Arylmethanes results

On the other hand, to constrain the cyclic, the blocking group (BG) has to be bulkier than the cavity of the cyclic molecule. Harrison found that a trityl group can only block cyclics up to 28-membered whereas the tris(p-t-butylphenyl)-methyl moiety can effectively constrain 42-membered cyclics [3, 13, 14]. These results have been widely applied as a guide in the preparation of polyrotaxanes e.g., monofunctional [18, 19] and difimctional [19—23] BG based on tri- and tetra-arylmethane derivatives were successfully prepared and used as end groups and in-chain units, respectively, to constrain crown ethers in a variety of polyrotaxanes. 

The well-known indicator phenolphthalein (8) is an example of tri-arylmethane dyes which represents a cyanine-type chromogen and does not fluoresce. If an oxygen bridge is introduced between the upper benzene rings, the intensely green fluorescing flourescein (1) results. 

The general explanation of this selectivity is the different stability of the product under acidic conditions. The more donating the aromatic compound the less selective the reaction. Very recently Anthony reported the hydroxyalkylation of benzene with zeolite over ZSM-5 zeolite [15], Although the results are modest, they indieate that the formation of the bis-arylmethane adduct can be avoided by use of a medium-pore zeolite. 

Low-symmetry molecular species can appear as a result of restricted reorientation and the interplay between stere-ogenic units. A tris-triarylmethane system containing chlorine atoms in all aromatic positions (3) has only three protons, one in each of the arylmethane units. However, the NMR spectrum of this species in solution (Figure 2) displays 16 proton signals. The unequal intensities of the peaks point out that they belong to different conformations in slow exchange. Analysis of the rather complex stereochemical properties of this system confirms the existence of six diastereomeric forms, five of which have Cl symmetry and therefore three proton signals, and one has C3 symmetry and contributes only one additional proton. 

The results obtained in this paper will now be discussed and compared with those previously reported [18, 49] for similar dendrimers terminated with bulky tetra-arylmethane groups (compounds and Tables 4.1 and 4.2). 

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