Iminium salts trimethylsilyl

The penicillin-N,S-acetal 479 reacts with N,N-bis(trimethylsilyl)formamide 22c and Hg(OAc)2, apparently via the iminium salt 480, to give the penicillin-N,N-acetal 481 in 65% yield [64]. On treatment of racemic y-ketoesters such as 482 with chiral silylated 1,3-mercaptoalcohols such as 483, in the presence of TMSOTf 20, at room temperature a kinetically controlled 2 1 mixture of the 0,S-acetals... 

This method fails, however, with bicyclic ketones such as 1-tetralones even in the presence of TsOH, affording only enol trimethylsilyl ethers such as 107 a [114, 115]. A subsequent investigation revealed that cyclohexanone reacts with equivalent amounts of N-trimethylsilyldimefhylamine 463 in the presence of TMSOTf 20 at -30 °C to give the enol silyl ether 107 a, whereas reaction of cyclohexanone, benzaldehyde, and chlorodimethyl ether with 463 and TMSOTf 20 or TCS 14 at 1-20 °C afforded the iminium salts 547, 548, and 549 in high yield [116-118]. Analogously, N-trimethylsilylpyrrolidine 550 and N-trimethylsilylmorphoHne 294 convert aldehydes such as benzaldehyde, at ambient temperature in the presence... 

Secondary and tertiary aminoalkylbenzotriazoles (108) react with pyrrole, indole, and their N-methyl analogues under mild conditions in the presence of a Lewis acid to afford selectively the corresponding secondary or tertiary aminoalkyl derivatives (Scheme 22) (92T4971). A,A-Bis(alk-oxymethyl)amines can also be used to give secondary alkylamines, which operate via reactive iminium salts formed by treatment with trimethylsilyl chloride (90TL4229). 

Iminium salt (39) was readily obtained by treatment of aminal (38) with trimethylsilyl iodide194. 

The quatemization of imines to form mote reactive iminium salts has had limited synthetic utility since the activating substituents are often not easily removed. For this reason Lewis acids have been utilized to activate imines. MacLean and cowotkers have also addressed this problem and found that si-lylation of 3,4-dihydroisoquinolines and 3,4-dihydro-P-carbolines with trimethylsilyl triflate (TMS-OTf) provides a reactive, yet labile, silyl iminium salt which undergoes nucleophilic addition (Scheme 9). With this procedure, 3,4-dihydro-6,7-dialkoxyisoquinoline (54) was converted to amidine (56) in 98% yield. In the absence of TMS-OTf no addition occurred. 

Iminium salts bearing a labile trimethylsilyl group can be generated in situ and undergo nucleophilic addition (see Sections 1.12.4.2 and 1.12.7.3). Bis(trimethylsilyl)methoxymethylamine (75), for example, has been used as a formaldehyde equivalent for the preparation of primary amines. Cyclic imines, such as 3,4-dihydroquinolines, react with trimethylsilyl triflate (TMS-OTf) to provide reactive labile iminium salts (55), which condense with picoline anions. The addition of nonstabilized Grignard and organolithium reagents to acyclic aromatic ketimines and aldimines, however, is often not facilitated by the presence of TMS-OTf ... 

We had previously demonstrated that the stable (phosphino)(silyl)carbene 16 [31] reacts with trimethylsilyl trifhioromethanesulfonate to give the phosphino-substituted carbocation 17 [32], one of the very few stable phosphorus analogues of iminium salts [33]. The same synthetic strategy was adopted to prepare the diphosphino carbocation 17 or its valence isomer 2"Pa. A dichloro-methane solution of bis(diisopropylamino)phosphenium triflate was added at 0°C to a pentane solution of the carbene 16. The adduct 2 rPa [34] was isolated in 66% yield as extremely air sensitive white crystals (melting point 89-90°C) by recrystallization from tetrahydrofuran at -5°C (Scheme 13). The symmetrical three-membered ring structure was evident from the spectroscopic data (31P NMR s, S + 7.3 ppm, 13C NMR t, <549.6 ppm, ]PC 7.3 Hz, 29Si NMR t, <5-10.7 ppm Jpsz 10 Hz). 

The notion that silyl groups and alkyl groups belong to different categories in the polarity sense is now established. The cyclization of an iminium salt by alkyne participation is dramatically controlled by the terminal substituent of the acetylene in being a methyl or a trimethylsilyl group [123]. 

Enolizable A -trimethylsilylaldimines can be generated in situ by the addition of organolithium reagents to bis(trimethylsilyl)formamide. These undergo addition reactions with enolates to form 3-lactams. Phosphonium salts used in catalytic amounts promote the reaction between aryl aldimines and silylketene acetals to form 3-amino esters. Mannich bases with N-2-hydroxyethyl-N-methyl substitution are prepared by the reaction of the iminium salt synthon, 3-methyl-1,3-oxazolidine, with enol silanes in the presence of chloromethylsilanes. ... 

A reversal of the reaction sequence, that is the introduction of the C-3 substituent first, is accomplished by starting with l,4-bis(trimethylsilyl)indole. Reaction of this compound with the usual iminium salt gives the product shown in equation (36) in variable yields. Once again, we observe that iminium salts are not electrophilic enough to effect ipso substitution of a silyl residue. 

Iminium salts. N-Silyl derivatives of secondary amines react with carbonyl compounds afford a-siloxyamines. On further treatment with Me,SiCl, iminium chlorides are obtained. Trimethylsilyl triflate is superior to Me SiCl since it can induce the transformation in the cases of enolizable aldehydes. a-Chloro ethers are more reactive than carbonyl compounds, enabling the preparation of vinylogous Viehe salts. 

Trimethylsilyl chloride. 15, 89 16. Iminium salts. The preparation i moted by MesSiCl. 

Trimethylsilyl trifluoromethanesulphonate is useful as a removable group for the activation of imines by conversion into iminium salts the reagent forms complexes with... 

Azizi and Saidi" found that in situ prepared iminium salts 88 [obtained via the treatment of aldehyde with (trimethylsilyl)dialkylamine] are very effective electrophiles in MBH reactions and react with methyl acrylate in the presence of a catalytic amount of DBU at ambient temperature to afford the corresponding adducts 89, which can undergo conjugated addition with (trimethylsilyl) dialkylamines to give diamines 90 in good yields (Scheme 2.38). 

Iminium salt 463, prepared in situ from the aldehyde and an excess of the (trimethylsilyl)dialkylamine (2.5 equiv.) in a concentrated ethereal solution of... 

Functionalized iminium salts 24 can be readily prepared by a number of methods and allows the reaction to be conducted at lower temperatures and in non-protic solvents thus allowing other, more sensitive active hydrogen species to participate in this reaction. These methods can include alkylation of existing imine 22, cleavage of aminals 25 or hemi-aminals 23 with acetyl chloride or trimethylsilyl iodide/chloride, or in a direct fashion from carbonyl derivative 26 with amine salts or with trimethylsilyl amines. 

A new approach (or modification) using preformed iminium salts and imines has been developed. The imines react with enolate (especially trimethylsilyl ethers) to give p-amino ketones. In this reaction TiCl was used as a promoter.The general scheme for the synthesis of P-aminoketones is given in Scheme 154. 

Photosensitized oxidation of ethers causes a-deprotonation in the case of methyl neopentyl ether, CC and CH fragmentation compete. The resulting radicals react with the sensitizer, viz. an aromatic nitrile or an iminium salt. As in the case of amines, a-trimethylsilyl ethers and thioethers are excellent precursors for the corresponding a-heteroatom-substituted alkyl radicals.The potassium salts of a-oxoalkyl and a-thioalkyl carboxylic acid decarboxylate upon photosensitization by phthahm-ides. Alcohols undergo, as expected, a-CH and no OH deprotonation, via the radical cation formed with sensitizers such as tetracyanobenzene (see Equation 4.24) and pyrrohnium salts. ... 

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