2-Silyl aziridines

Reaction with Further Electrophiles of Group IVA (Sl,Ge,Sn). IV-Silylated aziridines can be prepared from ethyleneimine by amination of chlorosilanes in the presence of an HC1 acceptor, by dehydrocondensation with an organosilicon hydride or by cleavage of a silicon—carbon bond in 2-furyl-, 2-thienyl-, benzyl-, or allylsilanes in the presence of an alkali metal catalyst (262—266). N-Silylated aziridines can react with carboxylic anhydrides to give acylated aziridines, eg, A/-acetylaziridine [460-07-1] in high yields (267). At high temperatures, A/-silylaziridines can be dimerized to piperazines (268). Aldehydes can be inserted... 

Trimethylsilylazide attacks citraconic anhydride with regiospecificity and catalytically adds to aldehydes in the presence of ZnCl2-18-crown-6. Silylazides react with vinyl and other alkenylsilanes to give bis(silyl)enamines through the thermal rearrangement of intermediate triazolines. Other azides give silyl-aziridines, which also result when silyl bromoazides, obtained from alkenylsilanes, are reduced with LiAlHi. ... 

Metalated Epoxides and Aziridines in Synthesis 5.2.4.2 Silyl-stabilized Lithiated Epoxides... 

Direct deprotonation/electrophile trapping of simple aziridines is also possible. Treatment of a range of N-Bus-protected terminal aziridines 265 with LTMP in the presence ofMe3SiCl in THF at-78 °C stereospecifically gave trans-a, 3-aziridinylsi-lanes 266 (Scheme 5.67) [96]. By increasing the reaction temperature (to 0 °C) it was also possible to a-silylate a (3-disubstituted aziridine one should note that attempted silylation of the analogous epoxide did not provide any of the desired product [81],... 

Fe(OTf)2-catalyzed aziridination of enol silyl ethers with PhlNTs followed by ring opening led to a-N-tosylamido ketones in good yields (Scheme 27) [81]. With silyl ketene ketal (R = OMe) as substrate, the N-tosyl-protected amino acid ester was obtained in 50% yield. In contrast, the copper (I) salt CuClOq was found not effective for this substrate [82]. 

Che et al. have reported that chiral Ru11(salen)s (54a) and (54b) are efficient catalysts for aziridination of alkenes (up to 83% ee) and amidation of silyl enol ethers (up to 97% ee), respectively (Scheme 39).163... 

In some cases 0-substituted oximes reacted with azomethine ylides. Thus, reaction of 0-substituted oxime (NC)2C=NOTs 139 with azomethine yhde derived from aziridine 140 afforded imidazoline 141 in 44% yield (equation 61). Addition of lithium derivative of silylated alkyne to oxime ethers 142 leads to 4-ethynyl-Af-hydroxy-2-imidazolines 143 in 49-72% yields (equation 62) . 

Lastly, Antilla has disclosed a novel asymmetric desymmetrization of a wide range of aliphatic, aromatic, and heterocyclic meso-aziridines with TMS-N3 promoted by 11 and related 12 (Scheme 5.31) [56]. Uniquely, this is one of only several reports of electrophilic activation of nonimine substrates by a chiral phosphoric acid. Mechanistic studies suggest that silylation of 11 or 12 by displacement of azide generates the active catalytic species A. Consequently, the aziridine is activated through coordination of it carbonyl with chiral silane A to produce intermediate B. Nucleophilic ring opening by azide furnishes the desymmetrized product and regenerates 11 or 12. 

O-Silyl-keten-O.O-acetale werden auch in nicht stereoselektiven a-Aminosaure-Synthesen mit Nitroso-benzol3, Phenyldiazonium-tetrafluoroborat4 oder Azido-ameisensaure-ethyl-ester5 umgesetzt. In letzterem Fall tritt intermediar ein Aziridin auf (s. S.640). 

Die Umlagerung eines intermediaren Aziridins ist der zentrale Schritt der Nitren-Addition an die 0-Silyl-keten-0,0-acetale verschiedener Alkansaure-ester zu a-Acylamino-car-bonsaure-estern5 (vgl. a. S. 614, 620ff.) ... 

Alkylation of aziridines amine synthesis.3 N-Substituted aziridines (alkyl, benzyl, silyl, Boc) react with lithium dialkylcuprates in the presence of BF3 etherate (excess) with opening of the ring to give primary or secondary amines. 

Alkenylsilanes react with silyl azide and other organic azides to give bissilylenamines and silylaziridines, respectively, via unstable triazoline intermediates.78 Heating trimethylsilyl azide with cyclohexene or vinyltrieth-ylsilane for 10 days affords a modest yield of the respective aziridine,178 but with carefully purified trimethylsilyl azide, even after 2 weeks of reflux, no olefin consumption is indicated.104 Unstable triazoline intermediates are also formed from the reaction of cyanogen azide,808 picryl azide,29 and arylsulfonyl azides808 with unactivated olefinic bonds. 

An IV-heterocyclic carbene (similar to that in Scheme 15) has proved to be an effective catalyst for the nucleophilic ring opening of IV-tosylaziridines by silylated nucleophiles (MesSiX, X = N3, Cl, I).45 Yields range from 89 to 99% for reaction at the least substituted carbon, except when a phenyl group on one of the carbons of the aziridine ring induces predominant attack at the benzyl carbon. The stereochemistry is consistent with the SN2 mechanism and THF was found to be the best solvent for the reaction. 

No direct alkylation of a secondary SMA has been reported. However, indirect methods have been described. Albeit the yield was not very high1, the condensation of A -benzyl MSMA with an aziridine in the presence of aluminum chloride, provided a route to a A-silyl-1,2-diamine.219... 

In the presence of trifluoroacetic acid, these aziridines undergo protonation at room temperature and ring opening at reflux in methanol, diethyl ether or hexane under nucleophilic attack of the trifluoroacetate anion followed by acyl exchange to give the corresponding a-silyl-(i-aminoalcoho] derivative.316... 

Noticeable is the absence of reaction when the aziridine ring does not bear at least one anion-stabilizing moiety. Noticeable also is the influence of the position of the trimethylsilyl group in the molecule as silyl group directly attached to the ring does not induce ring opening (see above). 

The nitrido complex was applied to the direct asymmetric animation with a silyl enol ether as a substrate. Although several examples for achiral aminations of silyl enol ethers have been reported [32], an asymmetric version of reagent-controlled reaction has not appeared except for the one recent example [33] and the diastereoselective reactions with silyl enol ethers having a chiral auxiliary [34], The amination, which is presumed to take place via an aziridine intermediate [5g, lid,32], proceeded smoothly to give the A-tosylated a-aminoketone in 76% yield with 48% ee. When the same silyl enol ether was treated with complex 15 under Carreira s condition, the TV-trifluoroacetylated a-aminoketone was obtained in 58 % yield with 79 % ee (Scheme 24). 

Despite this tendency, examples of alkylation, acylation, sulfonylation, halogenation, silylation, and phosphorylation of aziridines at nitrogen abound. Aziridinium salts can be prepared by further alkylation of the aziridine nitrogen. Aziridines can also be alkylated on nitrogen with epoxides producing -hydroxyamines. 

Wu J, Sun X, Ye S, Sun W (2006) N- Iclcrocyclic carbene. A highly efficient catalyst in the reactions of aziridines with silylated nucleophiles. Tetrahedron Lett 47 4813—4816... 

---