Equatorial position

In order to account for axial and equatorial positions of protons bonded to cyclo-hcxanc-likc rings, Eq, (19) was used, where 1 is an atom three non-rotatablc bonds (totally atoms) away from the proton and belonging to a six-membered ring, and is a dihedral angle in radians (Figure 10.2-6c). 

The halogens F Cl Br and I do not differ much in their preference for the equatorial position As the atomic radius increases in the order F < Cl < Br < I so does the carbon-halogen bond dis tance and the two effects tend to cancel... 

Branched substituents especially tert butyl have an increased preference for the equatorial position... 

The sulfoxide group tends to occupy an axial rather than an equatorial position in the thiane S-oxides. 

The 1,2,4-trioxolane ring prefers a half-chair conformation (131) the C—O—C portion of the ring forms the reference plane, and alkyl substituents prefer the equatorial positions. 

The isoxazolidine ring exists primarily as an envelope (77AHQ2l)207) and the nitrogen lone pair can occupy an axial or equatorial position. Photoelectronic spectroscopy is a useful tool to determine conformational analysis of molecules possessing vicinal electron lone-pairs. Rademiacher and Frickmann (78TL841) studied isoxazolidine and 2-methyl- and 2-t-butyl-isoxazolidine and found mixtures of equatorial and axial (e/a) compounds. The ratios of H, Me and Bu in the efa position were 1 3, 4 1 and 10 1, respectively. 

The multiplets and coupling constants of the axial) protons at = 3.15, 3.50 and 4.08 moreover confirm the equatorial positions of all three OH groups, as can be seen in formula B. Here the couplings from 10.0 to 11.5 Hz, respectively, identify vicinal protons in diaxial configurations, whilst values of 4.5 and 5.0 Hz, respectively, are typical for axial-equatorial relationships. As the multiplets show, the protons at 5 = 3.50 and 4.08 couple with two axial and one equatorial proton (triplet of doublets) respectively, whereas the proton at = 3.15 couples with one axial and one equatorial proton (doublet of doublets). 

Because AG° will be negative when the equatorial conformation is more stable than the axial, the value of —AG° is positive for groups which favor the equatorial position. The larger the value of —AG°, the greater is the preference for the equatorial position. 

The strong preference for a /-butyl group to occupy the equatorial position has made it a useful group for the study of conformationally biased systems. The presence of a /-butyl substituent will ensure that the equilibrium lies heavily to the side having the /-butyl group equatorial but does not stop the process of conformational inversion. It should be emphasized that conformationally biased is not synonymous with conformationally... 

Scheme 3.2 gives some data that illustrate the differences in reactivity between groups in axial and equatorial positions. It should be noted that a group can be either more or less reactive in an axial position as compared to the corresponding equatorial position. 

The stereochemistry of Mg and the heavier alkaline earth metals is more flexible than that of Be and, in addition to occasional compounds which feature low coordination numbers (2, 3 and 4), there are many examples of 6, 8 and 12 coordination, some with 7, 9 or 10 coordination, and even some with coordination numbers as high as 22 or 24, as in SrCdn, BaCdn and (Ca, Sr or Ba)Zni3. " Strontium is 5-coordinate on the hemisolvate [Sr(OC6H2Bu3)2(thf)3]. jthf which features a distorted trigonal bipyramidal structure with the two aryloxides in equatorial positions. ... 

Figure 12.13 Interchange of axial and equatorial positions by Berry pseudorotation (BPR).

Ring inversion, leading to interconversion of different ring conformers, is typically as facile a process as single-bond rotation. Particularly important are six-membered rings, where interconversion leads to interchange of axial and equatorial positions. 

For the kinetically controlled formation of 1,3-disubstituted tetrahydro-P-carbolines, placing both substituents in equatorial positions to reduce 1,3-diaxial interactions resulted in the cw-selectivity usually observed in these reactions." Condensation reactions carried out at or below room temperature in the presence of an acid catalyst gave the kinetic product distribution with the cw-diastereomer being the major product observed, as illustrated by the condensation of L-tryptophan methyl ester 41 with benzaldehyde. At higher reaction temperatures, the condensation reaction was reversible and a thermodynamic product distribution was observed. Cis and trans diastereomers were often obtained in nearly equal amounts suggesting that they have similar energies."... 

The intramolecular cyclization of l-(4-bromobutyl)-3-methoxycarbonyl-l,4,5, 6-tetrahydropyridine (140) and l-(3-bromopropyl)-3-methoxycarbonyl-l,4,5,6-tetrahydropyridine (143) (89T5269) resulted in the synthesis of quinolizidine ring system 141 and indolizidine ring system 144 in 43% and 72% yields along with the reduced tetrahydropyridines 142 and 145 in 21% and 8% yields, respectively. All the cyclized products appeared to be (ran.s-fused indolizidines or quinolizidines. The (ran.s -fused simple indolizidines are known to be some 2.4 kcal mol more stable than the d.s-fused isomers (68TL6191). In the and-isomer the methoxycarbonyl substituent occupies an equatorial position. 

In the conformationally biased cyclohexene ring, the two dioxolane oxygens must assume fixed axial and equatorial positions. Coordination of the zinc carb-... 

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