Reductive Lithiation of Methyl Substituted Diarylmethylsilanes: Application to Silanediol Peptide Precursors

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

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).

Silyllithium-Initiated Coupling of α-Ketoamides with tert-Butanesulfinylimines for Stereoselective Synthesis of Enantioenriched α-(Silyloxy)-β-amino Amides

A silyllithium-initiated coupling of α-ketoamides with tert-butanesulfinylimines was developed for the efficient, stereoselective synthesis of enantioenriched α-(silyloxy)-β-amino amides. Nucleophilic addition of silyllithium to α-ketoamides, followed by 1,2-Brook rearrangement, generates nucleophilic enolates, which are then intercepted by chiral imines to provide three-component coupling products. Use of α-ketoamides is critical for achieving high yields and diastereoselectivities in the resulting α-hydroxy-β-amino acid derivatives.

Enantioselective synthesis of α-aminosilanes by copper-catalyzed hydroamination of vinylsilanes

The synthesis of α-aminosilanes by a highly enantio- and regioselective copper-catalyzed hydroamination of vinylsilanes is reported. The system employs Cu-DTBM-SEGPHOS as the catalyst, diethoxymethylsilane as the stoichiometric reductant, and O-benzoylhydroxylamines as the electrophilic nitrogen source. This hydroamination reaction is compatible with differentially substituted vinylsilanes, thus providing access to amino acid mimics and other valuable chiral organosilicon compounds.

Enantioselective addition of silicon nucleophiles to aldimines using a preformed NHC-Copper(I) complex as the catalyst

A remaining major challenge in the asymmetric addition of silicon nucleophiles to typical prochiral acceptors, the enantioselective 1,2-addition to aldimines, is addressed. Activation of the SiB bond in the silicon pronucleophile by a copper(I) alkoxide with McQuade's chiral six-membered N-heterocyclic carbene as a supporting ligand releases the silicon nucleophile, which adds to various aldimines with high levels of enantioselectivity. The new method provides a catalytic asymmetric access to α-silylated amines.

Synthesis and structural study of precursors of novel methylsilanediols by IR and Raman spectroscopies, single-crystal X-ray diffraction and DFT calculations

On the way towards the development of a synthetic route aimed at obtaining new methylsilanediol derivatives with an aminocarbonyl group in β to silicon (which may have a potential biological interest), we have synthesized, isolated and purified five diphenylic possible precursors, namely chloromethyl(methyl)diphenylsilane, 2-{[methyl(diphenyl)silyl]methyl}-1H-isoindole-1,3(2H)-dione, N-[(methyl(diphenyl) silanyl)-methyl]-benzamide, N-[(methyl(diphenyl)silyl)-methyl]-acetamide and N-[(methyl(diphenyl)silyl)-methyl]-formamide. The conformational landscape of the five species in this study are explored by means of DFT calculations at the B3LYP/6-311++G(∗∗) level. The theoretical molecular structures predicted are confirmed by the reproduction of their respective IR and Raman spectral profiles, that are completely assigned. Some evidence in the vibrational spectra points to the occurrence of conformational mixtures in the samples. Further, single-crystal X-ray diffraction has allowed the elucidation of the crystalline structure of 2-{[methyl(diphenyl)silyl]methyl}-1H-isoindole-1,3(2H)-dione.

Efficient, enantioselective assembly of silanediol protease inhibitors

A five-step assembly of silicon-protected dipeptide mimics from commercially available reagents is described. This methodology makes silanediol protease inhibitors readily available for the first time. The sequence features asymmetric hydrosilylation, a novel reduction of a silyl ether to a silyllithium reagent, and addition of this dianion to a sulfinimine, to produce the complete inhibitor skeleton with full control of stereochemistry. Oxidation of the primary alcohol to an acid completes the synthesis.