Imines nitrogen nucleophiles

The suggested reaction mechanism involves a nucleophilic attack of the imine nitrogen at the activated triple bond, followed by a proton exchange, to give a benzimidazolinium system which, by intramolecular attack at the carbonyl group, leads to an epoxide that ring opens to the observed product. For the ethyl derivative (R = Et) a tub conformation could be established by X-ray crystallographic analysis.33... 

Recently, Ohe and IJemura reported a novel approach to the catalytic cyclopropanation of alkenes via 2-furyl178 179 or 2-pyrrolyl carbenoids180 that originate from the intramolecular nucleophilic attack of a carbonyl oxygen or an imine nitrogen (ene-yne-ketone and ene-yne-imine precursor, respectively) on a 7t-alkyne complex or a cationic cr-vinyl complex. Initially, the group 6 complexes like Cr(CO)s were used. Soon it was found that a series of late transition... 

An important prerequisite in these reactions is the nucleophilicity of the imine nitrogen. Indeed, electron-withdrawing substituents in the aryl ring of (159) retard the reaction with isothiocyanates, whereas such substituents in the R group of the isothiocyanates enhance the reaction. No reaction occurs under the above chosen conditions (reflux in chloroform), when the aryl ring of (159) is replaced by a sulfonyl group <88BSB83>. 

With five- (or four)-membered-ring cyclic imides (Scheme 16) 57 initially gives isolable eight- (or seven)-membered ring intermediates (58, 60, and 62), which can be converted readily to products 59,61, and 63. The key step appears to be transannular attack of the imine nitrogen of the intermediate on the amide carbonyl. This is followed by attack of a nucleophile and ring opening via a number of possible pathways. ... 

Naphthalen-1,4,imines, 16, 87 Naphthyridines, 11, 124 reactivity of, toward nitrogen nucleophiles, 33, 95... 

Asymmetric cycloaddition of 2-vinyloxiranes to carbodiimides proceeds in the presence of Pd2(dba)3-CHC13 and TolBINAP as the chiral ligand in THF at room temperature to yield 4-vinyl-l,3-oxazolidin-2-imines with up to 95% ee [74] (Eq. 8A.50). The enantio-determining step is assumed to be the nucleophilic attack of a nitrogen nucleophile on a 7i-allylpalladium intermediate. Reactions of 2-vinyloxiranes with isocyanates using the same catalyst system afford 4-vinyl-l,3-oxazolidin-2-ones with low enantioselectivity. 

Abstract The main computational studies on the formation of (3-lactams through [2+2] cycloadditions published during 1992-2008 are reported with special emphasis on the mechanistic and selectivity aspects of these reactions. Disconnection of the N1-C2 and C3-C4 bonds of the azetidin-2-one ring leads to the reaction between ketenes and imines. Computational and experimental results point to a stepwise mechanism for this reaction. The first step consists of a nucleophilic attack of the iminic nitrogen on the sp-hybridized carbon atom of the ketene. The zwitterionic intermediate thus formed yields the corresponding (3-1 actant by means of a four-electron conrotatoty electrocyclization. The steroecontrol and the periselectivity of the reaction support this two-step mechanism. The [2+2] cycloaddition between isocyanates and alkenes takes place via a concerted (but asynchronous) mechanism that can be interpreted in terms of a [n2s + (n2s + n2s)] interaction between both reactants. Both the regio and the stereochemistry observed are compatible with this computational model. However, the combination of solvent and substituent effects can result in a stepwise mechanism. 

It is often found that imines are stabilised towards hydrolysis by co-ordination of the nitrogen to a 7t-bonding transition metal. Of course, in the absence of significant 7C-bonding interactions, ligand polarisation is expected to have the opposite effect and activate the imine towards nucleophilic attack. 

More recently, Porta and co-workers [6] applied similar considerations of the polar effects to a new one-pot multicomponent process for the addition of nucleophilic radicals to aldimines, generated in situ in the presence of Ti(IV). In analogy with the Minisci reaction, Ti(IV), which acts as a Lewis acid, coordinates the nitrogen of the imine, strongly increasing the electron-deficient character of the carbon in the a-posilion and thus the reactivity of the imine toward nucleophilic radicals. This reaction, as well as the Minisci one, represents a useful route for the synthesis of a variety of poly-functionalized derivatives of chemical and biochemical relevance. 

S)-proline-catalyzed reaction using propionaldehyde as donor and the results showed that the imine reactivity was approximately sevenfold higher than that of the aldehyde [83]. Under basic conditions, it is generally accepted that nucleophilic addition to an aldehyde is typically faster than addition to an aldimine, but nucleophilic addition to an aldimine is faster than addition to an aldehyde when protonation of the imine nitrogen occurs [83]. In the (S)-proline-catalyzed three-component Mannich reactions in the absence of arylaldehyde, self-Mannich products were obtained with moderate to high diastereo- and enantioselectivities (Scheme 2.19) [71b, 82]. 

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