Chair and boat forms

Cycio-tetrametaphos-phoric acid (anions known in both boat and chair forms)... 

The atomization energy, electron affinity and ionization potential have been calculated for 1//-azepine. and a difference in energy between the boat and chair forms of 64.8 kJ mol -1 deduced.98 The calculated dipole moment for l//-azepine is 4.67 D.98 Hiickel-London theory has been applied to calculate the ring-current octopole hypersusceptibilities of l//-azepine."... 

Therefore the concept of puckered rings was introduced and the existence of boat and chair forms in cyclohexane. 

Fig. 39. Skeletal structures of the carbo cation (CgMegH) and the boat and chair forms of cyclohexane compared with the octadecahedron of (BuHu) -.

The possible conformations for these fused ring compounds are illustrated in Fig. 9. Any discussion of the relative stability of boat and chair forms depends on the axial/equatorial positions of the phosphorus substituents. It seems likely that ring inversion will be fast on the n.m.r. time-scale, so that the structures within a given group (i), (ii) or (iii) could all contribute (to varying degrees) to an observed spectrum. 

Among the possible conformations of oxepane the boat, twist-boat and chair forms were calculated to be of higher energy than the predominant twist-chair conformations (which were found to exist in approximately equal proportions). 

Figure 10.7 Molecular structures (a) [(Me2N)2SiNH]3 and (b) the boat and chair forms of (Me2SiNH)4.

The proton NMR spectra have been used for distinguishing coordinated and free ligands in crystals, and the spectra of the piperidine cluster complex have been analyzed in detail (37) and the peaks of each proton on the piperidine rings assigned using both 1-D and 2-D spectra. Conformational exchange between the boat and chair forms of the ligands was not observed. 

Figure 2.3. Boat and chair forms of P-D-glucuronic acid, (a) Cj chair conformation (left) and planar projection (right) (b) Boat conformation and (c) 1C chair conformation of P-D-glucuronic acid.The latter two conformations are energetically less stable than the Ci chair conformation.

Baeger-Villiger reaction, 311 Barbiturates, 358 Base peak in ms. 248 Bases, soft and hard. 121 Basicity and structure. 43 Basic Red. 428 Beckmann rearrangement. 406 Benzene, resonance structure. 192 structure, 189 Benzenonium ion, 205 Benzhydrol, 269 Benzidine rearrangement. 422 Benzyne. 217 Bicydic compounds. 162 Birch reduction, 200 Bischler-Napieralski reaction, 460 Boat and chair forms. 168 Boiling point, influences on, 212 Bond dissociation, 37 Bond order, 17 Bond stretching. 233 Bonding orbitals, 14 Bredt s rule. 168 Bromonium ion. 100 BrOnsted. 42... 

Semi-empirical and molecular mechanics calculations have been widely used in the study of eight-membered heterocycles with three heteroatoms. Theoretical studies have centered on conformations of this class of heterocines, which usually belong to four low-energy conformational families classified as boat-chair, crown, boat, and chair forms (see Section 14.08.4.2). 

Four main low-energy conformational families of eight-membered ring systems, consisting of the boat-chair, crown, boat, and chair forms, were surveyed in CHEC-II(1996). They are discussed in the foregoing sections of this chapter in conjunction with the theoretical and experimental structural studies of triheterocines (see sections 14.08.2 and 14.08.3). 

Interesting QOOH chemistry would also be favourable and include not only the compounds analogous to those observed for cyclohexane, but also two oxabicyclooctanes which would have some associated stability arising from the boat and chair forms of cyclohexanes. 

Conformational formulas for the boat and chair forms of pyranose. 

Another example that may be cited is the use of the rule in the confirmation of the stereochemistry of a- and ) -thujone. If we represent oe-thujone (22) in its two possible conformations, then it is to be expected that in the boat form (24A) a relatively high positive solvent shift will be observed for the protons of the 10-methyl group. In the chair form (24B) a negligible solvent shift is expected. The ) -isomer (23) will show the reverse of these trends in its boat and chair forms. 

Figure 9.13 shows that a pyranose, such as glucose, has two common conformational isomers, referred to as the "boat" and "chair" form. For glucose (and most sugars), the chair form is more stable because the hydroxyls of carbons 1 and 2 are further removed and thus have less steric interference with carbons 3, 4, and 5. 

Molecular orbital and molecular mechanical calculations have been used to investigate the conformational preferences of the various eight-membered heterocycles, which consist of four low-energy conformational families categorized as the boat-chair, crown, boat, and chair forms (see Section 9.25.4.1). The predicted energy minimized conformations in vacuo can be compared to those observed in the solid state by x-ray diffraction analysis. Molecular dynamics calculations are used to simulate solution conformations which in turn can be compared to those determined experimentally by NMR spectral studies. 

Conformational isomerism, which could arise if any of the above isomers were associated with more than one ring shape such as the boat and chair forms of (PNCl2)4. 

---