Efficient Generation and Synthetic Applications of Alkyl‐Substituted Siloxycarbenes: Suppression of Norrish‐Type Fragmentations of Alkanoylsilanes by Triplet Energy Transfer

Abstract

Acylsilanes have been knownt ou ndergo isomerizationt os iloxycarbenesu nder photoirradiation and the thus generated carbenes can be utilized for various synthetic reactions. But this carbene formation is not necessarily efficient with some alkanoylsilanes because Norrishtype fragmentations compete, which limit the synthetic utility of alkanoylsilanes as carbene precursors. In this study,g eneration of siloxycarbenes from alkanoylsilanes by visible-light-inducedenergyt ransfer was examined by using an Ir complex, [Ir{dF(CF 3 )ppy} 2 (dtbpy)]PF 6 ,a nd was successfully applied to the CÀCc oupling reactions with boronic esters or aldehydes. This methodology efficiently suppressed undesired Norrish-type reactions and broadened synthetic utility of alkanoylsilanes.Photocatalytic organic transformations have attracted significant attention in currents ynthetic chemistry.P hotoredox process involving ar adicali ntermediate is gaining importance in this field. [1] Another interesting aspect of photocatalytic reactions is triplet energy transfer processes which enableg eneration of triplet-excited states of organic substrates through energy transfer from photoexcited catalysts. [1b,c,e, 2] From as ynthetic point of view,t riplete nergy transfer is considered to have some benefits [3] as compared to direct photoexcitation of organic substrates:i )highly energetic singlet-excited states of substrates, whichs ometimes induce undesired reactions, can be avoided, and ii)ite nables excitationo fs ubstrates with longer wavelength light than the inherent absorption band of substrates.It has been known that triplet-excited states of acylsilanes 1 undergo a1 ,2-silyl migration to generatet ransienta nd nucleo-philic siloxycarbene intermediates 2 (Scheme 1a). [4] Since Brook and co-workers originally discoveredt his photoisomerization through reaction of siloxycarbene intermediates 2 with polar compounds such as alcohols over fifty years ago, [5] various synthetic reactions utilizingp hotochemically generated siloxycarbenes 2 have been developed by several groups [6] including us. [7] However, in contrast to the reactions of aroylsilanes (1, R 1 = aryl), the use of alkanoylsilanes (1,R 1 = alkyl) has been quite limited. [8] We have already reported intermolecularc oupling reactions of photochemically generated siloxycarbenes 2 with boronic esterso raldehydes under neutral or Lewis acidic conditions (Scheme 1b). [7] Duringt he course of these studies, we noticed that serious undesired photochemical processes competed with siloxycarbene formation in the reactions with somea lkanoylsilanes.F or example, UV irradiation( 365 nm) of am ixture of 4-phenylbutanoylsilane 1a [9] and boronic ester 5 gave styrene along with the desired coupling product 3a in low yield (Scheme 2). The formation of styrene clearly indicated that Norrish type II fragmentation [10,11] competed with the desired photochemical isomerization to siloxycarbene 2 from excited stateso falkanoylsilane 1a.Scheme1.a) Photochemical generation of siloxycarbenes. b) Photoi...

Silicon‐Derived Singlet Nucleophilic Carbene Reagents in Organic Synthesis

“…To circumvent this during the benzoin reaction outlined above, Kusama and co-workers have demonstrated that the desired carbene intermediates can be accessed via triplet-energy transfer using the photosensitiser [Ir(dF(CF 3 )ppy) 2 (dtbbpy)]PF 6 (which is commonly utilised to facilitate triplet-triplet energy transfer processes). [64] This strategy enables irradiation at a higher wavelength to be employed which suppressed Norrish-type fragmentations of alkanoyl silanes and afforded the desired product in higher yield (Scheme 32). Scheme 32.…”

“…To overcome this, the use of the photosensitiser and higher wavelengths to form the carbene via a triplet-triplet energy transfer process was successfully able to suppress the unwanted side reactions (Scheme 36). [64] Scheme 35. Nucleophilic addition of siloxy carbenes to boronates Scheme 36.…”

Visible Light Enabled Formal Cross Silyl Benzoin Reaction as an Access to α‐Hydroxyketones

“…[7,8] Recently, Kusama and coworkers have reported an elegant method on ZnI 2 assisted photoinduced coupling of acylsilanes with aldehydes with an irradiation of 500 W super-high-pressure Hg lamp. [9] In line with our continuing research interests in the reaction of aldehydes with reactive carbene intermediates, [10] we have unexpectedly uncovered an efficient protocol to prepare α-hydroxyketones through formal cross silyl benzoin reaction between acylsilanes with aldehydes under extremely mild conditions. Compared with Kusama's work, the reaction described in this work could proceed well without addition of any Lewis acid catalysts or photosensitizers.…”

Visible-Light-Induced Organocatalyzed [2+1] Cyclization of Alkynes and (Trifluoroacetyl)silanes

“…26 We proposed that the [2+1] cyclization reaction between acylsilane-derived carbenes and alkynes might provide a solution for the synthesis of cyclopropenols (Fig- ures 1b and 1c), because acylsilanes are known to be able to produce carbenes on excitation by light. [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] However, Bolm and co-workers found that the reactions of alkynes and acylsilanes afforded silylacylation products exclusively, with no [2+1] cyclization products (Figure 1b). 43 In addition, many less-reactive alkynes were tolerated in the intramolecular reactions, 29 but only activated internal alkynes were suitable substrates for the intermolecular reactions.…”

“…We commenced our study by investigating the conditions for the reaction of phenylacetylene ( 1a ) with the trifluoroacylsilane 2a under irradiation at 460 nm, but the conversion of acylsilane 2a was low, probably because of the low absorption of 2a at 460 nm (Scheme 1 , entry 1). Inspired by the work of the groups of Kusama 34 and Glorius 37 on the generation of siloxycarbenes through triplet-energy-transfer catalysis 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 from alkyl(acyl)silanes, we tested various photosensitizers. We found that 4CzIPN ( E T = 59.6 kcal/mol) 70,71 was a suitable catalyst, and the yield increased to 76% (entry 2).…”