Imines azomethines

The nitrogen analogs of ketones and aldehydes are called imines, azomethines, or Schiff bases, but imine is the preferred name and we use it here. These compounds can be prepared by condensation of primary amines with ketones or aldehydes.102 The equilibrium constants are unfavorable, so the reaction is usually driven forward by removal of water. 

This chapter is concerned with compounds containing the structure RR C=NR". These compounds are known as imines, azomethines, anils, or, more commonly, Schiff bases [la]. 

The imine-azomethine ylide tautomerization [Eq. (7)] is catalyzed by either protonic or Lewis acids (82CC384). For example, the cycloaddition of methyl Af-benzylidenephenylglycinate with A/-phenylmaleimide in toluene is catalyzed by 2,4-dinitrophenol and proceeds 40 times faster than the same reaction in the absence of the catalyst. The rate-determining step of N-protonation of the imine forming the iminium intermediate 64 is facilitated... 

It is interesting to compare the endo selectivities of two closely related types of dipoles. The ylides 47, derived by an imine-azomethine ylide tautomerism of the imines of a-amino esters (Section II,C), are typical dipoles that exhibit high endo selectivity in cycloadditions to cyclic olefins. On the other hand, N-substituted derivatives 79, derived by the deprotonation route (Section II,D) from N-substituted a-amino esters and aldehydes, are very poor in endo selectivity. The N-unsubstituted dipoles 47 are involved in the cycloadditions as ( , )-ylides (or syn-ylides), while the N-substituted dipoles 79 as ( ,Z)-ylidic forms (or anti -ylides). However, the difference in ylide geometry is not the only reason for the lowered endo selectivity of 79, since the anti forms of heteroaromatic N-ylides undergo exclusive endo-selective cycloadditions with cyclic cis-olefins (83H1907 84CL465 85BCJ3137). 

NR II II —c— imine (azomethine or Schiff base) diazepam, pralidoxime... 

If the multiple bonds are between two C-atoms of the ring, as, for instance, in 3,4-dihydro-2//-pyran, the compounds react essentially like alkenes or alkynes. The heteroatom can also be involved in a double bond. In the case of X = O", the compounds behave like oxenium salts, in the case of X = S", like sulfenium salts, and in the case of X = N, like imines (azomethines). 

The action of ammonia or primary amines and hydroxylamine-(9-sulfonic acid upon ketones also yields diaziridines. Likewise, the amination of imines (azomethines) with hydroxylamine-O-sulfonic acid yields diaziridines ... 

The following 1,3-dipoles will be considered (a) aryl azides (b) diazoalkanes (c) aryl nitrile oxides (d) nitrile imines (e) azomethine imines (/) azomethine oxides (g) azomethine ylides. (a) to (d) represent 1,3-dipoles with a double bond in their sextet structure, while the last three, from (e) to (g), are without a double bond . All of them have nitrogen as the central atom of the 1,3-dipole. They will be formulated as allyl-like systems, having their negative charge distributed (according to an unspecified balance) at the two sides of the positive nitrogen, e.g. 

Keywords 13-Dipoles Alkaloids Asymmetric synthesis Azides Azomethine imines Azomethine ylides Carbonyl ylides Cycloaddition Mesoionic systems Nitrile oxides Nitrones... 

Due to the high electrophilic nature of arynes, they are excellent dipolarophiles and can add to various 1,3-dipoles, leading to the construction of a range of benzo-fused five-membered heterocycles (Scheme 2). The commonly used 1,3-dipoles are nitrones, nitrile oxides, nitrile imines, azomethine imines, azides, diazo compoxmds, etc. Most of these reactions work under mild conditions in the absence of any transition-metal catalyst. 

Dipolar compounds (1,3-dipoles), first designated by Huisgen [1], are a class of 4tt-electron species that can be represented by zwitterionic (or ylide) forms with a separation of charge over three atoms. Basically, 1,3-dipoles are categorized into two subclasses, including the allyl type (carbonyl ylides, carbonyl imines, azomethine ylides, azomethine imines, and nitrones) and the propargyl/allenyl type (azides, diazoalkanes, nitrile ylides, nitrile imines, and nitrile oxides), as shown in Scheme 16.1. 

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