Triflate Moiety Research Articles

An ab Initio MO Study of Silver Triflate Complexation in [2.2.1]Cyclophane π-Prismands

Ab initio Hartree−Fock and DFT MO calculations have been used to study the conformations of six [2.2.1]cyclophane π-prismands and the formation of their π-complexes with silver triflate (AgSO3CF3). The lowest energy cyclophane conformations and their silver triflate π-complexes have been calculated with HF/3-21G* and B3LYP/3-21G* levels of theory. The nature of bonding in silver triflate π-complexes has been studied with natural bond orbital analysis (NBO). Energies of the calculated cyclophanes and complexes, together with the formation energies of those complexes, have also been discussed. The results have been compared to available X-ray crystal structures and also to results of the previously published ab initio calculations of slightly larger [2.2.2]cyclophanes and their silver complexes. Calculated and experimental X-ray structures agreed reasonably well, given the rather small 3-21G* basis set. Changes in the bonding between the ligand and Ag+ ion were observed, due to an enhanced bonding of the triflate moiety over the silver atom ion in the calculated models. In these complexes the silver ion is bonded to the cyclophane cavity by the bonds formed by σ donation and d−π* back-donation between the silver ion and the hydrocarbon skeleton resulting in variable bonding modes from η1 to η6 per aromatic ring depending on the ligand conformation. In this case the σ donation from the hydrocarbon to the silver ion is the main contribution to the metal−cyclophane bonding. Bonding of the silver ion to the present π-prismands relates by strength to a single moderately strong hydrogen bond and varies between 25 and 50 kJ/mol. Due to the smaller cavity, the more rigid hydrocarbon skeleton, and the presence of the triflate moiety [2.2.1]cyclophanes form different types of interactions with silver ion when compared to previously published [2.2.2]cyclophane π-prismand complexes with Ag+ ion.

Bismuth(III) Trifluoromethanesulfonate: A Chameleon Catalyst for the Friedel–Crafts Acylation

A mechanism for acylations catalyzed by bismuth(III) triflate (1) is proposed in the case of the benzoylation of benzene, toluene, and chlorobenzene. With Bz2O as a reagent, 1 acts as a Lewis acid and allows the benzoylation of toluene. It is almost completely recovered after the reaction. With BzCl, 1 promotes an exchange reaction which generates BzOTf, which is the active species of the benzoylation. In this latter case, the reaction leads to the formation of TfOH which finally reacts with BiCl3 to partially regenerate 1. The power of the Bz2O/1 system is less than that of BzCl/1, which allows not only the benzoylation of toluene but also that of benzene and deactivated chlorobenzene. The activity of 1 is much higher than that of other metallic triflates previously reported, and is comparable with that of TfOH, however it also has the advantage that the triflate moieties are more easily recoverable.

Enantiopure N-Acyldihydropyridones as Synthetic Intermediates: Asymmetric Syntheses of Indolizidine Alkaloids (-)-205A, (-)-207A, and (-)-235B.

Concise asymmetric syntheses of indolizidine alkaloids (-)-205A, (-)-207A, and (-)-235B were accomplished with a high degree of stereocontrol in eleven steps. Addition of 4-(1-butenyl)magnesium bromide to 1-acylpyridinium salt 5, prepared in situ from 4-methoxy-3-(triisopropylsilyl)pyridine and the chloroformate of (+)-trans-2-(alpha-cumyl)cyclohexanol, gave a 91% yield of diastereomerically pure dihydropyridone 6. Oxidative cleavage of 6 and subsequent reduction provided alcohol 7 in 81% yield. Removal of the chiral auxilliary and TIPS group (NaOMe; 10% HCl), N-acylation with BnOCOCCl, and treatment with NCS/Ph(3)P gave chloride 10. Methylation at C-3, copper-mediated conjugate addition of 4-(benzyloxy)butylmagnesium bromide, and vinyl triflate formation provided 13 in a stereoselective fashion. Catalytic reduction of the vinyl triflate moiety, simultaneous cleavage of the benzyl ether and Cbz groups, and cyclization to give amino alcohol 14 was effected via a one-pot reaction. Oxidation of 14 with the Dess-Martin reagent gave a 97% yield of amino aldehyde 4. Synthesis of each of the three title alkaloids was accomplished in one step from 4. The Seyferth-Gilbert reaction provided a 41% yield of (-)-205A. The appropriate Wittig olefination of 4 gave indolizidines (-)-207A and (-)-235B in 70% and 86% yield, respectively.

Synthesis and Further Reactivity of Functionalized Lactam-Derived Enol Triflates.

Pyrrolidinone- and piperidinone-derived enol triflates 2 were prepared in high yield (60-97%) from the corresponding lactams 1 using KHMDS and N-(5-chloro-2-pyridyl)triflimide. A structure-stability study on the less stable pyrrolidinone-derived triflates revealed that an N-tosyl group is essential, and an alpha-ethoxy substituent enhances thermal stability. Substituents at the 3- and 4-position are tolerated. Substitution of the triflate moiety by a wide variety of functional groups was achieved under mild conditions via metal-mediated reactions. Although cuprate couplings proceeded in only moderate yields, several palladium-catalyzed reactions gave good yields of interesting molecules for further synthetic operations (for example, Stille coupling with vinylstannanes, cross-coupling with arylzincs, and carbonylation processes). Preparation of the first enantiopure lactam-derived enol triflate 15 (from (S)-pyroglutamic acid) is described. Enamide hydrogenation of derivative 17 allowed the synthesis of a proline analogue 18 in excellent yield and diastereoselectivity (86% de).

Synthesis of 2-substituted 2′-deoxyguanosines and 6-O-allylguanines via the activation of C-2 by a trifluoromethanesulfonate group

A new general synthesis of 2-substituted 2′-deoxyguanosine and 6-O-allylguanine analogues is reported. 2-O-Trifluoromethylsulfonyl-6-O-allyl-3′,5 ′-bis-O-(tert-butyldimethylsilyl)-2′- deoxyxanthosine 8 can be converted into a 2-substituted 2′-deoxyguanosine analogue by substitution of the triflate (trifluoromethanesulfonate) moiety using a selected nucleophile, followed by deprotection. Therefore, a 2′-deoxyguanosine may overall be converted into a 2-substituted 2′-hypoxanthosine analogue in seven steps. Similar methodology has been used to synthesize 2-substituted 6-O-allylguanines which are of particular interest as potential resistance-modifying agents in cancer chemotherapy.

Synthesis of 2-exo-Methylenepenam by Reductive 1,2-Elimination and S–S Bond Fission in a PbBr2/Al Bimetal System

Abstract A convenient synthesis of 2-exo-methylenepenam was performed by reductive elimination of a 3,4-disubstituted chlorine atom(s) and/or a triflate moiety and S-S bond fission of the phenylsulfonylthio group of p-methoxybenzyl 3,4-dichloro or 4-chloro-3-(trifluoromethylsulfonyloxy)-2-[4-(phenylsulfonylthio)-2-oxoazetidin-1-yl]-2-butenoate derived from penicillin G in a lead(II) bromide/aluminum bimetal redox system.

Preparation and degradation of polysilylenes

Mechanistic aspects of preparation and degradation of polysilylenes (polysilanes) are discussed. Reductive coupling of disubstituted dichlorosilanes at ambient temperatures in the presence of ultrasound leads to monomodal polymers with relatively narrow molecular weight distributions (Mw/Mn from 1.2 to 1.5) and relatively high molecular weights (Mn from 50,000 to 100,000). Ring-opening polymerization of 1,2,3,4,-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane initiated with carbanions and silyl anions provides polymers with molecular weights from 10,000 to 100,000 and gives potential possibility of the microstructure control. The dearylation of phenyl containing polysilylenes with triflic acid provides polymers with strong electrophilic silyl triflate moieties. They can react with any nucleophiles such as alcohols, amines, carbanions, organometallics, etc., and produce various functional polysilylenes. Synthesis and solid-state transitions in random copolysilylenes are discussed. Thermal, mechanical, and chemical degradation of polysilylenes is described.