Three-Component Coupling of a Silylene, Aldehydes and Electron-Deficient Acetylenes via 1,3-Dipolar Cycloaddition

Abstract

Three-component coupling reactions of silylene, aldehydes and electron-deficient acetylenes proceed via 1,3-dipolar cycloaddition of silacarbonyl ylides with acetylenic dipolarophiles.The three-component coupling reaction 1 is a versatile method in synthetic organic chemistry because complex molecules can be constructed from readily available starting materials in a one-step operation. Among the methods studied, reactions involving 1,3-dipoles are privileged routes leading to various interesting and useful heterocyclic compounds. 2 Although silylenes are powerful species and afford unique compounds embedded in silicon units, 3 there are no examples of three-component coupling through 1,3-dipoles derived from silylenes. Woerpel and co-workers recently developed silylene chemistry using silacyclopropanes and silver catalysts, producing oxasilacyclopentane from the reaction of silacyclopropanes, aldehydes, and olefins. The mechanism of the reaction involved the copper-or zinc-catalyzed insertion of aldehydes into the C-Si bond of siliranes, generated from silacyclopropanes and olefins, catalyzed by silver salts. 4 On the other hand, previous studies described the novel cycloaddition of silacarbonyl ylides, generated from silylene and carbonyl compounds. 5 In these studies, dipolarophiles reacting with silacarbonyl ylides, were the aldehyde components for generation of the ylide (2:1 cycloaddition), 5a or were the moiety (substituent) of the carbonyl compounds (intramolecular cycloaddition). 5b As preliminary findings, the silacarbonyl ylide generated from a silylene with crotonaldehyde was trapped by the C=C bond of the aldehyde. 5a This result prompted the current investigation of the three-component coupling reaction of silylene, aldehydes and electron-deficient carbon-carbon unsaturated compounds (Scheme 1). Scheme 1The screening of a variety of olefinic dipolarophiles for use in the reaction did not lead to the desired cycloadducts. Therefore, a first experiment was performed to examine the ability of an acetylenic compound to act as a dipolarophile. A solution of trisilane 6 1 and but-3-yn-2-one (20 equiv) in diethyl ether was irradiated at -57°C for 20 hours in a cold bath under a low pressure mercury lamp. The resulting mixture was separated on a silica gel column to give 1-oxa-2-silacyclopentene 2 in a yield of 57% as a mixture of regioisomers. The structures of the cycloadducts were characterized by spectral data, including NOE measurement, as 1: 2 cycloadducts of bismesitylsilylene and the acetylenic ketone (Scheme 2). The formation of the cycloadduct prompted a further examination of the three-component coupling reaction of silylene, aldehyde, and acetylenic dipolarophiles. Scheme 2When a solution of trisilane 1, pivaldehyde (2 equiv), and methyl propiolate (5 equiv) in hexane was photolyzed at -57°C for 20 hours, three-component cycloadducts 3a and 3b were successfully obtained in a yield of 41%, as a mixture of regioisomers. Although the reaction did not proceed at all in dichloromethane, ethers were fo...

Diverse Reactivity of Hypersilylsilylene with Boranes and Three-Component Reactions with Aldehyde and HBpin

“…Minakata et al reported the photolysis of bis(trimethylsilyl)bis mesityl silane (SiMe 3 ) 2 SiMes 2 with aldehyde and an electron deficient alkyne to form cycloadducts, which were formed via in situ generation of silylene, Mes 2 Si:. 20 The reaction is similar to those reported by Calad and Woerpel, who reported the silver-catalyzed transfer of the silylene moiety to α,βunsaturated esters to access oxasila-cyclopentenes. 21 However, stable silylene involved in the multicomponent reaction is not known.…”

“…The three-component methodology is very attractive in organic chemistry as important complex molecules can be made using commercially available precursors in a single step. Minakata et al reported the photolysis of bis­(trimethylsilyl)­bis mesityl silane (SiMe 3 ) 2 SiMes 2 with aldehyde and an electron deficient alkyne to form cycloadducts, which were formed via in situ generation of silylene, Mes 2 Si: . The reaction is similar to those reported by Calad and Woerpel, who reported the silver-catalyzed transfer of the silylene moiety to α,β-unsaturated esters to access oxasila-cyclopentenes .…”

Reactions of an Isolable Dialkylsilylene with Ketones

“…Although 2 , 7 , and 10 are all 1:2 cycloadducts, the ring structure of 7 and 10 is isomeric to that of 2 . Recently, a number of interesting and useful synthetic reactions via the addition of dimesitylsilylene and di- tert -butylsilylene to various carbonyl compounds have been explored.…”

Siloxy-Substituted Cyclopentadiene Showing Aggregation-Enhanced Emission: An Application of Cycloaddition of Isolable Dialkylsilylene

“…In this aspect, cycloaddition of silylenes (silicon version of carbenes) may have an advantage for the synthesis of the AEE active molecules, because dialkylsilylene 1 (R H 2 Si:, Chart ) is now available as an isolable but reactive compound toward aromatic ketones and other substrates without transition metal catalysts. In addition, the bulky dialkylsilyl group may serve as a good isolator between aromatic cores of the silylene-incorporated AEE molecules. − Recently, dihydrocyclopenta[ a ]indene 2 , which shows AEE behavior, has been synthesized by palladium-catalyzed trimerization of diphenylacetylene . The skeletal structure of 2 is quite similar to 3 , synthesized by Belzner and co-workers using the reaction of a base-stabilized transient diarylsilylene [Ar 2 Si:, Ar = 2-(dimethylaminomethyl)phenyl] with 2,3,4,5-tetraphenylcyclopentadienone (tetracyclone), while its luminescence behavior has not been reported .…”