Seleniranium Ion-Triggered Reactions: New Aspects of Friedel−Crafts and N-Detosylative Cyclizations

Abstract

Seleniranium ions at low temperatures (-90 to -78 °C) will initiate effective Friedel-Carfts cyclization if a suitably placed arene is allowed to react even when the arene is unactivated. These intermediates generated from N-aryl-N-tosylamides, undergo a novel, surprisingly efficient, detosylative cyclization to form 5-or 6-membered nitrogen heterocycles. A debenzylation route is preferred if both benzyl and tosyl groups are present in the substrate.rajanbabu.1@osu.edu. Supporting InformationAvailable: Full experimental details of synthesis of substrates, cyclzation reactions, spectroscopic and chromatographic data for characterization of compounds. This material is available free of charge via the Internet at http://pubs.acs.org NIH Public Access Author ManuscriptOrg Lett. Author manuscript; available in PMC 2010 July 2. Published in final edited form as:Org Lett. Electrophilic cyclizations of alkenyl carboxylic acids, alcohols, amines, amides and functionalized dienes initiated by seleniranium ions have been broadly applied for the syntheses of diverse heterocyclic and carbocyclic compounds. 1 Intermolecular additions of carbon nucleophiles including electron-rich aromatic derivatives to seleniranium ions are also known and these reactions have been developed to a level that it is now possible to carry out highly diastereoselective additions, using enantiopure selenium reagents. 2 Even though mechanistically related intramolecular additions of carbon nucleophiles to putative seleniranium ion intermediates were among the earliest examples of selenium-induced cyclizations, 3 subsequent developments in this area have been sporadic. In one such rare example, Déziel in 1998 4 reported that 4-arylbutenes (1) with electron-rich aryl groups undergo competitive carbocyclization when the reaction is conducted in a mixture of CH 2 Cl 2 and methanol (Scheme 1). The major side product, the β-methoxyselenide, 4, is readily converted into the cyclic product 3 by protic or Lewis acids. Formation of the methoxylated product 4 cannot be avoided since in the absence of methanol the reaction gave poor yields. Yet another limitation of this potentially powerful Friedel-Crafts cyclization is that non-activated arenes (e. g., phenyl) do not participate in this reaction. 3j,4,5a While searching for a general route to 1-methylenetetralin and analogous heterocyclic compounds in connection with our asymmetric hydrovinylation approach to 2,3-pyrrolidinoindoles (Figure 1), we decided to revisit this area. We expected the alkene-forming elimination reaction to be more facile via a selenide as compared to an iodide 5 or sulfide 6 arising from alternate cyclizations, especially for the synthesis of the more sensitive N and O containing heterocycles. Several novel observations that were made during the course of these investigations form the basis of this paper.Our studies (Eq 1, 2; Table 1) started with cyclizations of two prototypical substrates, but-3-enyl 4-methoxyphenyl ether (5a) and N-phenyl-N-(but-3-enyl)-4-toluenesulfonam...

Seleniranium and Telluriranium Salts

“…Selectedbondparameters [pm, 8]: C1ÀC11149.0(5), SeÀC1 207.7(3), SeÀC112 06.6(3),S e ÀC21 196.7(4); C1-Se-C114 2.17 (13), C1-C11-Se 69. 30(18), C11-C1-Se6 8.52 (18),C1-Se-C21 104.33 (16),C 11-Se-C21 103.73 (15).…”

“…Their intermediacy has been assumed, for example, in additions of ArSeCl to alkenes, in reactions of PhSeF equivalents NPSP/Et 3 N ⋅ 3 HF with olefins, and in selenolactonizations of unsaturated carboxylic acids . Recently their reactivity as RSe + carriers to other olefins has been reported, also in Friedel–Crafts cyclizations . Some information about their structure exists already .…”

“…[14] Recently their reactivity as RSe + carriers to other olefinsh as been reported, [7] also in Friedel-Crafts cyclizations. [15] Some information about their structure exists already. [16] Seleniranium ions have become important as central intermediates in asymmetric synthesis, in the addition of chiral seleno electrophiles Ar*SeX to prochiral olefins.…”

Stereoselective Haliranium, Thiiranium and Seleniranium Ion‐Triggered Friedel–Crafts‐Type Alkylations for Polyene Cyclizations

“…An efficient intramolecular seleniranium ion‐induced Friedel–Crafts‐type alkylation with tethered arene‐alkene substrates 121 – 123 , at low temperature (−78 °C), was developed by RajanBabu and co‐workers (Scheme ). PhSeBr as electrophilic selenylating agent and AgSbF 6 as Lewis acid in stoichiometric amounts are used to generate the seleniranium ion intermediate at low temperature (−78 °C) which is subsequently trapped by tethered arene π ‐nucleophiles to afford seleno‐functionalized chromans, tetralins and tetrahydroisoquinolines, respectively, in good to excellent yields (Scheme ).…”

Fe(OTf) 3 -catalysed Friedel–Crafts reaction of benzenoid arenes with α,β-unsaturated carbonyl compounds: easy access to 1,1-diarylalkanes

“…in standard protocols to free the by-products from arylated products. Three-member cyclic and reactive intermediates, such as epoxides [24][25][26][27][28][29][30][31][32], aziridines [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] and halonium ions [48][49][50][51][52][53], or other unstable intermediates [54][55][56][57][58] derived in situ from alkenes have also been found as potential electrophiles to afford functionalized Friedel-Crafts alkylated products. Another important strategy to afford 1,1diarylalkanes is Friedel-Crafts-type hydroarylation reaction with olefins.…”