Visible Light Photocatalysis of 6π Heterocyclization

Article abstract of DOI:10.1002/anie.201705333

Photo-mediated 6π cyclization is a valuable method for the formation of fused heterocyclic systems. Here we demonstrate that irradiation of cyclic 2-aryloxyketones with blue LED light in the presence of an Ir^III complex leads to efficient and high yielding arylation across a panoply of substrates by energy transfer. 2-Arylthioketones and 2-arylaminoketones also cyclize effectively under these conditions. Quantum calculation demonstrates that the reaction proceeds via conrotatory ring closure in the triplet excited state. Subsequent suprafacial 1,4-hydrogen shift and epimerization leads to the observed cis-fused products.

Full text of DOI:10.1002/anie.201705333

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Visible light photocatalysis of 6π heterocyclization
Authors: Martin Derwyn Smith, Niels Münster, Nicholas Parker, Lucy
van Dijk, and Robert Paton
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201705333
Angew. Chem. 10.1002/ange.201705333
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10.1002/anie.201705333
Angewandte Chemie International Edition
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Photocyclization
Visible light photocatalysis of 6π heterocyclization**
Niels Münster, Nicholas A. Parker, Lucy van Dijk, Robert S. Paton* and Martin D. Smith*
Previous work: indoline synthesis (Chapman) and dihydrobenzofuran synthesis (Schultz)
Abstract: Photo-mediated 6π cyclization is a valuable method for
the formation of fused heterocyclic systems. Here we demonstrate
hυ
hυ
that irradiation of cyclic 2-aryloxyketones with blue LED light in the
presence of an Ir(III) complex leads to efficient and high yielding
arylation across a panoply of substrates by energy transfer. 2-
Arylthioketones and 2-arylaminoketones also cyclize effectively
under these conditions. Quantum calculation demonstrates that the
reaction proceeds via conrotatory ring closure in the triplet excited
state. Subsequent suprafacial 1,4-hydrogen shift and epimerization
leads to the observed cis-fused products.
N
N
O
O
H
H
O
O
This work: visible light mediated 6π photocyclization
F
F
O
O
H
Ir(Fppy)3 (cat.)
base
X
X
N
N
R2
R2
Ir
F
F
R1
Blue LED
N
R1
F
F
X = O, S, NR
d.r. >20:1
S
ix-electron cyclization reactions are of fundamental importance
within contemporary synthetic chemistry. Most investigations in this
field have focussed on thermal processes, with relatively little work
on the corresponding photochemical reactions.[1] Within this field,
Scheme 1. Previous work and approach to visible light mediated 6π cyclization
transfer[9] from an appropriately activated sensitizer could both offer
solutions to this problem. In considering the feasibility of these two
approaches, we rationalized that the estimated reduction potentials
photochemical 6π heterocyclization offers
a distinct strategic
approach to the synthesis of complex heterocyclic systems.
Chapman demonstrated that irradiation of N-aryl enamines led to the
formation of indolines bearing predominantly trans-stereochemistry
across the ring junction (Scheme 1). This reaction, which is
isoelectronic with the pentadienyl anion electrocyclization,[2] was
proposed to occur through excited state electrocyclization followed
by a suprafacial 1,4-hydrogen shift.[3] A related transformation was
disclosed by Schultz, who reported that photocyclization of
2-aryloxyketones and 2-arylthioketones could be achieved with
pyrex-filtered UV irradiation to yield fused dihydrobenzofurans and
dihydrobenzothiophenes respectively.[4-6] This reaction offers a
diastereoselective route to cis-fused heterocycles useful in a range of
synthetic endeavours, which we exploited in a recent total synthesis
of morphine.[7] Although the Schultz photocyclization reaction was
the key stereodefining step in our total synthesis, the reactivity of
the products under high intensity UV light and the sensitivity of
conversion to varying substitution on the aromatic ring motivated us
to find an alternative method to achieve this valuable transformation.
In principle, visible light photoredox catalysis[8] or triplet energy
1
of cyclic aryloxyketones (E 2 ≈ –2.2 V vs SCE) lie outside of the
reducing ability of most excited state photocatalysts.[10] In contrast,
the triplet energy of cyclic aryloxyketones is ≈ 60 kcal mol-1, which
is of a similar magnitude to the triplet energies of some excited state
iridium photocatalysts.[11] This principle has been elegantly applied
to the energy transfer mediated [2+2] cycloaddition of olefins[12] and
other processes.[13] Consequently, we prepared
a
test 2-
aryloxyketone substrate 1a through treatment of the epoxide derived
from 3-methylcyclohexenone with phenol in the presence of
potassium carbonate in acetonitrile,[14] and examined its reactivity in
the presence of a range of photocatalysts under irradiation with blue
LED light (Table 1).[15]
Table 1. Optimization: photocyclization of 2-aryloxyketones
O
O
visible light
H
O
O
photocatalyst
KOAc, MeCN
n
1a (n = 1); 1b (n = 0)
2
Entry
Catalyst
Conversion
Yield
[∗]
Dr Niels Münster, Nicholas A. Parker, Lucy van Dijk,
Prof. Dr Robert S. Paton, Prof. Dr Martin D. Smith
Chemistry Research Laboratory
1
2
3
4
5
6
Ru(bpy)3Cl2
Ru(phen)3Cl2
0%
0%
-
-
University of Oxford
[Ir(dtbbpy)(ppy)2]PF6
fac-Ir(ppy)3
<5%
45%
80%
100%
trace
44%
77%
95%
12 Mansfield Road, Oxford, OX1 3TA (UK)
E-mail: robert.paton@chem.ox.ac.uk; martin.smith@chem.ox.ac.uk
Homepage: http://msmith.chem.ox.ac.uk
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6
Ir(Fppy)3
[∗∗]
The ERC has provided financial support (grant agreement no.
259056). We are grateful to the Deutsche Forschungsgemeinschaft
(MU 3987/1-1) for a fellowship (to N.M.), the EPSRC Centre for
Doctoral Training in Synthesis for Biology and Medicine (EP/
L015838/1) for studentships (to N.A.P. and L.V.D.) and to John
Jolliffe for help with X-ray crystallography.
[a] Reaction conditions: 1a (0.05 mmol), KOAc (1.0 equiv.), catalyst (1 mol.%),
12 W blue LED, MeCN ([1a] = 0.05 mol dm-3), 60 ˚C, 16 h. Conversion and
yields measured by 1H NMR spectroscopy vs internal standard.
Ruthenium(II) catalysts proved to be ineffective (entries 1 and 2),[16]
and hence we examined photocatalysts with significantly higher
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201xxxxxx.
1
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10.1002/anie.201705333
Angewandte Chemie International Edition
F
F
Ir(Fppy)3
Ir(Fppy)3 (1 mol%)
KOAc (100 mol%)
O
O
6
9
H
X
X
7
8
N
N
R2
R2
Ir
F
MeCN (0.05 M)
Blue LED, 60 ˚C
16 h
F
R1
N
R1
F
X = O, S, NR
d.r. >20:1
F
O
O
F
O
O
O
O
O
O
Cl
O
Br
O
O
I
O
O
OMe
H
H
H
H
H
H
H
H
H
H
H
O
O
O
O
O
O
O
F
4
3
5
89%
6
92%
7
8
87%
2
94%
87%
90%
78%
OMe
O
O
H
H
H
O
O
O
(Me)H
O
H(NMe2)
O
OMe
O
CF3
O
O
N
Me(H)
NMe2(H)
OMe
CF3
10
93%
11
12
13
94%
14
15‡
9
87%
91%; r.r 2.2:1
68%; r.r. 2.3:1
92%
76%
O
O
O
O
O
H
O
O
H
O
O
O
O
O
H
H
H
H
H
H
O
O
O
O
O
CHO
CO2Et
CN
OMe
OBn
NHAc
SMe
17
18
19
90%
20
95%
21
22
88%
16
75%
95%
86%
90%
O
H
O
O
H
O
H
H
H
H
O
O
O
O
O
H
O
H
24
95%
25
93%
26
H3C
H3C
H
H
23
91%
28
29
80%; d.r. 15:1
88%a
87%a
27*
84%; d.r. 4:1
H
O
O
O
O
O
O
Br
O
H
O
O
O
O
H
H
H
H
H
H
H
O
O
O
Cl
O
OMe
O
Ph
CO2Et
O
H
H
H
H
Cl
33
56%a
OMe
34
85%a
30
80%a
31
87%a
32
76%a
35
87%b
Cl
Br
I
O
H
O
O
H
H
H
H
O
O
O
NMe2(H)
O
O
OMe
H(NMe2)
67%b; r.r 1.6:1
36
37
38
39
40
83%b
41
86%b
36%b (86%)
65%b (77%)
45%b (63%)
O
O
H
O
O
O
H
O
O
O
O
H
H
H
H
H
O
O
O
O
O
CHO
CO2Et
N
H
Ph
42
43
44
45
86%b
46
71%b
47
86%b
48
23%b (51%)
38%b (90%)
52%b (98%)
42%b
Ph
O
O
O
O
H
S
O
H
N
O
H
N
O
H
H
H
H
N
N
S
N
49
91%
50
87%b
51
77%
52
84%b
53
52%
54
88%b
55
77%
Table 2. Scope of triplet energy transfer mediated 6π heterocyclization. Reaction conditions: 0.3 mmol enone, 0.01 equiv. Ir(Fppy)3, 1 equiv. KOAc, 12 W blue LED,
MeCN ([enone] = 0.05 M); reaction time 16 h. Yields are for isolated material. d.r. (diastereoisomeric ratio) determined by 1H NMR spectroscopy. Positions around
the benzofuran core are indicated in red numerals. ‡Reaction time 72 h. *Reaction time 36 h. aReaction conducted in the presence of 8-methylquinoline (0.2 equiv.);
reaction time 2-7 h. bReaction conducted using NaOAc (1 equiv.) in EtOAc; reaction time 40 h. Numbers in parentheses indicate yields calculated on the basis of
recovered starting material. r.r. (regioisomeric ratio) determined by 1H NMR spectroscopy; minor regioisomer is indicated by parentheses.
triplet energies. [Ir(dtbbpy)(ppy)2]PF6 (entry 3)[17] afforded a trace of
product, but we observed significant reactivity with fac-
Ir(ppy)3leading to 2 in 44% yield (entry 4). Significantly higher
conversion was observed with [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (80%
yield), and Ir(Fppy)3 gave complete conversion and a 94% yield of
2; only the cis-dihydrobenzofuran isomer was isolated under these
reaction conditions.[18] Addition of triplet quenchers (TEMPO or
1,1-diphenylethylene) inhibited the reaction entirely and only
2
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10.1002/anie.201705333
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on the aromatic ring are similarly effective: 8-methoxy 40 (83%
yield) and 8-methyl 41 (86% yield). Electron withdrawing groups
are particularly challenging (8-formyl 42, 23% yield and ester 43,
38% yield), leading to lower conversions and yields. The fused
pyridine 44 can be generated in 52% yield. Variation in the β-
position of the cyclopentanone is also remarkably well tolerated:
ethyl 45 (86% yield), benzyl 46 (71% yield) and cyclohexyl 47
(86% yield) groups can be installed at the fully substituted centre
without issue. The cis-stereochemistry at the ring junction was
confirmed by single crystal X-ray diffraction of 40. Cyclization of
2-arylthioketones and 2-arylaminoketones is also effective under
these conditions. Dihydrothiophenes can be be formed in high yields,
to form both six (49, 91% yield) and five-membered fused ring
systems (50, 87% yield). In a related process, N-methylated 2-
arylaminoketones cyclize to afford dihydroindoles in good yields in
both five and six ring series (51, 77% yield; 52, 84% yield).
Different N-substituents are tolerated without incident as
exemplified by N-allyl 53 (52% yield) and N-benzyl 54 (88% yield)
derivatives. This approach can also be extended to generate fused
tetracylic systems such as 55 (77% yield) by cyclization of a
tetrahydroquinoline substrate. In general, the cyclization procedure
is highly effective and demonstrates remarkable insensitivity to
substitution, particularly in the cyclohexenone series. The scalability
of this process is demonstrated by a 5.00 mmol reaction (1.01 g) to
generate 2, which proceeds in 92% yield with a photocatalyst
loading of 0.05 mol%. A series of chemo-and diastereoselective
derivatizations were also performed on this compound.[15] In our
hands the reaction was also insensitive to the presence of moisture.
However, several substrates possessed distinct reactivity or were
surprisingly unreactive under these reaction conditions (Scheme 2).
Substrate 56, which contains an α,β,γ,δ-unsaturated ketone,
undergoes efficient intermolecular [2+2] cycloaddition (71% yield)
rather than the arylation reaction to afford cyclobutane 57, in which
stereochemistry was confirmed by single crystal X-ray diffraction.
This is consistent with previous observations on the rate of [2+2]
cycloadditions versus the arylation reaction.[21]
starting material was isolated, consistent with an energy transfer
mechanism.[19] With a working substrate synthesis and cyclization
procedure in hand, we examined the scope and limitations of this
visible light mediated process (Table 2). We initially explored
cyclization of cyclohexenone substrates with different substituents
on the aromatic ring. Substituents in the 6-position were well
tolerated including alkyl groups (3, 90%) and halogens (difluoro-
derivative 4, 87%; chloro-5, 89%; bromo- 6 92%; iodo- 7, 78%).
Substrates bearing electron-donating groups such as 8 and 9 both
cyclize effectively; in these cases the competing [2+2] cycloaddition
is likely disfavoured by geometrical constraints precluding close
approach of the enone and allyl groups. Introducing 7-substitution
does not have any deleterious effects on the reaction, and tetracycle
10 can be produced in 93% yield. meta-Substituents on
cyclohexenone-derived aryloxy substrates generally cyclize to give
regioisomeric ratios in which the 7-isomer is the minor product: 9/7-
methyl 11 (91% combined yield, ratio 2.2:1) and 9/7 dimethylamino
12 (68% combined yield, ratio 2.3:1). It is worth noting that electron
rich substrates efficiently cyclize to products such as 12 in high
yields; under high intensity UV irradiation this cyclization does not
proceed.[4e] Symmetrical disubstituted substrates cyclize effectively
whether electron donating (13, 94% yield) or electron withdrawing
(14, 92% yield). A similar trend is observed with groups in the 8-
position: pyridine 15 (76% yield), and both electron withdrawing
groups (aldehyde 16, 75% yield; ester 17, 95% yield; nitrile 18, 86%
yield) and electron donating groups (methoxy 19, 90% yield;
benzyloxy 20, 95% yield; amide 21, 90% yield; thioalkyl 22, 88%
yield; alkyl 23, 91% yield) are tolerated without incident. Using an
isophorone-derived substrate bearing a gem-dialkyl substituent on
the cyclohexane ring[4c] does not affect the reaction significantly,
and the product 24 an be isolated in 95% yield. Variation in the β-
position of the cyclohexenone is also possible: ethyl 25 (93% yield)
and isopropyl 26 (80% yield; 15:1 d.r.) groups can be
accommodated at the all-carbon quaternary stereocentre without
reduction in overall yield. Extension to a derivative of progesterone
was also possible, yielding 27 in 84% yield (d.r. 4:1). Cyclization of
substrates bearing hydrogen in the β-position of the enone is
possible, though these are more challenging substrates and can
O
Ir(Fppy)3 (1 mol%)
KOAc (100 mol%)
O
O
O
O
O
MeCN (0.05 M)
Blue LED, 60 °C
16 h
56
57
undergo competing oxidation to benzofurans.
This can be
O
O
O
circumvented somewhat by performing the reaction in the presence
of 8-methylquinoline. Under these conditions cyclization to afford
28 occurred in 88% yield, and introduction of groups around the
aromatic ring of this template was well tolerated as exemplified by
8-methyl 29 (87% yield) and 8-phenyl 30 (80% yield) products.
Introduction of an electron withdrawing group (31, 87% yield),
halogens (32, 76% yield and 33, 56% yield), or an electron-donating
group (34, 85% yield) in this position did not affect the efficacy of
the procedure. The cis-stereochemistry at the ring junction was
confirmed by single crystal X-ray diffraction of 2, 3, 22 and 28.[20]
We also examined the photocyclization of aryloxycyclopentenones,
which were significantly more challenging to work with than the
corresponding cyclohexenones. In general, longer reaction times are
required and the process is sensitive to the presence of electron
withdrawing groups on the aromatic ring, but high yielding
transformations can be achieved. Thus 35 can be generated in high
yield (87%) as a single diastereoisomer. Halogens can be tolerated
in the 6-position on the aromatic ring (chloro- 36, 36% yield;
bromo- 37 65% yield iodo- 38, 45% yield). In contrast to the
cyclohexenone series, meta-substituents on aryloxy substrates
generally cyclize to give regioisomeric ratios in which the 7-isomer
is the major product (9/7-dimethylamino 39; 67% combined yield,
ratio 1:1.6). Other electron donating substituents in the 8-position
O
unreactive
substrates
Ph
58
59
Scheme 2. 2-Aryloxyketone substrates with distinct reactivity profiles
β-Aryl substrate 58 and β-cyclopropyl substrate 59 were both
unreactive under the optimized reaction conditions and were re-
isolated unchanged. To probe the reaction mechanism (Figure 1A)
and divergent reactivity of some of the substrates investigated, we
turned to computations (M06-2X+SMD/def2-TZVP).[22] The
calculated energy required to promote cyclohexanone 1a from
singlet (S0) to π−π* triplet (T1) state is 58.2 kcal mol-1.[23] This is
well matched with the emissive energy of the Ir(Fppy)3 catalyst
(experimentally 60.1 kcal mol-1).[11] Triplet excitation of
cyclopentenone 1b, which required longer reaction times, requires
an additional 4.1 kcal mol-1 of energy. Substrate 58 forms a more
stable triplet state in comparison to 1a (48.8 kcal mol-1; Figure 1B).
However, heterocyclization is kinetically and thermodynamically
less favorable: TSAB has a higher barrier and this step is endergonic
3
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10.1002/anie.201705333
Angewandte Chemie International Edition
O
A
O
F
F
3[Ir]III*
ISC
O
O
O
3A
H
H
[1,4]
H-shift
N
N
O
O
O
epimerize
con
Ir
F
1[Ir]III*
TET
F
N
O
ISC
H
F
O
F
ISC
3B
1B
1[Ir]III
trans-C
intermolecular proton transfer
cis-C (35)
hυ
1[Ir]III
1A (1b)
B
TSAB (dis)
1B
TSAB (dis) 23.9
TSAB (con)13.5
S0
T
1
TSBC 10.8
1.0
2.13Å
TSBC
MECPB
MECPA
3.3
1.74Å
TSAB (con)
1.31Å
3A 0.0
3B –3.4 1B –5.0
2.14Å
trans-C
3A
trans-C –52.4
cis-C –67.7
mol-1
computed absorption: ΔG T →S0 / kcal
mol-1
experimental emission: ΔG T →S0 / kcal•
S0
1 A –62.5
φCCCC =115.0°
MECPA
•
1
1
1 A
Ir(ppy)3
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6
Ir(Fppy)3
55.2
60.1
60.1
cyclohexenone1a
cyclopentenone1b
β-phenylcyclohexenone 58
58.2
62.5
48.8
φCCCC = 93.3°
β-cyclopropylcyclohexenone 59 54.6
Figure 1. A Proposed mechanism of 6π heterocyclization of 1b to form 35. B M06-2X/def2-TZVP+SMD(EtOAc)//M06-2X/def2-TZVP computed Gibbs energy profile
(kcal mol-1) for the cyclization of 1b to form 35 taking place on singlet and triplet surfaces. ISC = intersystem crossing; TET = triplet energy transfer; MECP =
minimum energy crossing point; dis = disrotatory; con = conrotatory.
by 3.8 kcal mol-1. The intersystem crossing (ISC) from the triplet
states of unreactive substrates 58 and 59 involves smaller changes in
energy and in dihedral twisting relative to 1b. This is consistent with
more efficient relaxation from the triplet back to the singlet ground
state for these substrates. Heterocyclization proceeds via triplet
singlet ground state, with a computed barrier of 14.2 kcal mol-1. This
sigmatropic rearrangement is extremely exergonic and leads to
trans-fused C. The diastereomeric cis-fused product could arise
through base-mediated epimerization of C – it is significantly more
stable by 15.3 kcal mol-1
– or from the non-stereospecific
3
intermediate A,which preferentially occurs in a conrotatory fashion
intermolecular deprotonation/reprotonation of B. The extremely
large preference (ca. 10 kcal mol-1) for conrotatory closure was
obtained for all substrates and so it is implausible that the cis-fused
skeleton is the result of disrotatory cyclization.
(Figure 1B, with several other substrates shown in SI). Conrotatory
closure through TSAB, is favoured over disrotatory closure by more
than 10 kcal mol-1 (ΔG‡ 13.5 kcal mol-1). This preference is
consistent with the expectation that a conrotatory 6π electrocyclic
TS is Möbius aromatic in the triplet excited state.[24] However,
computed magnetic criteria of aromaticity (NICS, ACID, magnetic
susceptibility exaltation) do not show the shielding/cyclic ring
current characteristics of a pericyclic reaction (see SI). Ring-closed
intermediate 3B lies downhill by 3.5 kcal mol-1 (with a β-phenyl
group as in 58, this step is uphill by a similar amount, which may
also contribute towards lower reactivity) and must undergo
intersystem crossing (ISC) back to the singlet state. Interestingly we
In conclusion, we have described an operationally convenient
approach to the synthesis of fused dihydrobenzofurans,
dihydroindoles and dihydrothiophenes using an iridium(III)
sensitizer in the presence of visible light to mediate energy transfer.
This is a versatile reaction that enables the synthesis of fused
heterocyclic ring systems with a wide variety of substituents in good
yields. Quantum calculations demonstrate that this reaction proceeds
in the triplet state with a large preference for conrotatory ring
closure, but without the magnetic characteristics of an electrocyclic
mechanism.
1
find the open-shell singlet structure of B to be 7 kcal mol-1 more
stable than the closed-shell (e.g. ylide) structure. 1B and 3B
geometries, along with that of the minimum energy crossing point
(MECP) are very similar, which facilitates ISC. The subsequent
suprafacial [1,4]-H shift is Woodward-Hoffman allowed in the
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[21] J. P. Dittami, X. Y. Nie, H. Nie, H. Ramanathan, S. Breining, J.
Bordner, De.L. Decosta, J. Kiplinger, P. Reiche, R. Ware, J. Org.
Chem, 1991, 56, 5572–5578.
[22] For computational studies of photocatalysis see: (a) V. M. Fernańdez-
Alvarez, M. Nappi, P. Melchiorre, and F. Maseras Org. Lett. 2015, 17,
2676−2679; (b) L. Dell'Amico, V. M. Fernández-Alvarez, F. Maseras,
and P. Melchiorre Angew.Chem.Int. Ed. 2017, 56, 3304 –3308. For
computational details see the supporting information.
Keywords: visible light • photochemistry • mechanism • energy
transfer • catalyst • DFT
[23] The n-π* triplet structures were found to be uniformly less stable.
[24] (a) M. Rosenberg, C. Dahlstrand, K. Kilsa, H. Ottosson Chem. Rev.
2014, 114, 5379−5425; (b) In this case, there is one orbital phase
inversion, as opposed to a twisted topology with purely laticyclic
overlap: M. Mauksch,. S. B. Tsogoeva Angew. Chem. Int. Ed. 2009,
48, 2959–2963.
[1] For enantioselective approaches see: (a) K. Tanaka, O. Kakinoki and F.
Toda, J. Chem. Soc., Chem. Commun. 1992, 1053–1054; (b) T. Bach,
B. Grosch, T. Strassner, E. Herdtweck, J. Org. Chem. 2003, 68, 1107–
1116.
[2] C. W. Shoppee, G. N. Henderson, J. Chem. Soc. Chem. Commun.
1974, 561–562.
[3]
O. L. Chapman, G. L Eian, J. Am. Chem. Soc. 1968, 90, 5329–5330.
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[15] For a full list of all catalyst, solvent and base combinations explored,
and a range of derivatization reactions see supporting information.
Reaction temperatures of 60˚C are a consequence of heating from the
LED lamps; cooling reaction mixtures to RT under a stream of N2
extended reaction times but essentially complete conversion could still
be achieved.
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[18] Performing the reaction in dry MeCN (10 ppm H2O) in the absence of
base led to product in a cis:trans ratio of 10:1.
[19] Control reactions in the dark and in the absence of metal complexes
did not lead to formation of product.
[20] Single crystal X-ray data for 2, 3, 22, 28, 40 and 50 has been
deposited in the Cambridge Crystallographic database (CCDC
numbers 1538685–1538690 respectively).
5
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