73183-81-0Relevant articles and documents
Modified Tetrahelicene Systems, III: Doubly ortho-Bridged Triphenylamine Derivatives
Hellwinkel, Dieter,Schmidt, Werner
, p. 358 - 384 (2007/10/02)
The heterocyclic (2a, b, 19, 3b-f, and 9a-c) as well as the carbocyclic naphthanthracene derivatives (21-23), which can be represented by the skeleton types A and B (table 2), are helically distorted in the stereochemical ground state.They racemize so fast, however, that their free enthalpies of racemization (ΔG* * 21 (88) kcal(kJ)/mol) could be determined by standard DNMR methods.Only derivatives of compound type 3 with relatively large bridge Y (3a, Y = C(CH3)2, 3g', Y = S) exhibit higher racemization barriers (ΔG*160 = 24 (100.4) and 28.4 (119) kcal(kJ)/mol), which had to be evaluated by classical equilibration procedures.With the results obtained in this way the working hypothesis has been confirmed, according to which for compounds A and B enlargement of the bridges Y and/or X, as well as diminution of the center Z - in holding the periphery constant - should lead to an increase of nonbonding interactions in the planar transition state and therefore to an increase of the racemization barrier.It has furthermore been shown with numerous substitution products of compound type 1 that here frequently accidental coincidences of the 1H NMR signals of diastereotopic and also constitutopic groups do occur.
