Silanediol versus chlorosilanol: hydrolyses and hydrogen-bonding catalyses with fenchole-based silanes

Abstract

Biphenyl-2,2’-bisfenchyloxydichlorosilane (7, BIFOXSiCl2) is synthesized and employed as precursor for the new silanols biphenyl-2,2’-bisfenchyloxychlorosilanol (8, BIFOXSiCl(OH)) and biphenyl-2,2’-bisfenchyloxysilanediol (9, BIFOXSi(OH)2). BIFOXSiCl2 (7) shows a remarkable stability against hydrolysis, yielding silanediol 9 under enforced conditions. A kinetic study for the hydrolysis of dichlorosilane 7 shows a 263 times slower reaction compared to reference bis-(2,4,6-tri-tert-butylphenoxy)dichlorosilane (14), known for its low hydrolytic reactivity. Computational analyses explain the slow hydrolyses of BIFOXSiCl2 (7) to BIFOXSiCl(OH) (8, E a = 32.6 kcal mol−1) and BIFOXSiCl(OH) (8) to BIFOXSi(OH)2 (9, E a = 31.4 kcal mol−1) with high activation barriers, enforced by endo fenchone units. Crystal structure analyses of silanediol 9 with acetone show shorter hydrogen bonds between the Si–OH groups and the oxygen of the bound acetone (OH···O 1.88(3)–2.05(2) Å) than with chlorosilanol 8 (OH···2.16(0) Å). Due to its two hydroxy units, the silanediol 9 shows higher catalytic activity as hydrogen bond donor than chlorosilanol 8, e.g., C–C coupling N-acyl Mannich reaction of silyl ketene acetals 11 with N-acylisoquinolinium ions (up to 85% yield and 12% ee), reaction of 1-chloroisochroman (18) and silyl ketene acetals 11 (up to 85% yield and 5% ee), reaction of chromen-4-one (20) and silyl ketene acetals 11 (up to 98% yield and 4% ee).

Enantioselective dearomatization reactions of heteroarenes by anion-binding organocatalysis

“…36 Regarding to the hardly explored silanediol catalysts, it can be remarked the later development of biphenyl-2,2-bisfencholbased (BIFOL) silanediol by Goldfuss et al in 2019. 37 This alkoxysilanediol has an unexpectedly stable BIFOL backbone against hydrolysis, and is able to achieve the dearomatization of isoquinolines by N-acyl Mannich reaction in high yields, though only in low enantioselectivities (up to 12% ee).…”

Activation Modes in Asymmetric Anion-Binding Catalysis

“…14 The group of Goldfuss also investigated chiral fenchoned-based silanediols 10 in this reaction. 15 However, only low enantiomeric excesses could be obtained. Furthermore, other nucleophiles were applied in the coupling with N-acylisoquinoliniums.…”

Anion recognition by silanetriol in acetonitrile

“…[22][23][24][25][26][27] A silanol hydroxy group is another candidate as a hydrogen bond donor site for molecular recognition. Indeed, from our finding of di(1-naphthyl)silanediol as the first example of a silanol-based anion receptor, 28 various silanol derivatives, such as gem-silanediol, [28][29][30][31][32][33][34] 1,3-disiloxane-1,3-diol, 33,35,36 1,3disiloxane-1,1,3,3-tetraols, 37 incompletely condensed POSS derivatives, 38 orthosilicic acid and its oligomers, 39 and other derivatives, 40,41 have been applied as anion receptors in organic solvents such as MeCN-d 3 and CDCl 3 . 42 Increasing numbers of recognition sites should lead to more effective recognition.…”