Visible-light-driven metal-free aerobic synthesis of highly diastereoselective phosphinoylpyrroloindoles

Article abstract of DOI:10.1039/c9ob02730k

A visible-light-driven metal-free phosphorus radical mediated construction of 2-phosphinoyl-3H-pyrrolo[1,2,a]indoles is described. This mild tandem phosphinoylation/cyclization protocol utilizes air as a green oxidant and proceeds in a short span of time at room temperature with high functional group tolerance, and excellent chemo- A nd diastereoselectivity.

Full text of DOI:10.1039/c9ob02730k

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DOI: 10.1039/C9OB02730K
COMMUNICATION
Visible-Light-Driven Metal-Free Aerobic Synthesis of Highly
Diastereoselective Phosphinoylpyrroloindoles
Ramesh Gorre, +,a,b Damodar Enagandhula, +,a Sridhar Balasubramanian b,c and Srirama Murthy
Received 00th January 20xx,
Accepted 00th January 20xx
Akondi *a,b
DOI: 10.1039/x0xx00000x
A visible-light-driven metal-free phosphorus radical mediated more importantly, a transition metal reagent, which limit their
construction of 2-phosphinoyl-3H-pyrrolo[1,2,a]indoles is widespread application. From the stand point of sustainable
described. This mild tandem phosphinoylation/cyclization protocol chemistry, the realization of a transition-metal-free and toxic
utilizes air as a green oxidant and proceeds in a short span of time oxidant-free procedure to construct phosphorus containing
at room temperature with high functional group tolerance, pyrrolo[1,2-a]indole derivatives at room temperature is an
excellent chemo- and diastereoselectivity.
important challenge to meet.
Pyrrolo[1,2-a]indoles are an important class of indole
derivatives owing to their presence in drug molecules and
natural products. For instance, flinderoles and isoborreverine
have shown significant antimalarial activity and JTT-010 is a
protein kinase C (PKC) inhibitor (Figure 1).1 Additionally, indole
derivatives containing phosphorus functionality have attracted
attention in the synthetic community due to their occurrence in
the fields of pharmaceuticals, organic synthesis and material
science.2
H
H
N
O
N
O
H
H
R’
N
R
N
NH
R
N
H
N
H
N
N
H
N
NH
NH2CH3SO3H
R
R’
JTT-010
H
 Me
-
flinderole A
flinderole B
flinderole C
isoborreverine
Me  Me
-
Me  Me
-
Considering the importance of the pyrrolo[1,2-a]indole scaffold
and indolyl-based organophosphorus compounds, the
development of highly efficient methodologies to synthesize
phosphorus containing pyrrolo[1,2-a]indole frameworks that
combine both characteristics together is desirable and has
attracted attention in recent years. For instance, Tang and co-
Figure 1 Some biologically active compounds featuring pyrrolo[1,2-a]indole
framework.
Visible light driven organo-photoredox catalysis provides an
environmentally benign way of generating various reactive
radicals for the construction of complex carbocyclic and
polyheterocyclic skeletons. This technology has been utilized by
several research groups in recent years to construct useful
phosphorylated compounds by generating P-centered radicals
under mild conditions without using toxic reagents.7,8 However,
the construction of highly diastereoselective P-containing
pyrrolo[1,2-a]indole derivatives via mild and cost-effective
photoredox catalysis is still unexplored. Herein, we report a
visible light driven organic dye catalysed simultaneous C-P and
workers3 developed
a silver-mediated phosphinoylation-
cyclization-isomerization cascade for the preparation of various
2-phosphinoyl-9H-pyrrolo[1,2-a]-indoles (Scheme 1a). The
Song4 and Yue5 groups independently reported a silver-
catalysed carbon-phosphorus functionalization for the
preparation of 2-phosphinoyl-3H-pyrrolo[1,2-a]indoles with
excellent diastereoselectivity (Scheme 1b). And, a copper-
catalyzed tandem radical cyclization of 1-(3-phenylprop-2-yn-1-
yl)-1H- indoles leading to 2-phosphinoyl-9H-pyrrolo[1,2-
a]indoles was developed by Zhu and co-workers in 2017
(Scheme 1a).6 Despite these achievements, each of these
methods requires an external oxidant (either metal oxidant or
peroxide), elevated temperatures, long reaction times and
C-C
bond
formation
for
the
synthesis
of
2-phosphinoyl-3H-pyrrolo[1,2-a]indoles through exclusive 5-
endo-trig cyclization (Scheme 1c). This reaction utilizes air as a
green oxidant and proceeds at room temperature with just 2.5
mol% of eosin Y as a single catalyst with no use of toxic oxidants
and additives. These milder conditions offer excellent functional
group compatibility and diastereoselectvity.
a. Department of Organic Synthesis and Process Chemistry, CSIR-Indian Institute of
Chemical Technology (CSIR-IICT), Hyderabad 500007, India Email :
sriramakondi@iict.res.in; sriramiict@gmail.com
b. Academy of scientific and innovative research (AcSIR), Ghaziabad 201002, India
c. Department of Analytical Chemistry, CSIR-IICT, Hyderabad 500007, India
† These authors contributed equally
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Journal Name
R4
Table 1 Optimization of reaction conditionsa
Tang, 20163
R3
R4
View Article Online
DOI: 10C.1l039/C9OB02730K
R3
R2
AgOAc (3.0 equiv)
CH3CN, 100 oC, 24 h
Zhu, 20176
CuSO4 (25 mol%)
K2S2O8 (2.0 equiv)
CH3CN, 60 oC, 12 h
(a)
N
Ph2P(O)H
Ph
Ph
Ph
N
Cl
P
O
O
R1
photocatalyst
blue LED 450 nm
(PR m2)
O
R2
HP
R1
O
N
P
O
N
solvent, rt, air
Cl
Cl
Ph
Ph
Song, 20174
AgOAc (10 mol%)
Ph

Ar1
1a
2a
3aa
O
O
N
O
Ph
Zn(NO3)2.6H2O (50 mol%)
CH3CN, 100 oC, 20 h, Ar
Yue, 20175
AgOAc (20 mol%)
P
(b)
Ph
Ar1
N
R2
Ph2P(O)H
R2
R1
Mg(NO3)2.6H2O (50 mol%)
R1
CH3CN/THF (1:1)
80 oC, 24 h, O2
this work
Ar’
O
P
O
O
Ar
Ar
R1
R1
O
Penn PhD photoreactor m2 (PR m2)
N
eosin Y (2.5 mol%)
N
(c)
HPAr2
Ar’
blue LED 450 nm
Entry
Photocatalyst (mol%)
Solvent
Yield %b
DMSO, rt, 2-4 h
air
Ph
Ph

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
rose bengal (4)c
rose bengal (4)d
rose bengal (4)
rhodamine B (4)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
MeCN
toluene
i-PrOH
H2O
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
42
39
68
n.r.
n.r.
74
trace
23
n.r.
n.r.
n.r.
72
Scheme 1 Previous and present works for the synthesis of phosphorus containing
pyrrolo[1,2-a]indole.
Motivated by the desire to develop an environmentally benign
9,10-dicyanoanthracene (4)
eosin Y (4)
protocol
for
the
construction
of
2-phosphinoyl-3H-pyrrolo[1,2-a]indoles, we evaluated reaction
conditions employing 1a and 2a as model substrates using air as
an oxidant and a suitable organic dye as a photocatalyst. To our
delight, the anticipated product 3aa was obtained in 42% yield
when a solution of 1a (0.1 mmol), 2a (0.25 mmol) with 4 mol%
rose bengal in 0.1 M DMSO was irradiated with 14 W white LED
for 12 h in an open pot exposed to ambient air at room
temperature (Table 1, entry 1). We also examined the use of the
Sigma-Aldrich® SynLED parallel photoreactor (blue LED, 465-470
nm) instead of a white LED, though a slight reduction in yield
was observed (Table 1, entry 2). A remarkable improvement in
the yield as well as the rate was observed when the same
transformation was conducted using the Penn PhD
photoreactor m2 (blue LED, 450 nm).10 There was a 6-fold rate
enhancement, generating the desired product 3aa in 68% yield
after only 2 h (Table 1, entry 3). Encouraged by this result, we
further optimized the reaction conditions by exploring a range
of organophotocatalysts such as rhodamine B, 9,10-
dicyanoanthracene, eosin Y and [Acr-Mes] ClO4 under the same
conditions (Table 1, entries 4-7). These results indicated that
[Acr-Mes]ClO4 (4)
eosin Y (4)
eosin Y (4)
eosin Y (4)
eosin Y (4)
eosin Y (2.5)
eosin Y (1)
39
none
n.r.
trace
42
eosin Y (2.5)e
eosin Y (2.5)f
eosin Y (2.5)g
12
a The reaction was carried out with 1a (0.1 mmol), 2a (0.25 mmol) and eosin Y (2.5
mol%) in DMSO (1.0 mL) at room temperature under ambient air using PR m2 (blue
LED 450 nm) for 2 h. b Isolated yields. c Irradiated with 14 W white LED for 12 h. d
Irradiated in synLED photoreactor for 12 h. e No irradiation. f Under O2 (1atm).
Under N2
g
A screening of solvents of varying polarities revealed that DMSO
is the best solvent for this reaction system (Table 1, entries 8-
11). The optimum catalyst loading was found to be 2.5 mol%
since a further decrease to 1 mol% had a negative effect on the
yield of the reaction (Table 1, entries 12 and 13). No desired
product was detected in the absence of either photocatalyst or
light irradiation, indicating that the transformation proceeds
through a photoredox mechanism (Table 1, entries 14 and 15).
When the reaction was conducted under 1 atm of O2, the yield
of 3aa was reduced to 42% because of the faster decomposition
of 2a into its oxidative byproduct (Table 1, entry 16).8b,8d Finally,
the yield of the reaction was attenuated under an N2
atmosphere, giving the desired product 3aa in only 12% yield
(Table 1, entry 17). These optimization studies imply that eosin
Y, ambient air and visible light are all crucial for the progress of
this transformation.
eosin
Y is the most effective photocatalyst for this
transformation, affording 3aa in 74% yield. The structure of 3aa
was unambiguously confirmed by X-ray crystallographic
analysis (Figure 2), and exclusive trans diastereoselectivity was
observed.11
Using the optimized reaction conditions described in entry 12 of
Table 1, we then explored the scope and generality of this
protocol using various cinnamides and indoles 1a-1p. As shown
Figure 2 X-ray crystal structure (ORTEP) of 3aa.
2 | J. Name., 2012, 00, 1-3
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in Table 2, cinnamides bearing either electron-rich or electron- withdrawing group chloro (3ma) on the aromatiVciewrinArgticloefOntlhinee
DOI: 10.1039/C9OB02730K
deficient substituents at para, meta or on both positions of the indole, wherein the desired products were obtained in good
phenyl rings were well tolerated in this protocol, providing the yields. Notably, this transformation happened only when there
desired products (3aa–3la) in 51-74% yields with high was an aryl substituent at the 3-position of the indole ring and
diastereoselectivity.
no desired product was obtained when there was alkyl (3oa) or
electron-withdrawing (3pa) group. These observations might be
attributed to the poor stabilization of the radical, carbocation
intermediates by alkyl and electron withdrawing substituents
when compared to an aryl substituent.
Next, the scope of different H-phosphine oxides was examined
(Table 3). Simple diphenylphosphine oxide (DPPO) (3ab), 4-Me-
DPPO (3ac), 4-OMe-DPPO (3ad) and 4-F-DPPO (3ae) furnished
the corresponding products in good yields. Diphenylphosphine
oxide with an –NMe2 group in the para position (3af) furnished
lower yield due to the incomplete conversion of 1a and
formation of byproducts.
Table 2 Scope of cinnamides and indolesa
Cl
Cl
Ar
Cl
O
O
R1
Cl
P
R1
eosin Y (2.5 mol%)
N
O
N
blue LED 450 nm
DMSO, 2 h, rt
air
P
H
Ar
O
R
R
1a-1p
2a

3aa-3pa
Cl
Cl
Cl
Cl
Cl
Cl
O
O
O
N
P
P
P
O
O
N
N
O
Table 3 Scope of phosphine oxidesa
Ph
68% (3ba)
Ph
Ph
Br
OMe
72% (3aa)
61% (3ca)
Ph
O
Ar
O
Ar
Cl
N
Cl
Cl
P
Ar
N
Ar
eosin Y (2.5 mol%)
Cl
Cl
Cl
Cl
Cl
O
N
P
H
blue LED 450 nm
DMSO, 2 h, rt
air
O
Ph
O
O
O
N
Ph
P
P
P
Ph
3ab-3am
O
O
O
N

1a
2b-2m
Cl
Cl
Ph
74% (3da)
Cl
Ph
Ph
F
NO2
Me
MeO
52% (3ea)
58% (3fa)
Cl
Me
OMe
O
P
O
O
N
Cl
O
N
P
P
Cl
Cl
O
O
N
Ph
Ph
Ph
O
O
O
Ph
P
P
P
Ph
61% (3ac)
Me2N
Ph
O
O
O
N
N
N
69% (3ab)
F
OMe
OMe
62% (3ad)
O
Ph
Ph
54% (3ha)
Cl
Ph
F
O
Me
F
F
NMe2
69% (3ga)
62% (3ia)
O
N
O
O
N
Me
P
P
P
Cl
Cl
O
O
N
O
Cl
Cl
Ph
Ph
Ph
O
O
O
Ph
Ph
38% (3af)b
P
P
P
Ph
O
O
O
N
N
N
72% (3ae)
68% (3ag)
OCF3
F
OMe
OMe
Ph
Ph
71% (3ka)
Cl
Ph
OMe
OMe
Me
O
OMe
F
MeO
MeO
O
O
Me
65% (3ja)
51% (3la)
P
O
N
OMe
N
P
P
OMe
N
O
O
Cl
Cl
Ph
Ph
Ph
Cl
Cl
Cl
Ph
Ph
Ph
O
N
O
N
O
N
P
P
P
48% (3ai)b
71% (3ah)
65% (3aj)
O
O
O
Ph
Ph
Ph
Cl
MeO
OMe
Ph
Ph
X
MeO
O
O
X = Me (3oa)
X = CHO (3pa)
OMe
O
EtO
P
OEt
O
66% (3ma)
63% (3na)
0%
P
P
N
O
N
O
N
Ph
Ph
a
Ph
Reaction conditions: 1 (0.1 mmol), 2a (0.25 mmol) and eosin Y (2.5 mol%) in
DMSO (0.1 M) irradiated in Penn PhD PR m2 (blue LED 450 nm) at room
MeO
Ph
Ph
0% (3am)
Ph
51% (3al)b
temperature under ambient air. Isolated yields are reported.
60% (3ak)b
a
Reaction conditions: 1a (0.1 mmol), 2 (0.25 mmol) and eosin Y (2.5 mol%) in
A range of functional groups, such as methoxy (3ca, 3ha and
3la), methylenedioxy (3ia), trifluoromethoxy (3ja), halogens
(3ba, 3da, 3fa, 3ga and 3ka) and nitro substituents (3ea) were
well tolerated in this transformation. Compatibility with the
functional groups was further demonstrated by substituting the
electron-donating group methoxy (3na) and electron-
DMSO (0.1 M) irradiated in Penn PhD PR m2 (blue LED 450 nm) at room
temperature under ambient air. Isolated yields are reported. b Reaction continued
for 4 h.
Diphenylphosphine oxides bearing meta substituents (3ag, 3a h
and 3aj) reacted smoothly to furnish the anticipated products
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Journal Name
in moderate to good yields. Notably, dihenylphosphine oxides A plausible catalytic cycle has been proposed based on the
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having ortho substituents (3ai and 3ak) were also well tolerated above control experiments and precediDnOgI:li1t0e.1r0a3t9u/rCe9O(SBc0h2e73m0eK
under these reaction conditions despite the longer reaction 5).8b,8f,9
times (4 h). 1-Naphthyl-DPPO was also a suitable substrate for
Ph
O
Ph
O
Ph
this transformation, giving the product 3al in 51% yield. Diethyl
phosphite did not participate in the reaction, perhaps because
of its high oxidation potential and poor ability to undergo
tautomerization.8b,12
To demonstrate the practicability of this methodology, a one
mmol-scale experiment was then performed, employing 1a and
2a as substrates under optimized conditions (Figure 3). The
reaction took 6 hours for the completion to give the desired
product 3aa in a good yield of 65%.
Ph
1a
P
P
O
O
N
O
P
N
Ph
Ph
Ph
Ph
C
D
B
Ph
[O]
Ph
-H+
HO
EY
Ph
Ph
P
O
Ph
Ph
P
E EY*/EY
0.83 V
=
O2
A
O
N
vs. SCE
Ph
HO
Ph
P
Ph
E
Ph
Ph
Cl
2b’
EY*
O2
-H+
Ph
Cl
O
O
EY
blue LED (450 nm)
HP
O
O
P
N
eosin Y (2.5 mol%)

3ab
O
P
H
2b
N
Ph
blue LED 450nm
DMSO, rt, 6 h
air
Cl
Cl
Ph
Ph
Ph
Scheme 5 Plausible mechanism.

3aa
1a
323 mg, 1.0 mmol
2a
384 mg (65% yield)
(
)
Firstly, single electron oxidation of the diphenylphosphine oxide
2b by photoexcited eosin Y (EY*) generates radical cation A and
reduced eosin Y (EY). Radical cation A upon deprotonation
generates the phosphinoyl radical B which adds on to the highly
electron dense -position of the carbonyl group followed by 5-
endo-trig cyclization, leading to the radical intermediates C and
before
reaction
after
reaction
after
purification
D
respectively. Oxidation of intermediate D gives the
Figure 3 One mmol scale reaction for the synthesis of 3aa.
carbocation intermediate E, which upon deprotonation
produces the final product 3ab. The reduced photocatalyst
(EY) will be oxidized by O2 to complete the photocatalytic
cycle.
Some control experiments have been conducted to gain more
insights into this reaction mechanism (Scheme 4). When the
reaction was conducted in the presence of radical scavengers
such as butylated hydroxytoluene (BHT) and 2,2,6,6-
tetramethyl-1-piperidinyloxyl (TEMPO), the reaction was
completely shut down. Also, we identified the TEMPO-trapped
product 4 by high-resolution mass spectrometry and 31P nmr
spectroscopy. These control experiments clearly support the
phosphorus-centered radical reaction pathway. We also
conducted fluorescence quenching experiments and found that
Conclusions
In conclusion, we have developed an environmentally benign and
practical
method
for
the
synthesis
visible
of
light
2-phosphinoyl-3H-pyrrolo[1,2-a]indoles
under
conditions using only 2.5 mol% of organophotocatalyst eosin Y. This
the photoexcited eosin Y (EY*) was quenched by the phosphine transformation proceeded through a tandem C-P and C-C bond
oxide 2a (see the Supporting Information).
formation with excellent chemo (only 5-endo-trig instead of 6-endo-
trig) and diastereoselectivity. The efficiency of this protocol was
further enhanced by using air as green oxidant.
Ph
O
O
P
standard conditions
N
O
N
P
H
Ph
O
Ph
Conflicts of interest
Ph
1a
2b
3ab
There are no conflicts to declare
TEMPO (2.0 equiv)
BHT (2.0 equiv)
0%
0%
Ph
O
Acknowledgements
N
O
standard conditions
TEMPO (2.0 equiv)
N
P
O
P
H
RG thanks the Council of Scientific and Industrial Research (CSIR),
New Delhi, for a research fellowship. The authors thank Dr. S.
Chandrasekhar and Dr. Prathama S. Mainkar for their support and
valuable discussions (IICT/pubs./2019/383). ASR thanks prof. Lanny
S. Liebeskind for his constructive criticism.
O
Ph
1a
2b
4
detected by HRMS and
31P nmr
Scheme 4 Control experiments for mechanistic studies
4 | J. Name., 2012, 00, 1-3
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Notes and references
View Article Online
DOI: 10.1039/C9OB02730K
1
a) R. Vallakati and J. A. May, J. Am. Chem. Soc., 2012, 134,
6936–6939; b) M. Tanaka, M. Ubukata, T. Matsuo, K. Yasue, K.
Matsumoto, Y. Kajimoto, T. Ogo and T. Inaba, Org. Lett., 2007,
9, 3331–3334.
2
a) P. Gong, K. Ye, J. Sun, P. Chen, P. Xue, H. Yang and R. Lu, RSC
Adv., 2015, 5, 94990–94996; b) T. Jia, A. Bellomo, K. El Baina,
S. D. Dreher and P. J. Walsh, J. Am. Chem. Soc., 2013, 135,
3740–3743; c) X.-J. Zhou, R. C. Garner, S. Nicholson, C. J.
Kissling and D. Mayers, J. Clin. Pharmacol., 2009, 49, 1408–
1416; d) F. Rataboul, A. Zapf, R. Jackstell, S. Harkal, T.
Riermeier, A. Monsees, U. Dingerdissen and M. Beller, Chem.
- A Eur. J., 2004, 10, 2983–2990; e) F. R. Alexandre, A. Amador,
S. Bot, C. Caillet, T. Convard, J. Jakubik, C. Musiu, B. Poddesu,
L. Vargiu, M. Liuzzi, A. Roland, M. Seifer, D. Standring, R. Storer
and C. B. Dousson, J. Med. Chem., 2011, 54, 392–395; f) L.
Chen and Y. X. Zou, Org. Biomol. Chem., 2018, 16, 7544–7556.
S. Chen, P. Zhang, W. Shu, Y. Gao, G. Tang and Y. Zhao, Org.
Lett., 2016, 18, 5712–5715.
3
4
5
J. Xu, X. Yu and Q. Song, Org. Lett., 2017, 19, 980–983.
J. Liu, S. Zhao, W. Song, R. Li, X. Guo, K. Zhuo and Y. Yue, Adv.
Synth. Catal., 2017, 359, 609–615.
6
7
H. Zhang, W. Li and C. Zhu, J. Org. Chem., 2017, 82, 2199–
2204.
For selected reviews see; a) K. Luo, W. C. Yang and L. Wu,
Asian J. Org. Chem., 2017, 6, 350–367; b) B. G. Cai, J. Xuan and
W. J. Xiao, Sci. Bull., 2019, 64, 337–350.
8
For selected metal-free photoinduced phosphorylation
reactions, see; a) W. J. Yoo and S. Kobayashi, Green Chem.,
2013, 15, 1844–1848; b) P. Peng, L. Peng, G. Wang, F. Wang,
Y. Luo and A. Lei, Org. Chem. Front., 2016, 3, 749–752; c) J. G.
Sun, H. Yang, P. Li and B. Zhang, Org. Lett., 2016, 18, 5114–
5117; d) I. Kim, M. Min, D. Kang, K. Kim and S. Hong, Org. Lett.,
2017, 19, 1394–1397; e) D. Liu, J. Q. Chen, X. Z. Wang and P.
F. Xu, Adv. Synth. Catal., 2017, 359, 2773–2777; f) Y. Yin, W. Z.
Weng, J. G. Sun and B. Zhang, Org. Biomol. Chem., 2018, 16,
2356–2361; g) H. Wang, Y. Li, Z. Tang, S. Wang, H. Zhang, H.
Cong and A. Lei, ACS Catal., 2018, 8, 10599–10605; h) H. F.
Qian, C. K. Li, Z. H. Zhou, Z. K. Tao, A. Shoberu and J. P. Zou,
Org. Lett., 2018, 20, 5947–5951; i) M. Singsardar, A. Dey, R.
Sarkar and A. Hajra, J. Org. Chem., 2018, 83, 12694–12701; j)
Q. Fu, Z.-Y. Bo, J.-H. Ye, T. Ju, H. Huang, L.-L. Liao and D.-G. Yu,
Nat. Commun., 2019, 10, 3592.
9
S. Mitra, M. Ghosh, S. Mishra and A. Hajra, J. Org. Chem.,
2015, 80, 8275–8281.
10 C. C. Le, M. K. Wismer, Z. C. Shi, R. Zhang, D. V. Conway, G. Li,
P. Vachal, I. W. Davies and D. W. C. MacMillan, ACS Cent. Sci.,
2017, 3, 647–653.
11 CCDC 1960675 contains supplementary Crystallographic data
for the structure 3aa. This data can be obtained free of charge
at www.ccdc.cam.ac.uk/conts/retrieving.html.
12 B. G. Janesko, H. C. Fisher, M. J. Bridle and J. L. Montchamp,
J. Org. Chem., 2015, 80, 10025–10032.
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