Hewavitharanage et al.
may also undergo single-electron transfer (2) in both
thermal and photochemical reactions. Direct photochemi-
cal processes are limited to aryl and unsaturated borates
because borates are transparent to UV light longer than
220 nm only. The direct photolysis of tetraarylborates
(Ar4B-) was first studied by Williams and co-workers,23
and the photoinduced single-electron transfer from aryl
borates to neutral acceptors as well as to cation acceptors
is well-known.24,25 Electron transfer to positively charged
acceptors can either be by an intermolecular26 or an
intramolecular reaction.26-28 Schuster showed that car-
bocyanine alkyltriphenylborates undergo electron trans-
fer, yielding an alkyl radical and triarylborane if the
anion/cation is assembled as a tight ion pair.29 As a result
of these studies, the alkyltriphenylborates are probably
best known as free-radical initiators for vinyl photopo-
lymerization30 but they are also potential sources of Lewis
acids. Kochi first reported intramolecular alkyl transfer
from alkyltriphenylborates to pyridinium, quinolinium,
and isoquinolinium cations.27 However, phenyl-radical
formation was not indicated by his studies, and phenyl-
group transfer has not been observed in any aryl borates
before.
A particular advantage of tetrakis(pentafluorophenyl)-
borate is that, unlike the alkylaryl borate anions, it is
hard to oxidize (Eox ) 2.3 V vs SCE).31 Therefore, electron
transfer from tetrakis(pentafluorophenyl)borate is un-
likely to most electron acceptors (e.g., cyanine).31 Because
of its nonnucleophilic nature, the tetrakis(pentafluo-
rophenyl)borate anion is widely used as a counterion in
photoacid generators, some of which are commercial.32,33
Tris(pentafluorophenyl)borane is also used extensively
as an activator in Ziegler-Natta (Z/N) polymerization.34-37
It therefore becomes an objective to examine photochemi-
cal routes to it because there are no known photochemical
Z/N polymerization processes.
Our interest in photoreactions of isoquinolinium salts
results from attempts to photochemically generate poly-
olefins from simple olefin monomers and prepolymers.
A photo-Z/N polymerization might be achieved by gen-
erating an active cocatalyst in the presence of an acti-
vatable organometallic compound following irradiation
(Scheme 1).
Results and Discussion
Tris(pentafluorophenyl)borane, a commonly used co-
catalyst in Z/N polymerization, is widely recognized for
its superior catalytic properties.2-18 So, its photogenesis
takes on added importance. Irradiation of a pyridinium,
a quinolinium, or an isoquinolinium alkyltriphenylborate
leads to efficient alkyl transfer to the cation affording
nucleophilic adducts27 and triphenylborane in quantita-
tive yields. The basic questions we addressed were: can
tetrasubstituted pentafluorophenylborate salts be oxi-
dized using light, and if one finds that they can, does one
generate tris(pentafluorophenyl)borane? Unfortunately,
attempts to synthesize N-methylisoquinolinium methyl-
tris(pentafluorophenyl)borates failed, likely because of
the instability of the compound. The presence of the trace
water in B(C6F5)3 yielded, instead, salts containing hy-
droxytris(pentafluorophenyl)borate (Figure 1).
Upon irradiation at 350 nm in degassed CH2Cl2 or CH3-
CN, 1 yields 2-methyl-1-(2,3,4,5,6-pentafluorophenyl)-1,2-
dihydroisoquinoline (3) (48%, φ ) 0.21) presumably as a
result of C6F5 transfer to the isoquinolinium cation.
Because 3 is unstable on silica, the compound was only
partially purified by evaporating solvent after irradiation
and extraction with diethyl ether. The structure was
1
confirmed using GC/MS, H NMR, and 19F NMR. The
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resulted in bleaching of its absorption at 520 nm, and a
concomitant growth of the absorption at 350 nm (due to
protonated QR) was observed.39 When irradiated, 1 also
catalyzed the photopolymerization of either tri(ethyl-
eneglycol)divinyl ether (DVE-3) or cyclohexene oxide.
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