Cyclohexane half-chair

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Examine the geometry of norcarane. What is the conformation of the cyclohexane ring Choose a name (chair, twist boat, half-chair, etc. see Chapter 5, Problem 4) that accurately describes its shape. The bridgehead hydrogens in norcarane are cis. Do you think a trans stereoisomer is possible Explain. 

The annulation of 4//-thiopyran and cyclohexane rings in 50a results in the planarity of the heterocycle and a half-chair conformation of the carbocycle (81KGS1342). On the other hand, a boat conformation of the 2//-thiopyran ring was found in the crystal of224b [91JCS(P2)2061], Other geometrical parameters were within the limits of the expected values (Fig. 2). 

Figure 3. The conformational sphere for pyranoid rings. The perfect chairs are at the north and south poles (0=0 and 180 , respectively). The boat and skew (B and S designations) at the equator permit pseudorotation that is slightly hindered, at least for cyclohexane. The envelopes, E (also called sofas and half-boats), and half-chairs, H, are not observed for rings coiqposed of saturated carbon and oxygen atoms, but are iiqportant forms for rings with unsaturated carbon atoms. The aiqplitude of puckering corresponds to the radius of the sphere.

Note that a cyclohexane system will be forced into a similar half-chair conformation by fusing a planar aromatic ring onto a cyclohexane ring (a tetrahydronaphthalene system). 

Figure 8.9. One octant of a sphere on which the conformations of cyclohexane can be mapped. Special conformations C, chair B, boat TB, twist boat HB, half-boat and HC, half-chair. (From Cremer and Pople [1975a,b].)...

The value of q3 = (6) V2R (R is the CC bond length) is 0.63 A. Under pseudorotation the equatorial boat-shaped structures B (0 = 90°, = 0, 60°, 120°,.. . ) turn into a twist-boat structure TB (0 = 90°, = 30°, 90°,.. . ). The transitions between the chair and twist boat structures involve the intermediate formation of half boat (HB) and half chair (HC) structures. Quantum chemical calculations carried out by Dixon and Komornicki [1990] show that the axial structure C with symmetry D3d is stable. The energies of structures B and TB are 7.9 and 6.8kcal/mol higher than C. The barrier for transition from C to TB is 12.2-12.4 kcal/ mol. Because of the high barriers for pseudorotation, only thermally activated conformational transitions occur in cyclohexane. 

Fig. 5.45 One might have guessed that the chair cyclohexane conformations 1 and I are connected by a boat-shaped intermediate 2. However, this C2v structure shows an imaginary frequency it is a transition state which wants to twist toward 3 (arrows) or 3 (arrows in opposite directions, not shown), which are the actual intermediates (no imaginary frequencies) between 1 and 1. The chair conformation reaches the twist via a half-chair 4...

In 1937 Isbell published an important paper on the conformational analysis of aldopyranoses, in which several of the forms were depicted (see Fig. 2). They comprise a 4C, chair (I), afl03 boat (II), two half-chairs (III and IV), and a coplanar pyranose (V). He correctly favored the chair form (I) and predicted that, in the case of saccharides, the chair would tend to assume a somewhat flatter conformation than that of carbocycles such as cyclohexane because of the smaller bond angles of oxygen as compared to those of carbon (105° instead of 109.5°). We now know that such coplanarity is strongly avoided because of the strain that would be produced and because of the repulsive forces between the substituents. 

Conformational energy of cyclohexane. The chair conformation is most stable, followed by the twist boat. To convert between these two conformations, the molecule must pass through the unstable half-chair conformation. 

The conformations of the six-membered ring systems are better characterized than those of the less stable five-membered analogues. For example, the cyclohexane molecule can occur in two strainless forms, namely in the rigid chair form or in the flexible form (Fig. 2-9). The latter can exist in a variety of shapes of which only the boat and the skew boat (or twist) are regular and easily depictable on paper. The chair form is preferred energetically because it is usually free from steric interactions whereas the flexible forms are not. The half-chair conformation is possible, when a six-membered ring contains either a double bond or an oxiran ring. In the half-chair conformation four adjacent atoms are in the same plane. 

Fig. 2-9. Conformations of cyclohexane. 1, Chair 2, boat 3, skew boat 4, half-chair.

Six-membered rings with more than one sp2 C atom do lose their chair conformation—they become flattened to some degree when there are one or more double bonds included in the ring. Cyclohexene, with just one double bond in the ring, has a half-chair conformation similar to that of its related epoxide, cyclohexene oxide. The usual conformational diagram of cyclohexene is shown below. The barrier for ring inversion of cyclohexene is around 22 kj mol-1 (about half that for cyclohexane ). 

We discussed the conformation of some five-membered rings in Chapter 32 a saturated five-membered ring has a conformation variously called a half-chair or an envelope . It does look a bit like an opened envelope with one atom at the point of the flap, or it looks like most of (five-sixths rather than half ) a chair cyclohexane. 

The skeletal geometry of cyclohexene and 1,2-epoxy cyclohexane systems corresponds to a partially flattened chair or half-chair (8). The alternative "half-boat conformation has... 

In the half-chair conformation of cyclohexane, four ac//acent carbon atoms are in one plane with the fifth above this piane and the sixth beiow it. You will this conformation again later—it represents the energy minimum for cyclohexene, for example. 

As has been pointed out by Hart <84CHEC-I(7)185>, thiiranes fused to cyclopentanes adopt a boat conformation with C(3) and the sulfur atom on the same side of the C(1)C(2)C(4)C(5) plane. The angle of deviation from this plane is about 30° for C(3) and 60-70° for the S atom. Cyclohexane rings having fused thiiranes prefer a twisted half-chair conformation. Thiiranes fused to larger cycloalkanes can adopt a wide variety of conformations. 

---