(Trialkylsilyl)vinylketenes:  Synthesis and Application as Diene Components in Diels−Alder Cycloadditions

Abstract

New strategies for the synthesis of (trialkylsilyl)vinylketenes ("TAS-vinylketenes") are described based on the photochemical Wolff rearrangement of R′-silyl-R′-diazo-R,β-unsaturated ketones and the 4π electrocyclic ring opening of cyclobutenones. These remarkably robust vinylketenes undergo highly regioselective [4 + 2] cycloadditions with reactive olefinic and acetylenic dienophiles to produce highly substituted cyclohexenones and phenols in which the ketene carbonyl dominates in controlling the regiochemical course of the reaction. The stereochemical course of these cycloadditions follows the Alder endo rule, as illustrated in the reaction of nitropropene with TASvinylketene 22.

Aryl Trialkylsilyl Ketenes: Acid‐Catalyzed Synthesis from 1‐Aryl‐2‐diazo‐2‐trialkylsilylethanones and Their Conversion into 3‐Silyl‐1‐silyloxyallenes

“…In order to avoid protonation/desilylation of the product by the acidic ammonium salt, Danheiser et al have used the less polar reaction medium diethyl ether/hexane (1:1), in which the ammonium salt is less soluble, and in fact obtained the silyldiazo ketones in higher yields. [10] The silylation of diazomethyl ketones 1a-c with trimethylsilyl triflate or triethylsilyl triflate and diisopropylethylamine in ether or diethyl ether/pentane mixtures provided the corresponding α-silyl α-diazo ketones 2a-e (Scheme 1). The synthesis of 2b and 2d by this method has been described before, [4,8] but in the present study we prepared only concentrated solutions of 2a-e that were suitable for further transformations (see below).…”

“…Usually, silyl ketenes are prepared from silyldiazo ketones such as 2 in an additional step by a photochemical [10,14,15] or transition-metal (copper [14] or rhodium [4] ) catalyzed Wolff rearrangement. Attempts to isolate silyldiazo ketone 2c met with another surprise (Scheme 4): diazo ketone 1b was trimethylsilylated in ether in the presence of NEtiPr 2 as usual, the precipitated ammonium salt was filtered off, and the solvent was removed completely.…”

Ketenes in Polymer‐Assisted Synthesis

“…This is particularly true of intramolecular ketene reactions, [32] and especially of silylated ketenes, for which the scope and the set of possible reactions has been extended during the last decade. [46,47] The author expresses his deepest gratitude to Professor Thomas T. Tidwell of the Department of Chemistry at the University of Toronto for useful comments and suggestions, as well as his thorough review of the manuscript.…”

Stereoselective Synthesis of Highly Substituted Cyclopentenones through [4+1] Annulations of Trialkylsilyl Vinyl Ketenes with α‐Benzotriazolyl Organolithium Compounds

“…For example, TAS vinyl ketenes participate as electron-rich diene components in Diels-Alder and hetero-Diels-Alder reactions leading to cyclohexenones, phenols, and oxygen and nitrogen heterocycles. [11] In the case of reactions with carbenoid reagents, addition initially furnishes dienolate intermediates, which are believed to undergo ionization and subsequent 4p electrocyclization to generate cyclopentenone rings (see below). TAS vinyl ketenes are readily available through several routes, including the photochemical Wolff rearrangement of a'-silyl-a'-diazo-a,b-unsaturated ketones used for the preparation of 1 a-c in this study.…”