Imines and Iminium Salts

The application in the N-methylation of alkaloids has been published [SH3]. The N-methylation of amines by paraformaldehyde-NaBH4 in CF3COOH in the presence or absence of THF or by CH20-NaCNBH3 in AcOH has also been recommended [GN3]. However, this method is limited because it is not possible to make monomethylated amines from primary amines the transformation of the intermediate secondary amine to tertiary amine is very rapid [AC5]. The best way to prepare monomethylated amines is then via the carbamates (Section 3.2.8). 

The stereoselectivity of the reduction of the six-membered cyclic imines has been 

Another way to obtain achiral or chiral p-aminoalcohol derivatives stereoselec-tively is to use NaBH4 or Zn(BH4)2 to reduce a-hydroxyimines 3.233 or a-trimethyl- 

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The reduction of imines and iminium salts present a particular difficulty in that those which are N-substituted can exist in different geometrical isomers that are reduced at different rates and with different selectivities. One way to overcome this problem is to use cyclic imines that can exist only as cis isomers. Although these are good substrates, this is not a general solution. The cyclic amines produced by transfer hydrogenation, together with best reported enantiomeric excesses, are listed in Table 35.6. Primary amines are difficult to pre-... 

Imines and iminiums salts are oxidized by peracids into oxazidines and oxaziridinium ions, which are good reagents for oxidation of sulfides. The imine or iminium salt can, in principle,... 

This chapter will be divided into sections according to the electrophiles aldehydes and ketones, imines and iminium salts, carboxylic acid derivatives and finally a,P-unsaturated carbonyl compounds, which undergo conjugate additions. Further subdivision will be made according to the nature of the nucleophile, i.e. 0-, N-, S-, P- or C-nucleophiles. Finally, multicomponent heterocyclic syntheses will be mentioned, if they consist at least of one iron-catalyzed addition step to a carbonyl compound. 

Imines and iminium salts also undergo [3 + 2]-cycloaddition of allylsilanes to afford 3-silylpyrrolidines with high diastereoselectivity.121,177,178... 

REDUCTION OF IMINES AND IMINIUM SALTS OF AMMONIA AND AMINES I.22.J General Considerations and Comparisons to Carbonyl Reductions... 

Reduction of Preformed Imines and Iminium Salts of Ammonia, Primary and Secondary Amirtes... 

REDUCTION OF IMINES AND IMINIUM SALTS OF AMMONIA AND AMINES... 

Table 6 Stereochemistry of Reduction of 2-MethylcyclohexyI Imines and Iminium Salts with Hydride Reagents... 

The [3+2] cycloaddition of allylsilanes is applicable to imines and iminium salts [419, 484, 485], Highly substituted pyrrolidines can be synthesized with high diastereo and enantio control by reaction of homochiral crotylsilanes with carbon-nitrogen double bond generated in situ from acetals and methyl carbamate (Scheme 10.174) [419]. The cycloaddition to N-tosylaldimines of aromatic aldehydes proceeds with excellent 2,4-ds selectivity whereas the stereoselectivity with aliphatic aldimines is rather low [484]. With N-tosylaldimines, fhe formation of [2+2] adducts is not observed (vide infra). 

Schiff bases, imines, and iminium salts in synthesis of N-heterocycles ... 

Cydoadditions of Electron-Rich Dienes with Imines and Iminium Salts... 

One of the more important uses of sodium borohydride is the reduction of enamines, imines, and iminium salts, particularly useful in alkaloid and amino acid syntheses. Imine 98 was reduced to amine derivative 99 in... 

General but mild. Good for ketones, aldehydes, acid chlorides, imines and iminium salts... 

In addition, good results were reported for the synthesis of new mono-N-tosylated diamine ligands based on (/f)-(+)-limonene and their application to ATH of cyclic imines and iminium salts, with up to 98 % e.e. in some cases [69]. Reductions of iminium salts have been described with the use of 1.2 mol% of a Ru(II) catalyst, using sodium formate with cetyltrimethylammonium bromide (CTAB) in aqueous solution, with the addition of silver hexamethylantimonate [38]. 

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