The acetylenic iminium salts [(c-C3H5)C≡C-C(Ar)=N+Me2]OTf, Ar = phenyl (1a) or 2- thienyl (1b), both react in different ways with three mesoionic münchnones, namely 3-methyl-1,3-oxazolium-5-olate (2), 3-methyl-2-phenyl-1,3-oxazolium-5-olate (6) and 3-methyl-2-phenyl-4- trifluoroacetyl-1,3-oxazolium-5-olate (9). With 2, a [3+2] cycloaddition reaction followed by extrusion of CO2 yields pyrroles 5a, b. In the case of 6, the new münchnones 7a, b are obtained which result from an electrophilic substitution at C-4 by the acetylenic iminium cation. In contrast to 1a, b, the 4,4-but-2-yne 1-iminium salt 1c reacts with münchnone 6 to form pyrrole 8. Finally, the reaction of 1a, b and 9 affords (6-oxo-6H-pyran-3-yl)methylene iminium salts 10a, b under microwave heating conditions. The structures of pyrrole 5a, münchnone 7a, and trifluoromethyl-substituted pyranone 10a were established by single-crystal X-ray diffraction analysis.
The ambident cations of propyne iminium trifluoromethanesulfonates (triflates) 1 react with triphenyl-or tributylphosphane exclusively in a conjugate addition to form highly moisturesensitive [3-(dialkylamino)allenyl]phosphonium triflates 2 and 4. Allene 2f, bearing a methyl group at the enamine α-position, rearranges to form (3-morpholinobuta-1,3-dienyl)phosphonium salt 3 under the reaction conditions. The rather electron-rich allenes are easily protonated by triflic acid to form dicationic 3-phosphonio-prop-2-ene iminium bis(triflates) 5, one of which (5a) was characterized by X-ray crystal structure determination. Hydrolysis of the allenes yields (3-oxoprop-1-enyl)phosphonium triflates 6 (-+ PPh 3 ) or 8 (-+ PBu 3 ). Salts 6 can also be prepared by simultaneous addition of PPh 3 and triflic acid to acetylenic ketones. Salts 2 are converted thermally into 1,2,3,5-tetrasubstituted pyrroles 10; this reaction can be used for a PPh 3 -mediated conversion of propyne iminium salts 1 into pyrroles 10 without isolation of the allenylphosphonium salt intermediate.
Acetylenic iminium salts (propyne iminium salts) react with cyclic enol ethers in a thermal [2+2] cycloaddition reaction to afford condensed cyclobutene iminium salts. With acyclic enol ethers, a cycloaddition/electrocyclic ring-opening sequence occurs to give (1-alkoxybuta-1,3-dien-2-yl)methane iminium salts as mixtures of diastereoisomers. An analogous scenario is observed with 5-phenyl-2-vinylfuran as the nucleophilic olefinic component. With 1-methoxy-3-trimethylsilyloxy-1,3-butadiene, propyne iminium salts undergo a [4+2] cycloaddition. For all reactions, traces of acid must be quenched with solid K 2 CO 3 or CaO to avoid immediate polymerization of the enol ether.The [2+2] cycloaddition strategy appears to provide an obvious route to carbocyclic four-membered rings from two substrates containing carbon-carbon π bonds, and, indeed, this approach has been used to prepare a variety of substituted cyclobutanes 1 and cyclobutenes. 2 However, as the thermally induced concerted [ π 2 s + π 2 s ] cycloaddition is not allowed based on orbital symmetry conservation, 3 and the allowed [ π 2 s + π 2 a ] geometry is usually prohibited for steric reasons, the scope of this reaction type is limited to certain types of alkenes and alkynes; in particular, those that stabilize radical or zwitterionic reaction intermediates. 1a Thus, it is well known that electrophilic and nucleophilic alkenes combine to form cyclobutanes with rather moderate activation energies. Mechanistic investigations and product analyses by Huisgen and co-workers on the reaction of enol ethers with the extremely electron-poor olefins tetracyanoethene 4 and 1,2-bis(trifluoromethyl)ethene-1,2-dicarbonitrile 5 have provided compelling evidence for the two-step reaction course via 1,4-zwitterions. The synthetically useful synthesis of donor/acceptor-substituted cyclobutanes from silyl enol ethers (or dihydrofuran) and α,β-unsaturated esters was performed effectively under catalysis by Lewis acids or bis(triflyl)amide. 6,7
Propyne iminium triflates 1–6 react as dipolarophiles in thermal [3 + 2]-cycloadditions with sufficiently electron rich organoazides to form 1,4,5-trisubstituted 1,2,3-triazoles with iminium functionalization. The reactions require rather strong thermal activation, but can be accelerated by microwave irradiation. The regioselectivity of the cycloaddition at the internal acetylenic bond of 3-cyclopropylpropyne and 3-arylpropyne iminium ions (1–3 and 4, respectively) is very high, but is lowered in the presence of sterically demanding substituents at the opposite end of the iminium-substituted C,C triple bond. The iminium-functionalized triazoles can easily be transformed into neutral compounds; herein reported is the formation of triazolyl ketones 10 by hydrolysis and of tertiary triazolyldimethyl amines 12 by LiAlH4 reduction. When the reduction is performed with sodium boranate or sodium cyanoboranate, amine–borane complexes 15 and 16 are obtained.
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