Boat form

A modified boat conformation of cyclohexane, known as the twist boat (Fig. 1.8), or skew boat, has been suggested to minimize torsional and nonbounded interactions. This particular conformation is estimated to be about 1.5 kcal moE (6 kJ moE ) lower in energy than the boat form at room temperature. 

Stereochemistry. Cyclohexane can exist ia two molecular conformations the chair and boat forms. Conversion from one conformation to the other iavolves rotations about carbon—carbon single bonds. Energy barriers associated with this type of rotation are low and transition from one form to the other is rapid. The predominant stereochemistry of cyclohexane has no influence ia its use as a raw material for nylon manufacture or as a solvent. 

The twist form is also called the flexible form, because without angle strain a left-handed twist (c) can be converted into a right-handed one (d), by way of a third form ca. 7 kJ mol higher in energy (and actually not strictly an intermediate, since it occupies a maximum on the energy profile) which is the boat form (e). 

The reduction of 1 l-keto-9 -H steroids gives an lla-ol, due to the fact that ring B or C must be in the boat form. The presence of bromine at the 2a-, 5a- and 12a-positions does not afifect greatly the isomer composition resulting from reduction of 3-, 6- and 11-ketones respectively. When in a 7-keto steroid the bromine is in the 6a-position a mixture of la- and 7 -ols results when the bromine is in the 6 -position only the 7jS-hydroxy compound is obtained. " The presence of bromine in the 16a- or 16 -positions seems to reduce the amount of 17 -alcohol formed. ... 

The optical rotatory dispersion curves of steroidal ketones permit a distinction to be made between the conformations, and assignment of configuration is possible without resorting to chemical methods (see, e.g. ref. 36) which are often tedious. The axial halo ketone rule and, in the more general form, the octant rule summarize this principle and have revealed examples inconsistent with the theory of invariable axial attack in ketone bromination. 2-Methyl-3-ketones have been subjected to a particularly detailed analysis. There are a considerable number of examples where the products isolated from kinetically controlled brominations have the equatorial orientation. These results have been interpreted in terms of direct equatorial attack rather than initial formation of the axial boat form. 

In the absence of steric factors e.g. 5 ), the attack is antiparallel (A) (to the adjacent axial bond) and gives the axially substituted chair form (12). In the presence of steric hindrance to attack in the preferred fashion, approach is parallel (P), from the opposite side, and the true kinetic product is the axially substituted boat form (13). This normally undergoes an immediate conformational flip to the equatorial chair form (14) which is isolated as the kinetic product. The effect of such factors is exemplified in the behavior of 3-ketones. Thus, kinetically controlled bromination of 5a-cholestan-3-one (enol acetate) yields the 2a-epimer, (15), which is also the stable form. The presence of a 5a-substituent counteracts the steric effect of the 10-methyl group and results in the formation of the unstable 2l5-(axial)halo ketone... 

Ring inversions of 1H- and 4//-azepines between their two stable boat forms have been studied extensively by HNMR spectroscopy.37,38-40-76 82,85 A coalescence temperature of — 55 7 C and a AG value for ring inversion of 42.7 kJ mol have been determined for the two conformers 10 A and 10 B of A-phenyl-3//-azepin-2-amine.82... 

Of simple 1,5-benzothiazepines, only the 2-unsubstituted or 2-phenyl-substituted 4-methoxy-1,5-benzothiazepines are known. X-ray crystallography of the phenyl compound shows that the seven-membered ring exists in a boat form.51... 

For cyclohexane, there are two extreme conformations in which all the angles are tetrahedral. These are called the boat and the chair conformations and in each the ring is said to be puckered. The chair conformation is a rigid structure, but the boat form is flexible and can easily pass over to a somewhat more stable form known as the twist conformation. The twist form is 1.5 kcal mol (6.3 kJ mol )... 

In six-membered rings containing heteroatoms, the basic principles are the same that is, there are chair, twist, and boat forms, axial and equatorial groups, and so on. The conformational equilibrium for tetrahydropyridines, for example has been studied. In certain compounds, a number of new factors enter the picture. We deal with only two of these. ... 

With octalin (55), rotation to the trans chair form (59) followed by protonation from the outside face of the double bond at the less substituted carbon atom leads to the tertiary cation (60) with net equatorial introduction of the proton. Rotation to a trans boat form would lead to net axial protonation<84) ... 

Experimental structural data are available for two crystalline derivatives of c/s,cis-cyclo-octadiene-1,5 (86). The distorted-boat form occurs in both crystals (observed C—CH2—CH2—C torsion angles 65 and 74°, respectively). Hedberg and Hedberg have performed a gas-electron diffraction study and found that the distorted boat conformation predominates also in the gas-phase (87). For the dibenzo derivative, however, there are crystallographic indications that in the solid state the eight-membered ring has the chair conformation (88). 

The crystal structures of two nickel-complexes which contain cis.ds-cyclooctadiene as chelate ligands are known (89). For complex formation the 2v -symmetric boat-form is most favourable. The energetical compromise in the complexes is therefore such that the... 

From the pseudorotating transition state the inversion process proceeds via an intermediate minimum of D2-symmetry (twist-conformation) and across a symmetry-equivalent second pseudorotational transition state to the inverted chair-conformation. The symmetric boat-form of cyclohexane (symmetry C2v) corresponds to a one dimensional partial maximum, i.e. a transition state (imaginary frequency 101.6 cm-1). It links sym-... 

Fig. 18. Top transition coordinates (with symmetry species) of conformational transition states of cyclohexane (top and side views). Hydrogen displacements are omitted. The displacement amplitudes given are towards the C2v-symmetric boat form, and towards >2-symmetric twist forms (from left), respectively. Inversion of these displacements leads to the chair and an equivalent T>2-form, respectively. Displacements of obscured atoms are given as open arrows, obscured displacements as an additional top. See Fig. 17 for perspective conformational drawings. Bottom pseudorotational normal coordinates (with symmetry species) of the Cs- and C2-symmetric transition states. The phases of the displacement amplitudes are chosen such that a mutual interconversion of both forms results. The two conformations are viewed down the CC-bonds around which the ring torsion angles - 7.3 and - 13.1° are calculated (Fig. 17). The displacement components perpendicular to the drawing plane are comparatively small. - See text for further details.

Diazadiphosphacyclooctane (51) interacts with only two molecules of borane with participation of the phosphorus atoms [Eq. (50)]. The number of borane molecules added is likely to be determined by the conformation of the eight-membered ring, which exists in a distorted boat-boat form similar to that of the analogous sulfide. 

Two molecules with comparable geometry in an asymmetric unit were found for 3,4-bis(4-fluorophenyl)-l,2,5-oxadiazole 2-oxide. The bond length of the dipolar N-O bond is 1.107 (7) A <2006AXEo4827>. In the molecule of 5-(6,7-dimethoxy-l,2,3,4-tetrahydroisoquinolin-2-yl)-4-phenyl-l,2,5-oxadiazole Ar-oxide, the six-membered heterocyclic ring has a flattened boat form. Intermolecular C-H- O hydrogen bonds link the molecules into dimers, which may be effective in the stabilization of the crystal structure <2006AXEo3130>. 

Of all these (c) is much less likely to be formed because it involves twisting of the allyl system while (a) and (b) are strain free. Again of these (a) is more favoured because the six p lobes are in a quasi planar arrangement. Moreover the molecular orbital calculations made by Dewar have shown that the central p lobes of the two allyl systems in the boat form appear to have a slight destabilizing effect and this has also been confirmed by Woodward and Hoffmann with the aid of correlation diagrams. But both the chair and boat forms have been found in practice. 

The process of assessing the preferred conformation has become of importance of cyclic compounds, at least in six-membered rings because in their formation, almost all the strains are involved. We have seen that cyclohexane exists mostly in the chair conformation and the boat form occurs in negligible proportion because of higher energy which is of the order of >6 K cals/mole. 

The energy barrier between the chair and boat form of cyclohexane is about 35 k J/mole and this is not large enough to prevent their rapid interconversion at room temperature. This is why it is not possible to isolate each conformation. 

The distance between their centres at normal tetrahedral angles should have been only about 1.8 A, but the sum of the van der Waals radii of the two hydrogen atoms is 2.4A. This is sometimes called bowsprit-flagpole interaction and this too makes its contribution to the increased energy of the boat form. 

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