Flagpole interaction, cyclohexane boat

The boat conformation of cyclohexane has significant torsional strain (from eclipsing H s as well as flagpole interactions). The boat can alleviate some of its torsional strain by twisting, giving a conformation called a twist boat. 

Boat cyclohexane has torsional strain and flagpole interaction. 

Figure 4.15 (a) Illustration of the eclipsed conformation of the boat conformation of cyclohexane, (b) Flagpole interaction of the Cl and C4 hydrogen atoms of the boat conformation. 

Corey and co-workers also developed parameters to evaluate boat conformations, which are often important contributors to the conformational population of cyclohexane derivatives (see above). The energies for boat conformations with one or more substituents in the flagpole positions use a new term (b) (see 210 and 211) which is derived from the bowsprit-flagpole interactions (R-H in 210 or R -R in 211). Corey s examples include 212 and 213. In 212, the axial hydroxyl group receives a value (Aqh) of 0.8 kcal moT (3.35 kJ moT, see Table 1.5) and b] = 0 for bond a and b2 = 0.60 for bond c (see typical b values in Table 1.6. The conformational energy for 212 is (0.6 x 0.8) = 0.48 kcal moT (2.01 kJ moT ). 

There are many other nonplanar conformations of cyclohexane, one of which is the boat conformation. You can visualize the interconversion of a chair conformation to a boat conformation by twisting the ring as illustrated in Figure 3.9. A boat conformation is considerably less stable than a chair conformation. In a boat conformation, torsional strain is created by four sets of eclipsed hydrogen interactions, and steric strain is created by the one set of flagpole interactions. Steric strain (also called nonbonded interaction strain) results when nonbonded atoms separated by four or more bonds are forced abnormally close to each other—that is, when they are forced closer than their atomic (contact) radii allow. The difference in potential energy between chair and boat conformations is approximately 27 kj/mol (6.5 kcal/mol), which means that, at room temperature, approximately 99.99% of all cyclohexane molecules are in the chair conformation. 

The 1,6-addition to a,s, y, a-dienones is also subject to stereoelectronic effects. Addition on the bottom face of dienone 137 leads to a chair-like intermediate while that on the top face leads to a boat-like intermediate 140 in order to maintain maximum orbital overlap. Also, in 140 the R group encounters an eclipsed 1,2-R/H interaction and more importantly, a 1.4-CH3/R steric interaction which resembles the bowsprit flagpole arrangement of a twist-boat form of cyclohexane. This analysis of Marshall and Roebke (48) predicts that the trans product 139 should prevail over the cis product 141. 

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