Palladium‐Catalyzed Silylacylation of Allenes Using Acylsilanes

Abstract

We have developed a palladium-catalyzed addition of a CÀ Si bond of acylsilanes across a range of unactivated allenes. The reaction proceeds with complete regioselectivity, in which a silyl group binds to the central carbon of the allene, allowing for the straightforward access to functionalized alkenylsilane derivatives.Organosilicon compounds are essential components in diverse disciplines, encompassing synthetic organic, [1a-c] polymers, materials, [1d,e] bioorganic and medicinal chemistry. [1f,g] The most powerful and most widely used method, both commercially and in the laboratory, for their synthesis involves the transition metal-catalyzed hydrosilylation of unsaturated hydrocarbons, in which a HÀ Si bond is added across alkenes or alkynes. [2] If a CÀ Si bond in organosilanes could be added to unsaturated hydrocarbons in a similar manner, it would be a method of great value since it would allow access to elaborate organosilicon compounds from simpler organosilicon feedstocks with a 100 % atom economy (Figure 1). [3] However, realizing such an insertion reaction into a CÀ Si bond represents a daunting challenge, because, unlike a HÀ Si bond, the requisite oxidative addition of a CÀ Si bond is an uncommon organometallic process. [4] In fact, the cleavage of a CÀ Si bond by transition metal complexes primarily occurs through transmetallation with complexes bearing an X-type ligand, during which a silicon group is eliminated. [1b,c, 5] Therefore, insertion reactions into a CÀ Si bond, except for those using strained silacyclyes (i.e., silacyclopropanes [6] and silacyclobutanes [7] ), silyl cyanides [8] and intramolecular reactions, have not been reported (Figure 1a). [9,10] The Lewis acid-mediated carbosilylation of allylsilanes and related activated silanes have been reported, but, in such reactions, the CÀ Si bond cleavage occurs via a carbocation intermediate. [11] The photochemical addition of a CÀ Si bond of acylsilanes to activated alkynes has been reported, although the applicable alkynes are limited to acetylenedicarboxylates or intramolecular reactions (Figure 1b). [12] We report herein on the palladiumcatalyzed addition of a CÀ Si bond in acylsilanes across allenes, in which a CÀ C double bond can be functionalized with both acyl and silyl groups (Figure 1c). Narasaka and co-workers reported on the palladiumcatalyzed decarbonylative bis-silylation of dimethyl acetylenedicarboxylate using a bis-silyl ketone, which appears to proceed through the oxidative addition of a CÀ Si bond. [13] We also previously reported on the palladium-catalyzed siloxycyclopropanation of alkenes using acylsilanes, in which the oxidative addition of a CÀ Si bond presumably triggers the generation of a key siloxycarbene-palladium

Emerging applications of acylsilanes in organic synthesis and beyond

“…31 There were several merits of the reaction: (1) short reaction time, usually less than 1 h; (2) high efficiency (many reactions are quantitative); (3) stable products. These features were well fitted to the functionalization of complex molecules, for example, polymers with acylsilane-containing side chains (22). The method was also applied to chemically modify the copolymer containing the alkyne group (23), which is easily clickable with azide.…”

“…When the alkene was changed to allene, only silylacylation of allene was observed (Scheme 4b). 22 The reaction reacted regioselectivity on the internal CvC for mono-substituted allenes and 1,1dialkyl-substituted allenes, while for buta-2,3-dien-2-ylbenzene, the reaction regio-and stereo-selectivity occurred at the external CvC, forming valuable tetrasubstituted alkenyl silane (18) in a 73% yield.…”