Photoreactions of cyclic sulfite esters: Evidence for diradical intermediates

Article abstract of DOI:10.3762/bjoc.8.134

The photochemistry of a phenyl and 1, 2-diphenyl substituted sulfite ester is reported. The performance of photoreactions under relatively mild reaction conditions enables the detection of products that have not been observed in previous studies. It is concluded that, complementary to the initially proposed carbene intermediates, diradicals may also be considered.

Full text of DOI:10.3762/bjoc.8.134

Photoreactions of cyclic sulfite esters: Evidence for
diradical intermediates
Rick C. White*1, Benny E. Arney Jr.1 and Heiko Ihmels2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1208–1212.
1Department of Chemistry, Sam Houston State University, Huntsville,
TX 77341, USA and 2Department of Chemistry, Universität Siegen,
Adolf-Reichwein-Str. 2, 57068 Siegen, Germany
doi:10.3762/bjoc.8.134
Received: 31 May 2012
Accepted: 05 July 2012
Published: 30 July 2012
Email:
Rick C. White* - chm_rcw@shsu.du
This paper is dedicated to Professor Waldemar Adam on the occasion of
his 75th birthday.
* Corresponding author
Keywords:
Associate Editor: C. Stephenson
diradicals; mechanisms; photochemistry
© 2012 White et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The photochemistry of a phenyl and 1,2-diphenyl substituted sulfite ester is reported. The performance of photoreactions under
relatively mild reaction conditions enables the detection of products that have not been observed in previous studies. It is concluded
that, complementary to the initially proposed carbene intermediates, diradicals may also be considered.
Introduction
Photoinduced ring-opening or ring-fragmentation processes diaryloxiranes based on the photochemical formation of methyl
constitute an important type of reaction in organic photochem- ethers in methanol solution [8]. Mechanistically, stereoselec-
istry and have been examined both from a mechanistic and a tive oxygen scrambling was found in the photoextrusion of
synthetic point of view [1-5]. Mechanistic studies have revealed carbon dioxide from benzyl benzoate esters [9]. Furthermore,
information on reactive intermediates, solvent effects, and photodecarboxylation reactions in low-temperature matrices are
stereochemical outcomes, while synthetic work has pointed to the most efficient synthetic route to cyclobutadiene and its
novel approaches for preparing various molecules. For example, derivatives [10-12].
deazetation reactions in solvents of increasing viscosity were
employed to determine whether the loss of nitrogen from cyclic Mechanistic aspects of the photoinduced fragmentation of
azo compounds is stepwise or concerted [6]. The photoextru- carbonate esters have been discussed. Thus, Griffin et al.
sion of molecular nitrogen was employed also synthetically in reported that carbene intermediates are formed upon irradiation
the photoreaction of a benzothiadiazole to give an antiaromatic of cyclic carbonate esters such as 1b, as evidenced by the inter-
derivative of benzothiirene [7]. In another case, Griffin ception with appropriate trapping reagents; whereas the forma-
proposed carbene intermediates in the photochemistry of vicinal tion of persistent primary photoproducts has not been observed
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Beilstein J. Org. Chem. 2012, 8, 1208–1212.
Scheme 1: Photolysis of cyclic carbonate esters 1a and 1b in acetonitrile.
[13]. In contrast, we have found that under significantly milder however, in contrast to the results of the photoreaction of the
photochemical conditions (3 h versus 15 h), the cyclic carbon- cyclic carbonate ester 1a under similar conditions, the oxirane
ates 1a and 1b (Scheme 1) generate products derived from 1,3- 2a was not detected as a reaction product. The lack of oxirane
diradical intermediates [14].
formation may be explained by initial cleavage of the C–O bond
to give the biradical BR1, whose rotation about the C–C bond
Interestingly, the photoinduced fragmentation reactions of to give the relaxed biradical BR1rot is significantly faster than
nitrogen- and oxygen-containing functionalities have been the loss of sulfur dioxide (Scheme 3). As a consequence, the
studied intensively, whereas reports on photoextrusion reactions elimination of SO2 results in a biradical BR2 with a con-
of sulfur-containing functionalities are rather rare. For instance, formation that is unfavorable for oxirane formation, but enables
the photoextrusion of sulfur dioxide was carried out to facilitate efficient hydrogen migration to result in aldehyde 3a. Theoreti-
the preparation of cyclophane derivatives [15]. The photochem- cal calculations have confirmed that acetaldehyde derivatives
istry of the corresponding cyclic sulfite esters is even less well may be formed upon rearrangement of the 1,3-oxyethandiyl
known [16-18]. A report on the photochemistry of meso- [20,21] and that a subsequent decarbonylation results in an alkyl
hydrobenzoin sulfite undertaken about four decades ago stated radical. The latter reaction yields bibenzyl (4) and toluene (7) in
that these compounds are also carbene precursors [16], in the photoreaction of 8. It should be noted that the diradical BR2
analogy to the results obtained with carbonate 1b. To provide may be formed also upon photoinduced ring-opening reaction
further insight into the mechanism of the photoinduced cleavage of an intermediate oxirane 2a, which is not detectable in the
of the cyclic sulfites, we examined the styrene glycol sulfite (8) reaction mixture because of its rapid secondary reaction.
and the hydrobenzoin sulfite (9); and we herein demonstrate However, oxiranes have been observed as persistent intermedi-
that diradical intermediates are generated during the photoextru- ates under essentially identical conditions during photolysis of
sion processes.
cyclic carbonates 1a and 1b, thus it is unlikely that they are
formed to a significant extent in the photoreaction of 8.
Results and Discussion
Styrene glycol sulfite 8 was prepared from thionyl chloride and Similarly, we investigated the photochemistry of hydrobenzoin
styrene glycol in the presence of triethylamine [19]. The irradi- sulfite (9), which has been reported also to be a carbene
ation of the cyclic sulfite 8 for 3 h in acetonitrile gave phenyl precursor [16]. Because it has been shown in the case of cyclic
acetaldehyde (3a), bibenzyl (4) and toluene (7) in Scheme 2; carbonates that the application of milder reaction conditions
Scheme 2: Photoreactivity of styrene glycol sulfite (8).
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Beilstein J. Org. Chem. 2012, 8, 1208–1212.
Scheme 3: Photochemical pathway for photoextrusion of SO2 from cyclic sulfites.
enables the detection of reaction products that apparently [16]. The formation of 10 may be explained by the additional
decompose under harsher conditions, the original experimental phenyl substituent, as compared to the cyclic sulfite 8, which
conditions (t = 15 h) were changed. Thus irradiation of 9 in apparently facilitates the elimination of SO3 to give stilbene
CH3CN for 120 min gave 5 and 6, along with the so far unde- (10). Firstly, the phenyl substituent leads to a significant stabi-
tected products 3b, benzaldehyde (12), cis-stilbene (10) and lization of the resulting benzyl radical as opposed to the forma-
phenanthrene (11). The same products were found after irradi- tion of the oxylradical. Secondly, in the case of the cis-isomer
ation in cyclohexane. Such as in the case of the sulfite deriva- of the cyclic sulfite 9 the cleavage of the Cbenzyl–O bond may
tive 8, the products 3b, 5, and 6 may be formed by photoin- be energetically favorable, because it releases significant steric
duced elimination of SO2 through a biradical intermediate strain by changing from sp3 to sp2 hybridzation (Scheme 3).
(Scheme 3).
The cis-stilbene (10) is subsequently transformed into phenan-
threne (11) by photoinduced electrocyclization [22] as shown in
Furthermore, no oxirane photoproduct could be detected, Scheme 4.
presumably due to the relatively slow elimination of SO2, thus
resembling the photoreactivity of the derivative 8. To be noted Conclusion
is the formation of 10 as a reaction intermediate. Although it We have demonstrated that the performance of photoreactions
has been observed that 4,4,5,5-tetraphenyl-1,3,2-dioxothiolan- of cyclic sulfites under relatively mild reaction conditions
2-one undergoes photoinduced fragmentation of SO3 to give enables the detection of reaction products that have not been
9,10-diphenylphenanthrene as a product through an initially observed in previous studies. Notably, the analysis of the reac-
formed tetraphenylethylene, the latter has not been detected tion products shows that the initially proposed reaction mecha-
Scheme 4: Photoreactivity of meso-hydrobenzoin sulfite (9).
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Beilstein J. Org. Chem. 2012, 8, 1208–1212.
nism, which includes the formation of carbene intermediates, were added, and the spectra were recorded again to
needs to be reconsidered. Specifically, the results presented verify the presence of reaction products. Yields for
herein suggest complementary reaction pathways through the the reaction of 8 were calculated by adding redistilled
formation of biradical intermediates.
tert-butyl alcohol and comparing integrations of the
products with the methyl groups of the standard. Mass spectro-
metric data were recorded on photolysates immediately after
Experimental
General remarks: The NMR spectra were recorded on a reactions.
Bruker Avance 400 (1H NMR: 400 MHz; 13C NMR: 100
MHz). 1H NMR chemical shifts are relative to tetramethylsi-
Supporting Information
lane (δTMS = 0.00 ppm). Melting points were determined on a
Büchi 510K and are uncorrected. Phenyl benzyl ketone,
diphenyl acetaldehyde, diphenylmethane, benzil, phenyl
acetaldehyde, and bibenzyl were obtained commercially
(Aldrich) and used as received.
Supporting Information File 1
1H NMR spectra of photolysates.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-134-S1.pdf]
Styrene glycol sulfite was prepared by adding thionyl chloride
(2.5 g, 21 mmol) in diethyl ether (50 mL) dropwise to a solu-
tion of styrene glycol (2.8 g, 20 mmol) and triethylamine (4.6 g,
43 mmol) in ether (50 mL) with vigorous stirring. After being
stirred for 4 h, the mixture was poured into water (100 mL). The
ether layer was washed with water (4 × 50 mL) and dried
(MgSO4). After removal of the solvent, the oil was distilled
(148 °C at 2 mmHg) to give 2.29 g (60%) of pure styrene glycol
sulfite as a mixture of syn/anti isomers (syn = phenyl groups
and S=O bond on same side of ring); 1H NMR (400 MHz,
CDCl3) δ 4.45 (dd, 1H), 4.25 (dd, 1H), 4.20 (dd 1H), 4.92, (dd,
1H), 5.38, (dd, 1H), 5.90 (dd, 1H) and 7.5 (5H).
Acknowledgements
Support of this work by the Welch Foundation of Houston, TX
(grant x-0011) is acknowledged. Support from Universität
Siegen for a Visiting Professorship for RCW is also greatly
acknowledged.
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