Triptycene hydrogenation

The stable carbanions may belong in a special category since their stability is in most cases due to resonance, and the resonance has geometrical requirements that might or might not be the same as those of the bond hybridization of an ordinary carbanion. The central hydrogen of triptycene has none of the acidity of the central hydrogen of triphenylmethane.364... 

Exhaustive catalytic hydrogenation of triptycene affords an equilibrium mixture of perhydrotriptycene isomers. As expected, Boyd s force field (37) calculations, with a modified torsional constant, reproduced the observed composition fairly well (Table 6). All important conformations were taken into account for each isomer. The most stable conformations agree with the results of the X-ray analysis (131) and have the characteristic that the cyclohexane rings are invariably either boat or deformed chair. The most stable conformation of all is 20 (ttt). The predominant conformation of ccc, in which all cyclohexane rings are boat, has an enthalpy only 2.56 kcal/mol above that of 20. The difference is virtually all due to angle and torsional terms. 

However, preorganization by reversible interactions via hydrogen bonds does not necessarily need a further template such as an anion. Direct intermolecular hydrogen bonds between derivatives of isophthalic acid and 2,6-pyridine diacid were used to synthesize a series of new catenanes [17,18], rotaxanes [18c,19], and trefoil knots [20]. The interaction of dibenzylammonium cations and dibenzo[24]crown[8] [21] led to the preparation of rotaxanes by the attachment of stoppers to the thread ends (Scheme 5.5) [22], or similarly by clipping of the crown ether chain wrapped around the ammonium dumbbell [23]. Recently, several novel catenanes were synthesized via the macrocydic connection of three ammonium cations threaded through three crown ether rings attached to a triptycene core [24]. 

Fig. 1. Some of the theoretically possible disubstituted triptycenes. The X groups may or may not all be identical in the triptycenes known at the present time they are usually hydrogen, fluorine, or chlorine.

A triptycene-based tris(crown ether) has very recently been used to form a tris[2]pseudorotaxane precursor and this has in turn been converted into [4]pseudocatenane 183 by means of threefold olefin metathesis, in the presence of Grubbs catalyst 152, followed by hydrogenation <2005JA13158>. 

Another compound of unique structure related both to the triblattanes and to D3 perhydrotriphenylene mentioned above is the perhydrotriphenylene (221), m.p. 195°C, isolated by Farina et al. (183) from a mixture of stereoisomers obtained by medium-pressure catalytic hydrogenation (10% Pd/C, 60 atms., 150°C) of triptycene. Simple patterns observed in the l3C NMR and H NMR spectra of this isomer suggested its D3 symmetry, which was confirmed by X-ray analysis. 

The syntheses of other triptycene derivatives have been carried out similarly (308-314). These compounds have been characterized, in particular, by their IR spectra. For many of them the kinetics of hydrogenation on nickel and palladium have been measured simultaneously with the measurement of the electric potential of the catalyst, and for the quinones, in addition, polarographic reduction on a mercury electrode was investigated (315-317). The compounds which were synthesized by us (many of them for the first time) are listed in Table V. The substances which were subjected to catalytic hydrogenation are marked with an asterisk. 

It has been shown that in hydrogenation of triptycene quinones the more complicated structure of the quinone molecules leads to an increase of the rate, which changes with decreasing constant of formation of semiquinone. 

The multiplet theory permits the building of stereochemical models of active complexes of hydrogenation of the compounds (VII) and (VIII). Since the molecules of triptycene possess a rigid structure, except for flattening or inversion according to the Sn2 mechanism, the molecule cannot accommodate an index group on an even surface. Therefore, one should assume the existence of elevations on the surface of the catalyst. As the Cn=Ci6 bond is internal, the molecule must superimpose on the elevation that carries the (111) facet (see above). 

It could be expected that an increase in the number of benzene rings in the triptycene groups would bring about some spatial difficulties in hydrogenation according to the principle of structural correspondence... 

Investigation of the hydrogenation of quinones of the triptycene series (Table V) has shown that this reaction is characterized by an equation of the first order, which is proved both by the constancy of the... 

As stated above, the rigid triptycene moieties led to a relatively fixed cavity in the triptycene-derived calixarenes more than that, the triptycene was quite electron-rich, which further facilitated host-guest complexation via non-covalent interactions including hydrogen bonds, C H Jt and Jt- Jt stacking interactions. 

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