Atropisomerism in Compounds other than Biphenyls

Atropisomerism in Compounds other than Biphenyls: Phenomenon of atropisomerism in compounds other than biphenyls, Detection, rules and examples

1. DOS&R IN ORGANIC CHEMISTRY TUMKUR UNIVERSITY By PRUTHVIRAJ K Faculty DOS&R in Organic Chemistry KPR. DOS&R in ORGANIC CHEMISTRY TUT Atropisomerism in Compounds other than Biphenyls

2. Some of the molecules which different than biphenyl also show the atropisomerism . These molecules are linked together through a pivotal bond and rotation around the pivotal bond is restricted. The atoms joined through the pivotal bond are usually sp2 hybridized. One or both of the phenyl groups are replaced by other heteroaromatic or aromatic rings. Appropriately substituted molecules ( 15 ) are resolvable 3,3’-bipyridyl ( 16 ) can also resolved and it exits in two enantiomer due to atropisomerism KPR. DOS&R in ORGANIC CHEMISTRY TUT

3. Two phenyl groups (A and B) are introduced on p-substitution at biphenyls to form para terphenyl derivative and restricted rotation may arise around two pivotal bonds so two terminal phenyl rings (A and B) are co-planar as well as co-axial Molecule 17 is Cis , where both bromide on phenyl ring A and ring B are on same side. The molecule is C2 symmetric and also resolvable and chiral axis is passing through ring A and middle ring of the biaryl and another chiral axis is passing through ring B and middle ring of the biphenyl. The cis molecule has two chiral axes so it will have enantiomers as well as diasetreromers . If molecule B is trans than it will possess the inversion center and Ci point group and compound will be meso (same like in two chiral center tartaric acid) KPR. DOS&R in ORGANIC CHEMISTRY TUT

4. If one of the planar ring is replaced by an acyclic grouping (substituted alkene) which is two dimensionally. This type of molecules 19 may give atropisomerism if sufficient steric hindrance is created around the pivotal bond. The sp3-sp3 single bond is restricted through various extents but the energy barrier is too low so such type of molecules cannot be isolated. In the triptycene type molecules, however the barrier to rotation around a 9-substituted bond may be quite high and these atropisomers (20 and 20’) can be isolate at room temperature KPR. DOS&R in ORGANIC CHEMISTRY TUT

5. One or both of the phenyl groups are replaced by other aromatic or heteroaromatic rings, thus appropriately substituted N- Phenylpyrrole , N,N’- bipyrryl , 1,1’-binapthyl are resolvable. The two phenyl groups are interposed by a phenylene ring forming para terphenyl derivtaives and restricted rotation may arise around two pivotal bonds so that two terminal phenyl groups may coaxial as well as planar. This corresponds to a planar combination of two two-dimensional chiral units and both diastereomerism and enantiomerism may occur. KPR. DOS&R in ORGANIC CHEMISTRY TUT

6. Thus the terphenyl deriavatives in which all the 8 orthopositions are substituted exists in two achiral diastereomers which can be described as cis and trans with respected to two Br atoms, both the cis and trans isomers have C 2 axis, the cis has two σ v planes and the trans one σ h plane: the trans, in addition has centre of symmetry, belongs to C 2v & C 2h pount groups respectively. In the case of lower moieties cis belongs to C 2 and trans remains with Centre of symmtery and has meso like property of tartaric acid. KPR. DOS&R in ORGANIC CHEMISTRY TUT

7. One of the planar ring is replaced by an acyclic grouping which is two dimensionally chiral usually due to a dissymmetrical substituted trigonal atom, may exhibit atropisomerism if sufficient steric hindrance is created over pivotal bond. KPR. DOS&R in ORGANIC CHEMISTRY TUT

8. Atropisomerism in nonbridged and bridged biaryl compounds due to restricted rotation through pivotal bond. Biaryl compound with appropriate different ortho susbsituent on each aryl ring will so the atropisomerism . The bulkier groups on ortho position of the biaryl ring restrict the rotation through C-C bond gives two enantiomers and resolvable at room temperature. I n bridged biaryls effect of bridging on the restricted rotation of biaryl system is depends on the ring size. Some of the molecules which different than biphenyl also show the atropisomerism . The sp3-sp3 single bond is restricted through various extents and triptycene type molecules the barrier to rotation around may be quite high and these atropisomers can be isolate at room temperature. KPR. DOS&R in ORGANIC CHEMISTRY TUT