Leaving group effect on photochemistry of ortho-alkylphenacyl carboxylate

Full text of DOI:10.1016/j.tetlet.2018.10.042

Accepted Manuscript
Leaving Group Effect on Photochemistry of ortho-Alkylphenacyl Carboxylate
Seol Hee Kim, Mi Jang, Da Yoon Moon, Bong Ser Park
PII:
S0040-4039(18)31260-7
https://doi.org/10.1016/j.tetlet.2018.10.042
TETL 50352
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
13 August 2018
5 October 2018
22 October 2018
Please cite this article as: Kim, S.H., Jang, M., Moon, D.Y., Park, B.S., Leaving Group Effect on Photochemistry
of ortho-Alkylphenacyl Carboxylate, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.10.042
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Leaving Group Effect on Photochemistry of ortho-Alkylphenacyl Carboxylate
Seol Hee Kim, Mi Jang, Da Yoon Moon, and Bong Ser Park
Department of Chemistry, Dongguk University (Seoul campus), Seoul 04620, Korea
Leaving groups often play a significant role in determining reaction routes and/or product selectivity
in organic photochemistry.1 For example, in photochemistry of valerophenone, the reaction follows
totally different pathways depending on substituents at alpha position to the carbonyl of the ketone as
shown in Scheme 1. If the ketone has a bromine substituent, the C-Br bond cleavage reaction
becomes the predominant reaction pathway. As for chlorine substitution, instead of bromine, the
reaction follows the Norrish/Yang reaction route.2 In the case of a non-halogen leaving group,
sulphonate, another completely different reaction, spin center shift (SCS), occurs.3
Scheme 1. Photochemistry of -substituted valerophenones.
Among many photochemical reactions involving leaving groups, photoremovable protecting groups
(PPG) or photocages have been an hot research topic in organic photochemistry, which has recently
been reviewed several times.4 o-Alkylphenacyl PPG is one of them, which has been extensively
studied mainly by Klan and coworkers.5 This system also shows a very versatile mechanistic
scenario dependent upon leaving groups. In this o-alkylphenacyl PPG system, the leaving group
departs with the formation of indanone. Two different reaction mechanisms for the indanone
formation depending on leaving groups have been proposed as shown in Scheme 2. For chlorine or
carboxylate, the indanone is suggested to be formed by the cyclization of an o-xylylenol intermediate,
while for the mesylate leaving group, HX elimination concurrent with spin center shift is proposed.

Wessig suggested the intramolecular hydrogen bonded transition state for the elimination process as
shown in Scheme 2.6
Scheme 2. Photochemistry of -substituted o-alkylphenacyl photoremovable protecting groups.
Our research group has also studied photochemical behaviors of o-alkylphenacyl PPG and reported
that photolysis of o-alkylphenacyl carboxylates gave not only the indanone (IN) but also the
benzocyclobutenol (CB).7 The CB product was formed via conrotatory type of thermal electrocyclic
reaction of the o-xylylenol intermediate shown in Scheme 2. The IN to CB ratio was dependent on
various reaction parameters; solvents, temperature, size of o-alkyl group, reaction time, etc. When
the leaving group was Cl, Br, or mesylate, which are better leaving groups than carboxylates, CB
product was never detected. These results suggested that the IN to CB ratio is strongly dependent
upon leaving group properties. As continuing with our efforts to understand the relationship between
structure and photoreactivity, 8 we decided to investigate the effect of leaving group on
photochemistry of o-alkylphenacyl PPG more closely by using para substituted benzoate leaving
groups. 2,4,6-Trimethylphenacyl benzoates were chosen as backbone skeletons instead of o-
methylphenacyl benzoates, because the tri-methyl system gave much higher yields of CB products
than the mono-methyl system, therefore we could compare the IN to CB ratios with each other more
easily. It was our hope that this study could resolve the mechanistic ambiguity of this reaction as
shown in Scheme 2. In fact, several new insights on the reaction mechanism were made through this
study, which will be described below.
2,4,6-trimethylphenacyl benzoates 1-5 were prepared by simple substitution reaction of 2,4,6-

trimethylphenacyl bromide with corresponding benzoic acids. Each compound was dissolved in 0.75
mL of benzene-d6 to make ca. 0.01 M solution, which was degassed and sealed. Photolysis of the
ketone was done using Pyrex filtered light of a 450 W Hanovia medium pressure mercury arc lamp.
All the ketones gave the indanone (IN) and the benzocyclobutenol (CB), together with the
corresponding benzoic acid upon photolysis, and the ratios of IN to CB are given in Table 1.9
Table 1. Indanone to benzocyclobutenol ratios obtained from photolysis of 1-5. a
pKa of
d
+e
Compound
X
IN/CB ratiob
Hammett σp
Hammett σp
benzoic acidc
CN
F
7.3
4.2
1.7
2.4
3.55
0.66
0.062
0
0.67
-0.25
0
1
2
3
4
4.14
H
4.20
CH₃
4.37
-0.17
-0.26
OCH₃
2.2
4.50
-0.27
-0.65
5
1
c
a Photolysis was done at 21
℃ in benzene-d₆. b The ratio was taken from integration of H NMR spectra. J. J.
Christensen, R. M. Izatt, L. D. Hanse, J. Am. Chem. Soc. 1967, 89, 213. d D. H. McDaniel, H. C. Brown, J. Org. Chem.
1958, 24, 420. e C. G. Swain, E. C. Lupton, Jr., J. Am. Chem. Soc. 1968, 90, 4328.
As noted in our previous report7, the IN to CB ratios increased as photolysis proceeded due to
autocatalytic effect of the departing acid. Thus, in order to minimize interference of other factors, we
took the IN/CB ratios at the lowest conversion that we could measure by 1H NMR spectroscopy.10
Our data revealed a few intriguing features. All the substituted benzoates gave higher IN/CB ratios
than the unsubstituted one, 3. In addition, the benzoates with electron withdrawing groups (EWG) (1,
2) gave higher IN/CB ratios than the ones with electron donating groups (EDG) (4, 5). Our results do
not seem to show any correlation with Hammett substituent parameters as shown in the table.
Since only the IN forming process involves the departure of benzoic acid moiety, electronic effects

of para-substituents would be reflected mainly in the rate of IN forming route rather than that of CB
forming route. Leaving group power (or nucleofugality) can be evaluated by many different ways,
but one of the most popular and easiest way, is to correlate it by the acidity of conjugate acid of the
leaving group.11 If any reaction involves a rate determining departure of the leaving group, the
reaction would be favored by any factors increasing the acidity of conjugate acid of the leaving
group. Our results do not match with this trend, because the EDG containing benzoates show higher
IN/CB ratios than the unsubstituted one.
The fact that both EDG and EWG containing benzoates gave higher IN/CB ratios than the
unsubstituted one implies that the IN forming process does not depend entirely on the pKa of the
leaving benzoic acid. Another factor to be considered would be the basicity of the carbonyl oxygen
of the benzoate leaving group, which will be increased as the electron donor ability of the substituent
increases. Protonation of the carbonyl oxygen by any proton sources would eventually facilitate the
leaving group’s departure. Thus, a combination of two opposite electronic effects of substituents may
be involved in the leaving group departure. One of a few mechanistic scenarios to meet this dual
effect would be that the IN forming process goes through an intramolecular hydrogen bonded
transition state from E-xylylenol intermediate as shown in mechanism A of Scheme 3. The other
possible mechanism is mechanism B of the same Scheme, in which a biradical intermediate is
involved instead of the E-xylylenol.
Scheme 3. Two possible mechanisms of indanone formation from photolysis of 1-5.
Mechanism B resembles the one proposed for IN formation from a mesylate precursor by Wessig.6

In both mechanisms A and B, the C-O bond connecting the carboxyl leaving group will be loosened
and the hydrogen bonded O-H will be shortened as the reaction proceeds, which eventually will lead
to the departure of the leaving group. Considering standard electron flow, the C-O bond cleavage can
get more help from the electron withdrawing substituents, while the O-H bond formation will obtain
assistance from the electron donating groups, even though the effect may be much less in mechanism
B as compared to mechanism A. In order to obtain a clue to its mechanism, we have carried out a
quantum chemical calculation using Gaussian 09 programs.12 Energy minimized structures were
searched for reactants, products, and transition states (TS) of IN forming reactions for both
mechanism A and B.(Scheme 4) The optimization and frequency calculations were conducted at the
B3LYP/6-31G(d) level using density functional (DFT) method. The intrinsic reaction coordinate
(IRC) approach was performed to verify that the calculated TS, intermediates, and products were on
the potential energy surface. For simplicity, 2-methylphenacyl carboxylates were used for calculation
instead of 2,4,6-trimethylphenacyl carboxylates. The biradical intermediate of mechanism B is a
triplet state. Its singlet state is identical to the xylylenol intermediate of mechanism A.13 Table 2 and
3 show the summary of calculations.
Scheme 4. Two different energy paths for quantum chemical calculations showing each stationary
structure: (a) mechanism A, (b) mechnism B.
Table 2. Energy minimized structures of stationary points shown in Scheme 4 and some important

structural parameters
a, C=O—H-O (Å)
Structure Structure
b, C-O (Å)
c, C=O (Å)
Structure Structure
Structure Structure
A
B (TS)
1.42
1.34
1.31
1.27
1.20
A
B (TS)
2.51
2.46
2.42
2.38
2.26
A
B (TS)
1.28
1.28
1.28
1.29
1.29
Compound
Δ
Δ
Δ
1.86
1.84
1.84
1.83
1.83
-0.44
-0.50
-0.53
-0.56
-0.63
1.47
1.47
1.47
1.47
1.47
1.04
0.99
0.95
0.91
0.79
1.23
1.23
1.23
1.23
1.23
0.05
0.05
0.05
0.06
0.06
1
2
3
4
5
a, C=O—H-O (Å )
b, C-O (Å )
Structure Structure
c, C=O (Å )
Structure Structure
Structure structure
D
E (TS)
1.39
D
E (TS)
2.22
D
E(TS)
1.27
1.27
1.27
Compound
Δ
Δ
Δ
1.83
1.81
1.81
-0.44
-0.44
-0.44
1.49
1.49
1.49
0.73
0.70
0.69
1.23
1.23
1.23
0.05
0.04
0.04
1
2
3
1.37
2.19
1.37
2.18
1.81
1.80
1.37
1.36
-0.44
-0.44
1.49
1.48
2.17
2.16
0.68
0.66
1.23
1.23
1.27
1.27
0.04
0.04
4
5

Table 3. Activation barriers for mechanism A and B calculated using DFT method (B3LYP/6-31G(d)
level) of Gaussian 09 programs .
Mechanism A
ΔG⧧ (kcal/mol K)
19.73
Mechanism B
Compound
ΔG⧧ (kcal/mol K)
3.41
4.55
4.78
5.11
5.32
1
2
3
4
5
21.18
22.07
21.51
21.69
The result of our calculation shows that activation barriers of the departure of the benzoic acid from
xylylenol intermediates of ketones 1-5 correlate with our experimental CB/IN ratios remarkably well
as shown in Figure 1, while the barriers from biradical intermediates do not show such correlation at
all. The correlation between product ratios and computational results seem to support mechanism A
of Scheme 3 for the indanone formation from photolysis of o-alkylphenacyl carboxylates. The
calculation predicted not only that all the substituted benzoates gave lower barriers than the
unsubstituted one, but also that p-methoxy substituted ketone, 5, gave lower barrier than p-methyl
substituted one, 4, which all agreed well with experimental results shown by their CB/IN ratios. It
also predicted the lowest barrier of p-cyano ketone, 1, of all the ketones studied, which coincided
with the lowest CB/IN ratio of all for 1.
It is worthwhile to note that structural parameters shown on Table 2 show different trends from
mechanism A and B, especially in changes of bond length of hydrogen bonded O-H. The O-H bond
becomes more shorter in electron donating substituents than in electron withdrawing ones as the
reaction goes towards TS of mechanism A, while the bond is essentially unchanged towards TS of
mechanism B no matter what the substituent is. In contrast, C-O bond cleavage shows similar trends
for both mechanism A and B, where the C-O bond is more cleaved in electron withdrawing
substituents than in electron releasing ones towards TS. The result can be explained that effects of
basicity of carbonyl oxygen are almost neglected in mechanism B, whereas the effects certainly
become a determining factor to reaction barrier for mechanism A.

ΔG⧧ (kcal/mol K)
CB/IN ratios
26
25
24
23
22
21
20
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
ΔG⧧
CB/IN
19
1
2
3
4
5
Compound
Figure 1. Comparison of calculated reaction barriers of mechanism A (ΔG⧧ in kcal/molK) and
experimental CB/IN ratios in photolysis of ketones 1-5
As noted above, reaction mechanism of the indanone formation of o-alkylphenacyl PPG has been
studied for a long time, but there still have been debates over which of the xylylenol and the biradical
intermediate is responsible for the indanone formation. Our experimental and computational results
support that the xylylenol is the direct precursor for the indanone product, at least for carboxylate
leaving groups. We are planning to investigate other leaving groups such as sulphonates using a
similar approach under the reaction condition where the CB product can be formed as well as the IN
product such as at low temperature.
In summary, from our research on photochemical reaction mechanism of p-substituted o-
alkylphenacyl benzoates, we have discovered a very interesting substituent effect in which both
electron donating and electron withdrawing groups accelerate the departure of the p-substituted
benzoate leaving groups. Quantum chemical calculation on the reaction paths revealed that the
photochemical reaction forming the indanone product resulted from the xylylenol intermediate rather
than the biradical intermediate. The electron donating substituents increased the basicity of the
carbonyl oxygen of the carboxylate leaving group which helped the proton transfer and eventually
promoted the departure of the leaving group via an intramolecular hydrogen bonded transition state.
The electron withdrawing substituents also helped in the departure of the carboxylate leaving groups

in a more classical sense, whereby the more acidic the benzoic acids are, the better the leaving
groups. Our results added an evidence supporting the intermediacy of xylylenol on the way to
indanone formation from o-alkylphenacyl PPG, which has been debated.
References and notes
1
(a) Givens, R. S.; Rubina, M.; Stensrud, K. F. J. Org. Chem. 2013, 78, 1709-1717. (b) Abitelli, E.;
Protti, S.; Fagnoni, M.; Albini, A. J. Org. Chem. 2012, 77, 3501-3507; (c) An, H.-Y.; Kwok, W. M.;
Ma, C.; Guan, X.; Kan, J. T. W.; Toy, P. H.; Philips, D. L. J. Org. Chem. 2010, 75, 5837-5851. (d)
Ma, C.; Stelinmetz, M. G.; Kopatz, E. J.; Rathore, R. J. Org. Chem. 2005, 70, 4431-4442.
2
Cho, S.; Park, B. S. Bull. Kor. Chem. Soc. 2004, 25, 42-44.
3
Wessig, P. ; Muhling, O. Angew. Chem. Int. Ed. 2001, 40, 1064-1065.
(a) Solomek, T.; Wirz, J.; Klan, P. Acc. Chem. Res. 2015, 48, 3064-3072. (b) Klan, P.; Solomek, T.;
4
Bochet, C. G.; Blanc, A.; Givens, R. S.; Rubina, M.; Popik, V.; Kostikov, A.; Wirz, J. Chem, Rev.
2013, 113, 119-191.
5
(a) Klan, P.; Pelliccioli, A. P.; Pospisil, T.; Wirz, J. Photochem. Photobiol. Sci. 2002, 1, 920-923.
(b) Zabadal, M.; Pelliccioli, A. P.; Klan, P.; Wirz, J. J. Phys. Chem. A 2001, 105, 10329-10333. (c)
Pelliccioli, A. P.; Klan, P.; Zabadal, M.; Wirz, J. J. Am. Chem. Soc. 2001, 123, 7931-7932. (d) Klan,
P.; Zabadal, M.; Heger, D. Org. Lett. 2000, 2, 1569-1571.
6
(a) Wessig, P.; Muehling, O. Eur. J. Org. Chem. 2007, 14, 2219-2232. (b) Wessig, P.; Glombitza,
C.; Mueller, G.; Teubner, J. J. Org. Chem. 2004, 69, 7582-7591. (c) Wessig, P.; Muhling, O. Angew.
Chem. Int. Ed. 2001, 40, 1064-1065.
7
Park, B. S.; Ryu, H. Tetrahedron Lett. 2010. 51, 1512-1516.
(a) Jang, M.; Park, B. S. Bull. Korean Chem. Soc. 2016, 37, 1509-1512. (b) Jang, M.; Lim, B. H.;
8
Ryu, H. J.; Park, B. S. Tetrahedron Lett. 2013, 54, 7175-7179. (c) Ngoy, B. P.; Sebej, P.; Solomek,
T.; Lim, B. H.; Pastierik, T.; Park, B. S.; Givens, R. S.; Heger, D.; Klan, P. Photochem. Photobiol. Sci.
2012, 11, 1465-1475. (d) Sebej, P.; Lim, B. H.; Park, B. S.; Givens, R. S. Klan, P. Org. Lett. 2011, 13,
644-647.
9
Spectroscopic properties of 1: 1H NMR (CDCl3, 200 MHz) δ 8.23 (d, 2H, J = 8.4 Hz), 7.80 (d, 2H,
J = 8.4 Hz), 6.91 (s, 2H), 5.20 (s, 2H), 2.34 (s, 6H), 2.33 (s, 3H), 13C NMR (CDCl3, 50 MHz) δ 202.2,
164.5, 139.8, 134.8, 134.0, 133.3, 132.4, 130.5, 128.8, 118.0, 116.9, 70.1, 21.2, 19.1 ppm, IR (CCl4)
2230 (CN), 1725, 1715 (CO) cm-1, HRMS (C19H18NO3, M+H) calcd. 308.1287, found 308.1294.
Spectroscopic properties of IN: 1H NMR (CDCl3, 200 MHz) δ 7.10 (s, 1H), 6.94 (s, 1H), 3.06 (t, 2H,
13
J = 5.6 Hz), 2.68 (t, 2H, J = 5.6Hz), 2.61 (s, 3H), 2.40 (s, 3H), C NMR (CDCl3, 50 MHz) δ 207.5,
156.6, 145.1, 138.6, 132.4, 130.4, 124.5, 37.0, 25.3, 21.9, 18.3 ppm, IR (CCl4) 1698 (CO) cm-1, EI
MS (C 11H12O) 160 (M+)
1
Spectroscopic properties of CB-1: H NMR (CDCl3, 200MHz) δ 8.15 (d, 2H, J = 8.4 Hz), 7.74 (d,
2H, J = 8,4 Hz), 6.87 (s, 2H), 4.68 (s, 2H), 3.30 (AB quartet, 2H, J = 14.2 Hz), 2.80 (broad s, 1H,
OH), 2.32 (s, 3H), 2.26 (s, 3H), 13C NMR (CDCl3, 50 MHz) δ 165.1, 141.9, 140.8, 140.2, 133.8,
132.5, 132.2, 130.2, 129.6, 121.6, 117.9, 116.5, 78.5, 70.1, 43.8, 22.0, 17.1 ppm, IR (CCl4) 3446
(OH), 2233 (CN), 1727 (CO) cm-1, HRMS (C19H18NO3, M+H) calcd. 308.1287, found 308.1292.
Spectroscopic properties of 2: 1H NMR (CDCl3, 200 MHz) δ 8.18-8. 10 (m, 2H), 7.20-7.11 (m, 2H),
13
6.90 (s, 2H), 5.17 (s, 2H), 2.34 (s, 6H), 2.32 (s, 3H), C NMR (CDCl3, 50 MHz) δ 202.8, 166.1 (d,
2
1JCF = 253 Hz), 165.1, 139.6, 135.1, 134.0, 132.6 (3JCF = 9.5 Hz), 128.7, 125.7, 115.7 (d, JCF = 22

Hz), 69.7, 21.2, 19.1 ppm, IR (CCl4) 1729, 1720 (CO) cm-1, HRMS (C18H 18FO3, M+H) calcd.
301.1240, found 301.1244.
Spectroscopic properties of CB-2: 1H NMR (CDCl3, 200 MHz) δ 8.18-8.10 (m, 2H), 7.14 (t, 2H, J =
8.4 Hz), 6.86 (s, 2H), 4.66 (s, 2H), 3.30 (AB quartet, 2H, J = 14.2 Hz), 3.20 (broad s, 1H, OH), 2.34
13
1
(s, 3H), 2.28 (s, 3H), C NMR (CDCl3, 50 MHz) δ 165.9, 165.8 (d, JCF = 253 Hz), 142.2, 140.9,
140.0, 132.5, 132.2 (3JCF = 9.5 Hz), 129.5, 126.1, 121.5, 115.5 (d, JCF = 22 Hz), 78.6, 69.7, 43.6,
2
22.0, 17.1 ppm, IR (CCl4) 3455 (OH), 1728 (CO) cm-1, HRMS (C18H 18FO3, M+H) calcd. 301.1240,
found 301.1243.
Spectroscopic properties of 3: 1H NMR (CDCl3, 200 MHz) δ 8.12 (d, 2H, J = 8.2 Hz), 7.72-7.45 (m,
13
3H), 6.90 (s, 2H), 5.18 (s, 2H), 2.35 (s, 6H), 2.31 (s, 3H), C NMR (CDCl3, 50 MHz) δ 203,0, 166.1,
139.6, 135.2, 134.1, 133.5, 13.0, 129.4, 128.6, 69.7, 21.2, 19.2 ppm IR (CCl4) 1722, 1703 (CO) cm-1,
HRMS (C18H19O3, M+H) calcd. 283.1334, found 283.1329.
Spectroscopic properties of CB-3: 1H NMR (CDCl3, 200 MHz) δ 8.11 (d, 2H, J = 8.2 Hz), 7.68-7.43
(m, 3H), 6.87 (s, 2H), 4.69 (AB quartet, 2H, J = 11.8 Hz), 3.34 (AB quartet, 2H, J = 14.2 Hz), 3.10
(broad s, 1H, OH), 2.36 (s, 3H), 13C NMR (CDCl3, 50 MHz) δ 167.0, 142.3, 141.1, 140.1, 133.3,
132.7, 129.9, 129.6, 128.5, 121.6, 114.6, 78.8, 69.9, 43.8, 22.8, 17.3 ppm. IR (CCl4) 3463 (OH),
1726 (CO) cm-1, HRMS (C18H19O3, M+H) calcd. 283.1334, found 283.1337.
Specgtroscopic properties of 4: 1H NMR (CDCl3, 200 MHz) δ 8.01 (d, 2H, J = 8.2 Hz), 7.28 (d, 2H,
13
J = 8.2 Hz), 6.90 (s, 2H), 5.16 (s, 2H), 2.45 (s, 3H), 2.35 (s, 6H), 2.31 (s, 3H), C NMR (CDCl3, 50
MHz) δ 203.2, 166.1, 144.2, 139.5, 135.3, 134.1, 130.1, 129.3, 128.7, 126.7, 69.5, 21.8, 21.2, 19.2
ppm IR (CCl4) 1720, 1699 (CO) cm-1, HRMS (C19H21O3, M+H) calcd. 297.1491, found 297.1495.
1
Spectroscopic properties of CB-4: H NMR (CDCl3, 200 MHz) δ 8.03 (d, 2H, J = 8.4 Hz), 7.21 (d,
2H, J = 8.4 Hz), 6.89 (s, 1H), 4.63 (AB quartet, 2H, J = 11.5 Hz), 3.30 (AB quartet, 2H, J = 14.4 Hz),
2.32 (s, 3H), 2.27 (s, 3H), 2.13 (s, 3H), 13C NMR (CDCl3, 50 MHz) δ 166.5, 142.5, 141.0, 140.8,
132.5, 1131.7, 129.3, 124.2, 122.4, 121.6, 113.6, 78.8, 69.8, 43.7, 22.0, 21.3, 17.0 ppm IR (CCl4)
3450 (OH), 1727 (CO) cm-1, HRMS (C19H21O3, M+H) calcd. 297.1491, found 297.1498.
1
Spectroscopic properties of 5: H NMR (CDCl3, 200 MHz) δ 8.07 (d, 2H, J = 8.4 Hz), 6.99 (s, 2H),
13
6.91 (d, 2H, J = 8.2 Hz), 5.15 (s, 2H), 3.90 (s, 3H), 2.34 (s, 6H), 2.31 (s, 3H), C NMR (CDCl3, 50
MHz) δ 203.3, 165.7, 163.8, 139.5, 135.3, 134.0, 132.1, 128.7, 121.8, 113.8, 69.5, 55.6, 21.2, 9.1
ppm, IR (CCl4) 1725, 1701 (CO) cm-1, HRMS (C19H21O4, M+H) calcd. 313.1440, found 313.1435.
1
Spectroscopic properties of CB-5: H NMR (CDCl3, 200 MHz) δ 8.05 (d, 2H, J = 8.4 Hz), 6.95 (d,
2H, J = 8.4 Hz), 6.85 (broad s, 1H, OH), 4.65 (AB quartet, 2H, J = 11.6 Hz), 3.89 (s, 3H), 3.31 (AB
quartet, 2H, J = 14.4 Hz), 2.34 (s, 3H), 2.29 (s, 3H), 13C NMR (CDCl3, 50 MHz) δ 166.9, 163.7,
142.5, 141.1, 140.0, 132.7, 131.9, 129.6, 122.4, 121.6, 113.8, 78.9, 69.8, 55.5, 43.7, 22.1, 17.3 ppm.
IR (CCl4) 3445 (OH), 1726 (CO) cm-1, HRMS (C19H21O4, M+H) calcd. 313.1440, found 313.1446.
10
The CB products were formed at the lowest amount of all the photoproducts, so we tried to limit
the conversion using the NMR peaks of the CB, where their integration numbers were not too small.
See 1H NMR spectra given in the supporting information.
11
J. Bartl, S. Steenken, H. Mayr, R. A. McClelland, J. Am. Chem. Soc., 1990, 112, 6918–6928.
12
Gaussian 09 suite of programs M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. H. Petersson, M. C. Nakatsuji,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.
A. Jr. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J.
Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.

Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg,
S. Dapprich, A. D. Daniels, O. Parkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
Gaussian 09, revision A.02; Gaussian Inc.: Wallingford, CT, 2009.
13
Any attempts to get energy minimized structure of the singlet biradical intermediate always
resulted in the xylylenol structure.

Graphical Abstract
.
Leave this area blank for abstract info.
Leaving Group Effect on Photochemistry of ortho-Alkylphenacyl Carboxylate
Seol Hee Kim, Mi Jang, Da Yoon Moon and Bong Ser Park
Department of Chemistry, Dongguk University(Seoul Campus), Seoul 04620, Korea

Highlights
· Leaving group effects on photochemistry of o-alkylphenacyl PPG
· Unique way to evaluate substituent effect on a reaction involving carboxylate leaving group
· Re-evaluation of reaction mechanism of photochemistry of o-alkylphenacyl PPG