J. Am. Chem. Soc. 1997, 119, 11695-11696
11695
Scheme 1
Discrete High Molecular Weight Triarylamine
Dendrimers Prepared by Palladium-Catalyzed
Amination
Janis Louie and John F. Hartwig*
Department of Chemistry, Yale UniVersity
P.O. Box 208107, New HaVen, Connecticut 06520-8107
Albert J. Fry
Scheme 2
Department of Chemistry, Hall Atwater Laboratory
Wesleyan UniVersity, Middletown, Connecticut
ReceiVed August 11, 1997
Triarylamines are an important class of compounds because
they form stable aminium radical cations. Thus, triarylamines
can be building blocks for high-spin polyradicals that have
shown ferromagnetic coupling,1,2 as well as for conductive
polymers.3 Perhaps most commonly, triarylamines have been
used as the hole-transport layer in electroluminescent devices.4-7
Stable radical cations can also initiate pericylic reactions,8 act
as electrocatalysts,9,10 or act as mild and selective oxidizing
agents.11 The synthesis of arylamine macromolecules12 has been
limited by the modest yields of the copper-mediated Ullmann
chemistry, but new palladium-catalyzed methods to form
arylamines have emerged recently from our group13-17 and
Buchwald’s group.18-21 We report the first use of this pal-
ladium-catalyzed chemistry to form pure discrete arylamine
oligomers and the largest discrete arylamine dendrimer.
shown that anilines react with aryl bromides in the presence of
catalytic amounts of precursor Pd(DPPF)Cl2 (DPPF ) diphen-
ylphosphinoferrocene) to give diarylamines in high yields.15,19
We show here that DPPF-ligated palladium can produce
diarylamines or triarylamines. A combination of Pd(DBA)2
(DBA ) dibenzylideneacetone) and DPPF (2.5 mol %) cata-
lyzed the formation of tritolylamine in 89% yield from 4-tolui-
dine and 2.1 equiv of 4-bromotoluene in the presence of 2.1
equiv of sodium tert-butoxide.
Three new palladium-catalyzed reactions that form triaryl-
amines are shown in Scheme 1. Lithium ditolylamide and
4-bromotoluene reacted in the presence of 1 mol % Pd[P(o-
tolyl)3]2 (1) in toluene to produce tritolylamine quantitatively
after 1 h at 90 °C. Alternatively, ditolylamine and 4-bromo-
toluene reacted in the presence of stoichiometric amounts of
sodium tert-butoxide and 1 mol % Pd[P(o-tolyl)3]2 to give
tritolylamine in 92% yield. A qualitative half-life of 1 h at 90
°C was obtained for the latter reaction. We have previously
Most important for this work, these reactions suggested that
we could use this chemistry to produce electronically interesting
triarylamine dendrimers in high yields that were previously
prepared in modest yields. The first generation dendrimer
4,4′,4′′-tris(N,N-diphenylamino)triphenylamine (TDATA, 2) was
prepared from tris(4-bromophenyl)amine and 3.3 equiv of
lithium diphenylamide in the presence of 2 mol % catalyst 1
(Scheme 2) in 84% isolated yield. This molecule was prepared
in only 22% yield by copper chemistry.22
Low yields from Ullmann chemistry have prevented the
preparation of higher generation triarylamine dendrimers. We
demonstrate here that the palladium chemistry can produce
larger hyperbranched arylamines. Employing a convergent
strategy23 we prepared the largest triarylamine starburst den-
drimer, as shown at the top of Scheme 3. Reaction of N,N-
bis(4-bromophenyl)benzenemethanamine24 (3) and 2.2 equiv of
lithium ditolylamide in the presence of 2 mol % 1 in toluene
gave benzyl-protected 4 in 95% isolated yield by precipitation
and recrystallization from THF:EtOH (1:1). Debenzylation of
4 with 50 psi of H2 over Pd/C gave amine 5 in 88% yield after
purification by precipitation and recrystallization from THF:
EtOH (1:1). Reaction of lithium amide 6, generated from
lithiation of 5, and 0.3 equiv of tris(4-bromophenyl)amine in
the presence of 3.5 mol % 1 gave second generation dendrimer
7. Starburst 7 is soluble in benzene, toluene, and tetrahydrofuran
and was isolated in 80% yield by recrystallization from THF:
EtOH. This material was judged pure by both 1H and 13C NMR
spectroscopy. Silica gel chromatography gave analytically pure
material in 64% yield. The overall yield of starburst 7 from 3
was a remarkably high 70% after recrystallization and 56% after
chromatography.
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