Cyclobutadiene Metal Complexes: A New Class of Highly Selective Catalysts. An Application to Direct Reductive Amination

Article abstract of DOI:10.1021/acscatal.5b02916

A catalyst of a new type, cyclobutadiene complex [(C₄Et₄)Rh(p-xylene)]PF₆, was found to promote selective reductive amination in the presence of carbon monoxide under mild conditions (1-3 bar, 90 °C). The reaction demonstr

Full text of DOI:10.1021/acscatal.5b02916

Subscriber access provided by UNIV OSNABRUECK
Letter
Cyclobutadiene Metal Complexes: a New Class of Highly
Selective Catalysts. An Application to Direct Reductive Amination
Oleg I Afanasyev, Alexey A Tsygankov, Dmitry L. Usanov, Dmitry S. Perekalin,
Nikita V Shvydkiy, Victor I. Maleev, Alexander R. Kudinov, and Denis Chusov
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02916 • Publication Date (Web): 11 Feb 2016
Downloaded from http://pubs.acs.org on February 12, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 5
ACS Catalysis
1
2
3
4
5
6
7
8
9
Cyclobutadiene Metal Complexes: A New Class of Highly Selective
Catalysts. An Application to Direct Reductive Amination
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Oleg I. Afanasyev,† Alexey A. Tsygankov,† Dmitry L. Usanov,‡ Dmitry S. Perekalin,† Nikita V. Shvyd-
kiy,† Victor I. Maleev,† Alexander R. Kudinov† and Denis Chusov*,†.
† Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow 119991, Russia
‡ c/a: Department of Chemistry and Chemical Biology, Harvard University 12 Oxford Street, Cambridge, MA 02138 (USA)
ABSTRACT: A catalyst of a new type, cyclobutadiene complex [(C4Et4)Rh(p-xylene)]PF6, was found to promote selective reduc-
tive amination in the presence of carbon monoxide under mild conditions (1-3 bar, 90 °C). The reaction demonstrated perfect com-
patibility with a wide range of functional groups prone to reduction by conventional reducing agents. The developed system repre-
sents the first systematic investigation of cyclobutadiene metal complexes as catalysts.
KEYWORDS rhodium, cyclobutadiene, reductive amination, homogeneous catalysis, chemoselectivity, amines
Since the discovery of ferrocene in 1951,1 sandwich and
half-sandwich complexes with cyclopentadienyl ligands have
been extensively utilized in catalysis.2 Owing to its electronic
and steric properties cyclopentadienyl ligand provides effec-
tive stabilization of the catalytic species3 and in many cases
allows precise tuning of the catalytic activity. At the same
time, substantially less attention has been paid to other types
of cyclic π-ligands. For instance, catalytic applications of cy-
clobutadiene analogs so far have been restricted to their use as
intact planar chiral moieties in ligands for palladium and plati-
num.4 Surprisingly, despite the interesting electronic proper-
ties of cyclobutadiene ligands, cyclobutadiene metal complex-
es have never been used as catalysts themselves.
with standard laboratory equipment and would tolerate a broad
range of functional groups.
Classical catalyst:
R4
R3
H
R4
H
O
-2
0.4 mol%
20-90 bar CO, 120-140 o
Rh
,
[Rh]
2(OAc)4
+
N
R3
+
O
C
O
N
R1
R1 R2
C
R2
work):
Novel
O
of catalyst
(this
type
+
R4
R3
H
R4
H
-2
0.05 mol%
1-3 bar CO, 90 o
wide functional tolerance
2
N
+
O
C
O
N
R1
R1 R2
C
R3
R2
group
Scheme 1. Comparison of the catalyst described herein with
the best reported system for CO-mediated reductive amination.
In 2014, Chusov and List reported a novel approach to re-
ductive condensation processes.5 It was found that reductive
amination of aldehydes and ketones with primary and second-
ary amines can proceed without any external hydrogen source
and requires only carbon monooxide as a deoxygenative agent
and Rh2(OAc)4 as a catalyst. The developed system was nota-
ble for favorable green chemistry metrics, such as E-factors.6
Subsequent studies revealed that the concept could be applied
not only to reductive amination,7,8 but also to reductive
Knoevenagel condensation.9 No requirement for an external
hydrogen source rendered CO-assisted reductive condensa-
tions potentially suitable for substrates which are not compati-
ble with standard reductants, such as dihydrogen or hydrides.
However, the substrate scope investigations of the reported
studies5,7-9 did not involve systematic screening of potentially
labile functional groups, and therefore one of the key potential
advantages of the developed paradigm remained unexplored.
Moreover, the state-of-the-art methodology of CO-assisted
reductive amination still required high pressures (20-90 bar)
and temperatures (120-140 °C). 5
Herein we report a new type of catalyst, namely rhodium
cyclobutadiene complex 1 (Figure 1). We found complex 1 to
be especially well-matched for reductive amination, which
enabled highly selective synthesis of amines with reductively
labile functional groups (Scheme 1). The developed system
represents the first general application of cyclobutadiene metal
complexes as catalysts.
We began our studies by testing some commonly used rho-
dium catalysts in the model reductive amination reaction be-
tween p-anisidine and p-methylbenzaldehyde under substan-
tially milder conditions than those previously reported (3 bar
of CO, 90 °C, 6 h).5 Rh(III) complexes 2a and 2b did not show
any catalytic activity (Table 1, entries 1-2). On the other hand,
Rh(I) complex [(C2H4)2RhCl]2 (2c) gave product 3a in 6%
yield (Table 1, entry 3). Superior performance of 2c can be
attributed to the lower oxidation state of rhodium, which is
more favorable for the oxidative addition step of the plausible
catalytic cycle. Yet, low activity of 2c can be explained by fast
dissociation of the ethylene ligands followed by aggregation of
non-stabilized RhCl species. We therefore tested Rh(I) com-
plexes 2d and 2e with less labile ligands, however, no catalytic
activity was detected (Table 1, entries 4-5). At this point, we
decided to design a new type of Rh(I) catalyst with a carefully
tuned cyclic π-ligand, which would effectively stabilize the
metal center during the catalytic cycle.3 In our efforts to main-
In all reported systems the catalytic species were generated
from simple inorganic salts without introduction of any lig-
ands.5,7-9 Therefore, we envisioned that metal complexes could
be fundamentally superior catalysts for CO-mediated reductive
condensations, which would be active at pressures compatible
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 5
tain low oxidation state of rhodium, neutral cyclobutadiene
ligand appeared to be the best choice.
complex is air-stable and is robust enough to be compatible
1
2
3
4
5
6
7
8
even with 50 % aqueous sulfuric acid. The p-xylene ligand in
1 is readily replaced by two-electron ligands, such as CO or
amine, generating catalytically active species [(C4Et4)RhLx].12
To our delight, in the presence of complex 1 the model reac-
tion of p-anisidine with p-methylbenzaldehyde reached 74%
conversion over 6 hours under mild conditions (3 bar of CO,
90 °C; Table 1, entry 6). The amount of the catalyst could be
reduced to 0.05 mol% provided that the reaction time was
increased up to 100 hours (Table 1, entries 7-8). These results
demonstrated that the catalytic species generated from com-
plex 1 is robust enough to achieve turnover numbers of at least
1300.
Et
Et
Et
Et
Rh
l
C
l
C
Rh+
Rh2+
l
C
l
C
Rh
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
a
2
1
2b
Solvent screening (see supporting information) demonstrat-
ed that reactions conducted in aprotic solvents (except THF
and dioxane) gave low yields of the product. Whereas the pro-
cess was sluggish in water, reactions in various alcohols pro-
ceeded in good yields.13 Despite the fact that tert-butanol
showed the highest yield of the model reaction, we chose eth-
anol as a solvent for further screening for reasons of conven-
ience.
Rh
Rh
Rh
PPh3
l
C
l
C
l
l
C
C
Ph3P Rh
l
C
Rh
PPh3
c
2
e
2
2d
Figure 1. Conventional rhodium catalysts and the catalyst of a
new type (1).
Importantly, the reaction is fast enough under low pressure
(3 bar) which enables the use of Schlenk glassware instead of
autoclaves. Moreover, complex 1 was found to be active even
at atmospheric pressure of CO (see supporting information).
Notably, the amount of carbon monoxide used in the described
experiments did not exceed 1.5 equivalents, which is advanta-
geous in comparison with reductants conventionally used in
large excess.
Despite the wide application of rhodium complexes with
cyclopentadienyl ligands,10 their cyclobutadiene analogues are
almost unexplored. Several sandwich compounds of general
formulae CpRh(C4R4) and Cp*Rh(C4R4) have been prepared
by reactions of alkynes with CpRh(CO)2, [Cp*RhCl2]2 or
similar rhodium precursors.11 However, these compounds do
not undergo ligand exchange reactions and therefore are cata-
lytically inactive.
With the optimized conditions in hand, we proceeded to in-
vestigation of the substrate scope of the developed methodolo-
gy (Scheme 2). A wide range of aldehydes could be success-
fully employed, including variously substituted aromatic (3a,
3b), heteroaromatic (3c) and aliphatic (3d) substrates, which
underwent reductive amination in 70-83% isolated yields. The
method was also perfectly applicable to ketones (3e) and sec-
ondary amines (3f). Notably, a free phenol group was shown
to be suitable for this reaction; product 3g was isolated in 75%
yield. We were particularly interested in screening the devel-
oped catalytic system with the substrates containing functional
groups which often appear to be unstable under reductive con-
ditions. To our delight, high yields were obtained for the prod-
ucts containing such functional groups as carboxybenzyl (3j),
trifluoroacetamido- (3k), aromatic cyano- (3h), aromatic bro-
mo- (3i) and even aromatic nitro-group (3l). The lowered yield
of product 3l (60%) is due to crystallization of the correspond-
ing Schiff base from the reaction mixture (the nitro-group
stays intact under the reaction conditions).
Table 1. Screening of rhodium complexes as catalysts for
reductive amination.
.
H
H
O
mo
l
%
)
,
Rh 0 5
]
(
[
NR1R2
+
H
,
,
Et H 90 o 6 h
NR1R2
o
T l
ar
o
T l
CO 3 b
(
O
C
H
)
a
3
Catalyst loading,
[mol %]
Yield 3a,
Entry Catalyst
[%]
1[a]
2[a]
3[a]
4[a]
5[a]
6[a]
7[b]
8[b]
2a
2b
2c
2d
2e
1
0.25
0.5
trace
trace
6
0.25
0.25
0.2
trace
trace
74
Table 2 compares compatibility of a range of synthetically
important functional groups with a number of common reduc-
ing conditions, including the catalytic methodology reported
herein. Whereas the lability of functional groups is certainly
substrate- and condition-dependent, the general trends clearly
demonstrate the unique synthetic utility of the new catalytic
system.
0.5
1
0.05
0.1
67
1
93
Yields were determined by NMR. [a] HNR1R2 = p-anisidine
(1.2 equivalents). [b] HNR1R2 = 1.0 eq of dibenzylamine, 100 h
reaction time.
We were interested in the comparison of the developed
methodology with complex hydrides of high functional group
tolerance which are conventionally used for reductive amina-
tion, namely, sodium cyanoborohydride (NaBH3CN) and so-
dium triacetoxyborohydride (NaBH(OAc)3). Whereas the
convenience of these reagents on a small laboratory scale is
evident and widely exploited, a large number of disadvantages
We managed to synthesize the first cyclobutadiene-arene
rhodium complex [(C4Et4)Rh(p-xylene)]PF6 (1) in just one
step from commercially available starting materials: complex
2c, silver hexafluorophosphate, 3-hexyne and p-xylene. The
ACS Paragon Plus Environment

Page 3 of 5
ACS Catalysis
render these reagents highly inconvenient for scaled-up syn- stable under reductive conditions. A representative set of func-
1
2
3
4
5
6
7
8
thesis and barely compatible with industrial manufacturing in
light of ecological and economic impacts. For instance, pH
sensitive sodium cyanoborohydride reductions require prepar-
atively inconvenient work-up and specialized disposal of high-
ly toxic water-soluble reaction products,14 whereas sodium
triacetoxyborohydride generates even larger amounts of waste
and is unstable in protic solvents.15 Notably, the average E-
factors6 of CO-assisted catalytic preparation of Scheme 2
products are almost twofold and fivefold lower than those for
NaBH3CN and NaBH(OAc)3 respectively (0.18 vs. 0.32 and
0.92, if counted for stoichiometric ratios). Also, for an arbi-
trarily chosen subset of substrates from Scheme 2, we ob-
served considerably higher yields of the desired products using
our system (88-98% yields by NMR analysis) vs. sodium cya-
noborohydride (33-68%, see supporting information), which is
an inspiring example of the highly selective nature of the de-
veloped catalyst. Excellent performance of catalyst 1 together
with the atom-economical profile is promising for semi-
industrial and industrial applications.
tionalized amines was synthesized in 70-90% isolated yields.
In contrast to the best-performing method of CO-mediated
reductive amination reported to date,5 the developed catalytic
system is active at CO pressure compatible with standard la-
boratory glassware, such as Schlenk tubes. We believe that
cyclobutadiene complexes of rhodium and other metals have a
great potential in other types of reactions; studies in these di-
rections are currently ongoing in our group and will be report-
ed in due course.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. A generalized comparison of functional and pro-
tective group stability under reductive conditions.
H2/Ni H2/Rh LiAlH4 NaBH4 1/CO
[a]
[a]
[a]
[a]
[b]
R2N-Cbz
R2N-
[a]
[a]
[a]
[a]
[b]
COCF3
[a]
[a]
[j]
[a]
[a]
[h]
[h]
[g]
[a]
[a]
[j]
[a]
[a]
[c]
[d]
[f]
[b]
[b]
[b]
[b]
[b]
R2N-Bn
RO-Bn
Ar-NO2
Ar-CN
Ar-Br
R3
R1
R4
R2
-
mo
,
O
2 1 2
,
l
%
R3
R4
+
(
)
-
N
+
+
CO2
±
±
CO
ar
N
Et H 90 o 12 24 h
R1 R2
O
C
H
[h]
[i]
[d]
[e]
3 b
(
)
±
NHPMP
NHPMP
NHPMP
N
[a] Ref. 16. [b] this work. [c] Ref. 17. [d] Ref. 18, however reduc-
tion of nitriles with NaBH4 is known (Ref. 17). [e] Ref. 19. Ar-Br
bond is stable at room temperature but is reduced by LiAlH4 at
elevated temperature (Ref. 21). [f] Ref. 20. [g] Ref. 22. [h] Ref.
23. [i] Ref. 24. [j] Ref. 25.
e
M
F
,
,
,
,
c
a
3
3b
1
3
7
0
%
)
99 83
99
8
98
%
%
%
%
%
(
)
(
)
(
n
B
NHPMP
PMP
n
B
N
N
H
o
T l
,
,
e
3d
7
74
(
3
3f
8
88
8
1
96 90
%
%
%
%
%
%
)
)
(
)
(
ASSOCIATED CONTENT
H
Supporting Information.
O
NHPMP
NHPMP
is available free of charge via the Internet at http://pubs.acs.org.
e
M
O
o
T l
N
N
C
r
B
H
AUTHOR INFORMATION
,
,
,
3g
75
3h
5
3i
4
8
77
88
%
9
80
o
%
)
%
%
)
%
%
(
(
(
)
Corresponding Author
* Email: chusov@ineos.ac.ru; denis.chusov@gmail.com.
H
N
H
N
F
C
3
z
Cb
Present Addresses
‡ c/a: Department of Chemistry and Chemical Biology, Harvard
University 12 Oxford Street, Cambridge, MA 02138 (USA)
O
)
o
T l
N
T l
N
H
H
,
,
3
j
3k
7
7
9
% %
(
98
%
8
4
%
9
(
)
Funding Sources
HN
O
We thank the Russian Foundation for Basic Research (grant #15-
03-02548А) and the Council of the President of the Russian Fed-
eration (grant for young scientists No. MK-6137.2015.3) for fi-
nancial support.
o
T l
N
O2
,
a
[
]
3l
7
%
6
60
(
%
)
Scheme 2. Substrate scope studies of reductive amination
catalyzed by complex 1. NMR yields and isolated yields (in
parentheses). Cyan circles highlight the functional groups which
are poorly compatible with conventional reducing agents. [a] See
text.
ACKNOWLEDGMENT
We thank Prof. Valentine Ananikov for his support. High resolu-
tion mass spectra were recorded in the Department of Structural
Studies of Zelinsky Institute of Organic Chemistry, Moscow.
Dedicated to Professor Benjamin List on the occasion of his 47th
birthday.
In summary, we discovered a catalyst of a new type with
unique activity and selectivity. The use of complex 1 enabled
development of an unusually mild protocol for direct reductive
amination, which is compatible with a wide range of function-
al and protective groups, including those conventionally un-
REFERENCES
(1) a) Miller, S. A.; Tebboth, J. A.; Tremaine, J. F. J. Chem. Soc.
1952, 632-635; b) Kealy, T. J.; Pauson, P. L. Nature 1951, 168, 1039-
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 5
1040; c) Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward,
R. B. J. Am. Chem. Soc. 1952, 74, 2125-2126.
(19) Karabatsos, G. J.; Shone, R. L. J. Org. Chem. 1968, 33, 619-
621.
(20) Bergbreiter, D. E.; Walker, S. A. J. Org. Chem. 1989, 54,
1
2
3
4
5
6
7
8
(2) For selected reviews see: a) Ye, B.; Cramer, N. Acc. Chem. Res.
2015, 48, 1308-1318; b) Nishiura, M.; Guo, F.; Hou, Z. Acc. Chem.
Res. 2015, 48, 2209-2220; c) Toma, S.; Csizmadiova, J.; Meciarova,
M.; Sebesta, R. Dalton Trans. 2014, 43, 16557-16579; d) Song, G.;
Wang, F.; Li, X. Chem. Soc. Rev. 2012, 41, 3651−3678; e) Astruc, D.;
Ornelas, C.; Ruiz, J. Acc. Chem. Res. 2008, 41, 841-856; f) Alt, H. G.;
Licht, E. H.; Licht, A. I.; Schneider, K. J.; Coord. Chem. Rev. 2006,
250, 2-17; g) Trost, B. M.; Frederiksen, M. U.; Rudd, M. T. Angew.
Chem. Int. Ed. 2005, 44, 6630−6666; h) Atkinson, R. C. J.; Gibson,
V. C.; Long, N. J. Chem. Soc. Rev. 2004, 33, 313-328.
5138-5141.
(21) Lee, S. J.; Beak, P. J. Am. Chem. Soc. 2006, 128, 2178-2179.
(22) Wu, W.-L.; Burnett, D. A.; Spring, R.; Qiang, L.; Sasikumar,
T. K.; Domalski, M. S.; Greenlee, W. J.; O’Neill, K.; Hawes, B. E.
Bioorg. Med Chem. Lett. 2006, 16, 3668-3673.
(23) Takamizawa, S.; Wakasa, N.; Fuchikami, T. Synlett 2001,
1623-1625.
(24) Sanfilippo, D.; Rylander, P. N. Hydrogenation and Dehydro-
genation, Ullmann's Encyclopedia of Industrial Chemistry, 2009.
(25) Joseph, M. M.; Jacob, D. E. Ind. J. Chem. 2004, 43B, 432-
436.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(3) For the concept of protecting π-ligands in catalysis see: Maitlis,
P. M. Chem. Soc. Rev. 1981, 10, 1-48.
(4) a) Jones, G.; Richards, C. J. Organometallics 2001, 20, 1251-
1254; b) Anderson, C. E.; Overman, L. E. J. Am. Chem. Soc. 2003,
125, 12412-12413; c) Gomez Arrayas, R.; Garcia Mancheno, O.;
Carretero, J. C. Chem. Commun. 2004, 1654-1655; d) Kirsch, S. F.;
Overman, L. E.; Watson, M. P. J. Org. Chem. 2004, 69, 8101-8104;
e) Kirsch, S. F.; Overman, L. E. J. Am. Chem. Soc. 2005, 127, 2866-
2867; f) Nomura, H.; Richards, C. J. Chem. Eur. J. 2007, 13, 10216–
10224; g) Nomura, H.; Richards, C. J. Chem. Asian J. 2010, 5, 1726–
1740; h) Günay, M. E.; Hughes, D. L.; Richards, C. J. Organometal-
lics 2011, 30, 3901-3904; i) Singh, N.; Elias, A. J. Dalton Trans.
2011, 40, 4882-4891; j) Singh, N. E. M.; Elias, A. J. Chem. Sci. 2011,
123, 853-860; k) Cassar, D. J.; Ilyashenko, G.; Ismail, M.; Woods, J.;
Hughes, D. L.; Richards, C. J. Chem. Eur. J. 2013, 19, 17951-17962;
l) Cassar, D. J.; Roghzai, H.; Villemin, D.; Horton, P. N.; Coles, S. J.;
Richards, C. J. Organometallics 2015, 34, 2953-2961. m) For a re-
view see: Kumar, D.; Deb, M.; Singh, J.; Singh, N.; Keshav, K.; Elias,
A. J. Coord. Chem. Rev. 2016, 306, 115–170.
(5) a) Chusov, D.; List, B. Angew. Chem. Int. Ed. 2014, 53, 5199-
5201; b) Highlighted: Bergin, E. Nat. Chem. 2014, 6, 374.
(6) Sheldon, R. A. Green Chem. 2007, 9, 1273-1283
(7) a) Kolesnikov, P. N.; Yagafarov, N. Z.; Usanov, D. L.; Maleev,
V. I.; Chusov, D. Org. Lett. 2015, 17, 173-175; b) Yagafarov, N. Z.;
Usanov, D. L.; Kagramanov, N. D.; Maleev, V. I.; Chusov, D. Chem-
CatChem 2015, 2590-2593.
(8) Park, J. W.; Chung, Y. K. ACS Catal. 2015, 5, 4846–4850.
(9) Kolesnikov, P. N.; Usanov, D. L.; Barablina, E. A.; Maleev, V.
I.; Chusov, D. Org. Lett. 2014, 16, 5068-5071. For nitro group reduc-
tion see, for example Li, H.-Q.; Liu, X.; Zhang, Q.; Li, S.-S.; Liu, Y.-
M.; He, H.-Y.; Cao, Y.. Chem. Comm. 2015, 51, 11217-11220.
(10) Song, G.; Wang, F.; Li, X. Chem. Soc. Rev. 2012, 41, 3651-
3678.
(11) a) Kornmayer, S. C.; Hellbach, B.; Rominger, F.; Gleiter, R.
Chem. Eur. J. 2009, 15, 3380-3389; b) Lim, M. S.; Baeg, J. Y.; Lee,
S. W. J. Organomet. Chem. 2006, 691, 4100-4108; c) Han, W. S.;
Lee, S. W. J. Organomet. Chem. 2003, 678, 102-107; d) Lamata, M.
P.; José, E. S.; Carmona, D.; Lahoz, F. J.; Atencio, R.; Oro, L. A.
Organometallics 1996, 15, 4852-4856; e) Moreto, J.; Maruya, K.;
Bailey, P. M.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1982,
1341-1347; f) King, R. B.; Ackermann, M. N. J. Organomet. Chem.
1974, 67, 431-441.
(12) Perekalin, D. S.; Shvydkiy, N. V.; Nelyubina, Y. V.; Kudinov,
A. R. Chem. Eur. J. 2015, 21, 16344–16348.
(13) The possibility to use tert-butanol as a solvent excludes
Meerwein-Ponndorf-Verley type process.
(14) Gillmore, A. T.; Badland, M.; Crook, C. L.; Castro, N. M.;
Critcher, D. J.; Fussell, S. J.; Jones, K. J.; Jones, M. C.; Kougoulos,
E.; Mathew, J. S.; McMillan, L.; Pearce, J. E.; Rawlinson, F. L.; Sher-
lock, A. E.; Walton, R. Org. Process Res. Dev. 2012, 16, 1897−1904.
(15) Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff,
C. A.; Shah, R. D. J. Org. Chem. 1996, 61, 3849-3862.
(16) Green, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, Wiley-Interscience, New York, 1999.
(17) Kikugawa, Y.; Kuramoto, M.; Saito, I.; Yamada, S. Chem.
Pharm. Bull. 1973, 21, 1927-1937.
(18) Larkin, J. D.; Frimat, K. A.; Fyles, T. M.; Flower, S. E.;
James, T. D. New J. Chem. 2010, 34, 2922-2931.
ACS Paragon Plus Environment

Page 5 of 5
ACS Catalysis
1
2
3
4
5
Et
Et
Et
Et
R2
R2
+
R4
CO
R4
R1
N
HN
R3
Rh+
R1
O
–
CO2
R3
6
7
8
.
,
n
e c
,
,
,
,
:
z
Cb
n
B
O
o era es unc ona rou s
t l ti
t
l
N
O2
N NH
NHB
t
f
g
p
C
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
ACS Paragon Plus Environment