DIBORANE AS A REDUCING AGENT .8. REDUCTION OF TRIFLUOROACETYLINDOLES TO TRIFLUOROETHYLINDOLES AND HYDROXYTRIFLUOROETHYLINDOLES AND NOVEL DIMERIC 2-TRIFLUORO-1[1'-(3'-ALKYLINDOLINO)]ETHYLBORANES

Reduction of the trifluoroacetylindoles 1-5 with diborane affords the hydroxytrifluoroethylindoles 7-10 (20-70%), the trifluoroethylindoles 6 and 11 (8.4-18.5%) and the unique and dimeric indolino-trifluoroethy lboranes 16, 17a and 17b (0.6-12%). This appears to be the first repor t on the formation of alkylboranes in the reduction of a carbonyl chro mophore. The existence of indolinoalkylboranes in the form of the dime rs 16, 17a and 17b is confirmed by B-11 NMR and mass spectra, which sh ow molecular ions for both the monomers and the dimers. Compound 5 doe s not form dimers corresponding to 16 and 17,probably due to the elect ron-withdrawing effect of its phenyl group. The results have been disc ussed in relation to the mechanisms of diborane reduction and the orig in of different products.