Geometric E→Z Isomerisation of Alkenyl Silanes by Selective Energy Transfer Catalysis: Stereodivergent Synthesis of Triarylethylenes via a Formal anti‐Metallometallation

Abstract

An efficient geometrical E!Zi somerisation of alkenyl silanes is disclosed via selective energy transfer using an inexpensive organic sensitiser.Characterised by operational simplicity,s hort reaction times (2 h), and broad substrate tolerance,the reaction displays high selectivity for trisubstituted systems (Z/E up to 95:5). In contrast to thermal activation, directionality results from deconjugation of the p-system in the Z-isomer due to A 1,3 -strain thereby inhibiting re-activation. The structural importance of the b-substituent logically prompted an investigation of mixed bis-nucleophiles (Si, Sn, B). These versatile linchpins also undergo facile isomerisation, therebye nabling af ormal anti-metallometallation. Mechanistic interrogation, supported by at heoretical investigation, is disclosed together with application of the products to the stereospecific synthesis of biologically relevant target structures.

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

“…4C). Comparison of the starting isomers Z-8 and Z-16 confirmed the 35 expected planar chromophore in which the boron group is conjugated (Fig. 3D top).…”

“…In a broader sense, this strategic deficiency in alkene isomerization technology has been prominently highlighted by Baran and co-workers in the context of accessing dienophiles for cycloaddition chemistry (9). Terpene biosynthesis predicated on the sequential elongation of 35 small (C5) alkene-containing linchpins such as DMAPP and IPP are conceptually attractive blueprints in reconciling this disparity ( Figure 1A) (10)(11)(12). Under the auspices of enzyme catalysis, these fragments are unified with incredible fidelity thereby enabling the synthesis of well-defined polyenes by either stereodivergent biosynthesis (e.g.…”

“…Seminal studies by Hammond and Arai are synonymous with stilbenes and styrene isomerization through the use of a photosensitizer (26)(27)(28)(29), and triplet energy transfer-enabled selective isomerization of retinal in 10 the visual cycle has been simulated by Walker and Radda (30). More recently, the Weaver group (31), and our own research group (32)(33)(34)(35) has disclosed operationally simple isomerization protocols harnessing functionally diverse styrenes with various structural motifs in the β-position ( Figure 1B). However, the presence of an aryl alkene fragment is a pre-condition to act as a suitable chromophore.…”

Zinc‐Catalyzed Hydrocyanation of Ynamides and Photoisomerization, a General and Selective Access to E and Z Trisubstituted α‐Enamidonitriles

“…Thus, in the course of time, according to Le Chatelier's principle, the isomerization equilibrium is shifted towards the Z isomer yet similar in energy to the E one. Interestingly, the difference of molar absorptivity between E ‐ 2 a and Z ‐ 2 a can potentially be rationalized thanks to the DFT geometry optimization (TPSS−D3/def2‐TZVP) [19] that shows significative differences between both isomers in the dihedral angle between the olefin C=C and the aromatic ring ( E ‐2 a =0.5° vs. Z ‐ 2 a =28°) and in the less extend in the one between the olefin and the N‐(CO) bond of the oxazolidinone ( E ‐2 a =25° vs. Z ‐ 2 a =36°). Consequently, these two configurations represent two very different conjugated systems ( E ‐ 2 a being significantly less twisted than Z ‐ 2 a ) which could explain the difference in light absorption observed (Figure 5).…”

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

“…Seminal studies by Hammond and Arai are synonymous with stilbenes and styrene isomerization through the use of a photosensitizer (26)(27)(28)(29), and triplet energy transfer-enabled selective isomerization of retinal in the visual cycle has been simulated by Walker and Radda (30). More recently, the Weaver group (31), and our own research group (32)(33)(34)(35) has disclosed operationally simple isomerization protocols harnessing functionally diverse styrenes with various structural motifs in the β-position ( Figure 1B). However, the presence of an aryl alkene fragment is a pre-condition to act as a suitable chromophore.…”