Rotation in biphenyls with a single ortho-substituent.

Barriers to rotation in a range up to 15.4 kcal mol(-1) were determined by dynamic NMR spectroscopy for a series of biphenyl compounds 1a-1h and 2a-2d with a single ortho-substituent. Ab initio calculations reproduce these barriers satisfactorily and indicate the ground-state conformation of these molecules. Results are discussed in terms of the contribution of individual substituents to the barrier and of the buttressing of adjacent positions in a benzene ring by substituents.

Structure, conformation, and dynamic processes of the stereolabile atropisomers of hindered terphenyl hydrocarbons.

[reaction: see text] Ortho-substituted p-terphenyl hydrocarbons exist as trans and cis atropisomers that were identified by low-temperature NMR spectroscopy. The interconversion barriers increase with the dimensions of the ortho substituents, the experimental values being matched by ab initio calculations. X-ray diffraction shows that only the trans atropisomer is present in the solids. Spectra of a tert-butyl derivative in nonequilibrium conditions indicate that the cis is more populated than the trans atropisomer in solution, favored by attractive interactions.

Conformation of acetate derivatives of sugars and other cyclic alcohols. Crystal structures, NMR studies, and molecular mechanics calculations of acetates. When is the exocyclic C-O bond eclipsed?

[reaction: see text] A study of published crystal structures (of O-acetylated sugars for the most part) suggests that the exocyclic C-O bond in acetate esters of cyclic alcohols intrinsically prefers a staggered conformation, although the eclipsed conformation is only slightly less stable. When the acetate is flanked by two equatorial substituents the preferred conformation is close to eclipsed. Over 1500 C-OAc bonds have been analyzed. Diagnostic NMR criteria for torsion angles and MM3 calculations are reported and confirm these conclusions.

Why are silyl ethers conformationally different from alkyl ethers? Chair-chair conformational equilibria in silyloxycyclohexanes and their dependence on the substituents on silicon. The wider roles of eclipsing, of 1,3-repulsive steric interactions, and of attractive steric interactions.

An NMR study of the diaxial/diequatorial chair equilibrium in a range of silylated derivatives of trans-1,4- and trans-1,2-dihydroxycyclohexane is reported and discussed with a view to explaining unusually large populations of chair conformations with axial substituents, noted previously for some monosilyloxycyclohexanes and in some silylated sugars. X-ray diffraction studies of three bis-triphenylsilyloxycyclohexanes are reported and show both axial and equatorial silyloxy groups with the exocyclic bonds eclipsed. Eclipsing is also suggested by molecular mechanics (MM3) calculations on such derivatives. Both axial and equatorial tertiary silyl groups have 1,3-repulsive interactions with whatever substituents or hydrogen atoms are at the two adjacent equatorial positions, and these are relieved by rotation toward the eclipsed conformation of the exocyclic C-O bond. The three substituents on silicon interact attractively with the nine atoms at the 3, 4, and 5-positions of the cyclohexane ring and calculations suggest that these stabilizing interactions are significantly greater in the axial than in the equatorial conformation. An equatorial C-OSiR(3) bond with one or two equatorial neighbors has a restricted potential energy well that becomes much broader when the bond is axial without any equatorial neighbors in the alternative chair. Adjacent silyl groups in the 1,2-disubstituted series interact in a stabilizing way overall in all conformations, this being particularly marked in the diaxial conformation of the more complex ethers. These factors lead to unusually large axial populations.

Conformational studies by dynamic NMR. 95. Rotation around the adamantyl-alkyl bond. Remote substituent effect on conformational equilibrium.

Restricted rotation has been observed by NMR spectroscopy at very low temperature in isopropyladamantane, 1-tert-butyl-3-isopropyladamantane, and 1,3-diisopropyladamantane. The barriers for the corresponding dynamic processes were also determined. In the case of the disubstituted adamantane derivatives, two and four conformers, respectively, were observed and they were assigned on the basis of the symmetry properties. The relative populations deviate from the statistical distribution.

Conformational studies by dynamic NMR. 91. Conformational stereodynamics of tetraethylmethane and analogous C(CH2X)4 compounds.

Variable-temperature NMR studies of tetraethylmethane (1a), tetrapropylmethane (1b), tetrachloromethylmethane (1c), tetrabromomethylmethane (1d), tetracyclopropylmethylmethane (1e), and tetrabenzylmethane (1f) show a range of dynamic behavior. Separate signals for two types of conformation are observed for 1a, 1c, and 1d at low temperatures, with more than 95% of the molecules in a time-averaged D2d conformation, and the S4 conformation as the minor populated alternative. Compound 1e populates only S4-type conformations but equilibrates slowly between degenerate versions of these at low temperatures. Compounds 1b and 1f show a temperature-dependent spectrum but the low-temperature limit spectrum could not be observed. Ab initio calculations agree well with experiment on the conformational equilibria and suggest in particular that compounds 1b and 1f behave similarly to compounds 1a and 1e, respectively. A crystal structure of compound 1f is reported.

Conformational dynamics of tetraisopropylmethane and of tetracyclopropylmethane.

Tetraisopropylmethane (1) exists in solution as a mixture of two types of conformers (D2d and S4 time-averaged symmetry) in the ratio 93:7 at -110 degrees C, interconverting with a barrier of 9.7 kcal mol-1. Molecular mechanics calculations and the multiplicity of NMR signals at low temperature allow the assignment of these conformations. The only conformation populated in tetracyclopropylmethane (2) is the same type as the minor conformation (S4 time-averaged symmetry) populated in 1. 13C NMR spectra at about -180 degrees C show that degenerate versions of this conformation interconvert with a barrier of 4.5 kcal mol-1. Molecular mechanics calculations that characterize the six possible conformational types for these molecules, and the most important interconversion pathways, are reported. Calculated and experimental barriers match satisfactorily well.

New Models for the Study of the Racemization Mechanism of Carbodiimides. Synthesis and Structure (X-ray Crystallography and (1)H NMR) of Cyclic Carbodiimides.

The crystal and molecular structure of carbodiimides 2 (5,6,18,19-tetradehydro-5,12,13,18,25,26-hexahydrotetrabenzo[d,h,m,q][1,3,10,12]tetraazacyclooctadecine) and 3 (8,10,22,24-tetraazapentacyclo[23.3.1.1(3,7).1(11,15).1(17,21)]dotriaconta-1(29),3,5,7(32),8,9,11,13,15(31),17,19,21(30),22,23,25,27-hexadecaene) have been determined. The activation barriers for the racemization of carbodiimides 1 (6,7-dihydrodibenzo[d,h][1,3]diazonine), 2, and 3 have been determined. While 1 presents a relatively high barrier (17.4 kcal mol(-)(1)), 2 and 3 have very low activation barriers (between 5 and 7 kcal mol(-)(1)). We tentatively conclude that open-chain and large-ring carbodiimides racemize by nitrogen inversion or trans-rotation while medium-size cyclic carbodiimides racemize by cis-rotation.