Visible light photooxidative cyclization of amino alcohols to 1,3-oxazines

Article abstract of DOI:10.1016/j.tetlet.2013.02.031

1,3-Amino alcohols were prepared to examine how structure affects the oxidation of tertiary amines in visible light photocatalysis. These substrates were cyclized to produce oxazines following the photooxidative formation of iminium ions using catalytic Ru(bpy)₃Cl₂. Amino alcohol derivatives of tetrahydroisoquinoline, tetrahydroquinoline, pyrrolidine, and Δ³-piperidine all were found to be viable substrates. In the case of the unsaturated piperidines, cyclization is accompanied by addition of methanol across the alkene; most likely occurring via a conjugate addition to an intermediate α,β-unsaturated iminium ion prior to oxazine formation.

Full text of DOI:10.1016/j.tetlet.2013.02.031

Tetrahedron Letters 54 (2013) 2101–2104
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Visible light photooxidative cyclization of amino alcohols to 1,3-oxazines
⇑
Cheryl L. Mathis, Brandi M. Gist, Conerd K. Frederickson, Katie M. Midkiff, Christopher C. Marvin
Department of Chemistry, Hendrix College, 1600 Washington Ave., Conway, AR 72032, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3-Amino alcohols were prepared to examine how structure affects the oxidation of tertiary amines in
visible light photocatalysis. These substrates were cyclized to produce oxazines following the photooxi-
dative formation of iminium ions using catalytic Ru(bpy)3Cl2. Amino alcohol derivatives of tetrahydroiso-
quinoline, tetrahydroquinoline, pyrrolidine, and
the case of the unsaturated piperidines, cyclization is accompanied by addition of methanol across the
Received 24 January 2013
Revised 4 February 2013
Accepted 8 February 2013
Available online 17 February 2013
D
3-piperidine all were found to be viable substrates. In
alkene; most likely occurring via a conjugate addition to an intermediate
prior to oxazine formation.
a,b-unsaturated iminium ion
Keywords:
Visible light
Photoredox catalysis
Iminium ion
1,3-Oxazine
N,O-Acetal
Ó 2013 Elsevier Ltd. All rights reserved.
Photochemical reactions are attractive since they generally oc-
cur under mild conditions and can be considered green chemical
reactions.1 When a compound absorbs light radiation it enters an
electronically excited state that presents numerous opportunities
for chemical reactions. Unfortunately, most organic compounds
interact only with ultraviolet light. This necessitates the use of
high-energy UV lamps and quartz reaction vessels that are
transparent to this radiation. As a result of these requirements
for specialized equipment, photochemical reactions have been
underutilized in synthetic chemistry.
Recent reports have reinvigorated interest in synthetic photo-
chemistry by popularizing visible light photocatalysis.2 Photocata-
lysts, such as Ru(bpy)3Cl2 (1), are attractive as they become
powerful redox agents after absorbing visible light radiation
(Fig. 1).3 The capability of catalytically promoting photoinduced
electron transfer reactions with visible light is a powerful enabling
concept, as these reactions can be completed using household
lamps and standard laboratory glassware.4
Iminium ions are valuable synthetic intermediates as they are
readily captured by a variety of useful nucleophiles. Most of the
work involving the oxidative formation of iminium ions using vis-
ible light photocatalysis has involved N-aryl tetrahydroisoquino-
line substrates.2d,9 Iminium ions have been produced via visible
light photocatalysis from other benzylamines,10 dimethylani-
lines,9j,k,10a and N-arylglycines,11 as well as by the fragmentation
of N-arylated cyclopropylamines12 and TMEDA.13 Stephenson has
recently reported an impressive example where acyl iminium ions
are generated from dialkylamides, such as DMF, by use of persul-
fate in conjunction with Ru(bpy)3Cl2.14
We noticed that N-aryl groups are a common structural motif in
this chemistry and decided to study the formation of iminium ions
from trialkylamines using visible light photocatalysis. We imag-
ined that the intramolecular trapping of the iminium ion with a
nucleophile would minimize unproductive decomposition path-
ways. Therefore, we decided to prepare 1,3-amino alcohols and
study their photooxidative cyclization to oxazines. Although the
oxidation of such substrates to iminium ions is expected to be
more challenging due to the absence of an N-aryl group,15 we were
encouraged by precedented reactivity using stoichiometric photo-
sensitizers with UV radiation.16
We began by examining the reactivity of a simple carbinol ana-
log of N-phenyltetrahydroisoquinoline (5a) in several control reac-
tions (Table 1).17,18 Oxazine 6a was formed using catalytic
Ru(bpy)3Cl2 in methanol as expected, but was accompanied by
the formation of amide 7 (entry 1). It was verified that catalyst
and visible light irradiation were required to obtain product within
48 h. Furthermore, only 19% of product 6a was formed under
anaerobic conditions, which supports the involvement of oxygen
in catalyst turnover. We examined other catalysts and solvents
Many of the earlier reports featuring visible light photocatalysis
involved the reductive generation of free radicals. This paradigm
has been used to generate radicals from halides2b,c,5 and to pro-
mote [2+2] cycloadditions with enones.2a,6 In these methods the
addition of a tertiary amine serves as a terminal electron donor
and is necessary for catalytic turnover. Subsequent work in visible
light photocatalysis has involved the development of reactions
driven by the oxidative behavior of these systems,7 including the
oxidation of amines to
iminium ions.
a
-amino radicals,8 and their oxidation to
⇑
Corresponding author. Tel.: +1 501 505 2933; fax: +1 501 450 3829.
E-mail address: marvin@hendrix.edu (C.C. Marvin).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.02.031

2102
C. L. Mathis et al. / Tetrahedron Letters 54 (2013) 2101–2104
R
R
plays a more significant role in reactivity than the difference be-
tween oxidation potentials for N-aryl and N-alkyl amines. It was
recently reported that the a-C–H amination of N-aryl tetrahydro-
quinolines with azodicarboxylates occurs in good yield under vis-
ible light photoredox conditions with an Ir catalyst.8e That
amination reaction is postulated to occur via the addition of an
N
• 2 PF6
N
• 2 Cl
HO
R
O
O
R
N
N
N
N
N
N
N
N
N
N
CO2H
Ru2+
N
Ru2+
N
N
N
N
fluorescein (3): R = H;
E (S*/S ) = 0.74 V
a-amino radical to the azodicarboxylate. The more sluggish reac-
tivity for the cyclization of amino alcohol 5d to oxazine 6d in this
N
Ru(bpy)3Cl2 (1);
E (RuL32+*/RuL3+) = 0.77 V
Ru(bpz)3(PF6)2 (2);
eosin Y (4): R = Br;
E (S*/S ) = 0.83 V
E (RuL32+*/RuL3+) = 1.45 V
Table 2
Figure 1. Photoredox catalysts (Redox values vs SCE).
Photooxidation of amino alcohols to oxazinesa
Table 1
Photooxidative cyclization of 5a and control reactionsa
Entry Substrate
Product
Conversion
(time)b
Yieldc
(%)
Entry
Catalyst
Air
Visible light
Conversionb (%)
Ratiob 6a:7
1a
2c
3
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
None
Fluorescein
Fluorescein
Eosin Y
+
À
+
+
+
+
+
+
+
+
À
+
+
+
+
+
88
19
0
1.2:1
1:0
NA
NA
1:1
2.75:1
1:1
0:1
40 6a,
(40 7)
1
88 (48 h)
4
0
5
85
30
54
100
6d
7
2
>95 (24 h)
58
8d
Eosin Y
a
Conditions: a solution of 5a (0.42 mmol) and Ru(bpy)3Cl2 (5 mol %) in MeOH
(1.7 mL) was stirred open to the air and irradiated with a 23 W compact fluorescent
lamp.
b
Determined by 1H NMR integrations.
Degassed by three freeze–pump–thaw cycles and run under an Ar balloon.
DMF was used as a solvent.
>95 (24 h)
10:1 drc
c
3
52
d
for this reaction, including fluorescein (3) and eosin Y (4).9i–k,19
However, amide 7 was formed to varying extent in all cases.
We decided to examine the reactivity of other amino alcohols
with Ru(bpy)3Cl2, since it provided the best balance of reactivity
and selectivity for oxazine 6a. We also hypothesized that compet-
itive formation of the amide would be a lesser problem for amino
alcohols that lacked the stabilizing features of 6a. The results of
these experiments are shown in Table 2. As an initial benchmark,
6a and 7 each were isolated in 40% yield following the cyclization
of 5a (entry 1). Reactions of N-alkyl tetrahydroisoquinolines 5b9
and 5c were superior in comparison to the N-aryl compound 5a
(entries 2 and 3). These cyclizations were complete at 24 h and
were accompanied by traces (<5%) of an uncharacterized side-
product, which did not appear to correspond to the amides. It
was also noted that the formation of 6c was diastereoselective
(10:1 dr) and the major diastereomer could be isolated in 52% yield
after chromatography.
Tetrahydroquinoline 5d was then examined (entry 4). This ami-
no alcohol has an N-aryl group, but does not benefit from the pres-
ence of a benzylic amine as in 5a–c and should provide some
insight into the relative importance of these structural features.
This substrate was entirely unreactive to the standard conditions
using Ru(bpy)3Cl2 as the catalyst after 168 h. Slow cyclization to
oxazine 6d was eventually achieved by switching to Ru(bpz)3(PF6)2
(2), which is known to be a stronger oxidizing catalyst than 1
(Fig. 1).3a This reaction was allowed to progress to 56% conversion;
however, only a 12% yield of 6d was isolated after purification by
chromatography on neutral alumina. Comparing this result to the
8 (24 h)
56 (72 h)
4d
NDe12
5f
6
82 (96 h)
24g
NR
—
—
>95 (48 h)
1:1 dr
7
8
37g,h
90 (48 h)
1:1 dr
44g,h
a
See the Experimental section for detailed conditions.
Determined by 1H NMR integration of 5–6. In general it was found that longer
reaction times were required for reactions of more than 0.3 mmol 5.
b
c
Average of two or more experiments; isolated yields after column
chromatography.
d
Used 5 mol % of Ru(bpz)3(PF6)2 (2) as a catalyst in acetone.
e
Not determined.
f
This reaction was conducted in CH3CN on a 0.8 mmol scale.
Minor, uncharacterized, side products (approximately 10–20%) were observed
g
in the crude 1H NMR of this reaction.
h
The 1:1 mixture of diastereomers was inseparable by chromatography.
previous entries suggests that the nature of the a-amino C–H bond

C. L. Mathis et al. / Tetrahedron Letters 54 (2013) 2101–2104
2103
more, we are investigating the reaction of
D
3-piperidines in other
solvents and will report those results in due course.
Experimental
Photochemical reactions were conducted in
4 dram vials
(14.8 mL, 21 Â 70 mm, class B clear borosilicate glass), which were
equipped with a magnetic stir bar, septa, and opened to the air
with an 18 gauge needle. Ru(bpy)3Cl2 6H2O (11 mg, 0.015 mmol)
was added to a solution of substrate 5 (0.3 mmol) in methanol
(1.0–1.5 mL). This reaction was stirred at room temperature and
irradiated with a 23 W compact fluorescent lamp for 48 h. The
reaction mixture was then concentrated under reduced pressure,
whereupon it was dissolved in CDCl3 and analyzed by 1H NMR
(any insoluble material was removed by filtration through a plug
of cotton). Purification was accomplished by flash column
chromatography.
Scheme 1. A plausible mechanism to form 6g and 6h.
work can be explained in part by the fact that a second electron
transfer must occur to form the iminium ion intermediate.
Pyrrolidine amino alcohol 5e also proved to be a viable sub-
strate (entry 5). Amino alcohol 5e reacts slowly, but unlike quino-
line 5d, it is reactive with Ru(bpy)3Cl2. Curiously, 5e cyclizes to an
oxazine and undergoes further oxidation to give known pyrrolidi-
none 6e in modest yield.20 In contrast, acyclic amine 5f failed to re-
act despite the presence of a benzylic amine and an N-phenyl
group. The reason for the failure of 5f to react is unclear, but we
speculate that this may be due to an unfavorable conformational
equilibrium that disfavors initial oxidation to a radical cation. In
each attempt, 5f was unchanged according to 1H NMR analysis of
the crude reaction mixture.
Interesting results were obtained using unsaturated piperidines
5g and 5h (entries 7 and 8). In the case of these allylic, tertiary
amines, oxazine formation was accompanied by the addition of
methanol across the unsaturation to provide methoxy oxazines
6g and 6h as 1:1 mixtures of diastereomers. Whereas the conver-
sion of 5g and 5h was essentially complete by 1H NMR after 48 h,
isolated yields of 6g and 6h were moderate after chromatography
due to the acid sensitivity of these compounds. The lability of these
compounds during purification proved to be a general challenge
for the oxazine products derived from tertiary amines shown in Ta-
ble 2.21 A high mass balance was only obtained in the case of N-aryl
tetrahydroisoquinoline 5a, which also formed amide 7 as a major
side product.
Acknowledgments
This research was supported by an award from the Research
Corporation for Science Advancement. The authors also thank Hen-
drix College for generous start-up funds, and its Odyssey Program
for undergraduate summer research stipends (C.L.M., B.M.G., and
K.M.M.). The NMR facilities at Hendrix College are supported by
the National Science Foundation under Grant No. 1040470.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.02.
031.
References and notes
1. (a) Hoffmann, N. Chem. Rev. 2008, 108, 1052–1103; (b) Fagnoni, M.; Dondi, D.;
Ravelli, D.; Albini, A. Chem. Rev. 2007, 107, 2725–2756.
2. For selected leading references, see: (a) Ischay, M. A.; Anzovino, M. E.; Du, J.;
Yoon, T. P. J. Am. Chem. Soc. 2008, 130, 12886–12887; (b) Nicewicz, D. A.;
MacMillan, D. W. C. Science 2008, 322, 77–80; (c) Narayanam, J. M. R.; Tucker, J.
W.; Stephenson, C. R. J. J. Am. Chem. Soc. 2009, 131, 8756–8757; (d) Condie, A.
G.; Gonzalez-Gomez, J. C.; Stephenson, C. R. J. J. Am. Chem. Soc. 2010, 132, 1464–
1465.
3. (a) Haga, M.; Dodsworth, E. S.; Eryavec, G.; Seymour, P.; Lever, A. B. P. Inorg.
Chem. 1985, 24, 1901–1906; (b) Lazarides, T.; McCormick, T.; Du, P.; Luo, G.;
Lindley, B.; Eisenberg, R. J. Am. Chem. Soc. 2009, 131, 9192–9194.
4. For recent reviews on visible light photocatalysis, see: (a) Tucker, J. W.;
Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617–1622; (b) Shi, L.; Xia, W. Chem.
Soc. Rev. 2012, 41, 7687–7697; (c) Maity, S.; Zheng, N. Synlett 2012, 1851–1856;
(d) Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102–113;
(e) Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527–532; (f) Zeitler, K.
Angew. Chem., Int. Ed. 2009, 48, 9785–9789.
A plausible mechanism to account for the formation of these
products is shown (Scheme 1). Two pathways are possible after
the oxidative formation of iminium ion 8. In the first, cyclization
would occur to yield the expected product (9). Alternatively, con-
jugate addition of methanol to the
a
,b-unsaturated iminium ion
5. (a) Nagib, D. A.; Scott, M. E.; MacMillan, D. W. C. J. Am. Chem. Soc. 2009, 131,
10875–10877; (b) Shih, H.-W.; VanderWal, M. N.; Grange, R. L.; MacMillan, D.
W. C. J. Am. Chem. Soc. 2010, 132, 13600–13603; (c) Pham, P. V.; Nagib, D. A.;
MacMillan, D. W. C. Angew. Chem., Int. Ed. 2011, 50, 6119–6122; (d) Andrews, R.
S.; Becker, J. J.; Gagné, M. R. Angew. Chem., Int. Ed. 2010, 49, 7274–7276; (e)
Tucker, J. W.; Narayanam, J. M. R.; Krabbe, S. W.; Stephenson, C. R. J. Org. Lett.
2010, 12, 368–371; (f) Furst, L.; Matsuura, B. S.; Narayanam, J. M. R.; Tucker, J.
W.; Stephenson, C. R. J. Org. Lett. 2010, 12, 3104–3107; (g) Tucker, J. W.;
Nguyen, J. D.; Narayanam, J. M. R.; Krabbe, S. W.; Stephenson, C. R. J. Chem.
Commun. 2010, 4985–4987; (h) Tucker, J. W.; Stephenson, C. R. J. Org. Lett.
2011, 13, 5468–5471; (i) Nguyen, J. D.; Tucker, J. W.; Konieczynska, M. D.;
Stephenson, C. R. J. J. Am. Chem. Soc. 2011, 133, 4160–4163; (j) Courant, T.;
Masson, G. Chem. Eur. J. 2012, 18, 423–427; (k) Ju, X.; Liang, Y.; Jia, P.; Li, W.; Yu,
W. Org. Biomol. Chem. 2012, 10, 498–501; (l) Yasu, Y.; Koike, T.; Akita, M. Angew.
Chem., Int. Ed. 2012, 51, 9567–9571; (m) Iqbal, N.; Choi, S.; Kim, E.; Cho, E. J. J.
Org. Chem. 2012, 77, 11383–11387; (n) Iqbal, N.; Choi, S.; Ko, E.; Cho, E. J.
Tetrahedron Lett. 2012, 53, 2005–2008.
provides 10. Proton transfer leads to iminium ion 11, which then
cyclizes to 6g or 6h. Although oxazine 9 was not observed, it is ex-
pected to be in equilibrium with 8 under these conditions.
In conclusion we have found that iminium ions can be gener-
ated from a variety of N-alkylated heterocycles using visible light
photocatalysis and intramolecularly trapped by alcohols to pro-
duce 1,3-oxazines. While the presence of an N-aryl group is favor-
able, the nature of the
a-amino C–H bonds (benzylic, allylic, or
alkyl) and conformational effects appear to have a greater influ-
ence on amine reactivity. These reactions are operationally
straightforward; they can be conducted in standard laboratory
glassware, are tolerant of moisture and require air as a terminal
oxidant. The reactions of unsaturated piperidines (5g and 5h) are
particularly interesting, as oxazine formation is accompanied by
the addition of methanol across the alkene. We are working to im-
prove the modest yields of the isolated oxazine products. Further-
6. (a) Tyson, E. L.; Farney, E. P.; Yoon, T. P. Org. Lett. 2012, 14, 1110–1113; (b) Du,
J.; Espelt, L. R.; Guzei, I. A.; Yoon, T. P. Chem. Sci. 2011, 2, 2115–2119; (c) Du, J.;
Yoon, T. P. J. Am. Chem. Soc. 2009, 131, 14604–14605.
7. (a) Mitkina, T.; Stanglmair, C.; Setzer, W.; Gruber, M.; Kisch, H.; Konig, B. Org.
Biomol. Chem. 2012, 10, 3556–3561; (b) Rouch, W. D.; Zhang, M.; McCulla, R. D.

2104
C. L. Mathis et al. / Tetrahedron Letters 54 (2013) 2101–2104
Tetrahedron Lett. 2012, 53, 4942–4945; (c) Hajimohammadi, M.; Schwarzinger,
C.; Knor, G. RSC Adv. 2012, 2, 3257–3260; (d) Tucker, J. W.; Narayanam, J. M. R.;
12. Maity, S.; Zhu, M.; Shinabery, R. S.; Zheng, N. Angew. Chem., Int. Ed. 2012, 51,
222–226.
Shah, P. S.; Stephenson, C. R. J. Chem. Commun. 2011, 47, 5040–5042; (e) Su, F.;
Mathew, S. C.; Lipner, G.; Fu, X.; Antonietti, M.; Blechert, S.; Wang, X. J. Am.
Chem. Soc. 2010, 132, 16299–16301.
13. Cai, S.; Zhao, X.; Wang, X.; Liu, Q.; Li, Z.; Wang, D. Z. Angew. Chem., Int. Ed. 2012,
51, 8050–8053.
14. Dai, C.; Meschini, F.; Narayanam, J. M. R.; Stephenson, C. R. J. J. Org. Chem. 2012,
77, 4425–4431.
15. For the cyanation of tertiary amines using visible light produced by a 500 W Hg
lamp and 1800 W Xe lamps, see: (a) Cocquet, G.; Rool, P.; Ferroud, C. J. Chem.
Soc., Perkin Trans. 1 2000, 2277–2281; (b) Santamaria, J.; Kaddachi, M. T.;
Ferroud, C. Tetrahedron Lett. 1992, 33, 781–784; (c) Santamaria, J.; Kaddachi, M.
T.; Rigaudy, J. Tetrahedron Lett. 1990, 31, 4735–4738.
8. (a) McNally, A.; Prier, C. K.; MacMillan, D. W. C. Science 2011, 334, 1114–1117;
(b) Kohls, P.; Jadhav, D.; Pandey, G.; Reiser, O. Org. Lett. 2012, 14, 672–675; (c)
Miyake, Y.; Ashida, Y.; Nakajima, K.; Nishibayashi, Y. Chem. Commun. 2012,
6966–6968; (d) Miyake, Y.; Nakajima, K.; Nishibayashi, Y. J. Am. Chem. Soc.
2012, 134, 3338–3341; (e) Miyake, Y.; Nakajima, K.; Nishibayashi, Y. Chem. Eur.
J. 2012, 18, 16473–16477.
9. (a) Freeman, D. B.; Furst, L.; Condie, A. G.; Stephenson, C. R. J. Org. Lett. 2011, 14,
94–97; (b) Rueping, M.; Vila, C.; Koenigs, R. M.; Poscharny, K.; Fabry, D. C.
Chem. Commun. 2011, 47, 2360–2362; (c) Rueping, M.; Zhu, S.; Koenigs, R. M.
Chem. Commun. 2011, 47, 8679–8681; (d) Rueping, M.; Zhu, S.; Koenigs, R. M.
Chem. Commun. 2011, 47, 12709–12711; (e) Rueping, M.; Koenigs, R. M.;
Poscharny, K.; Fabry, D. C.; Leonori, D.; Vila, C. Chem. Eur. J. 2012, 18, 5170–
5174; (f) DiRocco, D. A.; Rovis, T. J. Am. Chem. Soc. 2012; (g) Xuan, J.; Feng, Z.-J.;
Duan, S.-W.; Xiao, W.-J. RSC Adv. 2012, 2, 4065–4068; (h) Zhao, G.; Yang, C.;
Guo, L.; Sun, H.; Chen, C.; Xia, W. Chem. Commun. 2012, 48, 2337–2339; (i) Hari,
D. P.; Konig, B. Org. Lett. 2011, 13, 3852–3855; (j) Pan, Y.; Kee, C. W.; Chen, L.;
Tan, C.-H. Green Chem. 2011, 13, 2682–2685; (k) Pan, Y.; Wang, S.; Kee, C. W.;
Dubuisson, E.; Yang, Y.; Loh, K. P.; Tan, C.-H. Green Chem. 2011, 13, 3341–3344.
10. (a) Rueping, M.; Vila, C.; Szadkowaska, A.; Koenigs, R. M.; Fronert, J. ACS Catal.
2012, 2, 2810–2815; (b) Xuan, J.; Cheng, Y.; An, J.; Lu, L.-Q.; Zhang, X.-X.; Xiao,
W.-J. Chem. Commun. 2011, 47, 8337–8339; (c) Jiang, G.; Chen, J.; Huang, J.-S.;
Che, C.-M. Org. Lett. 2009, 11, 4568–4571.
16. (a) Cossy, J.; Guha, M. Tetrahedron Lett. 1994, 35, 1715–1718; (b) Pandey, G.;
Gadre, S. R. Arkivoc 2003, 45–54.
17. We previously reported this reaction in poster format: Frederickson, C. C.;
Garimella, U.; Marvin, C. C. Abstracts of Papers, 243rd National Meeting of the
American Chemical Society, San Diego, CA, Mar 25–29, 2012; American
Chemical Society: Washington, DC; CHED-1073.
18. While we were working on this project, Xiao and co-workers reported the
cyclization of this substrate to an oxazine using Ir(ppy)2(dtb-bpy)PF6 as a
photocatalyst with irradiation from a 36 W fluorescent light. See Ref. 9g.
19. (a) Neumann, M.; Füldner, S.; König, B.; Zeitler, K. Angew. Chem., Int. Ed. 2011,
50, 951–954; (b) Hari, D. P.; Hering, T.; Koenig, B. Org. Lett. 2012, 14, 5334–
5337; (c) Hari, D. P.; Schroll, P.; König, B. J. Am. Chem. Soc. 2012, 134, 2958–
2961.
20. Cayley, A. N.; Gallagher, K. A.; Ménard-Moyon, C.; Schmidt, J. P.; Diorazio, L. J.;
Taylor, R. J. K. Synthesis 2008, 3846–3856.
21. Others have noted the difficulty of purifiying 1,3-oxazines by chromatography.
For example, see: Hoye, T. R.; North, J. T. Tetrahedron Lett. 1990, 31, 4281–4284.
11. (a) Wang, Z.-Q.; Hu, M.; Huang, X.-C.; Gong, L.-B.; Xie, Y.-X.; Li, J.-H. J. Org.
Chem. 2012, 77, 8705–8711; (b) Zhu, S.; Rueping, M. Chem. Commun. 2012, 48,
11960–11962.