Copper-Catalyzed Ring Opening of [1.1.1]Propellane with Alkynes: Synthesis of Exocyclic Allenic Cyclobutanes

Abstract

Despite the long history and interesting properties of propellanes, these compounds still have tremendous potential to be exploited in synthetic organic chemistry. Herein we disclose an experimentally simple procedure to achieve cyclobutane-containing allenes and alkynes through a copper-catalyzed ring opening of [1.1.1]­propellane and subsequent reaction with ethynes.

Rationalizing the diverse reactivity of [1.1.1]propellane through σ–π-delocalization

“…The reactions of 1 with cations is an underexplored eld; despite the observation of rapid protonation by acetic acid, 30 few cationpromoted reactions have been reported. 31,32 Cationic bicyclo [1.1.1]pentyl adducts are known to rapidly fragment to bicyclo [1.1.0]butyl-1-carbinyl cations (Fig. 7a), 76 which suggests that a signicant change in the electronics of the cage occurs as addition takes place.…”

“…A third reactivity mode of 1 involves facile cationic addition, a process that fragments the cage to an exo-methylene cyclobutane. [30][31][32] This diverse 'omniphilic' reactivity has traditionally been ascribed to the high strain energy of [1.1.1]propellane (total strain energy $100 kcal mol À1 ), 33 albeit only $30 kcal mol À1 of this strain is thought to be released on cleavage of the interbridgehead C1-C3 bond. 2,34 Nonetheless, this omniphilicity is not generally observed with other ring-opening reactions where relief of ring strain is a driving force (such as in cyclopropane, E strain ¼ $28 kcal mol À1 ), 35 which could suggest other factors govern the reactivity of 1.…”

Copper‐Catalyzed Enantioselective Sonogashira Type Coupling of Alkynes with α‐Bromoamides

“…As shown in Scheme 3 [Eq. (10)], the desilylation [24] by catalytic H 2 SiF 6 occurred smoothly to give free terminal alkyne e1 with maintenance of enantiopurity. Treatment of c2 with catalytic TBD led to a strained bridged polycyclic lactam f1, [25] however, in racemic form [Eq.…”

Copper‐Catalyzed Enantioselective Sonogashira Type Coupling of Alkynes with α‐Bromoamides

“…As shown in Scheme 3 [Eq. (10)], the desilylation [24] by catalytic H 2 SiF 6 occurred smoothly to give free terminal alkyne e1 with maintenance of enantiopurity. Treatment of c2 with catalytic TBD led to a strained bridged polycyclic lactam f1, [25] however, in racemic form [Eq.…”