Abstract
Introducing chiral silicon centers was explored for the asymmetric Rh-catalyzed cyclization of dihydrosilanes to enantiomerically enriched spirosilanes as targets to enable access to enantiostable pentacoordinate silicates. The steric rigidity required in such systems demands the presence of two naphthyl or benzo[b]thiophene groups. The synthetic approach to the expanded spirosilanes extends Takai’s method (Kuninobu et al. in Angew Chem Int Ed 52(5):1520–1522, 2013) for the synthesis of spirosilabifluorenes in which both a Si–H and a C–H bond of a dihydrosilane are activated by a rhodium catalyst. The expanded dihydrosilanes were obtained from halogenated aromatic precursors. Their asymmetric cyclization to the spirosilanes were conducted with [Rh(cod)Cl]2 in the presence of the chiral bidentate phosphane ligands (R)-BINAP, (R)-MeO-BIPHEP, and (R)-SEGPHOS, including derivatives with P-(3,5-t-Bu-4-MeO)-phenyl (DTBM) groups. The highest enantiomeric excess of 84% was obtained for 11,11′-spirobi[benzo[b]-naphtho[2,1-d]silole] with the DTBM-SEGPHOS ligand.
Highlights
Chirality is a cornerstone in chemistry, crucial to live, and increasingly important to materials science
Our approach is based on recent work by Takai and coworkers, who (a) reported on the synthesis of 9H-9sila-fluorenes using a rhodium catalyst for Si–H and C–H bond activation [26] and (b) showed that spirosilanes could be obtained in high enantiomeric excess from dihydrosilanes on using a Rh-catalyst with a chiral (R)BINAP diphosphane ligand (Scheme 2) [27]
The successful synthesis of the naphthalene and benzo[b]thiophene dihydrosilanes 3a and 3c and their conversion to the corresponding spirosilanes 4a and 4c showed that the Rh-catalyzed cyclization can introduce modest to high enantioselectivity
Summary
Chirality is a cornerstone in chemistry, crucial to live, and increasingly important to materials science. The synthesis of chiral organic molecules has become commonplace [1]. Simple examples of the numerous ones available are Knowles’s synthesis of l-DOPA and Sharpless’ asymmetric epoxidation resulting in compounds of high enantiomeric excess (ee) [2, 3]. Common approaches to introduce chirality are nucleophilic substitution of pro-chiral systems and asymmetric catalysis [4]. Once a chiral carbon center is formed, it Dedicated to the memory of George A.
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