in 7 to the -OH group followed by a rearrangement of the
aryl groups and subsequent dehydration catalyzed by proton
sources to afford 8. In order to avoid the use of LiAlH4 for
obtaining the -OH group in this highly bulky substrate (its
employment might have led to a simultaneous dehalogenation
of the substrate,12 forming impurities that are extremely
difficult to remove) we chose milder conditions for the
reductive step in the synthesis of 8.
We were pleased to find that reaction of 7 with one
equivalent of triethylsilane in trifluoroacetic acid at 80 °C
did not simply afford the ketone reduction product but the
target molecule 8, with unexpected high selectivity, in a fast
(3 h), high-yielding (80%) one-pot reaction. The product
could be straightforwardly purified and was fully character-
ized by NMR, IR, and elemental analysis. The selectivity of
this reaction is quite surprising, considering all the possible
byproducts that could be obtained during the reaction. It is
known that triethylsilane formally acts as a hydride source
for carbocations,13 and its versatility has been demonstrated
for the conversion of ketones to methylene groups.14 The
use of triethylsilane requires a protic source (trifluoroacetic
acid) necessary to generate the suitable carbocations.
For these reasons, it is realistic to consider two simulta-
neous reaction pathways for the obtainment of 8, as depicted
in Scheme 4.
carbocation 14. The latter intermediate would then undergo
the aryl shift yielding the more stable carbocation 15 and
subsequent aromatization leading to the target molecule 8.
On the other hand, another reaction pathway can start from
the rearrangement of 11 yielding the more stable carbocation
16, which, in turn, can act as hydride acceptor forming 17.
Dehydration of 17 leads again to the intermediate 15,
followed by aromatization through proton elimination, giving
the target molecule 8. The high selectivity of the reaction is
explained by assuming that the rearrangement of carbocation
14 and the aromatization of 15 are faster processes with
respect to reaction of these species with triethylsilane.
The advantages of having built up an iodophenanthrene
building block were demonstrated by carrying out poly-
merization tests for the obtainment of poly(2,7-phenan-
threne)s and poly(2,7-phenanthrylenevinylene)s.
As a first test, the polymerizaton of 8 by Ni(COD)2-
promoted reductive homocoupling was carried out affording
the corresponding polymer PPhen in 76% yield (Scheme
5). The obtained polymer was soluble in CHCl3, CH2Cl2,
Scheme 5. Synthesis of PPhen and PPhenV from 8
Scheme 4. Plausible Mechanisms for Obtaining 8
1
and THF and was characterized by H NMR, FT-IR, and
gel permeation chromatography (GPC).
Its number average molecular weight (against polystyrene
standards) was estimated as 31 000 Da (∼60 repeating units)
and, noteworthy, could be obtained with fairly low polydis-
persity (Mw/Mn ) 1.5).
In another polymerization experiment, monomer 8 was
submitted to a cascade Suzuki-Heck reaction, a method re-
cently reported by us for obtaining poly(arylenevinylene)s,15
which employs the use of potassium vinyltrifluoroborate16 as
ethylene equivalent. The macromolecule PPhenV was
obtained in a one-pot reaction with potassium vinyltrifluo-
roborate, palladium(II) acetate/tri(o-tolyl)phosphane as cata-
lyst, and triethylamine as base in toluene/DMF at 120 °C.
This polymer was obtained in 71% yield as a green powder,
(13) (a) Carey, F. A.; Tremper, H. S. J. Amer. Chem. Soc. 1968, 2578-
2583. (b) Carey, F. A.; Tremper, H. S. J. Org. Chem. 1971, 36, 758-761.
(14) West, C. T.; Donnelly, S. J.; Kooistra, D. A.; Doyle, M. P. J. Org.
Chem. 1973, 38, 2675-2681.
(15) (a) Grisorio, R.; Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.;
Suranna, G. P. Tetrahedron Lett. 2005, 46, 2555-2558. (b) Grisorio, R.;
Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.; Suranna, G. P.; Gigli, G.;
Piliego, C.; Ciccarella, G.; Cosma, P.; Acierno, D.; Amendola, E.
Macromolecules 2007, 40, 4865-4873.
Protonation of 7 leads to the carbocation 11, which can
act as hydride acceptor. The alcohol 12 can be further
protonated yielding, after loss of a water molecule, the
(16) (a) Molander, G. A.; Rivero, M. R. Org. Lett. 2002, 4, 107-109.
(b) Molander, G. A.; Bernardi, C. R. J. Org. Chem. 2002, 67, 8424-8429.
(12) Karabatsos, G. J.; Shone, R. L. J. Org. Chem. 1968, 33, 619-621.
Org. Lett., Vol. 9, No. 16, 2007
3151