Lewis Acid‐Assisted Photoinduced Intermolecular Coupling between Acylsilanes and Aldehydes: A Formal Cross Benzoin‐Type Condensation

Abstract

Intermolecular carbon-carbon bond-forming reaction between readily available acylsilanes and aldehydes was achieved under photoirradiation conditions with assistance of a catalytic amount of Lewis acid. Nucleophilic addition of photochemically generated siloxycarbenes to aldehydes followed by 1,4-silyl migration afforded synthetically useful α-siloxyketones. Electrophilic activation of aldehydes by Lewis acid is highly important to realize this reaction efficiently, otherwise the yield of the desired coupling products were significantly decreased. Noteworthy is that a formal cross benzoin-type reaction using acylsilanes was achieved under Lewis acidic conditions. This is the first example of Lewis acid-catalyzed reaction of photochemically generated siloxycarbenes with electrophiles.

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

“…Visible‐light irradiation of mixtures of alkanoylsilanes 1 and aldehydes in the presence of 1 mol % of the Ir complex 8 and 5 mol % of zinc iodide afforded the desired α‐siloxyketones 4 a – d in good yields. Again, yields of the products 4 a – d were comparable to those obtained under direct photoexcitation conditions …”

“…Yields in parentheses are those obtained under direct photoexcitation conditions (see ref. ). [b] 5 equiv of butyraldehyde were employed.…”

Preparation of Functionalized Acylsilanes by Diol Cleavage of Cyclic 1,2‐Dihydroxysilanes

“…Several reviews give a detailed and excellent overview of the versatile reactivity of acylsilanes . Furthermore, recent applications include photoredox‐catalyzed generation of acyl radicals and subsequent addition to electron‐deficient olefins, photoinduced acylsilane‐aldehyde coupling, enantioselective 1,2‐rearrangement to secondary alcohols, and stereoselective synthesis of allyl ethers from α,β‐unsaturated acylsilanes …”

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.…”