Multiple-photon ? chemistry in the laser-jet: Photochemistry of the 4-biphenylmethyl radical

Article abstract of DOI:10.1039/a606817k

In laser-jet (LJ) photolysis (high intensity) of the ether 4-(4-benzoylphenoxymethyl)biphenyl 1 a two-photon process is observed in CCl₄ to yield 4-(chloromethyl)biphenyl 5 as product (3%), while the one-photon product is 4-biphenylaldehyde 7 (100% in conventional photolysis versus 97% in laser-jet photolysis). In ethanol, the results suggest a three-proton reaction in which 4-(ethoxymethyl)biphenyl 6 is produced in appreciable amounts (18%) in addition to the one-photon product 1,2-bis(4-biphenyl)ethane 4, the latter as the major product (82%). It is proposed that under high-intensity LJ photolysis conditions the electronically-excited 4-biphenylmethyl radical 2* is photoionized to the 4-biphenylmethyl cation 2⁺ and that the latter is trapped by ethanol to give the 4-biphenylmethyl ether 6.

Full text of DOI:10.1039/a606817k

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Multiple-photon† chemistry in the laser-jet: photochemistry of the
-biphenylmethyl radical
4
Waldemar Adam and Katrin Schneider *
Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg,
Germany
In laser-jet (LJ) photolysis (high intensity) of the ether 4-(4-benzoylphenoxymethyl)biphenyl 1 a two-
photon process is observed in CCl to yield 4-(chloromethyl)biphenyl 5 as product (3%), while the one-
4
photon product is 4-biphenylaldehyde 7 (100% in conventional photolysis versus 97% in laser-jet
photolysis). In ethanol, the results suggest a three-proton reaction in which 4-(ethoxymethyl)biphenyl 6 is
produced in appreciable amounts (18%) in addition to the one-photon product 1,2-bis(4-biphenyl)ethane
4
, the latter as the major product (82%). It is proposed that under high-intensity LJ photolysis conditions
+
the electronically-excited 4-biphenylmethyl radical 2* is photoionized to the 4-biphenylmethyl cation 2
and that the latter is trapped by ethanol to give the 4-biphenylmethyl ether 6.
Introduction
The 4-biphenylmethyl radical 2 has for some time been the sub-
1
,2
ject of photophysical studies. This benzyl-type radical 2 has a
2
strong absorption band centred at 326 nm, which makes it a
3
suitable substrate for laser-jet experiments, especially at the 333
nm line available in the argon-ion laser (333, 351, 364 nm). For
other well-studied arylmethyl radicals, generated upon irradi-
ation of arylmethyl halides with pulsed lasers (two-colour
experiments), photoionization to aryl cations was demon-
4
,5
strated by transient UV spectroscopy.
Similar multiple-
photon chemistry is expected for the 4-biphenylmethyl radical
2
.
We report here that under high-intensity conditions of laser-
jet photolysis of 4-(4-benzoylphenoxymethyl)biphenyl 1 the
electronically-excited 4-biphenylmethyl radical 2* is produced.
Under further excitation 2* is transformed to the 4-biphenyl-
+
methyl cation 2 by electron transfer, a process that requires
several photons.
Results
The photolysis products, which are derived from the biphenyl-
methyl 2 and the 4-(benzoyl)phenoxy 3 radicals generated on
C–O homolysis of ether 1, are outlined in Scheme 1 and the
product studies summarized in Table 1. The photolysis of 4-(4-
benzoylphenoxymethyl)biphenyl 1 in CCl solution afforded
4
the chloride 5 and the aldehyde 7 as the products. A light inten-
sity dependence of the product distribution was observed for 4-
(chloromethyl)biphenyl 5, since it was absent at low intensity,
but detectable amounts for identification were found at high
intensities (Table 1, entries 1 and 2). Although the process is
inefficient (3%), these results indicate that multiple-photon
chemistry is operating under the laser-jet conditions (high
6
intensity). Aldehyde 7, essentially the exclusive product in
CCl , is observed both under low- and high-intensity conditions
4
(Table 1, entries 1 and 2).
To assess the origin of the aldehyde 7 in the low- and high-
intensity photolyses, the ether 1 was treated with the one-
electron oxidant bis(4-bromophenyl)(2,4-dibromophenyl)
aminium hexachloroantimonate, known as ‘Magic Green’
7
(
E = 1.42 V), in CDCl and a dry argon gas atmosphere. At
0
3
9
4
% conversion, the aldehyde 7 was the exclusive product besides
-hydroxybenzophenone 8.
In ethanol, the product composition of the ether 1 photolysis
†
A multiple-photon reaction refers to the sequential absorption of more
than one photon by intermediary chemical species, in contrast to a
multiphoton process in which two or more photons are absorbed simul-
taneously to produce a higher excited spectroscopic state.
was considerably simpler (entries 3 and 4) than in CCl
. The
4
J. Chem. Soc., Perkin Trans. 2, 1997
441

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Table 1 Product studies of the conventional (low intensity) and laser-jet (high intensity) photolyses of biphenyl ether 1 in carbon tetrachloride and
in ethanol
a,b
Product distribution (%)
a
a
Mode
Solvent
conv. (%)
mb (%)
4
5
6
7
Laser, widened beam
Laser-jet
Laser, widened beam
Laser-jet
CCl4
CCl4
EtOH
EtOH
33
4
62
29
78
95
69
79
—
—
100
82
—
3
—
—
—
—
—
18
100
97
—
—
a
Conversion (conv.) of ether 1, mass balance (mb) of products (deficit represents undefined and/or undetected material) and product distribution
1
b
were determined by H NMR spectroscopy, error 5% of stated value; 4-hydroxybenzophenone 8 was formed quantitatively. Normalized to 100%.
The necessity for electronically-excited arylmethyl radicals in
O
*
hν
1
Ph
C
H2
the photoionization with CCl was demonstrated by Johnston
4
CCl4
or
EtOH
Ph
and Scaiano through time-resolved laser-flash photolyses of
9
the 1-naphthylmethyl radical. Thus, the excited radical was
O
1
*
quenched by CCl and electron transfer was demonstrated by
4
9
its reaction with Methyl Viologen.
In ethanol as solvent, a completely different product com-
position is obtained for both conventional as well as laser-jet
photolysis (entries 3 and 4). Mechanistically significant is the
fact that in both the conventional and laser-jet photolysis
modes no 4-biphenylaldehyde 7 was observed. This underscores
Ph
C
H2
+ Ph
CH2Cl + Ph
CH2OEt
2
the fact that CCl is essential for the photo-induced electron
4
5
6
4
transfer, while alcohols do not serve as effective electron
acceptors. Moreover, it confirms that oxidation of the inter-
mediary radical 2 by molecular oxygen to the aldehyde 7 cannot
be a significant route because it should occur with nearly equal
HO
Ph
CHO
+
Ph
facility in CCl and EtOH.
4
O
The fate of the radicals 2 on laser-jet photolysis in ethanol is
that appreciable amounts (18%) of the new ether 6 (Table 1,
entry 4) are produced, besides the conventional dimer 4 as
major product (82%). Since electron transfer from the excited
7
8
Scheme 1
radical 2* to ethanol, as in the case of CCl , is not feasible, we
4
dimer 4 was formed under low- as well as high-intensity condi-
tions. It was essentially the only product in the conventional
photolysis (entry 3), while in the LJ experiment the ether 6 was
additionally formed as a minor product in substantial (18%)
amounts (entry 4).
propose that photoionization competes with dimerization of
+
the radical 2 to generate the cation 2 (Scheme 3), analogous to
3
+
our previous study on the benzhydryl radical. The cation 2 is
subsequently nucleophilically trapped by ethanol to afford the
ether 6. Such photoionization of arylmethyl radicals has been
4
,5
documented
and provides experimental support for this
mechanism. Thus, in laser-jet photolysis of the starting ether 1
Discussion
in ethanol at least three photons are required to generate the
+
The mechanism for the single-photon process (conventional
laser photolysis) is presented in Scheme 2, together with the
chemical oxidation of ether 1 by ‘Magic Green’, while the
multiple-photon reaction (laser-jet photolysis) is displayed in
Scheme 3.
4-biphenylmethyl cation (2 ) (Scheme 3). The first photon is
required for C᎐O bond homolysis in the ether 1 to afford the
benzyl-type radical 2, the second for excitation of the latter
radical, and finally at least a third photon for ionization of the
+
excited radical 2* to its cation 2 .
In the low-intensity experiments under conventional laser
This underscores the efficiency of the LJ technique in achiev-
ing high photon densities and in allowing detailed product stud-
ies for such multiple-photon reactions. Thus, the LJ mode of
operation constitutes a valuable complementary technique for
time-resolved laser flash photolysis with pulsed lasers. The
latter offers valuable mechanistic data in terms of absorption
spectra, lifetime and rate constants of short-lived transients,
but normally insufficient amounts of products are formed to
conduct the essential product studies.
photolysis in CCl solution, the electronically excited ether 1*
4
engages in electron transfer with CCl (Scheme 2) to produce
4
+
the radical cation 1 (ionization of the electron-rich biphenyl
π system) and the CCl4 radical anion. The latter has been
established to dissociate readily into chloride ions and trichloro-
methyl radicals. The radical cation 1 deprotonates and
᝽
Ϫ
᝽
8
+
᝽
further oxidation to its cation, followed by trapping with adven-
titious water, leads to 4-biphenylaldehyde 7 and the phenol
8
. The fact that chemical oxidation of the ether 1 by ‘Magic
Green’ afforded exclusively the aldehyde 7 and phenol 8 pro-
vides experimental support that the proposed photo-induced
electron-transfer process is operating.
Experimental
General aspects
4
-Biphenylaldehyde 7 also appears as the major product in
Ethanol and carbon tetrachloride were distilled. Immediately
CCl in the laser-jet photolysis (entry 2); however, in contrast
before photolysis, CCl was passed through an alumina column
4
4
to conventional photochemistry at low intensity (Scheme 2),
to remove traces of acid. All solutions were degassed by purging
with a slow stream of dry argon gas for 45 min before irradi-
ation. NMR spectra were recorded on a Bruker AC 200 or a
4
-(chloromethyl)biphenyl 5 was additionally observed under
high-intensity conditions, although in low (3%) yield. As
3
demonstrated previously for other arylmethyl radicals,
a
Bruker AC 250 spectrometer. Quantitative product studies were
1
multiple-photon process is involved, in which the inter-
determined by H NMR analysis (CDCl ) of the identified
3
mediary 4-biphenyl radical 2 is photoionized by electron
products directly on the product mixtures after removal of the
transfer with CCl (Scheme 3).
solvent under reduced pressure (ca. 20 ЊC, 18 Torr). The IR
4
4
42
J. Chem. Soc., Perkin Trans. 2, 1997

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O
•+
Ph
C
H2
Ph
O
hν
1•+
H+, e–
•
Cl –+CCl3
CCl4
Ph
CHO
O
O
Ph
C
H2
Ph
C
H
7
H2O
Ph
Ph
+
O
O
HO
1
Ph
O
Ar3N•+SbCl6–
CDCl3
+
–
H , e
8
O
•+
Ph
C
H2
Ph
O
1•+
Scheme 2
Laser-jet irradiations
The experimental set-up, as described previously,
O
*
hν
6a,b
1
Ph
C
was
H2
Ph
employed. The beam of the argon-ion laser (3.6 W over all UV
lines) was focused by means of a quartz lens (f = 80 mm) onto a
free-falling liquid jet of the photolysis solution. The free-falling
liquid jet was generated by passing the substrate solution to
be irradiated through a 50 µm capillary and maintained at a
constant flow rate by means of a Bischoff 2200 HPLC pump.
The irradiation chamber was kept under a positive argon gas
O
1*
•
O
Ph
3
O
pressure and the substrate solution was passed once (CCl ) and
4
four times (ethanol) through the focal region of the laser beam
for the irradiation.
2
2
•
Ph
C
H2
Ph
CH2
4
2
4
-(4-Benzoylphenoxymethyl)biphenyl 1
hν
A solution of 2.78 g (14.0 mmol) of 4-hydroxybenzophenone
and 1.82 g (13.2 mmol) of potassium carbonate in 50 cm of dry
3
acetone was stirred for 5 min at room temperature (ca. 20 ЊC)
3
and 3.00 g (12.1 mmol) of 4-bromomethylbiphenyl in 25 cm of
*
•
hν
+
dry acetone was added slowly and the reaction mixture was
allowed to reflux for 14 h. The solvent was removed under
reduced pressure (ca. 20 ЊC, 18 Torr) and 50 cm of distilled
Ph
CH2
Ph
CH2
3
e–
2+
2*
water was added. The aqueous solution was extracted with
CCl4
EtOH
H+
3
methylene chloride (3 × 20 cm ), the organic phase was
3
•CCl3
washed with saturated, aqueous Na CO (2 × 20 cm ), and with
2
3
3
saturated, aqueous NaCl (2 × 20 cm ), dried over MgSO and
4
the solvent removed under reduced pressure (ca. 20 ЊC, 18
Torr). After recrystallization from ethanol, 2.80 g (60%) of
Ph
CH2Cl
Ph
CH2OEt
ether 1 was obtained as colourless needles, mp 122–123 ЊC, IR
Ϫ1
5
6
(KBr) ν/cm 3000, 2920, 2840, 1640 (CO), 1580, 1280, 1240,
1
165, 1065, 990; UV (CH CN) λ/nm (log ε) 283 (3.77), 336
3
1
Scheme 3
(2.40); H NMR (CDCl , 250 MHz) δ 5.20 (s, 2 H), 7.08 (d, J 8.9
3
13
Hz, 2 H), 7.3–7.66 (m, 12 H), 7.78 (m, 2 H), 7.85 (m, 2 H);
C
spectra were recorded on a Perkin-Elmer 1420 and the UV
spectra on a Hitachi U-3200 spectrophotometer. Chloride 5,
aldehyde 7 and ketone 8 were prepared according to reported
NMR (CDCl , 50.3 MHz) δ 69.9 (t), 114.4 (d), 127.1 (d), 127.4
3
(s), 127.8 (d), 128.0 (d), 128.2 (d), 128.8 (s), 129.1 (d), 129.7 (d),
129.9 (s), 130.4 (d), 131.9 (d), 132.6 (d), 135.1 (s), 138.1 (s),
162.3 (s), 195.0 (s). Calc. for C H O (364.4): C, 85.69; H,
1
0a,b
procedures.
2
6
20
2
5
.53. Found: C, 85.26; H, 5.27.
Normal laser irradiations
These were carried out in a Schlenk tube by irradiation at the
1,2-Bis(4-biphenyl)ethane 4
3
33, 351 and 363 nm lines of the Coherent INNOVA 100 argon-
A suspension of 900 mg (4.44 mmol) of 4-chloromethylbiphenyl
3
ion laser (3.0 W over all UV lines), supplied with UV quartz
optics. The beam was widened with a quartz lens (f = 50 mm) to
a size of ca. 1 cm in diameter and the 20 cm sample was irradi-
5 and 603 mg (10.8 mmol) of iron powder in 20 cm of water was
3
allowed to reflux for 3 h. Methylene chloride (10 cm ) was
3
added and the organic phase was washed with water (3 × 10
3
ated for 15 min (ethanol) or 10 min (CCl ) under an argon gas
cm ), dried over MgSO and the solvent removed by distillation
4
4
atmosphere.
(42 ЊC, 760 Torr). Unreacted starting material was removed
J. Chem. Soc., Perkin Trans. 2, 1997
443

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3
under reduced pressure (bp 188 ЊC, 0.1 Torr), 20 cm of methy-
before use, was added 17.0 mg (19.3 mmol) of bis(4-bromo-
lene chloride was added to the residue, the solution was fil-
tered through a short silica gel (10 g) column, and the solvent
again removed by distillation (42 ЊC, 760 Torr). After recrystal-
lization from benzene, 300 mg (40%) of product 4 was obtained
phenyl)(2,4-dibromophenyl)aminium hexachloroantimonate
4
(Magic Green) under an argon atmosphere. The mixture was
1
well shaken and immediate analysis by H NMR spectroscopy
showed aldehyde 7 and ketone 8 as exclusive oxidation products
(conversion 9%, mass balance >95%).
Ϫ1
as colourless needles, mp 194–196 ЊC. IR (KBr) ν/cm 3025,
1
2
920, 2860, 1590, 1500, 1460, 1390, 1075; H NMR (CDCl , 200
3
1
3
MHz) δ 3.03 (s, 4 H), 7.21–7.83 (m, 18 H); C NMR (CDCl3,
Acknowledgements
5
0.3 MHz) δ 37.5 (t), 126.9 (d), 127.0 (d), 127.1 (d), 128.7 (d),
1
28.9 (d), 138.9 (s), 140.8 (s), 141.0 (s). Calc. for C H (334.5):
We thank the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie for generous financial support.
2
6
22
C, 93.37; H, 6.63. Found: C, 93.63; H, 6.35.
4
-(Ethoxymethyl)biphenyl 6
References
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in 10 cm of absolute ethanol was added to a solution of 127 mg
3
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under reduced pressure (ca. 20 ЊC, 18 Torr) and then 10 cm of
cold water was added to the residue. The solution was neutral-
ized by acetic acid, the aqueous phase was extracted with ether
5
3
6
3
(3 × 10 cm ) and the combined organic layers were dried over
6
MgSO . The solvent was removed by distillation (35 ЊC, 760
Torr) and fractional distillation of the residue under reduced
pressure afforded 610 mg (58%) of pure ether 6 as a colourless
oil, bp 102–103 ЊC/0.1 Torr. IR (NaCl) ν/cm 3010, 2840, 1475,
4
Ϫ1
7
1
1
450, 1365, 1345, 1090, 1000, 820; H NMR (CDCl , 200
3
MHz) δ 1.41 (t, J 7.0 Hz, 3 H), 3.70 (q, J 7.0 Hz, 2 H), 4.67 (s, 2
H), 7.53 (m, 5 H), 7.72 (m, 4 H); C NMR (CDCl , 50.3 MHz)
δ 15.1 (q), 65.6 (t), 72.2 (t), 126.7 (d), 126.9 (d), 127.1 (d), 128.0
1
3
3
1
1
950, 69, 1109; (b) G. Cavallini, E. Massranani, D. Nardi and
R. D’Ambrosio, J. Am. Chem. Soc., 1957, 79, 3514.
(d), 128.6 (d), 137.5 (s), 140.3 (s), 140.7 (s). Calc. for C H O
15 16
(212.3): C, 84.87; H, 7.60. Found: C, 84.62; H, 7.87.
Oxidation of ether 1 by Magic Green
To a solution of 20.0 mg (54.9 mmol) of ether 1 in 0.7 cm of
Paper 6/06817K
Received 4th October 1996
3
CDCl , passed through an alumina column (ca. 2 g) shortly
Accepted 25th November 1996
3
4
44
J. Chem. Soc., Perkin Trans. 2, 1997