Twist-boat

Twist boat (Section 3 7) Synonymous with skew boat... 

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. 

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. 

In addition to the chair conformation of cyclohexane, a second arrangement called the twist-boat conformation is also nearly free of angle strain. It does, however, have both sleric strain and torsional strain and is about 23 kj/mol (5.5 kcal/mol) higher in energy than the chair conformation. As a result, molecules adopt the twisl-boat geometry only under special circumstances. 

Twist-boat conformation (Section 4.5) A conformation of cyclohexane that is somewhat more stable than a pure boat conformation. 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

However, this is not true for the addition of carbonyl compounds to (Z)-2-alkenyltitanium, which also proceeds with moderate anti selectivity and requires a (twist) boat transition state16,51. 

The same reaction utilizing chlorotriisopropoxytitanium gives a lower yield and optical purity of the (Z)-anti product ( + )-4 (yield 33% 64% ee). Utilization of tetraisopropoxytita-nium causes complete racemization16. The reaction of (Z)-l-methylbutenyltitanium with both enantiomers of 2-( er/-butyldimethylsilyloxy)propanal proceeds only very sluggishly with approximately 20% yield99. The results are best explained by the assumption of a (twist)boat transition state. 

The stereoselectivity reverts in favor of the. rpn-isomer when bulky aldehydes such as 2,2-di-methylpropanal are employed8. This unusual feature is attributed to a higher reactivity of the (Z)-isomer in the equilibrating reagents15 or to the competition of the twist-boat transition states8. The diastereoselectivity decreases, when DMF is used as a solvent, or pyridine is added to the THF solution. Presumably, the complexation ability of the chromium reagent towards the aldehyde is decreased by these additives8. 

An interesting case of conformational analysis comes to play when we consider a six-membered ring (cyclohexane). There are many conformations that this compound can adopt. You will see them all in your textbook the chair, the boat, the twist-boat. The most stable conformation of cyclohexane is the chair. We call it a chair, because when you draw it, it looks like a chair ... 

According to molecular mechanics (MM) calculations, the minimum energy conformation of the enolate is a twist-boat (because the chair leads to an axial orientation of the f-butyl group). The enolate is convex in shape with the second ring shielding the bottom face of the enolate, so alkylation occurs from the top. 

A further test of the stereoelectronic theory of reactivity of phosphate esters has been attempted using measurements of the rates of displacement of 4-nitrophenate from the esters (23) and (24), their phosphorus epimers, and also (25), in aqueous methanol the introduction of the 4a-Me group into the system would, it was hoped, reduce the the flexibility of the bicyclic structures and so possibly eliminate the participation of twist-boat conformations. The presence of the 4a-Me group has no effect of the rate of displacement of the axial ArO group... 

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