenacyl group, OH attack gives the betainic zwitterion (Scheme 13). which is soluble in organic solvents (32). The stability of the C-betainic or ylid structure has been explained as an effect of resonance of the negative charge in the molecule (33, 34). 

When the migrating group is allyl, an additional concerted ([2,3] sigmatropic) pathway for rearrangement becomes available. In this an allylic shift must also occur. Nevertheless, the radical pathway is not always excluded. For example, rearrangement of ylids such as 36 (R = CHs.CO) leads to product 37 (R = CH3.CO) in which the allylic protons adjacent to the amido-nitrogen atom appear in emission (D. G. Morris, 1969). No polarization is observed in the much readier... 

For a review of nitrogen ylids, see Musker, W.K. Fortschr. Chem. Forsch., 1970,14, 295. For a monograph on sulfur ylids, see Trost, B.M. Melvin Jr, L.S. Sulfur Ylids Academic Press NY, 1975. For reviews, see Fava, A. in Bemardi Csizmadia Mangini Organic Sulfur Chemistry Elsevier NY, 1985, p. 299 Belkin, Yu.V. Polezhaeva, N.A. Russ. Chem. Rev., 1981, 50,481 Block, E. in Stirling The Chemistry of the Sulphonium Group,... 

A closely related reaction is formation of nitrogen ylids from quaternary ammonium salts (see 17-7) ... 

Although phosphorus ylids are most commonly used to alkenylation reactions, nitrogen ylids can occasionally be used. As an example, the reaction of A-benzyl-A-phenylpiperidinium bromide with base generated a A-ylid, which reacted with benzaldehyde to form styrene. ... 

It is reasonable to assume that the initial step in the cycloaddition reaction is an electrophilic attack by the carbene on the nitrogen atom to form the A -ylid. Where proton shift is possible, cyclization does not occur and the A-ylid produces the N-formyl compound (Scheme 7.30) [36]. 

Sydnones, which contain a cyclic ylid type structure [147], have one nitrogen and two possible oxygen protonation sites. Calculations quoted by Coulson (1952) show the largest negative ch urge on the carbonyl oxygen and protonation at that site has been observed by... 

This ylid can act as a nucleophile, and is also a reasonable leaving group. Addition to the carbonyl group of pyruvic acid is followed by decarboxylation, the positive nitrogen in the ring acting... 

Essentially the same mechanism is likely to be involved in an electrochemical version of the Stevens rearrangement. The mechanism of this reaction is not known with certainty however, it is known to be intramolecular and to involve deprotonation of quaternary ammonium salts to give nitrogen ylids which rapidly rearrange. The scheme set out in Scheme 9 is therefore entirety plausible. 

A summary of the in situ use of the azobenzene probases is given in Table 2. Apart from the generation of ylid, referred to above, the main applications have been for N- and C-alkylation of weak nitrogen and carbon acids, for the promotion of condensation and substitution reactions involving carbanions such as the cyano-methyl anion, for an interesting carboxylation reaction (entries 4 and 17), and for base-promoted cyclisations (entries 5 and 6). 

I. Zugravescu and M. Petrovanu, Nitrogen Ylid Chemistry, McGraw-Hill, New York, 1976. 

Although we do not know just how thiamine binds to the enzyme, the essential features of the reaction are quite well understood. Thiamine has an acidic hydrogen at the 2-position of the azathiacyclopentadiene ring, and you should recognize that the conjugate base, 10, is both a nitrogen ylid and a sulfur ylid (Section 16-4A) ... 

Although there is no direct evidence for the postulated intermediate 3 in this example, there is evidence for the existence of analogous ylids in the aziridine series (see Section II, B), and in some reactions of pyridine-type heterocycles with DMAD intermediates corresponding to 3 have been trapped. The formation of intermediates similar to 3 is the commonest first stage in reactions of nitrogen-containing heterocycles with activated acetylenes. 

When the aziridine nitrogen is substituted, Michael-type addition to the acetylene is not possible. Reactions are then usually preceded by ring cleavage with the formation of an azomethine ylid. The product obtained depends on the nitrogen substituent. Heine and co-workers 45,46 used m-l,2,3-triphenylaziridine (27) with DMAD, DEAD, EP, and EPP. The DMAD gave 98% of compound 28 after refluxing in toluene... 

Addition of DMAD to the carbonyl-stabilized nitrogen ylid 329 gave the furan 330 and the isoindolenine 331. This and similar reactions201 could involve either a symmetry allowed concerted cycloaddition or a nonconcerted reaction followed by elimination of the tertiary amine. 

The dithioester 83, which can be written as an ylid (84), with DMAD alone at 100°, gave 10% of 85.287 The nitrogen analog 86 apparently... 

DEAD exothermically in benzene or DMF to give initially 489, which has three contiguous saturated nitrogen atoms and undergoes N—N bond fission to give the highly stabilized ylid 490, which was isolated as the stable radical cation 491. Analogous reactions with 486 were carried out. 

Dihydro-l,3,5-oxazaphospholes (1 and 2) react with alkynes at temperatures between 100° and 140°C in toluene or xylene.826 Phosphate esters are eliminated and 1 1 adducts (4) of the resulting nitrogen ylids (3) are obtained.827 Similar reactions with MP have been described and the ratio of adducts 5 and 6 studied527 with 1 the ratio was 78 22, whereas 2 gave 70 30. The photoaddition of 1 to DMAD was carried out and 4 was again obtained.628... 

A3-Thiadiazolines (112) are unstable, losing nitrogen to yield thio-carbonyl ylids (113), which have been trapped with DMAD and other dienophiles.582 Hence, by allowing 112 to warm to room temperature in the presence of DMAD, 20% of 114 was obtained. Although... 

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