Imine oxidation

The perruthenate procedure (Entry 2) is also based on a general amine to imine oxidation. The iodosobenzene method (Entry 3) is an application of a... 

The anionic rhodium complexes [(PR3)2Rh(NHPh)2] Li (see above) have been shown to catalyze (TON = 21) the condensation of aniline with styrene to give the expected amine and the corresponding imine (oxidative amination) (Eq. 4.37, TOP 0-07 h- ) [161]. 

Reaction with Propargyl Halides. The indium-mediated coupling of propargyl bromide with a variety of imines and imine oxides afforded homo-propargylamine derivatives in aqueous media under mild conditions.78 Propargylation of glyoxylic oxime ether in the presence of a catalytic amount of palladium(O) complex and indium(I) iodide in aqueous media was also studied (Eq.11.47).79... 

Electron-rich 6-[(dimethyl(amino)methylene)amino uracil 82 underwent [4+2] cycloaddition reactions with various in situ generated glyoxylate imine and imine oxides to afford novel pyrinhdo[4,5-J]pyrimicline derivatives 83-84 after elimination of dimethylamine from the (1 1) cycloadducts and oxidative aromatization. This one-pot procedure yielded excellent yields when carried out in the solid state and under microwave irradiation <06BMCL3537>. 

Diaryl diselenides and benzisoselenazole-3(27/)-ones are used as efficient catalysts in the process of imine oxidation with hydrogen peroxide and tert-butyl-hydroperoxide (25). 

I.2. Oxidation of Amines Oxidation of primary amines is often viewed as a particularly convenient way to prepare hydroxylamines. However, their direct oxidation usually leads to complex mixtures containing nitroso and nitro compounds and oximes. However, oxidation to nitrones can be performed after their conversion into secondary amines or imines. Sometimes, oxidation of secondary amines rather than direct imine oxidation seems to provide a more useful and convenient way of producing nitrones. In many cases, imines are first reduced to secondary amines which are then treated with oxidants (26). This approach is used as a basis for a one-pot synthesis of asymmetrical acyclic nitrones starting from aromatic aldehydes (Scheme 2.5) (27a) and 3,4-dihydroisoquinoline-2-oxides (27b). 

The first tantalum nitrene was obtained in 1959 by thermolysis of [Ta(NEt2)]5-288 This class of compounds is presently accessible by several routes, including hydrogen abstraction from the mono- or di-alkylamides, reaction of metallacarbenes with organic imines, oxidation of low valent species by organic azides, or reductive coupling of nitriles (Table 13). The tantalum derivatives are usually stabler than those of niobium. 

Vinyl boronates 228 undergo 1,3-dipolar cycloaddition with aryl hydroximinoyl chloride 229 to furnish the imine-oxide 231. The boronate moiety is highly labile under the reaction conditions and it undergoes protonolysis to yield isoxazolines 232. Alternatively, the boronates could be oxidized with hydroperoxides or sodium percarbonate to furnish the 4-hydroxyisoxazolines 233 in good yield (Scheme 39) <2000T965>. 

Papaverine.—The biosynthesis of the simple benzylisoquinoline papaverine (89) is known to proceed via nor-reticuline (48) and tetrahydropapaverine (88).71 Dehydrogenation of the latter affords papaverine, and examination of the stereochemistry of the processes involved has led to the conclusion72 that loss of the proton at C-3 [in nor-reticuline (48)] is stereospecific (loss of the pro-S hydrogen atom) but removal of the C-4 proton is essentially non-stereospecific. These observations are perhaps best explained if enzyme-catalysed oxidation of (88) occurs to give (90), subsequent non-stereospecific imine-enamine isomerization occurring without enzyme participation to give (91). A further amine to imine oxidation then occurs to give papaverine (89).72... 

Acyl-aryl- E5, 1184 (Ald-imin + Oxid.) (Amino-thiocarbonyl)-aryl- E4, 517 (-NH-NH- -> —N = N —) Aryl-cyan- X/3, 595 Aryl-sulfonyl- X/3, 580 1 -Aryl-2-(tetrafluor-4-pyridyl)- -1-oxide E16d, 125 (Het.-NC12 + R-NO)... 

Oxa-3-thia-4-aza-tetracyclo-16.2.1.01 .04 undecan (+)-(lS,4S)-7,7-Dichloro-l 1,11-dimethyl- -3,3-dioxid E21f, 5870 (Imin-Oxid.)... 

This reaction depends on the type of nitrogen substitution and it is proposed that the reactivity may correlate with the ionization potential of the imine. Oxidized derivatives, i.e. the oxime (31 = OH) ... 

TABLE 7. CHIRAL SOLVENT EFFECTS ON IMINE OXIDATION... 

There is one report of the addition of a cyclopropene to an imine oxide to give a 2-oxa-3-azabicyclo[3.1.0]hexane 11. ... 

The mechanism of this transformation was not investigated, however a possible mechanism was proposed (Scheme 32). Transmetalation of the organoboron reagent to a rhodium(I) center could be followed by coordination of the imine, oxidative addition of the ortho C-H bond and reductive elimination to afford the ortho arylated product and a rhodium(I) hydride. Reoxidation would then follow through insertion of the imine in to the Rh-H bond followed by protonation with NH4C1. 

Various oxidants have been used to convert pyrrolidine and piperidine into nitrones (imine oxides), of value for 1,3-dipolar cycloadditions in one method hydrogen peroxide is the oxidant in the presence of a catalyst/... 

The monomeric structural unit of the 3-alkylpiperidine alkaloids is readily apparent in a number of simple 3-alkylpyridines isolated from marine sponges. In all known examples, the 3-alkyl component of the monomeric unit is attached to a primary amine, methyl amine, methoxy amine, methoxy methyl amine, imine oxide or oxime methyl ether functionality at the distal terminus. 

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