Imines dimerization

Thiazines heve been synthesized from the nitrile imine dimer 300 (Equation 105) <1980J(P1)2923>, 1,4-dithiin 1,1,4,4-tetraoxide 301 (Equation 106) <1982TL299>, 1,4-oxathiins (Equation 107) <1996JOC3894>, and the anthracene adduct 302 (Scheme 77) <1998S915>. 

Nitrenes have a short lifetime (only several microseconds)86- 8 and undergo stabilization by the following reactions isomerization to imines, dimerization to azo compounds, hydrogen abstraction followed by ring closure to heterocyclic compounds, bimolecular insertion into C-H bonds to secondary amines, addition to solvent yielding ylids, and addition to unsaturated systems yielding heterocyclic compounds. Table 117-106 includes the reaction products and references for the different classes of nitrenes. 

Toward this end, Woerpel and Nevarez examined the possibility of di-tert-butylsilylene transfer from cyclohexene silacyclopropane 58 to imine 169a (Scheme 7.48).123 Thermolysis produced a mixture of silaaziridine 170a and an imine-dimer byproduct (171). The results by Brook and coworkers suggested that if the temperature of silylene transfer were lowered, isolation of 170a without formation of byproduct 171 would be possible. As anticipated, exposure of cyclohexene silacyclopropane 58 to imine 169a in the presence of substoichiometric amounts of silver triflate produced only 170a. This silaazridine could be purified by bulb-to-bulb distillation to afford the product in 80% yield. Copper salts required... 

The pyrolysis of [dimethoxy(methyl)silyl]bis(trimethylsilyl)amine350 yields products which can be ascribed to an initial formation of N-(trimethylsilyl)-methoxy(methyl)silanimine (equation 165). The isolated dimers may be formed by the imine dimerization or possibly by an attack of the silanimine on the precursor instead. Additional evidence for the presence of some of the intermediates postulated to account for the results was obtained by a study of the copyrolysis of [dimethoxy-(methyl)silyl]bis(trimethylsilyl)amine and hexamethylcyclotrisiloxane, which yielded a fairly complex mixture of products351. The formation of product 88 most likely proceeds through the unsaturated eight-membered ring silanimine 89 and not by way of meth-ylsilanitrile, although the latter possibility was also considered by the authors (equation 166). 

Isoquinoline V-imines also have been made only by deprotonation of V-aminoisoquinolinium salts. Just as for the corresponding pyridinium derivatives (see Section II,A,1), the salts 43 can be produced by amination of isoquinoline with hydroxylamine-O-sulfonic acid75 or from V-(2,4-di-nitrophenyl) isoquinolinium chloride (44), 30 31 76 Substituted quaternary amino salts (46) are obtained by cyclization of 2-(2,4-dinitroanilino)-o-styrylaldehyde hydrazones (45) with ethanolic hydrochloric acid, and deprotonation to V-imines is easily effected by alkali. On liberation from the salts by alkali, unsubstituted isoquinoline V-imines dimerize to give... 

Although the Skraup/Doebner-von Miller reaction represents one of the most common reaction for the synthesis of quinoline core for more than a century, its mechanism is still dedebated. To date, both of the two more popular mechanistic explanations are involving fragmentation-recombination pathways. In the first one, initially the amine reacts with the aldehyde or ketone under acidic conditions to form an imine. Dimerization and Pictet-Spengler type cyclization forms a diazetine core. Protonation and subsequent cyclization-ring cleavage reaction assembles the isoquinoline nucleus. 

Folding-driven synthesis of oligomers and polymers of m-phenyleneethynylene has been reported in the reversible imine metathesis reaction of an N-terminal imine dimer and hexamer with a C-terminal imine hexamer, formation of the longer 12-mer was favored over other species in acetonitrile while lower oligomers were largely formed in chloroform. ... 

Rearrangement to an open chain imine (165) provides an intermediate whose acidity toward lithiomethylthiazole (162) is rather pronounced. Proton abstraction by 162 gives the dilithio intermediate (166) and regenerates 2-methylthiazole for further reaction. During the final hydrolysis, 166 affords the dimer (167) that could be isolated by molecular distillation (433). A proof in favor of this mechanism is that when a large excess of butyllithium is added to (161) at -78°C and the solution is allowed to warm to room temperature, the deuterolysis affords only dideuterated thiazole (170), with no evidence of any dimeric product. Under these conditions almost complete dianion formation results (169), and the concentration of nonmetalated thiazole is nil. (Scheme 79). This dimerization bears some similitude with the formation of 2-methylthia-zolium anhydrobase dealt with in Chapter DC. Meyers could confirm the independence of the formation of the benzyl-type (172) and the aryl-type... 

Two extreme mechanisms can be envisaged (Scheme 12), concerted [2 + 2] cycloaddition or the more generally accepted formation of a dipolar intermediate (164) which closes to a /3-lactam or which can interact with a second molecule of ketene to give 2 1 adducts (165) and (166) which are sometimes found as side products. In some cases 2 1 adducts result from reaction of the imine with ketene dimer. 

Isocyanides react with fluorine [75 to yield difluoromethylene imines, which tend to dimerize In an inert solvent, imines accept fluorine fioin trifluoromethyl hypofluorite [76, 77] (equation 11)... 

The addition of primary amines to fluoroolefins under anhydrous conditions yields imines The hexafluoropropene dimer, perfluoro-2-methyl-2-pcntcne, and ten butylamine react to yield a mixture of two compounds m a 9 4 ratio [4] (equation 3) rather than just the major keteiiimme-imine, as previously reported [5] It IS claimed that this result is possible by means of isomerization to the terminally unsaturated difluoromethylene isomer prior to nucleophilic attack Secondary amines add to fluoroolefins under anhydrous conditions to give fluonnated ternary amines m good yields If the fluoroolefin is added to the amine without cooling the reaction mixture, or if an excess of the secondary armne is used, there is a tendency toward dehvdrofluonnation of the ternary amine The products... 

The mechanism for the formation of ( )-169 is explained in terms of an intermolecular nucleophilic dimerization. Nucleophilic addition of C-3 of 19 to the 3 position of the initially generated cation 168 gives an imine-nitrone... 

Irradiation of the dimers 1 of quinoline 1-imines 2 gives stable 1H-1,2-benzodiazepines. Selected examples are given.122... 

In a similar type of process, imines have been dimerized to give 1,2-diamines, by a number of procedures, including treatment with TiCLj—In/aqueous EtOH, Zn/aqueous NaOH, Cp2VCl2/Zn/PhMe2SiCl, with Sml2, and I for silylated imines) NbCl4(thf)2. When electroreduction was used, it was even... 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

A range of products was obtained from aniline including those from oxidative coupling (azozybenzene, azobenzene, and benzidine), and phenazine by dimerization (Chan and Larson 1991). Oxidation of m-phenylenediamine was initiated by the oxidation of two molecules to produce an A-phenyl-2-aminoquinone-imine that reacted with m-phenylenediamine to produce 2-amino-5-phenylaminoquinone-imine after further oxidation (Kami et al. 2000). 

Under the same conditions, the hydroamination of acetylene with primary or secondary aromatic amines brings about the formation of dimerization-cyclization products since the generated imines or enamines, respectively, are not stable. 

However, with 1-hexyne or phenylacetylene, the thorium catalyst induces a dramatic inversion in regioselectivity giving imines with various amounts of dimerized alkyne (e.g., Eq. 4.84) [301],... 

Spectroscopic investigations of the lithium derivatives of cyclohexanone (V-phenylimine indicate that it exists as a dimer in toluene and that as a better donor solvent, THF, is added, equilibrium with a monomeric structure is established. The monomer is favored at high THF concentrations.110 A crystal structure determination was done on the lithiated A-phenylimine of methyl r-butyl ketone, and it was found to be a dimeric structure with the lithium cation positioned above the nitrogen and closer to the phenyl ring than to the (3-carbon of the imine anion.111 The structure, which indicates substantial ionic character, is shown in Figure 1.6. 

Reductive activation of the quinone shown in Scheme 7.9 and incubation in methanol afforded a complex mixture of products consisting mainly of head-to-tail coupling at C-5 or C-7 (Scheme 7.10). Minor reactions involve transfer of H2 from the hydroquinone to the ene-imine (internal redox reaction) and methanol trapping. The structures of the dimers and trimers in Scheme 7.10 were derived from H-NMR,... 

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