Ring Expansion of Aziridines

The photochemically induced addition of the dipolarophile methyl dithiobenzoate to 2,3-diphenylaziridine (106) in benzene produces, via the nitrile ylide (107), a mixture of the two stereoisomeric A -thiazolines (108a) and (108b), the latter as the major product (total 25%).  

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An intramolecular ring expansion of aziridine esters can be accomplished by installing an appropriate nucleophilic entity in these substrates. Conversion of the ester moiety into carboxamides derived from aminomalonate ester gives compounds 44 containing the requisite nucleophilic site in the malonate moiety (Scheme 35). 

Ring Expansion Via Carbonyiation Reactions 11.15.5.1 Ring Expansion of Aziridines... 

Since Alper and co-workers reported the first Rh-catalyzed carbonylative ring expansion of aziridines, yielding /3-lactams in 1989, this ring-expansion reaction catalyzed by Rh or Co complexes and its mechanism have been extensively studied. ... 

Dendritic rhodium catalyst systems on a resin have been developed for the carbonylative ring expansion of aziridines. " This catalyst system exhibited same high activity as the homogenous counterpart, that is,... 

For a review of a nitrogen analog, see Boeckman Walters Adv. Heterocycl. Nat. Prod. Synth. 1990,1, 1-41. "For reviews of ring expansions of aziridines, see Heine Mech. Mol. Migr. 1971, 3, 145-176 Dermer Ham Ethylenimine and Other Aziridines Academic Press New York, 1969, pp. 282-290. See also Wong et at.. Ref. 114, pp. 190-192. 

The ring expansion of aziridines has been reported in 2001 as a well established protocol [144] for preparing p-lactams in a regioselective manner [145]. A variety of aziridines with different substituents and stereochemistry was subjected to cobalt carbonyl-catalyzed carbonylation to give p-lactams. The ring expansion to... 

Scheme 56 Regioselective synthesis of P-lactams by cobalt-catalyzed ring expansion of aziridines...

A complete regioselectivity has been observed in 2002 in the carbonylative ring expansion of aziridines trimethylsilylsubstituted, using Co2(CO)8 as catalyst to give (5-lactams (Scheme 57), [146]. 

Lewis acid]+[Co(CO)4] complexes have been reported as a versatile class of catalysts for carbonylative ring expansion of aziridines to p-lactams [147]. For instance, catalysts such as [C5H5Ti(thf)2][Co(CO)4] and [(salph)Al(thf)2][Co (CO)4]1 [148] have been shown to efficiently carbonylate a variety of aziridines under mild conditions. Further, the authors proposed a mechanism for the CO insertion into aziridines. A theoretical investigation has been also reported for the [Co(CO)4] -catalyzed carbonylative ring expansion of (V-benzoyl-2-methylaziri-dine to p-lactams (Scheme 58), [149]. 

Rhodium-complexed dendrimers, supported on a resin, have been reported to show high activity for the carbonylative ring expansion of aziridines with carbon monoxide to give p-lactams (Scheme 59), [150]. 

Scheme 59 Carbonylative ring expansion of aziridines with rhodium-complexed dendrimers...

Theoretical studies have also been reported on the catalytic activity of the rhodium (I) in the carbonylative ring expansion of aziridines to (3-lactams [151]. 

The ring expansion of aziridines and azirines corresponds to this type of ring closure leading to A2-thiazolines. iV-Thioacylaziridines (366) rearrange under acid conditions to 2-aryl-A2-thiazolines (368) either by an S l or S 2 mechanism, depending on the nature of the acid catalyst and the solvent used (69JA5835,69JA5841). 

A palladium-catalyzed ring expansion of aziridines provided a novel route to highly substituted pyrroles <07TL2267>. Treatment of methylene aziridines 33 with Pd(0) in the presence of symmetrical 1,3-diketones 34 led to the formation of 3-ketopyrroles 35. Pyrrole-substituted nucleoside analogues were prepared using a vanadium-mediated transformation of 2-azirines in the presence of symmetrical 1,3-diketones <07SL2723>. 

In a similar vein, a resin-supported rhodium-complexed dendrimer 340 has been shown to promote the carbonylative ring expansion of aziridines to /3-lactams <1988CC710, 2006JHC11>, as illustrated by the conversion of the A-7-butyl aziridine 341 to the corresponding lactam 342 in almost quantitative yield. The supported catalyst, which shows reactivity comparable to the solution-phase variety, is easily recovered by filtration and exhibits no significant loss of activity upon recycling (Scheme 88). 

Later, /J-lactams have been obtained by regiospecific metal-catalyzed ring expansion of aziridines. Treatment of A -t ri-butyl-2-phenylaziridine (252) with carbon monoxide in benzene, employing chlorodicarbonylrhodium(I) dimer as the catalyst at 90°C and 20 atm, produced the azetidinone 253 in quantitative yield. Several other )S-lactams were also synthesized from the corresponding aziridine derivatives by the application of this method, and it was noted that this reaction was completely regiospecific. The possible reaction mechanism for the formation of fi-btciams is outlined in Scheme 47. 

Coldham, I., Collis, A. J., Mould, R. J., Rathmell, R. E. Ring expansion of aziridines to piperidines using the aza-Wittig rearrangement. Tetrahedron Lett. 1995, 36, 3557-3560. 

Ring expansion-carbonylation.1 The ring expansion of aziridines to /3-lactams by a Rh(l) catalyst (15,82-83) has been extended to expansion of pyrrolidines to piperidones by cobalt carbonyl-catalyzed carbonylation (equation 1). 

Ring expansion of aziridines by addition to double bonds... 

From Aziridines. The well-known synthesis of 2-thiazolines (see Volume 1, p. 397) and thiazolidines (see this Volume, p. 636) by the ring expansion of aziridines has been further studied and utilized. 

Alper and Hamel have reported an interesting case of Pd-catalyzed CO insertion reaction, involving ring expansion of aziridines to azetidinones (Scheme 47). It seems reasonable to assume that the reaction is initiated by oxidative addition of Pd(0) across the strained C—N bond, as shown below,... 

The carbonylative ring expansion of aziridines with carbon monoxide is accomplished in the presence of rhodium complexed dendrimers to afford j8-lactames 63 in quantitative yields . ... 

Carbonylative ring expansion of aziridines in the presence of palladium or cobalt catalysts has proved to be a useful reaction for the construction of the P-lactam ring of... 

SCHEME 17.13. Enantioselective synthesis of (—)-indolizidines 209B 57 and 209D 58 by an aza-[2,3]-Wittig rearrangement ring expansion of aziridine 55. 

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