Conformational rivalry of geminal substituents in silacyclohexane derivatives: 1-Phenyl vs. 1-OR, R=H or Me. Issue 22 (31st May 2019)

• Record Type: Journal Article
• Authors: Phien, Tran Dinh; Kuzmina, Liubov E.; Suslova, Elena N.; Shainyan, Bagrat A.; Shlykov, Sergey A.
• Is Part Of: Tetrahedron. Volume 75:Issue 22(2019)
• Page Start: 3038
• Page End: 3045
• Publication Date: 2019-05-31
• Subjects: 1-Methoxy-1-phenyl-1-silacyclohexane -- 1-Hydroxy-1-phenyl-1-silacyclohexane -- Conformational analysis -- Gas phase electron diffraction -- Quantum chemical calculations; Chemistry, Organic -- Periodicals; 547.005
• DOI: 10.1016/j.tet.2019.04.044
• Languages: English
• ISSNs: 0040-4020

Abstract

Abstract: Molecular structure and conformational equilibria of 1-methoxy-1 and 1-hydroxy-1-phenylsilacyclohexanes2 were studied by quantum chemical (QC) calculations and combined gas electron diffraction/mass spectrometry (GED/MS). Both molecules may exist in 5 or 6 forms, differing from each other by the substituents' position: (i) axial or equatorial and (ii) rotational orientation relative to the six-membered ring frame. The contribution of axial forms of both compounds varies from 35 to 60% depending on the theoretical method applied. From the GED data, the summarized molar fractions of the conformers were found to be Phax :Pheq = 70(15):30(20) and 50(20):50(20)% which corresponds to Δ G = G ax – G eq = −0.55(46) and 0.00(56) kcal/mol, for compound1 and2, respectively. The concentration of the Phax forms of 1-phenyl-1-(X)-silacyclohexanes (X = H, HO, Me, MeO and Me2 N) increases with the size of the second substituent at the silicon atom: 38(10)<50(20)<58(15)<70(15) <80(15)%. Graphical abstract: Image 1 Highlights: 1-methoxy- and 1-hydroxy-1-phenyl-1-silacyclohexanes were studied by GED and theoretical chemistry. The quantum chemical calculations give different conformational ratio for both compounds. The ratio Phax :Pheq = 70(15):30(20) and 50(20):50(20)% for 1-methoxy- and 1-hydroxy-1-phenylsilacyclohexanes from GED data. The rotation of the methoxy/hydroxy group induces the phenyl ring rotation in both Phax and Pheq forms, but not vice versa.