Rhodium‐Catalyzed Hydrosilylation of Internal Alkynes with Silane Reagents bearing Heteroatom Substituents. Studies on the Regio‐/Stereochemistry and Transformation of the Produced Alkenylsilanes by Rhodium‐Catalyzed Conjugate Addition

Abstract

Rhodium-catalyzed hydrosilylation of internal alkynes furnished (E)-1,2-disubstituted alkenylsilanes. The obtained alkenylsilane was subjected to reaction with a,b-unsaturated carbonyl compounds in the presence of a rhodium catalyst to undergo conjugate addition. One-pot hydrosilylationconjugate addition with a rhodium catalyst was also performed.Keywords: conjugate addition; 1,2-disubstituted alkenylsilanes; hydrosilylation; internal alkynes; rhodium Hydrosilylation of alkynes serves as an important tool for the synthesis of alkenylsilanes, which can be transformed into a variety of organic molecules by transition metal-catalyzed carbon-carbon bond-forming reactions with organic electrophiles.[1] We have been studying the use of rhodium complexes as a catalyst for the hydrosilylation of alkynes and found that a rhodium catalyst was effective for the regio-and stereoselective hydrosilylation of terminal alkynes.[2] Our further interest has turned to investigation of the reaction of internal alkynes, which form 1,2-disubstituted alkenylsilanes. [3] We herein report that with the use of a rhodium complex as a catalyst the hydrosilylation of internal alkynes took place at room temperature. Further reactions of the thus formed 1,2-disubstituted alkenylsilanes with several a,b-unsaturated carbonyl compounds in the presence of a rhodium complex were also studied.The reaction of diphenylethyne (1a) with triethoxysilane (2) was carried out in the presence of 0.5 mol % of [RhCl(cod)] 2 (cod ¼ 1,5-cyclooctadiene) at room temperature. The corresponding alkenylsilane (3a) was obtained after stirring for 3 h [Eq. (1)]. Hydrosilylation of 3-hexyne (1b) with 2 also proceeded stereoselectively to give (E)-3-triethoxysilyl-hex-3-ene (3b) in a quantitative yield. The reaction proceeded efficiently without solvent. Worthy of note is that the reaction took place at room temperature with a small amount of the rhodium catalyst (0.025 -0.5 mol %). By contrast, the similar reaction with a platinum catalyst, (n-Bu 4 N) 2 PtCl 6 , did not proceed at room temperature. ð1ÞThe stereochemistry of the product was found to be the E-form, which was confirmed by treatment of 3a with tetra-n-butylammonium fluoride (TBAF) in the presence of CuI to give stilbene (4a) (Z/E ¼ 9 : 1), suggesting that cis-addition of H À Si took place [Eq. (2)].(4) [4] To confirm the stereochemistry of 3b, whose desilylation led to the rather volatile 3-hexene, hydrosilylation of dodecyne (1c) was carried out and the desilylation of 3c with TBAF afforded (Z)-6-dodecene (4c) in 81% yield. The formation of 4c was also confirmed by comparison with the authentic sample, which was synthesized by a Wittig reaction of hexanal and the phosphonium salt of 1-bromohexane. The stereochemical outcome of hydrosilylation contrasts to that with a ruthenium catalyst reported by Trost to induce the trans-addition. [5,6] ð2Þ COMMUNICATIONS

Rhodium‐ and Palladium‐Catalyzed Asymmetric Conjugate Additions

“…The enantioselectivities observed were very close to those obtained with organoboronic acid, indicating a similar reaction pathway [17,60]. In a one-pot procedure, using the same cationic [Rh((S)-binap)(MeCN) 2 ]BF 4 complex, alkenylsilanes can be generated in situ by hydrosilylation of the corresponding alkyne, and then subjected to ECA [186,187]. Organosiloxanes were also used in the Rh-catalyzed asymmetric addition to α-substituted acrylic esters (for the reaction with organoboronic acids; cf.…”

Synthesis of 11‐cis‐Retinoids by Hydrosilylation–Protodesilylation of an 11,12‐Didehydro Precursor: Easy Access to 11‐ and 12‐Mono‐ and 11,12‐Dideuteroretinoids

“…The results obtained with rhodium catalysts were unexpected (Table 1). Reaction of 6 with the neutral dimer [{RhCl(cod)} 2 ] and triethoxysilane in CH 2 Cl 2 did not take place at all, despite this catalyst having been strongly recommended for hydrosilylation when heteroatom‐containing silanes are employed;19 after 6 h at RT, only starting material was recovered. Even more surprisingly, when this and other neutral rhodium catalysts were used with benzyldimethylsilane under the same experimental conditions, chemoselectivity was reversed, the major product being not the expected product 9 b but the O ‐silylated compound 11 b , the result of dehydrogenative coupling of the hydrosilane with the alcohol 29.…”

“…For syn ‐hydrosilylation, platinum and rhodium catalysts are the most frequently employed. For example, Mori and co‐workers19 reported the rhodium‐catalyzed room‐temperature hydrosilylation of internal alkynes with silane reagents that bear heteroatom substituents (e.g., [{RhCl(cod)} 2 ] (cod=cyclooctadiene), (EtO) 3 SiH, THF); Nakao et al 20. employed the homogeneous platinum catalyst ([Pt(dvds)( t Bu 3 P)] (dvds=divinyltetramethyldisiloxane; Karstedt’s catalyst) to prepare trisubstituted (2‐hydroxymethyl)phenylsilanes from symmetrical internal alkynes under very mild conditions (hexane, 0 °C); and Alami used a heterogeneous Pt catalyst (PtO 2 ) to synthesize ( Z )‐stilbenes from diarylalkynes by means of a one‐pot cis ‐hydrosilylation–protodesilylation ( n ‐tetrabutylammonium fluoride (TBAF)) sequence, although in this case heating was necessary 21.…”

Selective Hydrosilylation of Dienes, Enynes, and Diynes Catalyzed by a Platinum Complex with a Very Bulky NHC Ligand – The Crucial Role of Precise Tuning of the Reaction Conditions

“…A number of catalytic systems and procedures have been described in the literature to achieve high selectivities (> 95 %) for simple terminal or symmetrically 1,2‐disubstituted olefins, and acetylenes , . The literature describing selective transformations of polyunsaturated reagents is scarce and provides several examples of selective 1,4‐hydrosilylation and 1,2‐hydrosilylation[6b], of 1,3‐dienes.…”