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

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