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1486 J. Am. Chem. SOC.1987, 109, 1486-1495 Probing the Anomeric Effect. Trimethylsilyloxy and
 

Summary: 1486 J. Am. Chem. SOC.1987, 109, 1486-1495
Probing the Anomeric Effect. Trimethylsilyloxy and
tert-Butoxy Substituents in 1,4-Dioxane Derivatives: Theory
vs. Experiment'
Pinchas Aped,+Yitzhak Apeloig,*Aviyakar Ellencweig,+Benzion Fuchs,*+
Israel Goldberg,+Miriam Karni,l and Evgeny Tartakovskyt
Contribution from the School of Chemistry. Tel-Aviv University,
Ramat-Aviv 69 978 Tel-Aviv, Israel, and Department of Chemistry, Technion-Israel Institute of
Technology, 32 000 Haifa?Israel. Received April 7, 1986.
Revised Manuscript Received September 22, 1986
Abstract: A conformational analytical study of a series of trimethylsilyloxy- (TMSO) and tert-butoxy- (TBO) substituted
1,4-dioxanes was performed using NMR techniques. It was found that the TMSO and TBO groups (in contrast to MeO,
AcO, and PhO substituents2) alleviate the anomeric effect. Arguments based on molecular mechanics (MM2-parametrized
for the anomeric effect) and MO ab initio calculations of ROCOR' (R = CH,,H; R' = SiHJ at various levels help rationalize
this behavior, by isolating and identifying steric and electronic contributions. The electronic factors are (i) strong attractive
nonbonded OSi-.O interactions within the R-0-C-0-Si system, which reduce considerably the 0-C-0-Si dihedral angle
(down to 0') and the energy of the "ag" (equatorial) form vs. the "g'g'" one and (ii) inductive electron donation by SiMe,
(or CMe,) which lowers the electronegativity of TMSO (or TBO). Steric factors and (p -d ) r bonding are negligible. A
single-crystal X-ray analysis of 2,5-di-tert-butoxy- 1,4-dioxane revealed structural data in line with all the above.
The anomeric effect is recently an intensively studied and

  

Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology

 

Collections: Chemistry