Modern Friedel-Crafts chemistry. Part 31. † An efficient synthetic approach to mono-, di-and triphenylindane derivatives via direct Friedel-Crafts cyclialkylation of selected phenylated alkanols

Facile procedures for the synthesis of mono-, di-and triphenylindanee derivatives from the alcohols 1 - 4 are described thus treatment with 85% H 2 SO 4 , AlCl 3 /CH 3 NO 2 , H 3 PO 4 and/or PPA under varying conditions produced 1,1-dimethyl-3-phenylindanee 6 from 2-methyl-4,4-diphenyl-2-butanol 1 , 3,3-dimethyl-1,1-diphenylindanee 9 from 2-methyl-4,4,4-triphenyl-2-butanol 2 , 1-methyl-1,3-diphenylindanee 12 from 2,4,4-triphenyl-2-butanol 3 and 1,1,3-triphenylindanee 15 from 1,1,3,3-tetraphenyl-1-propanol 4 . The starting and final products were characterized by elemental, IR, 1 H NMR and MS analyses.

The failure of 85% H 2 SO 4 to induce complete closure of 5 is probably due to the greater tendency, under these conditions, to localize the positive charge on the tertiary center, so elimination to 3-methyl-1,1-diphenyl-2-butene 7 is favored.19).These results can be attributed to two factors: (i) the steric crowding of the two phenyls encountered in closure of tertiary carbocation 14 to 15 and (ii) the doubly benzylic nature of 14 causes extensive delocalization of the positive charge over the two phenyl groups resulting in a lower energy, and hence a less reactive reaction site in the intermediate cation.The relative retardation influence of these steric and electronic factors on the ring closure step is hard to measure, but it is believed that the steric factor plays the major part.

Conclusions
Commenting on the results of Table 1, it is useful to point out the following: (i) Friedel-Crafts cyclialkylations provide a facile route for the synthesis of mono-, di-and triphenylindane derivatives.
(ii) Intramolecular reactions are much favored over intermolecular ones as indicated by the fact that compound 9 was solely obtained in spite of the presence of nucleophilic benzene as solvent.
(iii) The results of Table 1, especially those with 85% H 2 SO 4 , reveal that intramolecular ring closure is highly dependent on steric factors.Thus, the ease of ring closure to indanes seems to follow the following order between the employed tertiary alcohols:

Experimental Section
General.Melting points were measured on a digital Gallenkamp capillary melting point apparatus and are uncorrected.Infrared spectra were determined with a Shimadzu 470 infrared spectrophotometer using KBr wafer and thin film techniques (ν cm -1 ). 1 H NMR spectra were recorded by 90 MHz Varian NMR spectrometer using the appropriate deuterated solvent with TMS as internal standard.Chemical shifts (δ) and J values are reported in ppm and Hz, respectively.Elemental analyses were performed on a Perkin-Elmer 2400 Series II analyzer.The mass spectra were performed by JEOL JMS 600 spectrometer at an ionizing potential of 70 ev using the direct inlet system.Reactions were monitored by thin layer chromatography using precoated silica plates (Kieselgel 60, F 254, E. Merck), visualized with UV light.Flash column chromatography (FC) was performed on silica gel (230-400 mesh, E. Merck).All reagents were purchased from Merck, Sigma or Aldrich Chemical Co. and were used without further purification.