Ligand-Controlled Regiodivergence in the Copper-Catalyzed [2,3]- and [1,2]-Rearrangements of Iodonium Ylides

Article abstract of DOI:10.1021/jacs.6b08624

Despite the importance of allylic ylide rearrangements for the synthesis of complex molecules, the catalyst control of [2,3]- and [1,2]-rearrangements remains an unsolved problem. We developed the first regiodivergent [2,3]- and [1,2]-rearrangements of iodonium ylides that are controlled by copper catalysts bearing different ligands. In the presence of a 2,2′-dipyridyl ligand, diazoesters and allylic iodides react via a [2,3]-rearrangement pathway. Alternatively, a phosphine ligand favors the formation of the [1,2]-rearrangement product. A series of α-iodoesters containing a broad range of functional groups were obtained in high yields, regioselectivities, and diastereoselectivities. Deuterium-labeling studies suggest distinct mechanisms for the regioselective rearrangements.

Full text of DOI:10.1021/jacs.6b08624

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Communication
Ligand-Controlled Regiodivergence in the Copper-
Catalyzed [2,3]- and [1,2]-Rearrangements of Iodonium Ylides
Bin Xu, and Uttam K Tambar
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b08624 • Publication Date (Web): 07 Sep 2016
Downloaded from http://pubs.acs.org on September 7, 2016
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Ligand-Controlled Regiodivergence in the Copper-Catalyzed [2,3]-
and [1,2]-Rearrangements of Iodonium Ylides
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Bin Xu and Uttam K. Tambar*
Department of Biochemistry, The University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Boulevard,
Dallas, Texas 75390ꢀ9038, United States
Supporting Information Placeholder
Scheme 1. Regiodivergent [2,3]- and [1,2]-
Rearrangements
ABSTRACT: Despite the importance of allylic ylide reꢀ
arrangements for the synthesis of complex molecules,
the catalyst control of [2,3]ꢀ and [1,2]ꢀrearrangements
remains an unsolved problem. We developed the first
regiodivergent [2,3]ꢀ and [1,2]ꢀrearrangements of ioꢀ
donium ylides that are controlled by copper catalysts
bearing different ligands. In the presence of a 2,2′ꢀ
dipyridyl ligand, diazoesters and allylic iodides react via
a [2,3]ꢀrearrangement pathway. Alternatively, a phosꢀ
phine ligand favors the formation of the [1,2]ꢀ
rearrangement product. A series of αꢀiodoesters containꢀ
ing a broad range of functional groups were obtained in
high yields, regioselectivities, and diastereoselectivities.
Deuteriumꢀlabeling studies suggest distinct mechanisms
for the regioselective rearrangements.
Formatted: Font: Arno Pro, 10.5 pt, Bold, Kern
at 10.5 pt
Due to the high reactivity of iodonium ylides, we hyꢀ
pothesized that the in situ generation of these substrates
from allylic iodides and diazoesters would provide the
ideal platform for developing regiodivergent metalꢀ
catalyzed rearrangements.4 This tandem ylide generaꢀ
tion/rearrangement would obviate the need for isolating
reactive iodonium ylides. Furthermore, the same metal
catalyst could be utilized to generate the iodonium ylide
and control the selectivity of the rearrangement. Altꢀ
hough examples of copperꢀ and rhodiumꢀcatalyzed ioꢀ
donium ylide generation/[2,3]ꢀrearrangement have been
reported,4 rhodium carbenoids are thought to dissociate
more readily from ylide intermediates generated from
diazoesters.5 Therefore, copper was selected to maximꢀ
ize the likelihood of observing unprecedented ligandꢀ
control of regioselectivity in the rearrangement step.6
Sigmatropic rearrangements of ylides represent a
powerful set of reactions for the synthesis of complex
and valuable products.1 Unfortunately a challenge reꢀ
mains, since this class of substrates can often access
multiple products via divergent reaction pathways. For
example allylic ylides undergo [2,3]ꢀ or [1,2]ꢀ
rearrangements depending on the sterics and electronics
of the substrate (Scheme 1a).2 To the best of our
knowledge, there are no ligandꢀcontrolled catalytic
methods for selecting one pathway over another,3 which
may be attributed to the high reactivity of allylic ylides
and the lack of any models for catalyst control over the
divergent processes. The ability to impart a high level of
control over this class of rearrangements would greatly
increase their synthetic utility.
We first optimized the copperꢀcatalyzed [2,3]ꢀ
rearrangement of iodonium ylides (Table 1). In the presꢀ
ence of commercially available [Cu(MeCN)4]PF6 and in
the absence of an external ligand, Eꢀcinnamyl iodide
(E)-1a and benzyldiazoester 2a yielded an unremarkable
mixture of [2,3]ꢀand [1,2]ꢀrearrangement products 3 and
4 (61:39 rr), and 3 was formed in poor diastereoselecꢀ
tivity (entry 1). The effect of various ligands on the regiꢀ
oselectivity and diastereoselectivity of this process was
examined (entries 2ꢀ6). [2,3]ꢀRearrangement product 3
was generated with low levels of selectivity in the presꢀ
In this Communication, we describe the first example
of a switch in selectivity from [2,3]ꢀrearrangements to
[1,2]ꢀrearrangements of allylic ylides that is controlled
by the ligand of the metal catalyst. We selected iodoniꢀ
um ylides4 to showcase this novel divergence in reactiviꢀ
ty (Scheme 1b). The products of these transformations
are αꢀiodoesters that are synthetically challenging to
access in high selectivity by other known methods.
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ence of catalytic dipyridinꢀ2ꢀylmethane (L1), 1,10ꢀ
phenanthroline (L2), 2,2′ꢀbipyridyl (L3), and 6,6′ꢀ
dimethoxyꢀ2,2′ꢀdipyridyl (L4). However, catalyst
composed of [Cu(MeCN)4]PF6 and 6,6′ꢀdibromoꢀ2,2′ꢀ
dipyridyl (L5) furnished the [2,3]ꢀrearrangement product
3 in high regioselectivity (>95:5 rr), albeit in poor diaꢀ
stereoselectivity (48:52 dr). The use of Zꢀcinnamyl ioꢀ
dide (Z)-1a enhanced the diastereoselectivity of the proꢀ
cess (entry 7) without affecting the regioselectivity. A
ing heteroaromatic rings were also accessible with high
regioselectivities (5j and 6j).
Table 1. Optimization of [2,3]-Rearrangementa
a
I
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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32
33
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CO2R
5 mol% [Cu]
6 mol% Ligand
R1
I
Ph
N2
3
4
R2
CO2R
CH2Cl2, T °C
I
(E)-1a, R1=Ph, R2= H
(Z)-1a, R1=H, R2= Ph
Ph
2a, R=Bn
2b, R=t-Bu
CO2R
screen
of
other
copper
sources
identified
N
N
[(MeCN)4Cu]PF6 as the optimal preꢀcatalyst for the reꢀ
action (entries 7ꢀ9). An examination of temperature reꢀ
vealed that a higher regioselectivity and yield are obꢀ
tained at lower temperatures (–20 °C) (entries 10ꢀ12). To
increase the diastereoselectivity to a more synthetically
useful level, we utilized more sterically encumbered tꢀ
butyldiazoester 2b to afford [2,3]ꢀrearrangement product
in 91% yield, >95:5 rr, and 94:6 dr (entry 13).
N
N
N
N
R
R
L3: R = H
L4: R = MeO
L5: R = Br
L1
L2
[Cu]
None [Cu(MeCN)4]PF6 25
Entry Ligand
T (°C) Yield (%) 3 : 4
anti:syn (3)
36 : 64
29 : 71
25 : 75
20 : 80
40 : 60
48 : 52
80 : 20
76 : 24
--
1
2
13
65
64
60
64
69
81
53
< 5
71
85
92
91
61 : 39
45 : 55
20 : 80
14 : 86
62 : 38
>95 : 5
>95 : 5
89 : 11
--
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
L1
L2
L3
L4
L5
L5
25
25
25
25
25
25
3
4
5
Next, we optimized the formation of [1,2]ꢀ
rearrangement product 4 by first identifying a class of
ligands that favor this alternate reaction pathway (Table
2). Bidentate phosphine ligands L6-L8 and tridentate
phosphine ligand L13 furnished almost exclusively
[1,2]ꢀrearrangement product 4 as a single Eꢀolefin isoꢀ
mer (entries 1ꢀ3, 8). In the presence of monodentate triꢀ
alkylphosphine ligand L12, an unfavorable mixture of
regioisomers was formed (entry 7). However, monodenꢀ
tate triarylphosphine ligands L9-L11 facilitated the forꢀ
mation of [1,2]ꢀrearrangement product 4 in >95:5 rr (enꢀ
tries 4ꢀ6). To emphasize the regiodivergence of these
ligandꢀcontrolled rearrangements, we utilized the same
tꢀbutyldiazoester 2b in the [1,2]ꢀrearrangement that was
optimal for the [2,3]ꢀrearrangement. This bulky diazꢀ
oester did not have any impact on the efficiency or seꢀ
lectivity of the [1,2]ꢀrearrangement (entry 9). By perꢀ
forming the reaction at the elevated temperature of 45
°C in the presence of monodentate phosphine ligand
L11, we observed a drastically improved yield of 92%
for the formation of [1,2]ꢀrearrangement product, which
was isolated in 100:0 rr (entry 10). Interestingly, when
pure Zꢀcinnamyl iodide (Z)-1a was subjected to these
optimized conditions, [1,2]ꢀrearrangement product was
isolated as a 37:63 mixture of Z:E olefin isomers (entry
11), which provides insight into the mechanism of this
process (vide infra).
6
7b
8b
9b
10b
11b
12b
13b,c
L5 (CuOTf)•1
/2PhMe 25
CuCl
L5
L5
L5
L5
L5
25
45
0
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
94 : 6
80 : 20
81 : 19
81 : 19
94 : 6
>95 : 5
>95 : 5
>95 : 5
[Cu(MeCN)4]PF6 –20
[Cu(MeCN)4]PF6
–20
Table 2. Optimization of [1,2]-Rearrangementa
I
Ph
CO2R
5 mol% [Cu(MeCN)4]PF6
6 mol% Ligand
4
R1
I
N2
I
R2
CO2R
CH2Cl2, T °C
CO2R
(E)-1a, R1=Ph, R2= H
(Z)-1a, R1=H, R2= Ph
2a, R=Bn
2b, R=t-Bu
Ph
3
Ph
P
PR3
Ph2P
n
PPh2
Ph2P
PPh2
L6: n = 2
L7: n = 3
L8: n = 4
L9: R = Ph
L13
L10: R = 4-FC6H4
L11: R = 4-MeOC6H4
L12: R = Cyclohexyl
T (°C)
Entry Ligand
Time (h) Yield (%)
3 : 4
Z:E (4)
25
25
25
25
25
25
25
25
25
45
45
1
L6
24
24
24
24
24
24
36
48
18
6
34
64
56
60
56
69
18
56
68
92
91
5 : >95
5 : >95
5 : >95
5 : >95
5 : >95
5 : >95
70 : 30
5 : >95
5 : >95
0 : 100
5 : >95
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
37: 63
2
L7
3
L8
4
L9d
L10d
L11d
L12d
L13
L11d
L11d
L11d
5
With optimized reaction conditions in hand for both
the copperꢀcatalyzed [2,3]ꢀ and [1,2]ꢀrearrangements of
iodonium ylides, we explored the functional group comꢀ
patibility of these regiodivergent rearrangements (Table
3). Both reaction conditions exhibited similar tolerance
to aromatic systems with a broad range of substituents
(5a-5i and 6a-6i, Table 3A). Products were universally
generated in good yields, high regioselectivities, and
high diastereoselectivities. Substitution at the orthoꢀ,
metaꢀ, and paraꢀ positions of the aromatic rings did not
affect the efficiency of the reactions. Products containꢀ
6
7
8
9c
10c
11b,c
6
a
Reaction conditions: allylic iodide (E)-1a (0.4 mmol),
benzyldiazoester 2a (1.2 equiv), copper catalyst (5 mol%),
c
ligand (6 mol%), CH2Cl2 (0.1M). b (Z)-1a was utilized. tꢀ
Butyldiazoester 2b was utilized. d 12 mol% ligand.
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We expanded the scope of the [2,3]ꢀ and [1,2]ꢀ
rearrangements to alkyl substituted allylic iodides (5k-
5o and 6k-6o, Table 3B). Products were generated in
high yields, regioselectivities, and diastereoselectivities
in the presence of several functional groups, including a
silyl ether, chloride, azide, and protected amine.
would be difficult to access by traditional methods (5p-
5t and 6p-6t, Table 3C). These examples highlight the
potential of iodonium ylide rearrangements to generate a
diverse collection of products from the same set of startꢀ
ing materials.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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30
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60
Gratifyingly, the efficiency of [2,3]ꢀ and [1,2]ꢀ
rearrangements was maintained on gram scale, as we
formed 1.28 g of 5a (90% yield) and 1.37 g of 6a (94%
yield) under the optimized conditions.
Notably, we subjected polyꢀsubstituted allylic iodides
to the regiodivergent rearrangements, which furnished
products with significantly different architectures that
Table 3. Substrate Scope of Copper-Catalyzed [2,3]- and [1,2]-Rearrangements of Iodonium Ylides
OMe
5 mol% CuPF6
6 mol% L5
R3
I
R3
I
R3
5 mol% CuPF6
12 mol% L11
N2
R1
I
R1
N
N
CO2t-Bu
P
CO2t-Bu
CH2Cl2, –20 °C, 6h
(cis-allylic iodides)
CO2t-Bu
CH2Cl2, 45 °C, 6h
R2
R1 R2
R2
Br
Br
L5
5
2b
6
1
(trans-allylic iodides)
MeO
OMe
L11
A. Aryl-Substituted Allylic Iodides
B. Alkyl-Substituted Allylic Iodides
I
I
NC
I
I
CO2t-Bu
I
CO2t-Bu
n-Bu
CO2t-Bu
CO2t-Bu
CO2t-Bu
n-Bu
X-ray
5a, 90% yield, 1.28 gram
>95:5 rr, 94:6 dr
6a, 94% yield, 1.37 gram
100:0 rr
5f, 78% yield
>95:5 rr, >95:5 dr
6f, 84% yield
100:0 rr
5k, 89% yield
>95:5 rr, 95:5 dr
6k, 92% yield
>95:5 rr
I
I
I
I
CO2t-Bu
F
I
CO2t-Bu
I
CO2t-Bu
CO2t-Bu
TBSO
CO2t-Bu
CO2t-Bu
OTBS
F
5b, 97% yield
>95:5 rr, 95:5 dr
6b, 86% yield
100:0 rr
5g, 87% yield
>95:5 rr, 95:5 dr
6g, 91% yield
100:0 rr
5l, 78% yield
>95:5 rr, 92:8 dr
6l, 92% yield
100:0 rr
I
I
F
I
CO2t-Bu
I
CO2t-Bu
I
CO2t-Bu
I
Cl
CO2t-Bu
CO2t-Bu
Cl
CO2t-Bu
F
5c, 86% yield
95:5 rr, 94:6 dr
6c, 93% yield
100:0 rr
5h, 92% yield
>95:5 rr, 93:7 dr
6h, 96% yield
100:0 rr
5m, 80% yield
>95:5 rr, 95:5 dr
6m, 90% yield
100:0 rr
I
I
I
CO2t-Bu
Ph
F
I
CO2t-Bu
F
I
I
CO2t-Bu
N3
CO2t-Bu
CO2t-Bu
CO2t-Bu
N3
Ph
5d, 84% yield
>95:5 rr, 89:11 dr
6d, 88% yield
100:0 rr
5i, 89% yield
>95:5 rr, 91:9 dr
6i, 90% yield
100:0 rr
5n, 66% yield
>95:5 rr, 93:7 dr
6n, 78% yield
100:0 rr
I
I
I
NC
CO2t-Bu
I
MeO2C
CO2t-Bu
I
CO2t-Bu
Ph
O
I
Ph
N
CO2t-Bu
O
CO2t-Bu
N
CO2t-Bu
Ts
Ts
CN
CO2Me
5e, 80% yield
>95:5 rr, 95:5 dr
6e, 93% yield
100:0 rr
5j, 48% yield
>95:5 rr, 82:18 dr
6j, 56% yield
100:0 rr
5o, 86% yield
>95:5 rr, 95:5 dr
6o, 94% yield
100:0 rr
C. Poly-Substituted Allylic Iodides
Ph
I
I
I
I
I
CO2t-Bu
CO2t-Bu
CO2t-Bu
CO2t-Bu
CO2t-Bu
Me Me
Ph
5p, 92% yield
100:0 rr
5q, 67% yield
94:6 rr
5r, 68% yield
92:8 rr, 92:8 dr
5s, 88% yield
5t, 71% yield
>95:5 rr, 100:0 dr
>95:5 rr, >95:5 dr
I
I
I
Ph
I
I
Me
CO2t-Bu
CO2t-Bu
6p, 96% yield
CO2t-Bu
CO2t-Bu
CO2t-Bu
Me
Ph
6q, 73% yield
100:0 rr
6r, 84% yield
>95:5 rr
6s, 94% yield
100:0 rr
6t, 80% yield
100:0 rr
100:0 rr
We propose two possible mechanisms for the copperꢀ
catalyzed regiodivergent [2,3]ꢀ and [1,2]ꢀrearrangements
of iodonium ylides (Scheme 2a). In both scenarios, we
surmise that the diazoester and allylic iodide combine to
form copperꢀcoordinated iodonium ylide 7. This interꢀ
mediate can undergo a concerted chargeꢀinduced rearꢀ
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rangement (path a). Alternatively, the rearrangement
switching selectivity from [2,3]ꢀrearrangements to [1,2]ꢀ
rearrangements of allylic ylides that is controlled by the
ligand of the metal catalyst. Both reactions demonstrate
a broad substrate scope and functional group tolerance
of aryl and aliphatic allylic iodides. Mechanistic studies
are consistent with different mechanisms for the reꢀ
giodivergent rearrangements. We are currently exploring
enantioselective versions of these processes and their
application to the synthesis of complex target molecules.
products can be formed through a stepwise oxidative
addition/reductive elimination mechanism (path b),
which proceeds through interconverting copper(III)ꢀallyl
complexes 8, 9, and 10.7
10
11
12
13
14
15
16
17
18
19
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We devised deuteriumꢀlabeled experiments to distinꢀ
guish between these two possible mechanisms (Scheme
2b). Diazoester 2b and deuterated allylic iodide 11 were
subjected to both reaction conditions. Terminal allylic
iodide 11 was selected to avoid any steric bias that may
exist for a substituted allylic iodide. Whereas the optiꢀ
mized [2,3]ꢀrearrangement conditions led to the forꢀ
mation of product 13 with fidelity in the transposition of
deuterium, the optimized [1,2]ꢀrearrangement conditions
yielded a mixture of deuterated products 12 and 13.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and
characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Scheme 2. Mechanistic Studies
Corresponding Author
uttam.tambar@utsouthwestern.edu
ACKNOWLEDGMENT
Financial support was provided by W. W. Caruth, Jr. Enꢀ
dowed Scholarship, Welch Foundation (Iꢀ1748), National
Institutes of Health (R01GM102604), National Science
Foundation (1150875), and Sloan Research Fellowship.
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ton, D. A. J. Am. Chem. Soc. 2015, 137, 14244ꢀ14247. (c) West, T.
H.; Spoehrle, S. S. M.; Kasten, K.; Taylor, J. E.; Smith, A. D. ACS
Catal. 2015, 5, 7446ꢀ7479. (d) Hoffmann, R. W. Angew. Chem. Int.
Ed. 1979, 18, 563ꢀ572. (e) Brückner, R. In Comprehensive Organic
Synthesis; Fleming, I., Ed.; Pergamon: Oxford, 1991, pp 873ꢀ908.
Based on these experiments, we propose that the copꢀ
perꢀcatalyzed [2,3]ꢀrearrangement in the presence of
2,2′ꢀbipyridyl ligand L5 proceeds through a concerted
chargeꢀinduced rearrangement mechanism (path a), and
the copperꢀcatalyzed [1,2]ꢀrearrangement with monoꢀ
dentate phosphine ligand L11 proceeds through a copꢀ
per(III)ꢀallyl complex that is formed via a stepwise oxiꢀ
dative addition/reductive elimination mechanism (path
b). We hypothesize that the diminished σꢀdonicity of
2,2′ꢀbipyridyl ligand L5 decreases the reactivity of metꢀ
alꢀcoordinated iodonium ylide 7 toward oxidative addiꢀ
tion, which favors the formation of the [2,3]ꢀ
rearrangement product via the concerted chargeꢀinduced
rearrangement mechanism. Alternatively, the strong σꢀ
donating phosphine ligand L11 facilitates oxidative adꢀ
dition of metalꢀcoordinated iodonium ylide 7 to form
copper(III)ꢀallyl complexes 8ꢀ10,7 which leads to the
formation of the [1,2]ꢀrearrangement product via subseꢀ
quent reductive elimination.
3
Isolated examples of controlling regioselectivity for [2,3]ꢀor
[1,2]ꢀrearrangements by modifying substrates or reaction conditions.
For example: (a) Pine, S. H.; Munemo, E. M.; Phillips, T. R.; Bartoliꢀ
ni, G.; Cotton, W. D.; Andrews, G. C. J. Org. Chem. 1971, 36, 984ꢀ
991. (b) Tanaka, T.; Shirai, N.; Sugimori, J.; Sato, Y. J. Org. Chem.
1992, 57, 5034ꢀ5036. (c) Narita, K.; Shirai, N.; Sato, Y. J. Org.
Chem. 1997, 62, 2544ꢀ2549. (d) Tayama, E.; Kimura, H. Angew.
Chem. Int. Ed. 2007, 46, 8869ꢀ8871. (e) Tayama, E.; Takedachi, K.;
Iwamoto, H.; Hasegawa, E. Tetrahedron 2010, 66, 9389ꢀ9395.
4
(a) Giddings, P. J.; D. Ivor, J.; Thomas, E. J. Tetrahedron Lett.
1980, 21, 395ꢀ398. (b) Doyle, M. P.; Tamblyn, W. H.; Bagheri, V. J.
Org. Chem. 1981, 46, 5094ꢀ5102. (c) Giddings, P. J.; John, D. I.;
Thomas, E. J.; Williams, D. J. J. Chem. Soc., Perkin Trans. 1 1982,
2757ꢀ2766. (d) Doyle, M. P.; Forbes, D. C.; Vasbinder, M. M.; Peterꢀ
son, C. S. J. Am. Chem. Soc. 1998, 120, 7653ꢀ7654. (e) Doyle, M. P.;
Hu, W.; Phillips, I. M.; Moody, C. J.; Pepper, A. G.; Slawin, A. G. Z.
Adv. Synth. Catal. 2001, 343, 112ꢀ117. (f) Krishnamoorthy, P.;
Browning, R. G.; Singh, S.; Sivappa, R.; Lovely, C. J.; Dias, H. V. R.
Chem. Commun. 2007, 731ꢀ733. (g) Deng, Q.ꢀH.; Chen, J.; Huang, J.ꢀ
S.; Chui, S. S.ꢀY.; Zhu, N.; Li, G.ꢀY.; Che, C.ꢀM. Chem. Eur. J. 2009,
15, 10707ꢀ10712.
In conclusion, we have developed regiodivergent copꢀ
perꢀcatalyzed [2,3]ꢀ and [1,2]ꢀrearrangements of iodoniꢀ
um ylides. These results represent the first example of
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Liang, Y.; Zhou, H.; Yu, Z.ꢀX. J. Am. Chem. Soc. 2009, 131,
17783ꢀ17785.
Soc. 2012, 134, 15497ꢀ15504. (b) Guo, X.; Hu, W. Acc. Chem. Res.
2013, 46, 2427ꢀ2440. (c) Xu, B.; Zhu, S.ꢀF.; Zhang, Z.ꢀC.; Yu, Z.ꢀX.;
Ma, Y.; Zhou, Q.ꢀL. Chem. Sci. 2014, 5, 1442ꢀ1448. (d) Ref. 5.
6
For discussions of the role of both structures of metal bound
7
ylides depicted in Scheme 1b, see: (a) Li, Z.; Boyarskikh, V.; Hansen,
J. H.; Autschbach, J.; Musaev, D. G.; Davies, H. M. L. J. Am. Chem.
Yoshikai, N.; Nakamura, E. Chem. Rev. 2012, 112, 2339ꢀ2372.
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