Photolytic decomposition of dibenzylic sulfites

Article abstract of DOI:10.1016/j.tetlet.2012.06.137

The photolytic decay of a library of para-substituted dibenzylic sulfites has been evaluated by UV radiation in a Srinivasan-Griffin-Rayonet photochemical reactor in various deuterated solvents. The decay for each dibenzylic sulfite was examined with respect to Swain and Lupton's field constant, F. The rate of photolytic decay varies depending on the identity of the benzyl substituents. Furthermore, it has been observed that the solvent affects both the rate of sulfite photolytic decay as well as final product distribution.

Full text of DOI:10.1016/j.tetlet.2012.06.137

Tetrahedron Letters 53 (2012) 4933–4937
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photolytic decomposition of dibenzylic sulfites
Paolo N. Grenga b, Eric G. Stoutenburg b, Ronny Priefer a,b,
⇑
a College of Pharmacy, Western New England University, Springfield, MA 01119, USA
b Department of Chemistry, Biochemistry, and Physics, Niagara University, NY 14109, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The photolytic decay of a library of para-substituted dibenzylic sulfites has been evaluated by UV radia-
tion in a Srinivasan–Griffin–Rayonet photochemical reactor in various deuterated solvents. The decay for
each dibenzylic sulfite was examined with respect to Swain and Lupton’s field constant, F. The rate of
photolytic decay varies depending on the identity of the benzyl substituents. Furthermore, it has been
observed that the solvent affects both the rate of sulfite photolytic decay as well as final product
distribution.
Received 14 June 2012
Revised 26 June 2012
Accepted 28 June 2012
Available online 15 July 2012
Keywords:
Dibenzylic sulfite
Ó 2012 Elsevier Ltd. All rights reserved.
Photolytic decay
Swain and Lupton’s field constant
Photolysis, or photolytic decay, is a chemical process by which
molecules are degraded in the presence of light. Perhaps most
notably, photolysis is illustrated in the series of oxidation events
that occur during photosynthesis, the impetus of which is sunlight.
In what has been reported as the first organic photochemical reac-
The photochemical stability of sulfites is thus of interest, as it may
be possible for it to be altered by light during storage.
Little is known regarding the photolytic decay of sulfites, be-
yond a 1983 paper by Olsen et. al. on benzyl and 3-methylbenzyl
sulfites,10 as well as photocycloelimination of cyclic sulfites by
Griffin and Manmade.11 Previous work from our lab has shown
that the photolytic decay of closely related dibenzyloxy disulfides
follows a parabolic relationship with Swain and Lupton’s field con-
stant, F, depending on their para-substituents.12 The objective of
this project was to evaluate the photolytic decay of bis(para-
substituted benzyl) sulfites with respect to Swain and Lupton’s
field constant, F, as well as examine the decay products.
tion, changes in a-santonin crystals exposed to sunlight have been
observed.1 A technique exploiting photolytic decay, flash photoly-
sis, was first explored by the 1967 Nobel Prize laureates, Eigen,
Norrish, and Porter.2 Experiments utilizing flash photolysis of
caged compounds have also allowed for analysis of potassium
channels in neurons.3 Additionally, photolytic decay has been
investigated as a possible removal mechanism of steroid hormones
in waste water treatment processes.4 The photolysis of particular
classes of compounds has also been of interest. For example, a
measure for light intensity has been examined based on the pho-
tolysis of airborne NO2.5
We initially synthesized the bis(benzyl) sulfites with various
substituents in the para position based upon past procedures13
which consisted of reacting 2 equiv of a benzyl alcohol with 1 equiv
of thionyl chloride and 2 equiv of pyridine for 5 h in dichlorometh-
ane. The reaction mixture was quenched with distilled water and
washed twice with brine. The organic layer was collected, dried
on MgSO4, and concentrated under reduced pressure. The crude
residue was then purified via column chromatography in 5:1 hex-
anes/ethyl acetate. A library of eight bis(para-substituted benzyl)
sulfites was successfully synthesized in this manner (Scheme 1).
All were obtained in moderate to good yields, {1: bis(benzyl) sul-
fite; 2: bis(p-nitrobenzyl) sulfite; 3: bis(p-phenylbenzyl) sulfite;
4: bis(p-methylbenzyl) sulfite; 5: bis(p-t-butylbenzyl) sulfite; 6:
bis(p-chlorobenzyl) sulfite; 7: bis(p-cyanobenzyl) sulfite; 8; bis(p-
trifluoromethylbenzyl) sulfite}, and stored in the freezer in the dark
until use. All the benzyl alcohols were synthesized from commer-
cially available benzyl alcohol with the exception of 7 and 8 which
were initially reduced from their corresponding acids as previously
reported.14 We also looked to synthesize bis(p-methoxybenzyl) sul-
fite and bis(phenoxybenzyl) sulfite however in both cases we were
Many practical applications exist for molecules containing the
sulfite moiety. Sulfites have been studied as possible lithium ion
battery additives.6 Sulfites and sulfiting agents are also used to in-
hibit enzymatic and non-enzymatic browning, inhibit and control
microorganisms, as well as to control pH, and act as stabilizing
agents in food.7 There is concern, however, about the adverse
health effects of sulfites in foods, which include anaphylactic
shock, asthmatic attacks, urticaria and angioedema, nausea,
abdominal pain and diarrhea, seizures, and death.8 Many pharma-
ceuticals, including bronchodilators, local anesthetics, injectable
antibiotics, and anti-shock agents contain sulfites as antioxidants.9
⇑
Corresponding author. Tel.: +1 413 796 2416; fax: +1 413 796 2266.
E-mail address: ronny.priefer@wne.edu (R. Priefer).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.137

4934
P. N. Grenga et al. / Tetrahedron Letters 53 (2012) 4933–4937
O
S
SOCl2, pyridine
CH2Cl2
OH
O
O
X
X
X
X = H (1), NO2 (2), Ph (3), Me (4), tBu (5), Cl (6), CN (7), CF3 (8)
Scheme 1. Synthesis of library of bis(benzyl) sulfites.
O
S
O
S
Cl
Cl
Cl
Cl-
OH
O
Cl
R
R
R
R
+
O
-SO2
-HCl
O
O
O
Scheme 2. Possible mechanism for the formation of benzyl chlorides opposed to bis(benzyl) sulfites.
Figure 1. Half-life of photolytic decomposition of the library of dibenzylic sulfites in DMSO-d6 versus Swain and Lupton’s field constant, F.
Table 1
Half-life for library of bis(benzyl) sulfites in various solvents and their Swain and Lupton’s, F value
Bis(benzyl) sulfite
Swain and Lupton’s, F
DMSO
MeCN
Toluene
Benzene
Acetone
1 (H)
0
24.5
52.4
11.2
26.4
33.8
15.4
24.6
11.4
76.2
478.2
12.4
115.4
252.4
41.4
144
82.1
924.7
15.6
136.4
498.4
30
11.4
52.2
8.8
19.6
29.2
5.4
2 (NO2)
3 (Ph)
4 (Me)
5 (tBu)
6 (Cl)
1.00
0.25
ꢀ0.01
ꢀ0.10
0.72
0.90
0.64
410.2
15.4
411.9
511.4
81.1
164.8
9.4
7 (CN)
8 (CF3)
175.4
7.4
204.1
6.4
14.4
3.6
unsuccessful. This was attempted in a variety of ways, including
altering temperature, solvent, reaction time, and base. What were
obtained in all cases were their corresponding benzyl chlorides.
We hypothesized that the electron-donating ability of the oxygen
causes the liberation of SO2 prior to possible dimerization (Scheme
2). Subsequent attack by chloride would afford the benzyl chlorides.
With the other benzyl alcohols, benzyl chlorides were always ob-
served although only as a minor by-product; in particular with

P. N. Grenga et al. / Tetrahedron Letters 53 (2012) 4933–4937
4935
O
S
O
O
X
X
·
O
O
·
H
H
O
X
O
S
·
·
O
H
O
X
O
O
X
X
X
X
X
·
O
O
·
OH
O
H
O
X
X
X
X
X
X
O
O
Scheme 3. Possible mechanisms for photolytic decomposition of dibenzylic sulfite to form benzyl alcohol, benzaldehyde, benzyl benzoate, and dibenzyl ether.
Table 2
Table 3
Photolytic decay products and relative percentages of abundance as compared to
percentage of sulfite in DMSO
Photolytic decay products and relative percentages of abundance as compared to
percentage of sulfite in acetonitrile
75% Sulfite
remaining
50% Sulfite
remaining
0% Sulfite
remaining
75% Sulfite
remaining
50% Sulfite
remaining
0% Sulfite
remaining
H (1)
H (1)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
14
4
5
26
8
11
5
65
14
14
7
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
20
4
1
36
4
6
75
6
10
9
2
0
4
NO2 (2)
NO2 (2)
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
16
4
5
34
9
7
77
15
8
Benzyl alcohol
Benzaldehyde
23
2
45
5
91
9
Ph (3)
Ph (3)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
20
4
1
34
4
9
64
5
21
10
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
15
6
4
29
5
16
74
3
23
0
3
Me (4)
Me (4)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
19
3
2
32
3
10
5
61
9
19
11
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
19
2
4
32
8
10
67
7
26
1
tBu (5)
tBu (5)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
15
6
4
32
10
8
77
8
15
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
14
5
6
25
5
20
65
1
34
Cl (6)
Cl (6)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
17
7
1
33
17
10
4
53
10
22
15
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
15
6
4
33
13
17
37
28
35
0
CN (7)
CN (7)
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
17
6
2
20
15
23
41
26
33
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
17
6
2
23
15
12
64
14
22
CF3 (8)
CF3 (8)
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
24
0
1
45
2
3
86
4
10
Benzyl alcohol
Benzaldehyde
23
2
44
6
87
13

4936
P. N. Grenga et al. / Tetrahedron Letters 53 (2012) 4933–4937
Table 4
Table 5
Photolytic decay products and relative percentages of abundance as compared to
percentage of sulfite in toluene
Photolytic decay products and relative percentages of abundance as compared to
percentage of sulfite in benzene
75% Sulfite
remaining
50% Sulfite
remaining
0% Sulfite
remaining
75% Sulfite
remaining
50% Sulfite
remaining
0% Sulfite
remaining
H (1)
H (1)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
11
10
3
25
7
13
5
52
3
29
16
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
12
9
2
21
11
10
8
42
17
22
19
1
2
NO2 (2)
NO2 (2)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
13
12
0
22
21
2
46
27
27
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
13
12
0
27
21
2
55
28
17
Ph (3)
Ph (3)
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
15
10
0
29
17
4
55
24
21
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
13
11
1
25
21
4
48
38
14
Me (4)
Me (4)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
13
11
1
31
14
5
64
22
14
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
16
9
0
30
17
3
55
36
9
tBu (5)
tBu (5)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
13
12
0
27
21
2
55
39
6
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
17
6
2
35
10
5
68
15
17
Cl (6)
Cl (6)
Benzyl alcohol
Benzaldehyde
14
11
29
21
57
43
Benzyl alcohol
Benzaldehyde
19
6
40
10
79
21
CN (7)
CN (7)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
11
11
3
21
18
11
49
22
29
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
14
10
1
26
18
6
54
31
15
CF3 (8)
CF3 (8)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
9
6
5
5
18
7
12
13
42
6
25
27
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
10
7
5
22
11
12
5
40
17
26
17
3
the strong electron withdrawing group (i.e. NO2, Cl, CN, and CF3)
where as little as 2% was detected with 1H NMR.
range was 9.4 h for 3 (Ph) up to 511.4 h for 5 (tBu). Although not
universal, it appears that the less polar solvents increase the stabil-
ity of the sulfites under photolytic condition. Regardless, when the
time to ½ life is compared to Swain and Lupton’s field constant, F,
a parabolic relationship is seen for all solvents (Table 1).
Each bis(benzyl) sulfite was dissolved in DMSO-d6 (0.01 g/
0.50 mL) in a NMR tube flushed for 30 min with argon, and placed
in a Srinivasan–Griffin–Rayonet photochemical reactor, then con-
tinually irradiated (350 nm) and maintained at 35 °C. At various
time intervals, the samples were taken out and analyzed via 1H
NMR. Each sample was then placed back into the reactor until
the next time point. All bis(benzyl) sulfites were run until complete
disappearance of starting material. It was quickly determined that
there was a mixture of products formed during this decay (vide in-
fra), thus we examined the time to ½ life of the starting sulfites. For
this initial study it was determined that the decomposition fol-
lowed first order reaction kinetics with respect to the disappear-
ance of the starting bis(benzyl) sulfites. We next wished to
determine whether the rate was related to the Swain and Lupton’s
field constant, F,15 Indeed we did see the same parabolic relation-
ship (Fig. 1) as was reported for the bis(benzyl) disulfides,12 with
In addition to analyzing the rate of decomposition of the
bis(benzyl) sulfites, we also wished to examine the product ratio.
We had initially hypothesized that the fragmentation of the
sulfites would occur in a simple homolytic fashion yielding the cor-
responding alcohol starting materials; however, this was not the
case. Indeed, the benzylic alcohols were observed, in addition to
benzaldehydes, dibenzyl ethers, and benzyl benzoates. [Note: If
the sample is not flushed with argon prior to irradiation, benzoic
acid is also observed.] The exact mechanism for the formation of
the four aforementioned products is not completely understood
however, Scheme 3 illustrates possible mechanisms for the forma-
tion of benzyl alcohol, benzaldehyde, dibenzyl ether, and benzyl
benzoate. The benzyl alcohol most likely is formed by the homo-
lytic cleavage of the O–S bond and subsequent protonation from
either the solvent or another benzyloxy radical; as has been seen
with the bis(benzyl) disulfides.16 This route would thus afford
the aldehyde which could be deprotonated by another benzyloxy
radical. Subsequent attack with another equivalent of the alkoxy
radical would yield the benzyl benzoate. Yet another possible route
of decay would be the attack of a benzyloxy radical to a starting
sulfite, producing the dibenzyl ether. As many of these compounds
can themselves undergo further photolytic decomposition, we ana-
lyzed the ratio of products when decomposition of the sulfite was
25%, 50%, and 100% completed Tables 2–6.
2
an equation of t1/2 = 134.9 F ꢀ117.3 F +28.1 (R2 value = 0.858).
By examining the slope of the curve versus the F-value, we obtain
a linear correlation of dt1/2/dF = 269.8 F ꢀ117.3.
We next explored the photolysis of dibenzylic sulfites in other
deuterated solvents to see how the rate of the decay was affected.
Interestingly, decay was either zero, first, or second order depend-
ing upon solvent. In addition, within deuterated acetonitrile the
decay was autocatalyzed for all sulfites tested. Drastic, different
times of decay were observed when comparing solvents. When
irradiated in DMSO-d6, the ½ life ranged from a low of 11.2 h for
2 (NO2) to a high of 52.4 h for 5 (tBu). However, in d-toluene, the

P. N. Grenga et al. / Tetrahedron Letters 53 (2012) 4933–4937
4937
Table 6
of the decay. This latter result correlates well with this compound
also having the least amount of benzyl alcohol at the conclusion of
the decay. It can be assumed that the chloro benzyloxy radical re-
acts with a starting sulfite, yielding the ether (Scheme 3). This im-
plies that the chloro has a higher propensity to leave the solvation
cage. This phenomenon had been previously observed with the
dibenzyloxy disulfides which also afforded benzyloxy radicals in
their decomposition.16
Photolytic decay products and relative percentages of abundance as compared to
percentage of sulfite in acetone
75% Sulfite
remaining
50% Sulfite
remaining
0% Sulfite
remaining
H (1)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
15
4
6
32
5
13
56
8
36
In summation we have examined the photolytic decay of a
range of para-substituted dibenzyl sulfites. Their rate of decompo-
sition is parabolically correlated with Swain and Lupton’s field con-
stant, F in all solvents tested. In addition, the decay products that
were produced are dependent upon solvent used. Examination of
the ortho- and meta-substituted derivatives of dibenzyl sulfites
may provide additional information concerning this pathway.
NO2 (2)
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
17
4
4
31
9
10
60
21
19
Ph (3)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
17
5
3
33
9
8
62
16
21
Me (4)
Acknowledgements
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
20
2
3
32
4
10
4
58
7
21
14
The authors would like to thank the Niagara University Aca-
demic Center for Integrated Sciences for their financial support.
P.N. Grenga would like to thank the Barbara S. Zimmer Memorial
Research Award for financial aid.
0
tBu (5)
Benzyl alcohol
Benzaldehyde
15
10
32
18
62
38
References and notes
Cl (6)
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
19
5
1
29
12
9
55
25
20
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CN (7)
Benzyl alcohol
Benzaldehyde
Benzyl benzoate
Dibenzyl ether
17
5
3
26
7
12
5
48
11
26
15
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CF3 (8)
Benzyl alcohol
Benzaldehyde
Dibenzyl ether
16
5
4
30
11
9
55
24
21
No clear trend was observed in the distributions and identities
of decay products as compared to solvents. In all cases benzyl alco-
hol was the predominate product, ranging from a low of 37% with 6
(Cl) in DMSO-d6 (Table 2) to a high of 91% with 2 (NO2) in CD3CN
(Table 3). In cases where benzyl benzoate was a product, there
was a non-linear increase in its abundance as the decay proceeded.
Compound 1 (H) when irradiated in acetone-d6 (Table 6) afforded
the highest amount of benzyl benzoate at 36% after all the sulfite
had decayed. The non-linear increase was also observed when
the dibenzyl ether was formed and compound 6 (Cl) in DMSO-d6
(Table 2) produced a high of 35% of this product at the conclusion
15. Swain, C. G.; Unger, S. H.; Rosenquist, N. R.; Swain, M. S. J. Am. Chem. Soc. 1983,
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