X-ray Crystal Structures and Conformational Analysis of Bicyclo[5.3.1]undec-1(11)-enes:  Twisting versus Pyramidalization in Anti-Bredt Olefins

Abstract

The synthesis of several 9,11,11-trihalo[5.3.1]propellanes and their 4-dimethylsila analogues is described. They solvolyze under formation of the corresponding isomeric 7,9,11- trihalobicyclo[5.3.l]undec-1(11)-enes which are "anti-Bredt" olefins with a strained trans double bond in a bridged eight-membered ring; in the presence of nucleophiles such as water or ethanol, the corresponding 7-hydroxy or 7-ethoxy derivatives, respectively, are obtained. On the basis of the X-ray crystal structures of four of these compounds (1a, 9a, 15, 17b), the effect of strain and of the substitution pattern on the degree of twisting and pyramidalization of the double bond is discussed.

Structure and Reactivity of an Isolable Seven‐Membered‐Ring trans‐Alkene

“…The overall strain of the CC double bond is measured by ξ , the sum of the twisting and pyramidalization angles ( ϕ and χ , respectively). Conversely, η , the difference between ϕ and χ , indicates which of the two plays a larger role in relieving strain 13. Similar to trans ‐silacycloheptene 5 and the calculated structure of trans ‐cycloheptene 6 , trans ‐oxasilacycloheptene 3 relieves strain mostly by pyramidalization, but to a larger extent than the others.…”

“…The amount of strain in alkene 3 can be assessed by examining the deformation of the sp 2 ‐hybridized centers of the CC double bond (Figure 2). 13 This deformation can be quantified by several different parameters. The misalignment of the p orbitals that results from constraining a trans double bond in a medium‐sized ring is measured by both the torsion angle ( θ ) and the twisting angle ( ϕ ; Figure 2).…”

[4+2] Cycloadditions of Seven‐Membered‐Ring trans‐Alkenes: Decreasing Reactivity with Increasing Substitution of the Seven‐Membered Ring

“…It was hypothesized that fusing a second ring to the seven‐membered ring would lead to heightened reactivity. The fused ring system would limit the ability of the double bond to alleviate strain by altering bond and torsion angles , . Bicyclic trisubstituted alkenes 15 – 17 were synthesized to examine the effect of the fused ring and to determine how the size of the fused ring impacts reactivity.…”

“…Bicyclic alkene 17 did not react with diene 5 after 24 hours [Equation ]. The decreased reactivity of alkene 17 compared to alkenes 15 and 16 likely results because the larger fused ring allows the trans ‐alkene to alleviate strain by altering torsion and bond angles, thereby decreasing the distortion of the double bond , . Steric effects are also likely to be more significant with the additional seven‐membered ring, leading to decreased reactivity.…”

Strained organosilacyclic compounds: synthesis of anti-Bredt olefins and trans-dioxasilacyclooctenes

“…Because they contain a bridgehead double bond that is the equivalent of having a trans double bond in an eight-membered ring, these alkenes are anti-Bredt olefins 30 and are related to medium-ring trans-cycloalkenes. 19,20,31 Scheme 1 anti-Bredt olefins from vinyl spiro-epoxide.…”