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|Title:||Compounds of Pentacoordinate (10-Boron-5) and Hexacoordinate (12-Boron-6) Hypervalent Boron|
|Author(s):||Lee, David Yen|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Efficient synthetic routes to 2-substituted pyridines and 2,6-disubstituted pyridine 1-oxides are described, involving direct ortho lithiation with the sterically hindered base, lithium 2,2,6,6-tetramethylpiperidide, in the presence of electrophiles that are compatible with the base, e.g., trimethylsilyl chloride and hexafluoroacetone.
Syntheses of the first isolable hypervalent 10-B-5 species 17, 25, and 28, and 12-B-6 species 30 and 35 are described. The ('1)H, ('13)C, and ('19)F NMR spectra support the assigned hypervalent structures. The ('11)B NMR downfield chemical shifts resulting from protonation of the 10-B-5 species 17 (-20.1 ppm) + H('+) (--->) 18 (-0.1 ppm, 25 (-41.0 ppm) + H('+) (--->) 26 (+5.7 ppm), and 28 (-35.7 ppm) + H('+) (--->) 29 (+12.0 ppm) fall in the range associated with 8-B-4 species with only first-row elements bonded to boron (-17.5 to +50 ppm). The ('11)B NMR chemical shifts resulting from monoprotonation of the 12-B-6 species 30 (-123.0 ppm) + H('+) (--->) 31 (-70.4 ppm) and 35 (-233.2 ppm) + H('+) (--->) 36 (-61.0 ppm) fall in the range near those of the other 10-B-5 species 17, 25, and 29. The ('11)B NMR chemical shifts of the doubly protonated species 32 + H('+) (--->) 33 (+4.7 ppm) and 36 + H('+) (--->) 37 (+11.5 ppm) fall in the range of ordinary borates. Hypervalent 10-B-5 species 36 is deprotonated by using 2,2,6,6-tetramethylpiperidine to recover 12-B-6 species 35. The spin-lattice (T(,1)) relaxation results indicate that the geometries of the hypervalent 10-B-5 and 12-B-6 species are more symmetrical than that of tetracoordinate borates. The values of T(,1) fall, with some exceptions, in the order 12-B-6 > 10-B-5 > 8-B-4.
Two electrons of the three-center four-electron 0-B-0 bond are delocalized into the diequatorial five- or six-membered ring of hypervalent 10-B-5 species 6 and 11, making them 6-(pi) Huckel aromatic systems that we call bis ipso aromatic. The proposed bis ipso aromaticity is evidenced by the great similarity of the electronic spectra of the hypervalent 10-B-5 species (6 and 11) to their carbon analogues (9-phenyl fluorenyl anion and anthracene). The hypervalent boron atom appears to have electronic properties almost identical to those of a carbon atom in the analogous (pi)-system with near identity of electronic spectra.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1985.
|Date Available in IDEALS:||2014-12-15|