Twist Isomers

The twisted boat form (twist boat) of the cyclohexane molecule is a simple example of a helical alicyclic hydrocarbon skeleton. It is a conformation of the cyclohexane skeleton with low conformational stability (Fig. 22) 

By bridging of suitable carbon atoms of cyclohexane, the twist boat conformation is fixed. Two ethano bridges in the 1,4- and 1,5-positions of cyclohexane as clamps lead to tricyclo[4.4.0.0 ]decane (twistane, 3a, b). In addition to four equally sub- 

According to Brewster a net P-helicity (P,P,M,P,M) and a net M-helicity (M,M,P,M,P) can be assigned to P-twistane 3b) and M-twistane 3a), respectively. The first synthesis of optically active twistane has been reported by Nakazaki . He postulated — as did shortly afterwards Tichy — an absolute configuration for (-l-)-twistane, which was corrected later by Tichy as follows (-l-)-5Z corresponds to iR,3R,6R,8R), )-3b corresponds to (1S,3S,6S,8S) But even today there are some uncertainties with respect to the absolute configuration in the twistane series . Numerous aliphatic and heterocyclic molecules having helical twistane skeletons have been synthesized and characterized stereotopologically. Some of them are shown in Table 5 together with some chiroptical data. 

The high rotational strengths of molecules like 3 and 34-50 are remarkable. The helicity of the skeleton may account mainly for this. Molecules exhibiting the same rotational sign show the same helicity too, e.g. the enantiomer rotating counter clockwise is correlated to minus (M) helicity 

The dioxa analogues 37-40 of the twistanes 3a, b and of 2,7-twistadienes 34, 35 show lower rotation strengths compared to the corresponding carbocyclic skeletons. 

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Stanton ° confirmed the assignment of type n as the twist isomer by computing the NMR shifts of 39, 42, and 44, shown in Table 3.27. The heart isomer (44) gives rise to six signals, inconsistent with the five observed in the experiment. The differences between the experimental chemical shifts and the computed values for isomer 39 are less than 1.1 ppm but 13.1 ppm for 42. The relative spacings between the peaks also match much better with 39. This still leaves open for question the exact nature of Masamune s type I isomer. 

Diamagnetic dinitrogen tetroxide (AHf(g) = - -ll.l(l)kJ mol ) has a planar (D2h) geometry (66). The activation energy for internal rotation is estimated to be 40kJmol , the twisted isomer has been detected under nonequihbrium conditions. Table 24 shows a comparison of the bonding parameters of N2O4 in the solid state and in the gas phase. 

A series of remarkable transformations takes place when 95 is irradiated with 254-nm light. As has been shown by X-ray structure determination, this radialene adopts the chair configuration c-95 in its ground state. On irradiation c-95 is first converted into the twist-isomer t-95, whose structure has been determined by X-ray analysis again . In a second step, two photochemical 1,5-hydrogen shifts take place leading to the ara-xylylenes 173 and 174 (equation 22). Normally, fiiese all-carbon analogues of /7-benzoquinone are very reactive and cannot be obtained in substance. In the present instance they can be handled easily under normal laboratory conditions . Most likely this stability is caused by the complete substitution of the ara-xylylene core of these hydrocarbons. 

Scheme 33 The probable twist isomers of the R-D-mannopyranose boronate esters, where R=m-nitro phenylboronic acid. ...

The photochromism of spirooxazines has been investigated by Chu, Schneider, and many other researchers. Their application potential seems to be very powerful. Ring opening of the Sp form of SO occurs via the Sj state and takes place in the psec domain. " The quantum yield is rather large in solvents of either low or large polarity. A biexponential thermal decoloration was observed, which is consistent with fully established equilibria between the Sp form and two photoisomers. Therefore, two separate steps leading to the ring-closure were considered for SO. In addition, a more twisted isomer was spectroscopically and kinetically observed upon irradiation of the most stable... 

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

The Z,Z,Z,Z,Z-isomer is required by geometry to have bond angles of 144° to maintain planarity and would therefore be enormously destabilized by distortion of the normal trigonal bond angle. The most stable structure is a twisted form of the , Z,Z,Z,Z-isomer. MO (MP2/DZd) calculations suggest an aromatic stabilization of almost ISkcal for a conformation of the , Z,Z,Z,Z-isomer in which the irmer hydrogens are twisted out of the plane by about 20°, but other calculations point to a polyene structure. ... 

The syn isomer can achieve a conjugated system with angles of up to 35° between adjacent p orbitals. The anti isomer is much more twisted. An X-ray crystal structure of the syn isomer shows C—C bond lengths between 1.368 and 1.418 A for the conjugated system (Fig. 9.3). ° The spectroscopic properties of the syn isomer are consistent with considering it to be a delocalized annulene. B3LYP calculations indicate that both the syn and anti structures are stabilized by delocalization, the syn (17.6kcal/mol) more so than the anti (8.1 kcal). ... 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Thus, sulfur extrusion from the twisted naphtho[3,4-c/]thiepin is rapid, because in this case less energy is required to form the thianorcaradiene isomer than for a total loss of aromaticity. 

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. 

Although the —CH2— group could be inserted in other places, the free rotation about single C—C bonds in hydrocarbons allows the resulting molecules to be twisted into one or the other of these two isomers. Both compounds are gases, but butane (24) condenses at —1CC, whereas methylpropane (25) condenses at — 12°C. Two molecules that differ only by rotation about one or more bonds may look different on paper, but they are not isomers of each other they are different conformations of the same molecule. Example 18.3 illustrates how to tell if two molecules are different isomers or different conformations of the same isomer. 

Molecules (b) and (c) are identical. The atoms of molecule (e) are joined together in the same arrangement as that of (b) and (c), but the structure looks different because it has been rotated 180° and twisted so (b), (c), and (e) are the same. There are therefore only three distinct isomers with formula C5H12 (a), (b), and (d). 

In polysulfide solutions there exist chainlike dianions with practically arbitrary chain lengths n. Berghof, 8ommerfeld, and Cederbaum have investigated the onset of stabihty in the 8 (n=2-8) series of dianions based on 8CF calculations with the DZPD basis set [89]. The isolated chainlike 8 isomers were found to exhibit twisted structures, and the onset of electronic stabihty (with respect to electron auto detachment) was predicted to occur at 71=7. Branched isomers were found to be electronically more stable than the... 

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