Keteneimine

Chiral oxazolines have also been utilized for the synthesis of ehiral ketones bearing quaternary earbon stereoeenters. As shown below, reaetion of substituted oxazoline 30 with 2 equiv PhLi followed by treatment with benzyl bromide gives ketone 33 upon aeidie hydrolysis. This reaetion is believed to proeeed via addition of PhLi to keteneimine 31 to afford metalated enamine 32, whieh undergoes alkylation at the nueleophilie earbon to provide 33 after aqueous workup. ... 

All attempts to isolate efficient process giving a white solid polymer which appears to have repeating keteneimine units. This assignment is consistent with the very strong absorption at 2140 cm.-1 in the infrared spectrum. ... 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

The only carbene-like reaction reported so far is the low-temperature addition of tert-butyl isocyanide to carbenes 2d and 2u.78 From the P-hydrogenophosphonio carbene 2d, the heterocycle 89 was isolated in high yield. It is believed that the initial coupling product 87d rapidly inserts a further equivalent of isocyanide into the P-H bond, leading to the intermediate 88, which then undergoes rapid elimination of diisopropylamine. When the same reaction was performed with the P-chloro(phosphonio)car-bene 2u, a 1/1 mixture of keteneimine 90 and phosphinonitrile was obtained. This result can be explained by the cleavage of the carbene-isocyanide coupling product 87u by residual HCN, inherently present in the f-BuNC. 

Scheme 5.3 The reaction of a benzylidene tungsten complex with keteneimine.

MNDO results suggest that the activation energies are similar for the gas phase thermal isomerization of isoxazole to oxazole via either a nitrile ylide or a keteneimine, through an azirine intermediate. The first step is rate limiting, which is in good agreement with experimental results (90JPO611). 

Lithio TMS-diazomethane reacts with ketenimines to produce 4-TMS-1,2,3-triazoles or 4-amino-3-TMS-pyrazoles87 88, depending on the keteneimine substitution pattern. 

Keteneimines will also undergo electrochemical hole-catalyzed cycloaddition reactions, producing dimers and even trimers as shown below (Scheme 78)181. Adventitious water or the replacement of aryl a-hydrogens leads to somewhat different products (Scheme 79)182. 

Bicyclic amidines were obtained when the keteneimine 318 was heated in toluene solution to produce a formal [2+2] cycloaddition between the imine group and the C=C bond to give an approximately equal mixture of the diastereoisomeric azeto[l,2- ]imidazoles 153 and 319 in about 50% overall yield (Equation 36) (see Section 2.04.6.5) <2005T1531>. 

The nucleophile initially adds at the multiple bond, forming carbanion A. Further transformations of A occur in line with electronic and steric effects, depending on the reaction conditions and on the use of nucleophilic catalysis. Several routes are possible, leading to different reaction products. Note that the use of nucleophilic catalysis is a general technique in the chemistry of compounds with electrophilic multiple bonds in particular, it is widely employed for dimerization and trimerization of activated olefins, keteneimines, etc. 

In the presence of alkaline metal fluorides or tertiary amines, perfluoro-olefins react with fluorine-containing keteneimines to form azetidines 25 in mild conditions (76IZV1813). The role of the catalyst seems to be the transformation of the electrophilic keteneimine into the nucleophilic mesomeric anion capable of adding at the multiple bond of the perfluoro-olefin. The addition is accompanied by cyclization generating a catalyst. 

Azetidines are also formed in the reaction of octafluoroisobutylene with N-sulfinyl-amines in the presence of CsF (79IZY396). The initial reaction is vinyl substitution and -elimination of SOF2 rather than cycloaddition. The forming keteneimine 26 adds one more octafluoroisobutylene molecules, generating azetidine 27. 

The rates of [2- -2]cycloaddition reactions between di-tert-butylthioketene and azomethines [141], thiobenzophenones and keteneimines [552] as well as [2- -2-1-2]... 

Isoxazolo[3,4-c/]pyrimidine 39 when treated with cyanoolefins 40, in the presence of TEA as catalyst, gave biologically interesting pyrido[2,3-c/]pyrimidine oxides 41 in excellent yields. Probably, a [4+2] cycloaddition of the azadiene moiety of 39 with keteneimine intermediates, derived from 40 and TEA, is involved in the process <03TL1847>. 

The reaction of isoxazolopyrimidine and cyanoalkenes, in the presence of triethylamine as a catalyst, afforded pyridopyrimidine V-oxides in excellent yields. The reaction likely involves a [4-1-2] cycloaddition of the starting isoxazole as azadiene on keteneimine intermediates, derived from cyanoalkenes and NEts (Equation 5) <2003TL1847>. 

Photolysis and pyrolysis of vinyl azides leads to the production of 3H-azirines and unstable keteneimines ... 

Vinyl nitrenes usually do not produce keteneimines. However, in the case of l-azido-2,2-dicyanoethylenes (57) the major products were the dicyanoketeneimines 58. Due to their instability only their addition products [i.e. with ethanol (59)] were isolated ". 

Another example in which a Grignard base generates a reactive intermediate by elimination is depicted in Eq. (32) [39]. a-Cyanoeneamines are deprotonated by MeMgl, giving the reactive trialkyl ketenimine. The ketenimine subsequently reacts with primary amines, yielding amidines. The critical step is cyanide elimination from the deprotonated eneamine. The keteneimines could be isolated either by careful distillation or could be directly reacted with amines. 

Acrylonitrile. Acrylonitrile is usually considered to be monofunctional in polymerization—Le., it reacts normally with free-radical and anionic catalysts. It has been reported, however, that irradiation with x-rays at —78°C. results in partial formation of keteneimine groups (7), as shown in Reaction 21. 

Whether carbene, germylene, and silylene are justifiable terms for stabilized versions of the reactive species is a very debatable question [28], However, for synthetic chemists, the most important issue is to know whether these stable species feature the reactivity of transient carbenes. Formation of azaphospholidines upon thermolysis (intramolecular carbene insertion into a carbon-hydrogen bond), cyclopropanation reactions, and [l + l]-addition to isocyanides giving keteneimines... 

Azirines can be transformed to imino esters (259) and (260) (Scheme 37) by addition of alcohols or alkoxy carbenes respectively. By sodium methoxide catalyzed addition of methanol to the ketene imine (261 Scheme 38) the imino ester (262) is obtainable. Keteneimines undergo photoinduced addi-... 

A new and very neat synthesis of evodiamine and rutaecarpine is described by the authors as a retro mass spectral synthesis , since the original conception was derived from the mode of fragmentation of these alkaloids in the mass spectrometer this involves a familiar retro Diels-Alder fission of ring c. Evodiamine (16a) was thus constructed by a 27r + 47r cycloaddition of 3,4-dihydro-/8-carboline with the keteneimine (17), prepared in situ by elimination of sulphur dioxide from the sulphinamide anhydride (18) (Scheme 7). When the anthranilic acid derivative (19) was used the product was rutaecarpine (20) itself, the initially formed dihydro-... 

In contrast to ketenes, keteneimines readily undergo metal-catalyzed [3+21-cycloadditions with methylenecyclopropanes. Depending on the substituents in both educts, pyrroles, a-methylene-A3-pyrrolines or iminocyclopentenes can be synthesized selectively in good to excellent yields. The substituent of the imino group determines... 

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