focusing on the direct detection of intermediates by means
of laser flash photolysis (LFP). Likewise, the dependence
of CR regioselectivity on the thietane structure as a mecha-
nistic fingerprint has not been investigated.
With this background, the aim of the present work is to
study in detail the behavior of model thietanes under PET
conditions, with special attention to mechanistic elucidation
by LFP measurements. Specifically, 2,2,3-triarylthietanes 1
were chosen as models and thiapyrylium salt 2 as PET
photosensitizer5,6 (Figure 1).
Table 1. Photoinduced Reactions of 1 in the Presence of
Thiapyrylium Salt (2)a
product distributionb, c (%)
entry
thietane
3
4
5a
5′a
5b
5′b
1
2
3
1a
1b
1c
1a
1b
48
52
26
100
28
85
33
39
4d,e
5d,f
38
37
39
41
11
10
12
12
51g
a 1, 0.05 M; 2, 0.002 M, CH3CN, nitrogen atmosphere. Irradiation for
30 min with a high-pressure mercury lamp at room temperature (effective
irradiation wavelength between 300 and 500 nm). b Conversions were nearly
100%, except in the crossover experiments, where it was kept below 50%.
Mass balance was in general higher than 95%. c Determined by HPLC.
d Determined by GC analysis, using biphenyl as internal standard. e In the
presence of 0.05 M of anethole, 3b. f In the presence of 0.05 M of trans-
ꢀ-methylstyrene, 3a. g Yield of anethole, 3b.
that the yield of [4 + 2] cycloadduct decreased with
increasing charge and spin delocalization in the purported
alkene radical cation intermediate.
Figure 1
2.
.
Chemical structures of thietanes 1a-c and photosensitizer
In the absence of photosensitizer, CR occurred with the
reverse regioselectivity, leading always to the corresponding
2-aryl-1,1-diphenylethenes (see Scheme 1 as well as the
Supporting Information, Scheme S1 and Table S1).
To investigate the possible involvement of ion-molecule
complexes, such as (3/4)•+, as opposed to free ions (3•+ or
4•+), crossover experiments were performed. Thus, when a
mixture of 1a and 2 was irradiated in the presence of 3b
under the usual conditions, 5a and 5′a were still formed
together with lower amounts of the crossed products 5b and
5′b (Table 1, entry 4). Conversely, in the analogous experi-
ment using a mixture of 1b and 2 in the presence of added
3a (Table 1, entry 5), the crossed products 5a and 5′a were
now the major ones, whereas the direct products 5b and 5′b
were obtained in smaller amounts.
Reactions under study are shown in Scheme 1. Trans
4-methyl-2,2,3-triphenylthietane (1a) was totally consumed
Scheme 1. Direct and Sensitized Photolysis of 1a-c
In order to obtain direct mechanistic evidence based on
detection of possible reaction intermediates, LFP measure-
ments were carried out on 2 in the presence of 1a-c, using
355 nm as the excitation wavelength. As the intersystem
crossing quantum yield of 2 is very high (ΦISC ) 0.94), the
triplet excited state (detectable as a broad T-T band with a
flat maximum at ca. 450 nm) was assumed to be involved
in the CR process.5 Indeed, triplet quenching was observed
in all cases; rate constants were determined by application
of the Stern-Volmer equation8 and found to be 1.2 × 109
M-1 s-1 (1a), 2.6 × 109 M-1 s-1 (1b), and 1.4 × 109 M-1
s-1 (1c). From these data, it was established that thietanes
1a-c quench the triplet excited state of 2 at a nearly
diffusion-controlled rate.
after 30 min irradiation in the presence of thiapyrylium salt
(2), giving cyclic products 5a and 5′a in high yield (Table
1, entry 1). The presence of an electron-releasing group in
trans-2,2-diphenyl-3-(4-methoxyphenyl)-4-methylthietane
(1b) was associated with a decreased yield of 5b and 5′b,
together with formation of anethole (3b) and thiobenzophe-
none (4) (Table 1, entry 2). Products 5a,b and 5′a,b,
previously described,7 were analyzed by HPLC. Interestingly,
trans-2,2,3,4-tetraphenylthietane (1c) gave cleanly the frag-
ments trans-stilbene (3c) and 4 under the same reaction
conditions (Table 1, entry 3). Thus, although C2-C3 bond
scission occurred in all cases, the ratio (3 + 4)/(5 + 5′)
strongly depended on the thietane structure. It is remarkable
Concomitantly with the disappearance of the T-T band,
a new transient peaking at 500 nm was detected in the
presence of 1a and 1b (Figure 2). This species was tentatively
assigned to the radical cations of 5a and 5b, respectively. In
the case of 1b, an absorption band around 600 nm was also
(5) Miranda, M. A.; Garc´ıa, H. Chem. ReV. 1994, 94, 1063–1089
.
(6) (a) Clennan, E. L.; Liao, C. J. Am. Chem. Soc. 2008, 130, 4057–
4068. (b) Bonesi, S. M.; Manet, I.; Fagnoni, M.; Albini, A. Eur. J. Org.
Chem. 2008, 2612–2620. (c) Bonesi, S. M.; Manet, I.; Freccero, M.; Fagnoni,
M.; Albini, A. Chem.sEur. J. 2006, 12, 4844–4857
.
(7) Argu¨ello, J. E.; Pe´rez-Ruiz, R.; Miranda, M. A. Org. Lett. 2007, 9,
(8) IUPAC Compendium of Chemical Terminology, 2nd ed.; McNaught,
A. D., Wilkinson, A., Eds.; Royal Society of Chemistry: Cambridge, 1997.
3587–3590.
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