Imines carbene transfer

The aziridination of imines catalyzed by the CpFe(CO)2+ fragment by carbene transfer is a most remarkable reaction [26], The imine is consumed first to form both the cis and trans products. The catalyst then coordinates predominantly to the trans product and leads to its decomposition, leaving the cis product untouched [27]. 

Diels-Alder additions in water. It also catalyzes aldehyde olefination with N2C(C02Et)2 and PPhs as oxygen sink. Its catalyzed decomposition of N2C(C02Et)2 leads to carbene transfer reactions allowing for the formation of epoxides from aldehydes and ketones, aziridines from imines and cyclopropanation. 

However, at this stage relatively little progress has been made in research on asymmetric catalytic carbene transfer to imines. In 1995, Jacobsen and Jorgensen reported independently that reaction of ethyl diazoacetate with selected imines can be catalyzed by copper salts [27,28]. In the former case [27], moderate levels of enantioselection were found to be imparted by bisoxazoline ligands associated with the copper catalyst (Scheme 11). The observation of racemic pyrrolidine byproducts in the reaction was taken to support a mechanism of catalysis involving initial formation of a copper-bound azomethine yhde intermediate (Scheme 12 ). Collapse of this intermediate to the optically active aziridine apparently competes with dissociation of the copper to a free azomethine ylide. The latter can react with fumarate formed by diazoester decomposition in a dipolar cycloaddition to afford racemic pyrrolidine. 

So far aziridination reactions have, in some ways, had more in common with cyclopropanation reactions (see Section 9.1) than with epoxidation reactions. Nevertheless, the aziridination reaction is more synthetically akin to epoxidation, and on that basis, is included in the present chapter. Aziridines maybe prepared by nitrene transfer to alkenes or by carbene transfer to imines and both approaches have been performed in an enantioselective sense using enantiomerically pure metal-based catalysts. 

Ylide generation via carbene transfer also works weU for the aziridination of N-sulfonyl-imines [37]. With 20mol% sulfide 18b as the catalyst, the reactions of SES-protected imines with diazomethylbenzene provided the aziridine products... 

Complexation of [Cp IrCl2]2 with iV-heterocyclic carbenes has led to complexes such as 25, developed by Peris and coworkers [107, 108], and 133, developed by Crabtree and coworkers [12]. Complex 24 is activated by the addition of silver triflate and is effective for the iV-alkylation of amines with alcohols and for the iV-alkylation of anilines with primary amines. Complex 25 has also been shown to couple benzyl alcohol 15 with a range of alcohols, including ethanol 134, to give ether products such as ether 135 (Scheme 31). Complex 133 was an active hydrogen transfer catalyst for the reduction of ketones and imines, using 2-propanol as the hydrogen source. It was also an effective catalyst for the iV-alkylation of amines... 

A large number of reports have concerned transfer hydrogenation using isopropanol as donor, with imines, carbonyls-and occasionally alkenes-as substrate (Scheme 3.17). In some early studies conducted by Nolan and coworkers [36], NHC analogues of Crabtree catalysts, [Ir(cod)(py)(L)]PF,5 (L= Imes, Ipr, Icy) all proved to be active. The series of chelating iridium(III) carbene complexes shown in Scheme 3.5 (upper structure) proved to be accessible via a simple synthesis and catalytically active for hydrogen transfer from alcohols to ketones and imines. Unexpectedly, iridium was more active than the corresponding Rh complexes, but... 

The use of the Rh complex 29 bearing a tridentate carbene ligand has also been described for the transfer hydrogenation of ketones and imines [74], The catalyst was found to be highly active, needing only 0.001 mol% (up to 68 000 TON) to completely reduce to substrates. 

These spacer-free Janus bis-carbene complexes were successfully employed in the intramolecular cyclisation of acetylenic carboxylic acids (4-pentynoic acid and 5-hexynoic acid) as well as the transfer hydrogenation of ketones and imines. 

The addition of carbenes and carbenoids to imines and nitriles continues to be a popular approach to aziridines and 2//-azirines. These processes have been well reviewed by Deyrup (B-83MI101-01). Dichlorocarbene and other dihalocarbenes have been added to a wide variety of imines to provide dihaloaziridines. A recent example is shown in Equation (64), illustrating the use of phase-transfer conditions <93H(36)69i). The treatment of azides with excess dichlorocarbene results in the formation of the 2,2,3,3-tetrachloroaziridines (92TL2339). Presumably, the azide is converted by dichlorocarbene to the imidoyl dihalide RN=CC12 which reacts further with dichlorocarbene. Transition metal-promoted reaction of a-diazoesters with imines provides 2-(alkoxycarbonyl)aziridines [Pg.46]

The catalytic transfer of carbenes to imines (Fig. 4) represents a method for the preparation of aziridines complementary to the imido transfer strategies summarized above. The synthetic accessibility of imines and diazocarbonyl compounds, combined with the relative cleanliness of diazo chemistry and the inherent convergence associated with the coupling of two potentially complex reaction partners offer considerable incentive for developing this approach. 

Single-electron Transfer Carbene Mechanisms. The vast majority of reactions of LDA owe their mechanism to the basic character of the reagent some recent experiments have shown that LDA may behave very differently in the presence of alkyl halides and dihalides (specifically iodides), 7r-deficient aromatic heterocyclic compounds (Scheme 8, eq 7), a-bromo imines, conjugated acetylenes, alkyl sulfonates, or benzophenone. In these instances, it has been shown that LDA may act as a single-electron transfer donor as in the case of anthracene and perylene, or via carbene mechanism in the case of benzyl halides. Some substrates exhibit competition between multiple mechanisms (Scheme 8, eq 8, ref 160). 

Cationic carbyne complexes, such as [(CO)5CrsCNEt2] and [Cp(CO)2M =C-Aryl]+ (M = Mn, Re), were shown to readily add neutral and anionic nucleophiles to the carbyne carbon atom to give the corresponding cationic or neutral carbene complexes. Carbene complexes have also been obtained in the reaction of [Cp(CO)2Mn = C-Tol]+ with Ph(H)C=NMe and Ph(H)C=N-N=C(H)Ph [6]. The products result from insertion of the imine between the carbyne carbon and a carbon of the Cp ligand. We now report on the reactions of [Cp(CO)2Mn = C-Ph] + and substituted derivatives thereof with electron-rich NsC and C = C compounds. The aim of our investigations was to study the possibility of transferring the carbyne ligand to these multiple bonds. 

Several examples of imino-functionalized carbene complexes of palladium have been described. " The imidazolium salt 97 was prepared and coordinated to Pd by an Ag-transfer reaction. Tautomerization of the carbene-imine bond occurs during the transfer reaction to give the the enamine complex 98. Neutral and cationic Pd complexes of a second closely related chelating iminocarbene ligand, 99, were also prepared via an Ag-transfer route. " In this case, no tautomerization was reported. However, evidence of solvent-dependent hemilability was observed. 

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