Formation and Utility of Azasilacyclopentadienes Derived from Silacyclopropenes and Nitriles

Abstract

The copper-catalyzed insertions of nitriles into the Si-C bonds of silacyclopropenes provide azasilacyclopentadienes, which can be converted to allylic amines after reduction and protodesilylation. The enamine functionality of azasilacyclopentadienes also participates in 1,4-addition reactions and undergoes a hydroboration and oxidation sequence to form an allylic 1,2-amino alcohol.Allylic amines are useful intermediates in organic synthesis, and a number of methods have been developed for preparing these compounds. 1 Because the insertion of carbonyl compounds into silacyclopropenes provides a method for the synthesis of allylic alcohols, 2 we considered that reactions of silacyclopropenes with C-N multiple bonds could lead to a synthesis of allylic amines. Although the photochemical reactions of nitriles with a silacyclopropene have been reported, 3,4 the insertion products underwent further reactions in modest yields, and applications of these reactions in synthesis were not described. 3 In this Letter, we report the copper-catalyzed insertions of nitriles into the Si-C bonds of silacyclopropenes to form azasilacyclopentadienes. These compounds can be functionalized by reductions, 1,4-additions, and hydroborations to form allylic amines and allylic amino alcohols. 5Copper salts proved to be efficient catalysts for the insertion of nitriles into silacyclopropenes. When a 1:1 mixture of acetonitrile and silacyclopropene 1a in C 6 D 6 was treated with 5 mol % of Cu(OTf) 2 , silacyclopropene 1a disappeared over 24 hours, and enamine 2a was formed as a single regioisomer (Scheme 1). Preference for the 1,2-insertion product, which was confirmed by a 1 H-1 H NOESY experiment, is consistent with the regioselectivity of insertions of carbonyl compounds into silacyclopropenes and silacyclopropanes. 2,6 The imine tautomer of 2a was not observed in the product mixure or at any time during the transformation. A subsequent catalyst screen for the acetonitrile insertion of silacyclopropene 1a showed that Cu(OTf) 2 and (CuOTf) 2 ·tol were more efficient catalysts than CuBr 2 or CuI. When the transformation was performed on a one mmol scale, (CuOTf) 2 ·tol gave higher yields than Cu(OTf) 2 (Table 1, entry 1).The insertion of a nitrile into a monosubstituted silacyclopropene was general for a number of nitriles and silacyclopropenes (Table 1). Nitriles with alkyl, aryl, and silyloxy groups ( were tolerated by the reaction conditions. 7 In all cases, the 1,2-insertion product and the Zenamine were observed exclusively. 8 The imine tautomer of the azasilacyclopentadiene was only observed for the insertion of isobutyronitrile into the Si-C bond of silacyclopropene 1a (Scheme 2). The formation of the enamine tautomer in this case may be disfavored because it would be destablized by interactions between the resulting isopropylidene group and the phenyl substituent.The copper-catalyzed insertion of nitriles was also successful for disubstituted silacyclopropenes (Table 2). These transformations required mild heating but proc...

Silver‐Catalyzed Silylene Transfer

“…Isolation of the air-sensitive silacyclopropene was avoided by development of a two-step, one-flask procedure, which transformed alkynes into the desired azasilacyclopentadienes (Scheme 7.29). 99 For terminal alkynes, silver phosphate was employed for di-tert-butylsilylene transfer and copper(I) triflate was used to promote nitrile insertion. These conditions successfully transformed phenylacetylene into azasilacyclopentadiene 106b.…”

“…99 Reduction of the imine and methyl carbamate protection of the resulting amine produced vinylsilane 107; protodesilylation of 107 afforded amine 108. Woerpel and Anderson also exploited the inherent nucleophilicity of the enamine to participate in conjugate addition and hydroboration/oxidation reactions; 1,4 addition of azasilacyclopentadiene 106a to methyl acrylate afforded 109 as a single tautomer, and hydroboration of enamine 106b followed by oxidation with H 2 O 2 and NaOH produced allylic 1,2-aminoalcohol 110 as a single stereoisomer.…”

Formation of silacycles via metal-mediated or catalyzed Si-C bond cleavage

“…They also developed a copper-catalyzed insertion of C≡N bond into the SiC bond of silacyclopropene to provide azasilacyclopentadiene 70 (eq. (24)) [76]. They found the copperor zinc-catalyzed reaction of silacyclopropanes with carbonyl compounds being highly stereo-, regio-, and chemoselective.…”

Recent Advances in Activation of Silicon Bonds by Fluoride Ion / Jaunākie Sasniegumi Silīcija Saišu Aktivācijās Ar Fluorīda Jonu

“…Interesting elimination of silyl group in vinyl silanes 19 leading to alkynes 20 in the presence of fluoride ion was presented (Scheme 8) [31]. Beside this, protiodesilylation of silyl group in vinyl silanes was described [32,33]. For example, reaction of silanol 21 in the system Bu 4 NF / tert-BuOK afforded alkene 22 (Scheme 9) [33].…”

“…Beside this, protiodesilylation of silyl group in vinyl silanes was described [32,33]. For example, reaction of silanol 21 in the system Bu 4 NF / tert-BuOK afforded alkene 22 (Scheme 9) [33].…”