Half-chair

In fhe envelope confermafion four of fhe carbon afoms are coplanar The fiflh car bon IS ouf of fhe plane of fhe ofher four There are fhree coplanar carbons m fhe half chair confermafion wifh one carbon atom displaced above fhaf plane and anofher below if In bofh fhe envelope and fhe half chair confermafions m plane and ouf of plane carbons exchange posifions rapidly Equihbrafion befween confermafions of cyclopenfane is very fasf and occurs af rafes similar to fhaf of rofafion abouf fhe carbon-carbon bond of efhane... 

FIGURE 3 12 The (a) planar (b) envelope and (c) half chair conformations of cyclopentane... 

Cyclopentane has two nonplanar conformations that are of similar sta bility the envelope and the half chair... 

Half chair (Section 3 6) One of the two most stable conforma tions of cyclopentane Three consecutive carbons in the half chair conformation are coplanar The fourth and fifth carbon he respectively above and below the plane... 

Saturated five-membered heterocyclic compounds are non-planar, existing in half-chair or envelope conformations. The far-IR spectra of THE and 1,3-dioxolane (127) show both to have barriers of ca. 0.42 kJ moP ... 

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

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. 

Dithiolane (132) derivatives also possess non-planar skeletons the most important conformation is probably of symmetry C2 (half-chair). The dithiolane ring may be quite flexible and a minimum energy. conformation is only well defined if there is a bulky substituent at the 2-position. 

The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffiaction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. ... 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

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. 

Bond orders, charges on the atoms in 1 l//-pyrido[2,l-Z)]quinazolin-l 1-one and its protonated form were calculated by quantum chemical calculations by the semiempirical AMI method. According to the results, the equilibrium conformation of the ring in 1 l//-pyrido[2,l-Z)]quinazolin-l 1-one is planar, while l//-pyrimido[l,2-u]quinolin-1-one adopts a conformation close to a half-chair due to the unfavorable interactions between the oxygen atom of the carbonyl group and the ring C-10 atom in the pen-position (97MI22). 

The vibrational spectrum of 1,4-dioxin was studied at the MP2 and B3-LYP levels in combination with the 6-3IG basis set [98JST265]. The DPT results tend to be more accurate than those obtained by the perturbational approach. The half-chair conformation of 4//-1,3-dioxin 164 was found to be more stable than the corresponding conformations of 3,4-dihydro-1,2-dioxin 165,3,6-dihydro-1,2-dioxin 166, and of 2,3-dihydro-1,4-dioxin 167 (Scheme 114) [98JCC1064, 00JST145]. The calculations indicate that hyperconjugative orbital interactions contribute to its stability. 

No annular tautomeric equilibrium transformations in compounds of the diox-ane series have been reported yet recently (97JCC1392), however, the optimized geometries and total energies of unsubstituted isomeric 3,4-dihydro-1,2-dioxin 22 and 3,6-dihydro-1,2-dioxin 23 were calculated using ab initio 3-21G, 6-31G, and MP2/6-31G //6-31G methods. All the methods applied revealed that the total energies for half-chair conformations of 22 and 23 are approximately the same. 

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). 

The coordinated cyclohexenones react from half-chair conformations A and B in order to show a maximum of tr-overlap. The 4-methyl-2-cyclohexenone prefers, for stereoelectronic factors, the half-chair A, which leads to the ci.s-product on (2-propenyl)silane addition (path a), even if this is not the sterically least hindered approach. 

The 5-methyl-2-cyclohexenone prefers, for stereoclectronic reasons, the half-chair B. Now, both steric- and stereoelectronic factors work in the same direction, to yield in all cases the Irani-products (path d). 

FIGURE 16. The half-chair and envelope models of sulfolane. 

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