Generation and Trapping of a Cage Annulated Vinylidenecarbene and Approaches to Its Cycloalkyne Isomer

Abstract

A novel cage-annulated (bis-homocubyl) vinylidenecarbene has been generated and successfully trapped without any intermediacy of its cycloalkyne isomer. The greater kinetic and thermodynamic stability of the vinylidenecarbene vis-à-vis its cycloalkyne isomer has been predicted by DFT B3LYP/6-31G* calculations. The calculated results suggest the prospects of the cycloalkyne becoming amenable for trapping, if generated under suitable experimental conditions, owing to the substantial kinetic energy barrier associated with its possible ring contraction via 1,2-shift to the vinylidenecarbene isomer and marginal ground state energy difference. However, all of our attempts to directly generate and trap the cycloalkyne yielded unsatisfactory results. Attempted generation and trapping of a C2-symmetric bis-vinylidenecarbene from a bis-vinylidenedibromide met with unexpected failure.

Generation and Capture of Both Sides of the 7-Norbornylidene — Bicyclo[2.2.2]oct-2-yne Fritsch–Buttenberg–Wiechell Rearrangement

“…The combined ltrates were concentrated in vacuo, then suspended in 10% aqueous sodium thiosulfate (185 mL) and stirred 1 h. The mixture was then extracted with diethyl ether (3 x 100 mL), and the combined organic phases were dried (Na 2 SO 4 ), ltered, and concentrated in vacuo to afford 7-norbornol (21) as an orange waxy solid (2.20 g, 98%): 1 7-norbornone (22): Following the procedure of Rosenkoetter et al, 49 to a suspension of pyridinium chlorochromate (5.39 g, 25 mmol) in dichloromethane (300 mL) was added a solution of 7-norbornol (21, 1.12 g, 10.0 mmol) in dichloromethane (150 mL). The reaction mixture was stirred until 7-norbornol (21) was completely consumed, as determined by GC-MS. Diethyl ether (150 mL) was then added and the mixture was stirred 30 min, then transferred to a separatory funnel and extracted with diethyl ether (150 mL). The organic phase was washed with water (3 x 400 mL), dried (Na 2 SO 4 ), ltered, and concentrated in vacuo.…”

“…10,2 The generation of alkynes via FBW rearrangements of alkylidene carbenes is generally considered to depend upon a thermodynamic equilibrium that favors the target alkyne over the corresponding carbene. [17][18][19][20][21] While this is the case for most monocyclic alkynes, 14,9,8 with the exceptions of cyclobutyne 17 and cycloheptyne, [22][23] the additional geometric constriction present in many polycyclic alkynes can potentially tip equilibrium in favor the carbene. In such cases, even direct synthesis of the strained alkyne will result in rearrangement to the alkylidene carbene through the reverse 1,2-shift, known as the Roger Brown rearrangement.…”

“…17,[24][25] Attempts to detect a number of polycyclic alkynes, including 6, 8, and 10, through the FBW rearrangement of their respective alkylidene carbenes have been unsuccessful, purportedly due to unfavorable differences in the relative stabilities of the two chemical species (Scheme 2). [18][19][20][21] A number of experimental factors affect the successful detection of a cycloalkyne that is in equilibrium with an alkylidenecarbene, factors that can confound subsequent interpretations of the relative thermodynamic stabilities of these intermediates. Preparation of alkylidene carbenes for the generation of polycyclic alkynes has primarily involved the deprotonation of bromoethylenecycloalkanes, [14][15]26,16 or the lithiation of dibromomethylenecycloalkanes, 23,[18][19][20][21] both of which initially generate an alkylidenecarbenoid species.…”

“…[18][19][20][21] A number of experimental factors affect the successful detection of a cycloalkyne that is in equilibrium with an alkylidenecarbene, factors that can confound subsequent interpretations of the relative thermodynamic stabilities of these intermediates. Preparation of alkylidene carbenes for the generation of polycyclic alkynes has primarily involved the deprotonation of bromoethylenecycloalkanes, [14][15]26,16 or the lithiation of dibromomethylenecycloalkanes, 23,[18][19][20][21] both of which initially generate an alkylidenecarbenoid species. Alkylidenecarbenoids exhibit different patterns of reactivity compared to free alkylidenecarbenes, 27 and can react through unique pathways 3 that involve dimerization, 28 decomposition, [29][30][31] and FBW rearrangements.…”

Ring Expansion of Alkylidenecarbenes Derived from Lactams, Lactones, and Thiolactones into Strained Heterocyclic Alkynes: A Theoretical Study

“…Alkylidenecarbenes ( 1 ), and surrogate carbenoids ( 2 ), often undergo 1,2-shifts to furnish alkynes ( 3 ) by a process commonly referred to as the Fritsch-Buttenberg-Wiechell (FBW) rearrangement (Scheme 1a) [1,2,3,4,5,6]. By extending this method to cyclic alkylidenecarbenes ( 4 ) and carbenoids ( 5 ), in which the formally sp 2 -hybridized carbon connected to the carbene center is part of a ring, strained cycloalkynes ( 6 ) have also been generated (Scheme 1b) [4,7,8,9]. In recent years, our laboratory has reported that cyclopropanated phenanthrene systems such as 7 [10,11,12,13] and 8 [14,15] can serve as photochemical precursors to 1 and 4 respectively (Scheme 2), and thus provide light-induced routes to the corresponding linear and cyclic alkynes.…”