B(C₆F₅)₃-Promoted Tandem Silylation and Intramolecular Hydrosilylation: Diastereoselective Synthesis of Oxasilinanes and Oxasilepanes

Abstract

B(C 6 F 5 ) 3 promotes regio-and stereoselective cyclizations of unsaturated alkoxysilanes to generate oxasilinanes and oxasilepanes. The same products are available directly from alkenols via tandem silylation and hydrosilylation.Intramolecular hydrosilylation of alkenes is an important transformation in organic synthesis.1 Initially investigated for unsaturated silanes,2 the methodology is now often applied to unsaturated alkoxy-and aminosilanes,3 where stereospecific oxidative cleavage of the newly formed C-Si bond enables stereodefined synthesis of diols and aminoalcohols. 4 , 5 The majority of examples involve metal-catalyzed 5-endo or 5-exo ring closures, although six-membered cyclizations have been reported.1 , 3 , 6 We now report regio-and stereoselective formation of oxasilinanes and oxasilepanes via formation and cyclization of unsaturated alkoxysilanes in the presence of a nonmetal catalyst.In the course of investigations into the influence of Lewis acids on the ozonolysis of unsaturated silanes, we found that addition of B(C 6 F 5 ) 3 to a solution of unsaturated alkoxysilane 1-Pr resulted in regioselective formation of oxasilinane 2-Pr with high 3,5-trans diastereoselectivity (Table 1).7 , 8 The cyclization proceeded efficiently at −78 °C or RT and in the presence of either stoichometric or catalytic B(C 6 F 5 ) 3 . Cyclization was also observed for the dimethylsilyl ether (not shown),9 but the hydrolytic instability of this class of reactants led us to abandon this thread following the discovery of the tandem cyclizations discussed later. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptThe cyclization, apparently the first intramolecular example of a known intermolecular hydrosilylation,10 was investigated further using alkoxysilanes prepared as illustrated in Scheme 1. 6-Endo cyclization onto an α-substituted styrene (3-Pr) proceeded slowly but in high yield and with high trans selectivity (Scheme 2). Cyclization onto a cyclobutene (5-Pr) proceeded much more slowly through a 6-endo pathway to furnish a modest yield of the cisfused adducts (6-Pr) as a 5:1 mixture of sidechain epimers. A bishomoallyl substrate, 7-Pr, reacted very slowly through a 6-exo pathway to furnish a trans-3,6-disubstituted-2-oxa-1-silinane (8-Pr).B(C 6 F 5 ) 3 also catalyzes the reductive silylation of alcohols,11 and we became intrigued by the possibility of tandem silylation/hydrosilylation (Table 2). B(C 6 F 5 ) 3 -promoted reaction of alkenol 1 with stoichometric Et 2 SiH 2 or Ph 2 SiH 2 generated oxasilinanes 2-Et or 2-Ph with very similar regio-and stereoselection as observed in the stepwise cyclizations. Although alcohols 3 and 5 decomposed under the tandem conditions, cyclohexenol 9 reacted to selectively furnish the 3,5-trans diastereomer of cis-fused octahydrobenzooxasilinanes 10-Et and 10-Ph; the lower yield for the Et 2 SiH 2 reaction is likely related to undesired reductive deoxygenations (vida infra). Alkenol 11, which generates an intermediate siloxane capable of undergoing cy...

Stereoselective Isoxazolidine Synthesis Via Copper-Catalyzed Alkene Aminooxygenation

“…3-Methyl-1-phenylbut-3-en-1-ol 15 was converted to 1l (105 mg, 51% yield, yellow solid). mp = 83–85 °C; 1 H NMR (400 MHz, CDCl 3 ) δ 7.79 (d, J= 8.0 Hz, 2H), 7.33-7.27 (m, 7H), 6.62 (s, 1H), 5.15 (dd, J = 5.6 Hz, 8.0 Hz, 1H), 4.83 (s, 1H), 4.73 (s, 1H), 2.59 (dd, J = 8.4 Hz, 14.4 Hz, 1H), 2.41 (s, 3 H), 2.35 (dd, J = 5.2 Hz, 14.4 Hz, 1H), 1.81 (s, 3H); 13 C NMR (75.5 MHz, CDCl 3 ) δ 144.7, 141.3, 140.0, 133.7, 129.6, 128.7, 127.1, 113.5, 88.2, 44.1, 23.4, 21.7; IR (thin film), 3223, 2923, 1597, 1454, 1339, 1167, 812, 727 cm −1 ; HRMS (ESI) calcd for [M+Na] + C 18 H 21 O 3 N 1 Na 1 S 1 : 354.1134, found: 354.1134.…”

Chlorine‐Free Catalytic Formation of Silsesquioxanes with Si‐OH and Si‐OR Functional Groups

“…[3] Examples for synthetic application of transition metal complexes in alcohol and phenol dehydrocoupling with silanes were widely reported. [46,47,52,53] It has also been recently reported that non-metal based oxidizing catalysts, such as NaOH, [58] NH 4 I [59] or B(C 6 F 5 ) 3 , [60][61][62][63][64] have been successfully used.…”

Chemoselective Silylative Reduction of Conjugated Nitriles under Metal‐Free Catalytic Conditions: β‐Silyl Amines and Enamines

“…Fori nstance,s ubstrates bearing ethyl, tert-butyl, and phenyl substituents at the para position underwent the silylative reduction in satisfactory yields (3-5;T able 2). [8c, 13] Cinnamonitriles having multiple substituents also reacted smoothly to give the desired products in moderate to good yields (14)(15)(16). In the case of sterically demanding substrates,s uch as orthomethylcinnamonitrile,t he reaction with diphenylsilane (Ph 2 SiH 2 )w as carried out at 65 8 8Ct oo btain ar easonable yield of 7.I nterestingly,t he use of diethylsilane instead of diphenylsilane allowed the reaction of ortho-phenylcinnamonitrile to proceed at room temperature,t hus giving an analogous product (8)i ng ood yield.…”

“…Theo btained amine salt was converted into its sulfonamide derivative,which was subsequently oxidized under the Tamao conditions [14] to deliver the b-hydroxy N-sulfonamide 22 in good yield (90 %i nt wo steps). When 1-cyano-1butene (10 mmol) was reacted with diphenylsilane under the optimal reaction conditions,2 -silyl-1-pentylamine was obtained as the HCl salt in 81 %y ield (19;2 .47 g) using 3mol %o ft he B(C 6 F 5 ) 3 catalyst at 25 8 8Ci nc hloroform.…”