Regioselective Intermolecular Allylic C?H Amination of Disubstituted Olefins via Rhodium/π-Allyl Intermediates

Article abstract of DOI:10.1002/anie.201707021

A method for catalytic intermolecular allylic C?H amination of trans-disubstituted olefins is reported. The reaction is efficient for a range of common nitrogen nucleophiles bearing one electron-withdrawing group, and proceeds under mild reaction conditions. Good levels of regioselectivity are observed for a wide range of electronically diverse trans-β-alkyl styrene substrates.

Full text of DOI:10.1002/anie.201707021

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Accepted Article
Title: Regioselective Intermolecular Allylic C-H Amination of
Disubstituted Olefins via Rh-π-allyl Intermediates
Authors: Jacob S Burman and Simon B. Blakey
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707021
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10.1002/anie.201707021
Angewandte Chemie International Edition
COMMUNICATION
Regioselective Intermolecular Allylic C–H Amination of
Disubstituted Olefins via Rh-π-allyl Intermediates.
Jacob S. Burman and Simon B. Blakey*[a]
A) π-Allyl Complexes from Preoxidized Precursors
Abstract: A method for catalytic intermolecular allylic C–H amination
R
Nu
of trans-disubstituted olefins is reported. The reaction is efficient for a
or
or
R
R
X
R
R
Ln
M+2
or
MLn
NuH
range of common nitrogen nucleophiles bearing one electron
withdrawing group and proceeds under mild conditions. Good levels
of regioselectivity are observed for a wide range of electronically
diverse trans-ß-alkyl styrene substrates.
M = Pd0,
RhI, IrI
Nu
R
•
R
Metal dependent regiochemistry for linear or branched products in allylic substituion
B) Pd-Catalyzed Oxidative Allylic C–H Functionalization of
Terminal Olefins
Catalytic allylic substitution reactions proceeding through
transition metal π-allyl intermediates are well established as
powerful tools for the synthesis of medicinally relevant natural
products and drug candidates.1 The allyl fragment is typically
obtained from olefins bearing allylic leaving groups, although
more recently alkynes, allenes, and conjugated dienes have all
been developed as suitable pre-oxidized alternatives for entry into
π-allyl chemistry (Scheme 1A).2
Ln
PdII
H
PdIILn
NuH
R
R
R
Nu
Nu = RCO2H, CH2(EWG)2, TsNH(CO2Me)
C) This Work: Allylic C–H Amination of trans-Disubstituted Olefins
R1
R2
Cp*
Ln
H
N
Rh
Cp*RhIIILn
R1R2NH
The direct oxidative allylic functionalization of readily
available simple alkenes has long been recognized as an
attractive alternative to allylic substitution chemistry. In 2004
White and co-workers reported a major advance with the
development of a novel catalyst capable of generating palladium
π-allyl species from terminal olefins, and in a series of reports
showed that these intermediates could be intercepted by a variety
of stabilized carbon, nitrogen and oxygen nucleophiles (Scheme
1B).3 Despite these significant developments, challenges remain
to fully develop allylic C-H functionalization as a robust strategic
alternative to allylic substitution chemistry. Specifically, catalyst
Ar
R'
Ar
R'
Ar
R'
Scheme 1. Transition Metal Catalyzed π-Allyl Chemistry is a Powerful Synthetic
Tool.
To test this hypothesis, we studied the reaction of simple
nitrogen nucleophiles with trans-1,3-diphenylpropene (1) (chosen
to eliminate regioselectivity as a complicating factor during the
initial reaction development). Exploratory reactions with
TsNHCO2Me as the nucleophile, under the conditions described
by Cossy, did not provide any amination products (Table 1, entry
1). When Cu(OAc)2 was replaced with a more potent oxidant
(AgOAc), product 2a in which the carbamate had been lost was
obtained in 31% yield. Utilizing TsNH2 as the nucleophile resulted
in a significant improvement and when the reaction temperature
was reduced to 60 °C, an 80% yield was observed (entry 4).
Surprisingly, lowering the catalyst loading to 1.0 mol % showed a
positive effect on yield (85%), suggesting that the catalyst might
also be involved in product degradation pathways (entry 7). No
product was detected when 2.5 equivalents of AgOAc were used
without any AgBF4 (entry 8). We note that chloride/acetate
exchange in rhodium Cp* complexes is reported to be slow,8 and
suggest that the requirement for AgBF4 is to provide kinetic
access to the active catalyst under the reaction conditions. In an
additional control, no product was observed when the rhodium
complex was omitted from the reaction (entry 9). Allylic C–H
amination could be accomplished at 40 °C, but required extended
reaction times (24 h) to ensure full conversion of the olefin (entry
10-14). DCM was also found to be an effective solvent at lower
reaction temperatures (entry 14). Under these optimized
conditions, the product 2a could be isolated in 88% yield. We note
that at elevated catalyst loading, with TsNH2 as the nucleophile,
Cu(OAc)2 is a moderately effective oxidant (entry 15).
systems
capable
of
promoting
regioselective
allylic
functionalization of internal olefins, and of utilizing a broad array
of nucleophiles (both stabilized, and unstabilized) remained
elusive.4,5 In this communication we describe the development of
a regioselective intermolecular allylic C-H amination reaction of
1,2-disubstituted olefins.
Our investigations were founded on a report from Cossy
demonstrating that Rh(III)Cp* complexes catalyzed intramolecular
cyclyzation by allylic C-H amination of terminal olefins with simple
N-Ts-alkyl amines.6 In a subsequent mechanistic study, Tanaka
and coworkers isolated a Rh(III)CpE-π-allyl complex, obtained by
C-H functionalization of a simple 1,2-disubstituted olefin.7 These
observations suggested that a Rh(III)Cp* derivative might catalyze
allylic functionalization reactions of internal olefins.
[a]
J. S. Burman, Prof. Dr. S. B. Blakey
Department of Chemistry
Emory University
1515 Dickie Dr., Atlanta, GA 30322, U.S.A.
E-mail: sblakey@emory.edu
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201707021
Angewandte Chemie International Edition
COMMUNICATION
Table 1. Reaction Optimization for Allylic C–H Amination of Disubstituted
Olefins.
derivative 2j was isolated in just 26
stereoinduction was observed in this reaction and the product was
obtained as a 1.6:1 mixture of diastereomers.
% yield. Minimal
TsNH2 (equiv)
[RhCp*Cl2]2 (mol %)
AgBF4 (mol %)
Table 2. Scope of Nitrogen Nucleophiles for Allylic Amination.
H
NHTs
Ph
Ph
Ph
Ph
Oxidant (2.1 equiv)
DCE, Temp, Time
R'R''NH (2.5 equiv)
1
2a
R'
R''
[RhCp*Cl2]2 (1 mol %)
AgBF4 (4 mol %)
H
N
TsNH2
AgBF4
(equiv) (mol %) (mol %)
[Rh]
Temp Time
% Yielda
Entry
Oxidant
Ph
Ph
Ph
Ph
(° C)
(h)
AgOAc (2.1 equiv)
1
2
Solvent, Temp, Time
1b
2b
3
2.5
2.5
2.5
2.5
1.25
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
5.0
5.0
5.0
5.0
5.0
2.5
1.0
5.0
0
Cu(OAc)2
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
Cu(OAc)2
80
80
80
60
60
60
60
60
60
40
40
40
40
40
80
n.d.
31
20
20
20
20
20
10
4
16
16
4
Ts
Me
Ts
Ph
Ph
Ts
Bn
N
N
N
75
Ph
Ph
Ph
Ph
Ph
4
4
80
2b, 88%a
o-Ns
2c, 41%c
2d, 41%c
4
5
74
O
O
O
O
4
6
80
H
Ph
S
H
S
H
N
O
N
Cl3C
O
N
4
7
85
2
8c
9
0
0
4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2f, 80%d
2e, 76%b
2g, 82%b
20
20
20
20
4
4
0
Me
4
10
11
12
13
14d
15
5.0
5.0
5.0
1.0
1.0
5.0
55
Cbz
H
Cbz
Ph
Cbz
Ph
8
60
N
MeO2C
Ph
N
MeO2C
Ph
N
24
24
24
16
77
Ph
85
2j, 26%d,e,f
2h, 95%d
2i, 78%d
93(88)e
46
4
20
Yields are reported for isolated product. [a] Reaction was run in DCM at 40 °C
for 24 h. [b] Reaction was run in DCM at 40 °C for 48 h. [c] Reaction was run in
DCE at 60 °C for 24 h. [d] Reaction was run in DCE at 60 °C for 48 h. [e] 5.0
equiv. of nucleophile was used. [f] Product was isolated as a 1.6:1 mixture of
diastereomers.
[a] Yields are determined by 1H NMR using 4,4’-di-tert-butylbiphenyl as an
internal standard and are the average of two reactions. [b] TsNHCO2Me was
used as the nucleophile. [c] 2.50 equiv of AgOAc was used. [d] Reaction was
run in DCM in place of DCE. [e] Isolated yield is reported in parentheses.
To further understand the potential utility of this reaction, we
investigated the efficiency and regioselectivity of the amination for
a range of unsymmetrical disubstituted olefins (Table 3). In these
reactions 2 mol % of the rhodium catalyst was used to ensure
complete reaction within a 24 h time frame for all substrates.
Amination of 3a with tosylamine gave an 86% combined yield of
amination products, favoring the conjugated isomer 4a over the
benzylic amine 5a in a ratio of 6.3:1. This sense of regioselectivity
was observed for all the substrates investigated in the study.
Introduction of an ortho-methyl substituent on the styrene
derivative had little impact on the reaction, with products 4b and
5b isolated in 84% yield with 6.1:1 regioselectivity. Electron
withdrawing groups were tolerated on the aryl ring (3c-3f)
although elevated temperatures (60 °C) were often required to
complete the reaction within 24 h. Regioselectivities were
generally moderate (4:1 – 5:1), although in the case of the para-
fluoro-substituted substrate 3c a ≥20:1 selectivity was observed
(vide supra). Electron donating substituents were tolerated in the
ortho, meta, and para positions of the aryl substituent (3g-3i), and
in all cases good to excellent regioselectivities were observed
(9.5:1 - ≥20:1). During our studies of nitrogen nucleophile
reactivity we observed that unhindered sulfonates were generally
more efficient nucleophiles than hindered carbamates. Based on
these observations we predicted that the N-Boc-aniline 3j would
not act as a competitive nucleophile, and that the N-Boc-aniline
functionality would be tolerated in the reaction. This was indeed
the case, and 4j was selectively formed as the major regioisomer
in 74% yield. In addition to styrene derivatives 3a-j, 2-
Having identified effective conditions for the intermolecular
allylic amination reaction of a disubstituted olefin, we explored the
limits of this system with various nitrogen nucleophiles (Table 2).
The simple N-methyl-tosylamide was an effective reagent and the
product 2b was isolated in 88% yield. However, larger N-
substituents such as phenyl (2c) and benzyl (2d) required
elevated temperatures (60 °C) and only moderate yields of
product were obtained (44% and 41% respectively). In both cases
a product arising from competing acetate addition was observed.
The more electron deficient o-nosylamide gave the product 2e in
good yield (76 %) at 40 °C but required an extended reaction time
of 48 h. Sulfamate esters were also found to be suitable
substrates providing 2f and 2g in good yields (80% and 82%
respectively). Benzyl carbamate required an elevated reaction
temperature (60 °C), but was an excellent nucleophile giving the
amination product 2h in 95% yield. However, tert-butyl carbamate
was not an effective nucleophile, and the product arising from this
reaction was only isolated in 15% yield (see supporting
information). We note that under these reaction conditions simple
amides such as acetamide and trifluoroacetamide did not produce
any amination products. The scope of simple protected amines
that are compatible with these reaction conditions suggested that
the method could prove useful for the direct incorporation of more
complex amines. In this context we examined N-Cbz-amino acid
derivatives. Glycinate ester 2i was formed in good yield (78 %) at
60 °C. However, we again observed that increased steric
hindrance resulted in reduced reaction efficiency, and the alanine
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10.1002/anie.201707021
Angewandte Chemie International Edition
COMMUNICATION
vinylthiophene derivative 3k and 3-vinylindole derivative 3l were
also investigated. In both cases the oxidatively sensitive
heterocycles were tolerated, and allylic amination products were
formed in moderate yields (42% and 52% respectively).
During the reaction scope study, we observed that
amination of fluoro-substituted styrene derivative 3c gave a 4:1
ratio of product regioisomers (75%) after 24 h when the reaction
was run at 40 °C, but that an improved selectivity (≥20:1, 82%)
was observed after 24 h when the reaction was run at 60 °C. To
assess whether these results reflected an unusual difference in
kinetic selectivity, or an equilibration to the more
thermodynamically stable regioisomer at elevated temperatures,
we conducted a reaction in which substrate 3c was exposed to
the standard reaction conditions at 40 °C for 24 h (Scheme 2). An
aliquot was removed, and a mixture of regioisomers 4c (60%) and
5c (15%), allylic acetate 13 (10%), and starting material (3%) was
observed by 1H NMR. The reaction was then heated to 60 °C for
an additional 24 h. Subsequent analysis of this reaction mixture
showed an increased yield of thermodynamic product 4c (78%),
with only trace 5c (3%) and allylic acetate 13 (1%) present at this
point. Based on these data, we conclude that the initial mixture of
products can equilibrate under the reaction conditions providing
increased yields of the thermodynamically favored product. A
reexamination of other substrates represented in Table 3 showed
that while this behavior was not unique to substrate 3c, it was also
not uniformly applicable. For instance, ortho-methoxy-substrate
3g exhibited similar behavior, with the reaction providing 4g:5g in
a 3.4:1 ratio at 40 °C (24 h) and an improved ≥20:1 ratio when the
reaction was conducted at 60 °C (24 h). However, reactions of
para-chloro-substrate 3d showed identical product ratios after 24
h at both 40 °C and 60 °C (4d:5d, 5:1).
Table 3. Functional Group Tolerance for Allylic Amination of trans-ß-Alkyl
Styrene Derivatives.
NHTs
OTBDPS
Ar
TsNH2 (2.5 equiv)
[RhCp*Cl2]2 (2 mol %)
3
H
4a-l
OTBDPS
+
Ar
AgBF4 (8 mol %)
AgOAc (2.1 equiv)
NHTs
3
OTBDPS
3a-l
Ar
3
5a-l
Me
F
Cl
a, 86%
(6.3:1)a
b, 84%
(6.1:1)a
d, 91%
(5.0:1)a
c, 82%
(≥20:1)b
MeO
F3C
MeO2C
OMe
e, 56%
(4.0:1)b
f, 51%
(4.0:1)b
g, 79%
(≥20:1)b
h, 65%
(9.5:1)b
S
N
MeO
BocHN
Ts
i, 87%
(≥20:1)a
l, 52%
(≥20:1)c
j, 74%
(≥20:1)b
k, 42%
(5.4:1)b
NHTs
OTBDPS
TsNH2 (2.5 equiv)
[RhCp*Cl2]2 (2 mol %)
AgBF4 (8 mol %)
Ar
H
3
Yields are reported for isolated product. Ratios of regioisomers 4:5 were
determined by 1H NMR analysis of the crude reaction mixture and are shown in
parentheses. [a] Reaction was run in DCM at 40 °C for 24 h. [b] Reaction was
run in DCE at 60 °C for 24 h. [c] Reaction was run in DCE at 80 °C for 48 h.
4c
OTBDPS
+
Ar
NHTs
3
AgOAc (2.1 equiv)
DCE
Ar = (p-F-C6H4)
OTBDPS
3c
Ar
3
5c
In addition to aryl-alkyl disubstituted olefins with linear alkyl
chains, we investigated substrates with additional branching at
the homoallylic position (eq. 1). The cyclohexyl substituted
substrate 6, gave amination product 7 in excellent yield and
regioselectivity (84%; ≥20:1). Although the tert-butyl substituted
substrate 8 was competent in the reaction, elevated temperature
(80 °C) was required and the reaction gave only moderate yield
of products with reduced regioselectivity (9, 41%, 2.1:1).
+
OAc
Reaction composition after 24 h at 40 °C:
3c:4c:5c:6c = 3:60:15:10%a
OTBDPS
Ar
Reaction composition after additional 24 h at 60 °C:
3c:4c:5c:6c = 0:78:3:1%a
3
13
[a] Yields determined by 1H NMR analysis of
1,4-dinitrobenzene as an internal standard.
a
reaction aliquot using
Scheme 2. Product Equilibration is Observed Under Reaction Conditions at
Elevated Temperature.
H
NHTs
R1
TsNH2 (2.5 equiv)
[RhCp*Cl2]2 (2 mol%)
AgBF4 (8 mol%)
Ph
R1
Ph
(1)
In an additional mechanistic experiment to probe the
reversibility of the C-H functionalization step, we conducted a
reaction using deuterated tosylamine (eq. 3). AcOD (10 equiv)
was added to the reaction to increase the concentration of
available deuterons in the reaction mixture. The reaction was run
to partial completion, and no deuterium incorporation was
observed in either the recovered starting material (3i), or the
product amine (4i). These results indicate the initial C–H
functionalization step is irreversible under the reaction conditions.
6, R1= cHex
8, R1 = tBu
7, R1= cHex, 84% (≥20:1) (60 °C)
9, R1 = tBu, 41%, (2.1:1) (80 °C)
AgOAc (2.1 equiv)
DCE, 24hr
TsNHR2 (2.5 equiv)
[RhCp*Cl2]2 (2 mol%)
AgBF4 (8 mol%)
R2
H
Ph
N
(2)
Ts
Ph
AgOAc (2.1 equiv)
DCE, 40 °C, 24hr
11, R2 = H, 40%
12, R2 = Me, 51%
10
We also note that the simple terminal olefin allyl benzene
(10) is a viable substrate, providing linear products 11 and 12 in
moderate yields (40% and 51% respectively, eq. 2).
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10.1002/anie.201707021
Angewandte Chemie International Edition
COMMUNICATION
H/D H/D
Keywords: C-H functionalization • π-allyl • rhodium • amination •
TsND2 (2.5 equiv)
[RhCp*Cl2]2 (2 mol %)
AgBF4 (8 mol %)
OTBDPS
H
Ar
Ar
intermolecular
3
OTBDPS
+
(3)
3i, 30%
Ar
3
AgOAc (2.1 equiv)
AcOD (10 equiv)
DCE, 40 °C, 1 h
Ar = (p-MeO-C6H4)
H/D
[1]
a) T. Graening, H.-G. Schmalz, Angew. Chem. Int. Ed. 2003, 42, 2580;
b) B. M. Trost, M. L. Crawley, Chem. Rev. 2003, 103, 2921; c) B. M. Trost,
M. L. Crawley, In Transition Metal Catalyzed Enantioselective Allylic
Substitution in Organic Synthesis; Kazmaier, U., Ed.; Springer Berlin
Heidelberg: Berlin, Heidelberg, 2012, p 321; d) J. S. Cannon, L. E.
Overman, Acc. Chem. Res. 2016, 49, 2220.
H/D NHTs
OTBDPS
3i
3
4i, 29%
0% D incorporation observed in both recovered 3i and product 4i
Finally, we note that Z-olefin 14 does not provide the
corresponding Z-amination product 15. Instead, isomerized
E-olefin product 4a (50%) and regioisomer 5a (6%) were isolated
(eq. 4). Trisubstituted olefin 16 is a competent substrate and
reacts selectively to give the E-amination product 17 in moderate
yield (39%, eq. 5).
[2]
a) P. Koschker, B. Breit, Acc. Chem. Res. 2016, 49, 1524; b) X. H. Yang,
V. M. Dong, J. Am. Chem. Soc. 2017, 139, 1774; c) F. A. Cruz, V. M.
Dong, J. Am. Chem. Soc. 2017, 139, 1029; d) F. A. Cruz, Z. W. Chen,
S. I. Kurtoic, V. M. Dong, Chem. Commun. 2016, 52, 5836; e) Q. A. Chen,
Z. W. Chen, V. M. Dong, J. Am. Chem. Soc. 2015, 137, 8392; f) N. J.
Adamson, E. Hull, S. J. Malcolmson, J. Am. Chem. Soc. 2017, 139,
7180; g) T. M. Beck, B. Breit, Angew. Chem. Int. Ed. 2017, 56, 1903; h)
N. Thieme, B. Breit, Angew. Chem. Int. Ed. 2017, 56, 1520; i) P. A.
Spreider, A. M. Haydl, M. Heinrich, B. Breit, Angew. Chem. Int. Ed. 2016,
55, 15569; j) Z. Liu, B. Breit, Angew. Chem. Int. Ed. 2016, 55, 8440.
a) M. S. Chen, M. C. White, J. Am. Chem. Soc. 2004, 126, 1346; b) M.
S. Chen, N. Prabagaran, N. A. Labenz, M. C. White, J. Am. Chem. Soc.
2005, 127, 6970; c) K. J. Fraunhoffer, M. C. White, J. Am. Chem. Soc.
2007, 129, 7274; d) A. J. Young, M. C. White, J. Am. Chem. Soc. 2008,
130, 14090; e) S. A. Reed, M. C. White, J. Am. Chem. Soc. 2008, 130,
3316; f) S. A. Reed, A. R. Mazzotti, M. C. White, J. Am. Chem. Soc. 2009,
131, 11701; g) C. C. Pattillo, J. I. Strambeanu, P. Calleja, N. A.
Vermeulen, T. Mizuno, M. C. White, J. Am. Chem. Soc. 2016, 138, 1265;
h) A. N. Campbell, P. B. White, I. A. Guzei, S. S. Stahl, J. Am. Chem.
Soc. 2010, 132, 15116.
R
TsNH2 (2.5 equiv)
[RhCp*Cl2]2 (2 mol %)
OTBDPS
3
OTBDPS
3
[3]
Ph
R'
AgBF4 (8 mol %)
AgOAc (2.1 equiv)
DCE, 80 °C, 24 h
(4)
(5)
H
TsHN
15, R = H, R' = Ph, 0%
4a, R = Ph, R' = H, 50%
+ 5a, 6%
14
TsNH2 (2.5 equiv)
[RhCp*Cl2]2 (2 mol %)
Ts
Ph
H
Ph
N
H
AgBF4 (8 mol %)
AgOAc (2.1 equiv)
DCE, 80 °C, 24 h
Me
Me
17, 39%
16
In conclusion, we have developed a new intermolecular
[4]
[5]
a) L. T. Pilarski, N. Selander, D. Bose, K. J. Szabó, Org. Lett. 2009, 11,
5518; b) L. T. Pilarski, P. G. Janson, K. J. Szabó, J. Org. Chem. 2011,
76, 1503.
allylic C-H amination reaction. The reaction is efficient for a variety
of common nitrogen nucleophiles and good regioselectivities
were observed for a range of aryl-akyl-disubstituted olefins.
These reactions represent the first examples of intermolecular
allylic C-H amination of internal olefins via π-allyl intermediates.
This process significantly expands both the types of olefins and
the nitrogen nucleophiles that can be utilized in these oxidative
amination reactions.3e-g Although it is reasonable to expect that
this reaction proceeds via a Rh(III)-π-allyl intermediate,6,7 our
preliminary experiments have indicated that the details of the
reaction mechanism are complex. Additional experimental and
computational studies to further understand the energy landscape
are ongoing in our laboratory.
For alternative approaches to allylic amination see a) J. L. Roizen, M. E.
Harvey, J. Du Bois, Acc. Chem. Res. 2012, 45, 911; b) N. S. Dolan, R. J.
Scamp, T. J. F. Berry, J. M. Schomaker, J. Am. Chem. Soc. 2016, 138,
14658; c) C. G. Liang, F. Collet, F. Robert-Peillard, P. Muller, R. H. Dodd,
P. Dauban, J. Am. Chem. Soc. 2008, 130, 343; d) Y. Wu, F. Y. Kwong,
P. Li, A. C. Chan, Synlett 2013, 24, 2009; e) S. Clark, C. Roche, Chem.
Commun. 2005, 5175; f) R. T. Gephart III, T. H. Warren Organometallics
2012, 31, 7728; g) T. W. Liwosz, S. R. Chemler, Chem. Eur. J. 2013, 19,
12771; P. Xiong, F. Xu, X.-Y. Qian, Y. Yohannes, J. Song, X. Lu, H.-C.
Xu, Chem. Eur. J. 2016, 22, 4379; h) H. Bao, U. Tambar, J. Am. Chem.
Soc. 2012, 134, 18495;
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[7]
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Acknowledgements
a) P. M. Boyer, C. P. Roy, J. M. Bielski, J. S. Merola, Inorg. Chim. Acta
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This research was supported by the National Science Foundation
under CHE-1362502 (CCHF) and CHE-1531620 (NMR), and the
Laney Graduate School of Emory University (J.S.B.).
This article is protected by copyright. All rights reserved.

10.1002/anie.201707021
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28 examples, 26-95% yield
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