Intramolecular Binaphthyl Formation
J . Org. Chem., Vol. 63, No. 18, 1998 6259
aryl cation succeeding the intramolecular rearrangement
to the electron-withdrawing aryl group. Triplet-sensi-
tized reactions of the di- and trinaphthyl esters by
benzophenone also occur to produce the binaphthyl.
These intramolecular biaryl formations are attributed to
both the formation of an intramolecular excimer or
exciplex and the electron-withdrawing character of the
P(O) group of the O-P(O)-O spacer between two aryl
groups. It has been reported that an intramolecular
excimer forms in 1,3-diarylpropanes and bis(4-methoxy-
1-phenoxy)dimethylsilane and decays without the forma-
tion of the biaryl.6e,7 No intramolecular excimer forms
in the di- or triaryl esters such as carbonate, borate, and
sulfite.6e
The photoinduced electron-transfer reactions of orga-
nophosphorus compounds have been studied for trivalent
phosphorus compounds such as trialkylphosphines and
trialkyl phosphites in which radical cations of these
compounds are generated and oxidized by methanol to
give trialkylphosphine oxides and trialkyl phosphates.1d
We have recently reported that 9,10-dicyanoanthracene
(DCA)-sensitized photoinduced electron-transfer reaction
of tri-1-naphthyl phosphate occurs and initially forms the
tri-1-naphthyl phosphate radical cation and 1,1′-binaph-
thyl as the final product.8
Although photoinduced electron-transfer reactions of
organic compounds have been extensively investigated,9
little is known about organophosphorus compounds.
Studies of the electron-transfer reactions of aryl phos-
phates and phosphonates are of particular importance
for caged compounds and organic syntheses. In this
paper, we report the mechanistic study of the DCA-
sensitized photoinduced electron-transfer reaction of
trinaphthyl phosphates (1) and dinaphthyl methylphos-
phonates (2) in which the initially formed radical cations
of 1 and 2 change to the intramolecular π-dimer radical
cations between two naphthyl groups and a binaphthyl
radical cation is subsequently eliminated. As a compari-
son, the DCA-sensitized photoinduced electron-transfer
reaction of mononaphthyl phosphates (3) is reported.
It has been reported that intramolecular π-dimer
radical cations form between two aromatic groups in
various diarylalkanes on the basis of the absorption
spectra, although there is no data available for the
dimerization rate constant or equilibrium constant be-
tween the monomer and dimer radical cations.10 We have
estimated the rate constant and equilibrium constant
between the radical cations of 1 and 2 and the intramo-
lecular π-dimer radical cations because the rate of
formation of binaphthyl radical cation from the intramo-
lecular π-dimer radical cations can be directly measured
using the transient absorption measurements. The
reactivity of the intramolecular π-dimer radical cations
of 1 and 2 can be compared with that of the other
intramolecular π-dimer radical cations and is discussed
with respect to the character of the spacer between the
two aromatic groups.
Resu lts
P r od u ct An a lyses in P h ot oir r a d ia t ion . Photo-
irradiation (cutoff < 355 nm) of an argon-saturated
acetonitrile solution containing 5.0 × 10-5 M DCA and
1.0 × 10-2 M tri-1-naphthyl phosphate (1a ) for 1 h gave
1,1′-binaphthyl (4a ) and 1-naphthyl phosphate (5a ) in a
1:1 ratio with a quantum yield of 0.0013 (Scheme 1).
Upon photoirradiation of a mixture of DCA and di-1-
naphthyl methylphosphonate (2a ) in a manner similar
to that already described, 4a and methylphosphonic acid
(5′) were produced. Photoirradiation of tris(4-methoxy-
1-naphthyl) phosphate (1b) or bis(4-methoxy-1-naphthyl)
methylphosphonate (2b) with DCA in acetonitrile also
gave 4,4′-dimethoxy-1,1′-binaphthyl (4b) and 4-methoxy-
1-naphthyl phosphate (5b) or 5′, respectively. Table 1
summarizes the product yields and quantum yields of 4.
Trace amounts of other products were also obtained,
although they were not identified. Therefore, the main
product of the photosensitized reactions of 1 and 2 was
4. The material balances showed that the naphthyl
groups of the consumed 1a and 2b transformed almost
quantitatively into 4a and 4b, respectively. No reaction
occurred upon the photoirradiation of the diethyl naph-
thyl phosphates (3) with DCA. The photoirradiation of
a mixture of DCA and di-1-naphthoxymethane (6) was
also carried out for comparison with 1a and 2a ; however,
no reaction occurred. These results suggest that the
formation of 4 intramolecularly occurs.
La ser F la sh P h otolyses. A time-resolved spectro-
scopic study using the laser flash photolysis technique
was carried out to obtain kinetic information about the
photoreactions. The transient absorption spectra were
measured by the laser flash photolyses (355 nm) of a
mixture of 1a (1.0 × 10-2 M) and DCA (5.0 × 10-5 M) in
argon-saturated acetonitrile. Figure 1 shows the tran-
sient absorption spectrum with peaks at approximately
430 and 710 nm in the range of 430-750 nm observed
20 ns after a 4 ns flash (open circle).
The peak of the transient absorption at 430 nm
disappeared in the presence of 5.0 × 10-4 M trans-4,4′-
dimethoxystilbene (DMS) as a radical-cation scavenger
used to form the DMS radical cation (DMS•+) of which a
transient absorption appeared at 530 nm, while the peak
at 700 nm was not quenched.11 The peak at 700 nm was
quenched in the presence of oxygen, while that at 430
nm was not influenced (closed circle in Figure 1). The
peaks at 430 and 700 nm are assigned to the radical
cation of 1a (1a •+) and the DCA radical anion (DCA•-),
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