Synthesis of Alkynylmethylidene‐benzoxasiloles through a Rhodium‐Catalyzed Cycloisomerization Involving 1,2‐Silicon and 1,3‐Carbon Migration

Abstract

It is shown that a cationic rhodium(I)/biphep complex catalyzes the cycloisomerization of 2-(alkynylsilylethynyl)phenols, leading to alkynylmethylidene-benzoxasiloles through concomitant silicon and carbon migration. This unprecedented cycloisomerization presumably proceeds via the formation of rhodium vinylidenes through 1,2-silicon migration, followed by 1,3-carbon (alkyne) migration via the formation of hypervalent silicon centers.

Rhodium‐Catalyzed Cascade Synthesis of Benzofuranylmethylidene‐Benzoxasiloles: Elucidating Reaction Mechanism and Efficient Solid‐State Fluorescence

“…The reaction of the thus obtained bis(2‐ethynylphenol)silane 1 a to give benzofuranylmethylidene‐benzoxasilole 2 a was examined as shown in Table (see Figure for ligand structures). In this study, active catalysts were prepared through removal of the cod ligand by hydrogenation, as reported in our previous report . We were pleased to find that the desired cascade cycloisomerization proceeds to give 2 a by using a cationic rhodium(I)/( S )‐binap complex (20 mol %) at 60 °C, whereas the yield of 2 a was moderate because of the formation of various unidentified by‐products as well as incomplete conversion of 1 a (entry 1).…”

“…Although there are several examples of transition‐metal‐catalyzed cycloisomerization reactions involving silicon or carbon migration, no transition‐metal‐catalyzed cycloisomerization reaction involving both the silicon and carbon migrations has been reported to date. Pleasingly, the thus obtained trimethylsilylethynylmethylidene‐benzoxasilole was transformed into benzofuranylmethylidene‐benzoxasilole 2 c , which showed high fluorescence quantum yields in both solution and powder states (Scheme , top) . However, the synthesis of 2 c requires many reaction steps (eight steps from commercially available chlorodiisopropylsilane), although different substituents can be introduced on two benzene rings of benzofuranylmethylidene‐benzoxasiloles 2 , and details of the mechanism of the 1,2‐silicon migration that proceeds under surprisingly mild conditions has not been elucidated.…”

“…Subsequently,w er eported the synthesis of alkynylmethylidene-benzoxasiloles [8] from 2-(alkynylsilylethynyl)phenols through 1,2-silicon migration to form rhodium vinylidenesf ollowed by 1,3-carbon (alkyne) migration (Scheme 1, top). [9] Althought here are several examples of transition-metal-catalyzed cycloisomerization reactions [10,11] involving silicon [12,13] or carbon migration, [14,15] no transitionmetal-catalyzed cycloisomerization reaction involving both the silicon andc arbon migrations has been reported to date. Pleasingly, the thus obtainedt rimethylsilylethynylmethylidenebenzoxasilole [16] was transformed into benzofuranylmethylidene-benzoxasilole 2c,w hich showed highf luorescenceq uantum yields in both solution and powder states (Scheme 1, top).…”

“…As previously reported, whereas reactions involving metal vinylidene formation through 1,2‐silicon migration typically require high temperature (≥90 °C), it is noteworthy that the above reaction proceeds under mild conditions (room temperature to 80 °C). Subsequently, we reported the synthesis of alkynylmethylidene‐benzoxasiloles from 2‐(alkynylsilylethynyl)phenols through 1,2‐silicon migration to form rhodium vinylidenes followed by 1,3‐carbon (alkyne) migration (Scheme , top) . Although there are several examples of transition‐metal‐catalyzed cycloisomerization reactions involving silicon or carbon migration, no transition‐metal‐catalyzed cycloisomerization reaction involving both the silicon and carbon migrations has been reported to date.…”

“…Pleasingly, the thus obtainedt rimethylsilylethynylmethylidenebenzoxasilole [16] was transformed into benzofuranylmethylidene-benzoxasilole 2c,w hich showed highf luorescenceq uantum yields in both solution and powder states (Scheme 1, top). [9] However, the synthesis of 2c requires many reaction steps (eights teps from commerciallya vailable chlorodiisopropylsilane), although different substituents can be introduced on two benzene rings of benzofuranylmethylidene-benzoxasiloles 2,a nd detailso ft he mechanismo ft he 1,2-silicon migration that proceeds under surprisingly mild conditions hasn ot been elucidated. Furthermore, although the introduction of bulky substituents on the silicon atom would suppress undesired intermolecular interactions in the solid state, no detailed reasonsh ave been presented to explain the observed efficient solid-state fluorescence of 2c.…”