Boron-Enabled Geometric Isomerization of Small Alkene Fragments via Selective Energy Transfer Catalysis

Abstract

The mammalian visual cycle epitomizes the importance of complex polyenes in biology. However, isomerization-based strategies to enable the sterodivergent construction of these important biomolecules from geometrically defined alkene linchpins remain conspicuously underdeveloped. Mitigating the thermodynamic constraints inherent to isomerization is further frustrated by the considerations of atom efficiency in idealized low molecular precursors. Herein, we report a general ambiphilic C3 scaffold that can be isomerized and bi-directionally extended. Predicated on highly efficient triplet energy transfer, the selective isomerization of β-15 borylacrylates is contingent on the participation of the boron p-orbital in the substrate chromophore. Rotation of the C(sp2)-B bond by 90° in the product renders re-excitation inefficient and endows directionality. This subtle stereoelectronic gating mechanism enables the stereocontrolled syntheses of well-defined retinoic acid derivatives.<br>

Selective E to Z isomerization of 1,3-Dienes Enabled by A Dinuclear Mechanism

“…2a). The structures of these isomers were assigned by 1 H-NMR and 13 C-NMR analyses, where the vicinal coupling constant J H3-H4 of the transoid-antifacial isomer (J H3-H4 = 10.4 Hz) is larger than that of cisoid-antifacial one (J H3-H4 = 6.8 Hz), and the η 1 -bound 13 C atom, which is located at the α-position with respect to the COOMe group, appeared at the high-field region (δ = 31 ppm for 3-transoidantifacial). The molecular structure of the major isomer 3transoid-antifacial was confirmed by X-ray structure analysis (Fig.…”

“…Despite its potential usefulness, however, the E to Z geometric isomerization of alkenes is not straightforward. Although photoirradiation of several alkenes leads to the E to Z isomerization 1,[11][12][13] , Z stereoselection under photoirradiation is sometimes incomplete, mainly because a photochemical E/Z ratio depends on a photostationary state derived from an excited state structure 1,14,15 . Moreover, it has been difficult to undergo E to Z geometric isomerization without photoirradiation, because an equilibrium of a reversible E/Z isomerization lies largely to the side of E-alkenes (Fig.…”

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

“…In a related context, Gilmour disclosed an elegant strategy for isomerizing alkenes via selective energy transfer ( Figure 22 C). 85 , 86 A boronate ester is introduced at the β-position of an α,β-unsaturated carbonyl substrate as a handle to induce directionality of the isomerization equilibrium toward the Z -isomer. The photosensitizer thioxanthone can engage in triplet energy transfer with an alkene, which causes cleavage of the π-bond by generating a delocalized biradical intermediate wherein C–C rotation is possible.…”

Coumarins by Direct Annulation: β‐Borylacrylates as Ambiphilic C3‐Synthons

“…β‐Borylacrylates ( II ) [12] were conceived as attractive coupling partners for a Suzuki–Miyaura/substitution cascade due to their ease of preparation and structural tenacity [13] . Moreover, the BPin unit in this modular acrylate platform serves as both a traceless linker for C(sp 2 )–C(sp 2 ) coupling, and as a chromophore antenna to enable inversion of the alkene geometry [14] via selective energy transfer catalysis [15] . In the absence of base‐mediated isomerisation, [16] regulating alkene geometry is necessary to facilitate lactonisation to access 3‐substituted coumarins.…”

“…Although attractive from the perspective of divergence, a solution was required to expand the substrate scope. To that end, a photocatalytic isomerisation of the β‐borylacrylate was performed under the auspices of selective energy transfer catalysis to generate substrates Z ‐20 and Z ‐21 [15] . Exposure to the general condition reported above enabled formation of the desired coumarins 24 and 25 (55 % and 90 %, respectively).…”