Twist conformations

Figure 8-6. Comparison of the radial distribution function of the ctiair, boat, and twist conformations of cyclohexane (hydrogen atoms are not considered).

Conformation. Neutron diffraction studies of sucrose revealed the presence of two strong intramolecular hydrogen bonds 0-2—HO-1 and 0-5—HO-6 in the crystal form (7,8). These interactions hold the molecule in a weU-ordered and rigid conformation. The two rings are disposed at an angle close to 90°, with the glucopyranosyl and fmctofuranosyl residues adapting chair and T" twist conformations, respectively. 

Trioxane 210 has been used as a model system by Gu and coworkers to study the antimalarial drug artemisinin 211 (Scheme 137) [97CPL234, 99JST103]. It is the boat/twist form rather than the chair conformer of 210 that describes the subunit in 211. Moreover, geometric parameters and vibrational frequencies can only reliably be computed at the DFT level and by post-Hartree-Fock methods. B3-LYP/6-31G calculations on the conformers of 3,3,6,6-tetramethyl-1,2,4,5-tetroxane show that the chair conformer is stabilized with respect to the twisted conformer by about -2.8 kcal/mol [00JST85]. No corresponding boat conformer was found. 

The results described thus far sketch the synthetic demands for being able to prepare processable, structurally defined PPPs, in which the 7r-conjugalion remains fully intact or is even increased as compared to that of the parent PPP 1 system. The key step in the realization of this principle is the preparation of a PPP in which the aromatic subunits can be obtained in a planar or only slightly twisted conformation in spite of the introduction of substituents. 

On page 132, atropisomerism was possible when ortho substituents on biphenyl derivatives and certain other aromatic compounds prevented rotation about the bond. The presence of ortho-substituents can also influence the conformation of certain groups. In 88, R= alkyl, the carbonyl unit is planar with the trans C=0 - F conformer more stable when X=F. When X=CF3, the cis and trans are planar and the trans predominates. When R = alkyl there is one orthogonal conformation but there are two interconverting nonplanar conformations when R=0-alkyl. In 1,2-diacylbenzenes, the carbonyl units tend to adopt a twisted conformation to minimize steric interactions. " ... 

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 some molecules, the twist conformation is actually preferred. In all cis-2,5-di-fert-butyl-l,4-cyclohexanediol, hydrogen bonding stabilizes the otherwise high-energy form and 1,3-dioxane 89 exists largely as the twist conformation shown. Of course, in certain bicyclic compounds, the six-membered ring is forced to maintain a boat or twist conformation, as in norbornane or twistane. 

Deoxy-3-fluoro- -D-xylofuranosyl)cytosine (821) was prepared by condensation of 2,5-di-0-benzoyl-3-deoxy-3-fluoro-D-xylofuranosyl bromide with bis(trimethylsilyl)cytosine (773). It was proposed that 1 -(3 1eoxy-3-fluoro-)S-D-arabinofuranosyl)uracil (823), prepared from l-(2,3-anhy-dro-)S-D-lyxofuranosyl)uracil (822) with anhydrous HF ( 8% in 1,4-dioxane 116 °, 41 h, 3% yield), has a twist conformation of the furanose ring(°r, acetone- 4-D20 . Jn-vf 13.5, Hz). Reexamination ... 

Each of the three tetrakis(trialkylsilyl)disilenes 22-24 has an absorption band near 420 nm but compound 24, which bears the largest substituents, also exhibits a rather strong absorption at exceptionally low energy, max 480 nm.21 The likely explanation of this anomaly is that a strongly twisted conformation of 24 may exist in solution. Among the tetraalkyldisilenes, the unusually long wavelength absorption at 433 nm for 21 may also result from a twisted conformation.4... 

Thermochromism was already noted for the first stable disilene l.2a In dialkyl- or bis(trimethylsilyl)diaryldisilenes, the size of the aryl groups affects the thermochromism Tip-substituted ones (9,10) are thermochro-mic,10 while Mes-substituted ones (3,4) are not.9 Interestingly, tetrakisftri-alkylsilyl)disilanes 22 and 23 are highly thermochromic hexane solutions of 22 and 23 are light yellow below 0°C but dark red above 50°C.21 These observations were interpreted in terms of a thermal equilibrium between the bent and twisted conformations. Compound 24, having an absorption band at 480 nm, is dark red in solution even at room temperature. 

A number of fluorescent dyes with internal charge transfer mechanism allow the molecule to twist (rotate) between the electron donor and electron acceptor moieties of the fluorescent dipole. In most cases, the twisted conformation is energetically preferred in the excited Si state, whereas the molecule prefers a planar or near-planar conformation in the ground state. For this reason, photoexcitation induces a twisting motion, whereas relaxation to the ground state returns the molecule to the planar conformation. Moreover, the Si — So energy gap is generally smaller in the twisted conformation, and relaxation from the twisted state causes either a... 

Vogel M, Rettig W (1985) Efficient intramolecular fluorescence quenching in triphenyl-methane dyes involving excited states with charge separation and twisted conformations. Ber Bunsenges 89(9) 962-968... 

Perfluoro-substituted four- and six-membered (Si-N) ring systems (3, 5) can be obtained by reacting dilithiated silylamines with SiF4. Cyclodisilazanes of the type 3 are the smallest known four-membered (Si-N) rings. Transannular Si Si ring distances of 237 pm are found [1,3]. For the first time cyclotrisilazane crystallizes in twist conformation [1]. 

Depending on the molar ratio, 8 reacts with SiF4 to give the acyclic or cyclic compounds 9 or 10. 10 represents the first halofunctional hydrazine ring system. The ring is far from planar and has a twist conformation [4,5]. 

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-... 

By flexing to the twist conformation, the boat conformation can relieve some of its torsional strain and reduce the flagpole interactions. 

Figure 4.16 (a) Carbon skeleton and (b) line drawing of the twist conformation of cyclohexane. 

The energy barrier between the chair, boat, and twist conformations of cyclohexane are low enough to make separation of the conformers impossible at room temperature. 

Molecular rotors are fluorophores characteristic for having a fluorescent quantum yield that strongly depends on the viscosity of the solvent [50], This property relies on the ability to resume a twisted conformation in the excited state (twisted intramolecular charge transfer or TICT state) that has a lower energy than the planar conformation. The de-excitation from the twisted conformation happens via a non-radiative pathway. Since the formation of the TICT state is favored in viscous solvents or at low temperature, the probability of fluorescence emission is reduced under those conditions [51]. Molecular rotors have been used as viscosity and flow sensors for biological applications [52], Modifications on their structure have introduced new reactivity that might increase the diversity of their use in the future [53] (see Fig. 6.7). 

Structural investigations of individual stereoisomers and equilibrium mixtures revealed some general features (83M11). A phenyl group at phosphorus is predominantly in an axial position when there is no steric hindrance. The introduction of bulky substituents onto carbon atoms of the ring shifts the equilibrium toward an equatorial form, and one more twist conformer appears. The influence of equatorial substituents on the equilibrium position is clearly pronounced in the case of P(III) derivatives as well as for a P(IV) compound. The substitution of a phenyl group by a... 

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