Aziridines, carbonylation

Scheme 58 Proposed catalytic cycle for aziridine carbonylation...

All types of electrophiles have been used with 2-lithio-l,3-dithiane derivatives, including alkyl halides, sulfonates, sulfates, allylic alcohols, arene-metal complexes, epoxides, aziridines, carbonyl compounds, imines, Michael-acceptors, carbon dioxide, acyl chlorides, esters and lactones, amides, nitriles, isocyanates, disulfides and chlorotrialkylsilanes or stannanes. The final deprotection of the dithioacetal moiety can be carried out by means of different types of reagents in order to regenerate the carbonyl group by heavy metal coordination, alkylation and oxidation184 or it can be reduced to a methylene group with Raney-nickel, sodium or LiAIII4. 

Scheme 60 Catalytic oxirane and aziridine carbonylative ring expansion...

With the same concept, but using the more reactive Ti(III) cationic radical [Cp2TiCl(THF)2] or a cationic salphen aluminum complex in combination with the cobalt anion [Co(CO)4] , Coates et al. succeeded to make the epoxide or aziridine carbonylative ring expansion reaction catalytic (Scheme 60) [149]. For both substrates, it is proposed a nucleophilic attack of the cobalt anion at the least-substituted carbon atom of the three-membered ring, the latter being activated by the Lewis acidic part of the catalyst. Of note, catalysts 106 and 107 used in this reaction are described as ion pairs rather than M-Co bond containing complexes. 

A new, stereoselective, one-pot synthesis of A -heterocyclic bicycles has been developed, based on the Michael addition of R C=CCH2CH(CN)2 and another nucleophile (e.g. R NHOH) to R CH=CHCHO, followed by a condensation/cycloaddition to furnish aziridine carbonyls, j8-lactams, or octahydrobenzo[c]isoxazoles, depending on the reaction conditions and the nucleophile. ... 

Types of Catalysts and Scope of Substrates for Aziridine Carbonylation... 

Efforts to develop aziridine carbonylation have occurred in parallel with efforts to develop epoxide carbonylation. The 3-lactams that are formed by this process are important in medicinal, organic, and polymer chemistry. Important contributions to aziridine carbonylation have been made by Alper, Davoli and Prati, and Coates. " A series of carbonylations of aziridines were developed using [Rh(CO)jCl]j and COj(CO)jas catalyst. More recently, faster rates and expanded scope have been found with catalysts that combine a Lewis acid cation with the [Co(CO)J" anion. 

Primary and secondary amines also react with epoxides (or in situ produced episulfides )r aziridines)to /J-hydroxyamines (or /J-mercaptoamines or 1,2-diamines). The Michael type iddition of amines to activated C—C double bonds is also a useful synthetic reaction. Rnally unines react readily with. carbonyl compounds to form imines and enamines and with carbo-tylic acid chlorides or esters to give amides which can be reduced to amines with LiAlH (p. Ilf.). All these reactions are often applied in synthesis to produce polycyclic alkaloids with itrogen bridgeheads (J.W. Huffman, 1967) G. Stork, 1963 S.S. Klioze, 1975). 

Seienazolidines can also be obtained by first treating aziridine with hydrogen selenide and condensing the intermediate product with a carbonyl compound (Scheme 70) (Methode II). 

Oxidative cleavage of P-aminoacyl complexes can yield P-amino acid derivatives (320,321). The rhodium(I)-catalyzed carbonylation of substituted aziridines leads to P-lactams, presumably also via a P-aminoacyl—metal acycHc compound as intermediate. The substituent in the aziridine must have 7T or electrons for coordination with the rhodium (322,323). 

Cyclic Carhene Complexes. The reaction of aziridines with carbonyl, thiocarbonyl, or isonitrile ligands in Mn, Re, Fe, Ru, Pd, or Pt complexes leads to formation of cycHc carhene complexes (324—331). 

Azomethine ylides are also frequently obtained by ring opening of aziridines, and the analogous carbonyl ylides from oxiranes. These aspects are dealt with in Section 3.03.5.1. A variety of five-membered heterocycles can also function as masked 1,3-dipoles and this aspect is considered in Section 3.03.5.2. 

Simple aziridines are optically transparent in the UV region of the electromagnetic spectrum (B-69MI50401). In more highly substituted aziridines, such as the 2-aroyl-3-arylaziridines (9), there is an interaction between the carbonyl and aryl ring substituents... 

Cycloadditions of aziridines to diphenylcyclopropenone lead to 4-oxazolines (36) (70CJC89). A mechanism involving initial addition to the cyclopropenone carbonyl group followed by ring opening and recyclization was suggested. 

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

Transition metals have also been inserted into the aziridine ring affording derivatives (295). Stereochemical studies suggest that transfer of a proton is followed by bimolecular attack on the ring with subsequent closure on the carbonyl group (76AG(E)495). 

The Corey-Chaykovsky reaction entails the reaction of a sulfur ylide, either dimethylsulfoxonium methylide (1, Corey s ylide, sometimes known as DMSY) or dimethylsulfonium methylide (2), with electrophile 3 such as carbonyl, olefin, imine, or thiocarbonyl, to offer 4 as the corresponding epoxide, cyclopropane, aziridine, or thiirane. ... 

It is well known that aziridination with allylic ylides is difficult, due to the low reactivity of imines - relative to carbonyl compounds - towards ylide attack, although imines do react with highly reactive sulfur ylides such as Me2S+-CH2-. Dai and coworkers found aziridination with allylic ylides to be possible when the activated imines 22 were treated with allylic sulfonium salts 23 under phase-transfer conditions (Scheme 2.8) [15]. Although the stereoselectivities of the reaction were low, this was the first example of efficient preparation of vinylaziridines by an ylide route. Similar results were obtained with use of arsonium or telluronium salts [16]. The stereoselectivity of aziridination was improved by use of imines activated by a phosphinoyl group [17]. The same group also reported a catalytic sulfonium ylide-mediated aziridination to produce (2-phenylvinyl)aziridines, by treatment of arylsulfonylimines with cinnamyl bromide in the presence of solid K2C03 and catalytic dimethyl sulfide in MeCN [18]. Recently, the synthesis of 3-alkyl-2-vinyl-aziridines by extension of Dai s work was reported [19]. 

Ring-opening of diastereomerically pure vinylaziridine 131, prepared by azir-idination of butadiene with 3-acetoxyaminoquinazolinone 130 [52], yielded acetate 132 with inversion of configuration, together with amino alcohol 133 with retention (Scheme 2.34) [53]. The formation of 133 can be explained by assuming participation by the quinazolinone carbonyl oxygen, which produces an intramolecular reaction with the aziridine carbon with retention of configuration. 

Azirines (three-membered cyclic imines) are related to aziridines by a single redox step, and these reagents can therefore function as precursors to aziridines by way of addition reactions. The addition of carbon nucleophiles has been known for some time [52], but has recently undergone a renaissance, attracting the interest of several research groups. The cyclization of 2-(0-tosyl)oximino carbonyl compounds - the Neber reaction [53] - is the oldest known azirine synthesis, and asymmetric variants have been reported. Zwanenburg et ah, for example, prepared nonracemic chiral azirines from oximes of 3-ketoesters, using cinchona alkaloids as catalysts (Scheme 4.37) [54]. 

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