Allenylketenes: Versatile Substrates in Nucleophilic, Electrophilic, and Cycloaddition Reactions

Abstract

4-Methylene-2-trimethylsilyl-3-phenylcyclobutenones 3 are obtained from the corresponding cyclobutenedione 7 by Tebbe and Peterson methylenation reactions, and upon photolysis give 3-phenyl substituted allenylketenes R 1 CH=C=C(Ph)C(SiMe 3 )=C=O 4. These are more reactive in ring closure back to 3 than are the 3-trimethylsilyl analogs 1, as predicted on the basis of molecular orbital calculations. Reactions of allenylketenes 1 and 4 include addition of H 2 O followed by cyclization to form butenolides 13, addition of aniline to give allenyl carboxamide 14 and lactam 15, addition of CF 3 CO 2 H to form allenyl carboxylic acid 17, addition of Br 2 in a 1,4-fashion to give penta-2,4-dienoic acid derivatives 18, and reaction with m-chloroperbenzoic acid to give the unique allenyl acylsilane 23.[4+1]-Cycloaddition reactions of 1 and 4 with Me 3 SiCHN 2 give methylenecyclopentenones 24 and 25 while acetaldehyde undergoes BF 3 ∑OEt 2 catalyzed [2+2] cycloadditions forming b-lactones.[4+2] Cycloadditions occur with benzaldehyde to form the highly crowded exomethylene pentenolactone 29, and with TCNE forming methylenecyclohexenones 31. IntroductionWe have reported the formation of stabilized and persistent allenylketenes 1 from photolysis of 2,3-bis(trimethylsilyl)-substituted methylenecyclobutenones 2 (Eq. 1). 1a The allenylketenes were isolated as long-lived species at room temperature, but at thermal equilibrium both 1 and 2 were present in significant amounts. The stabilization of these ketenes by silyl substituents is a manifestation of the b-silicon effect, and arises from s-p hyperconjugative electron donation of the Si-C bond to the electron-deficient p-orbital of the ketene carbonyl. This stabilization has been quantified using ab initio molecular orbital calculations, and for different substituents is correlated with group electronegativities, so that the less electronegative groups are the more stabilizing. 1b,c Stabilization parameters SE for substituents on ketenes 1b,c and allenes 1c relative to hydrogen were derived from ab initio molecular orbital calculations and isodesmic reactions (Eq 2) and together with the calculated relative energies of the parent compounds give accurate predictions of the relative amounts of 1 and 2 present at thermal equilibrium. 1a The predicted preference for the depicted anti-planar geometry of 1a (R 1 = Ph) has been confirmed by X-ray crystallography. 1aAllenylketenes 1 were shown to react with nucleophilic water to form lactones (Eq. 3) 1a and with nitroxyl radicals with oxygen atom transfer, to give butenolides (Eq. 4), 1dand have considerable promise as synthetic reagents. The related 1,2-bis(ketenes), 2 vinylallenes, 3 and 1,2-bis(allenes) 4 have also attracted a great deal of recent attention in synthetic and mechanistic study. Equation 3Equation 4 Danheiser and co-workers 5 have prepared a-silylalkenylketenes 5 by Wolff rearrangements, by dehydrochlorinations, and by cyclobutenone ring opening (Eq. 5). Unsubstituted vinylketene is calculated to be of approximately equa...

Stereoselective [4 + 1] Annulation Reactions with Silyl Vinylketenes Derived from Fischer Carbene Complexes

“…In contrast, silyl vinylketenes have been demonstrated as viable precursors to cyclopentenones 6 and other cycloadducts . The silyl substituent provides significant stabilization to the ketene moiety and suppresses the dimerization pathway, thereby facilitating their participation in [4 + 1] cycloaddition reactions with carbenoid reagents.…”

Ketenes in Polymer‐Assisted Synthesis

“…For instance, the photochemical ring-opening of methylenecyclobutenones is an efficient method for the generation of allenyl ketenes. [37] Using MPEGOH as a support, the Wittig reagent 77 was treated with 3,4-bis(trimethylsilyl)cyclobut-3-ene-1,2-dione (78) to give the polymer-bound methylenecyclobutenone 79 (Scheme 17). [15,36] Photolytic ringopening gave the stable polymer-bound silylated allenylketene 80.…”

Tebbe’s Reagent

“…Tebbe's reagent (1) is an organometallic compound and has found diverse applications in organic synthesis such as methylenation of carbonyl compounds, 1 synthesis of C-glycosides, 2 1,6-disaccharides 3 and in the synthesis of intermediates, for example vinyl silanes 4 and allenylketenes. 5 It is readily prepared by reacting titanocene dichloride and trimethylaluminum in toluene at r.t. (Scheme 1). 1 When Tebbe's reagent is treated with a Lewis base, for example pyridine or THF, a highly reac-tive titanocene methylidene is generated.…”

“…4 (H) Allenyl ketene is synthesized from cyclobutenedione by Tebbe methylenation. 5 The allenyl ketene can then undergo different nucleophilic and electrophilic additions and cycloaddition reactions.…”

“…5 It is readily prepared by reacting titanocene dichloride and trimethylaluminum in toluene at r.t. (Scheme 1).…”