Chiral Pincer Carbodicarbene Ligands for Enantioselective Rhodium-Catalyzed Hydroarylation of Terminal and Internal 1,3-Dienes with Indoles

Article abstract of DOI:10.1021/jacs.7b08575

Catalytic enantioselective addition of N-heteroarenes to terminal and internal 1,3-dienes is reported. Reactions are promoted by 5 mol % of Rh catalyst supported by a new chiral pincer carbodicarbene ligand that delivers allylic substituted arenes in up to 95% yield and up to 98:2 er. Mechanistic and X-ray evidence is presented that supports that the reaction proceeds via a Rh(III)-η³-allyl.

Full text of DOI:10.1021/jacs.7b08575

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Chiral Pincer Carbodicarbene Ligands for Enantioselective Rhodium-
Catalyzed Hydroarylation of Terminal and Internal 1,3-Dienes with Indoles.
Justin S. Marcum, Courtney C. Roberts, Rajith S Manan, Tia N Cervarich, and Simon J. Meek
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Oct 2017
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Chiral Pincer Carbodicarbene Ligands for Enantioselective
Rhodium-Catalyzed Hydroarylation of Terminal and Internal
1,3-Dienes with Indoles.
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Justin S. Marcum,‡ Courtney C. Roberts,‡ Rajith S. Manan, Tia N. Cervarich, and Simon J. Meek*
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599ꢀ3290.
Supporting Information Placeholder
ABSTRACT: Catalytic enantioselective addition of Nꢀ
heteroarenes to terminal and internal 1,3ꢀdienes is reꢀ
ported. Reactions are promoted by 5 mol % of Rh cataꢀ
lyst supported by a new chiral pincer carbodicarbene
(CDC) ligand that delivers allylic substituted arenes in
up to 95% yield and up to 98:2 er. Mechanistic and Xꢀ
ray evidence is presented that supports that the reaction
proceeds via a Rh(III)ꢀη3ꢀallyl.
Metalꢀcatalyzed olefin hydroarylation is an important,
atom economical C–C bond forming strategy for the
synthesis of functionalized arenes. Of particular sigꢀ
nificance, is the development of chiral catalysts that
render this process enantioselective. Enantioselective
olefin hydroarylation methods are scarce (>95:5 er),
and are generally limited to intramolecular variants or
,
electron deficient alkenes.1 2
internal 1,3ꢀdienes in up to 98:2 er (Scheme 1b).7
Transformations are facilitated by an in situ generated
(CDC)ꢀRh complex at 35–60 °C, and structural data
indicates the importance of a tridentate CDC scaffold.
We recently reported the siteꢀselective addition of Nꢀ
heteroarene nucleophiles to dienes catalyzed by pinꢀ
cer carbodicarbene (CDC) Rh complex 1 that operates
,
with a Lewis acid coꢀcatalyst.3 4 Carbodicarbenes beꢀ
long to an emerging class of Nꢀheterocyclic carbon(0)
donor ligands for transition metal catalysis that also
include cyclic bentꢀallenes (e.g., 2),5 and carbodiꢀ
phosphoranes (Scheme 1a).6 In light of the above
studies, related efforts in our laboratories have been in
connection to the design, synthesis, and development
of new chiral carbon(0) ligands and their ability to
effect enantioselective olefin hydrofunctionalizaꢀ
tions.3,6b Herein, we describe the synthesis, structure,
and activity of the first chiral, optically pure CDC
ligands that can be used to efficiently promote enanꢀ
tioꢀ and siteꢀselective hydroarylation of terminal and
Selection of a chiral pyrazoliumꢀbased CDC pincer
(3) over a chiral diazapeniumꢀbased CDC (e.g., 1)
was based on the idea that a pyrazolium ligand scafꢀ
fold would offer increased conformational flexibility,
while maintaining a wellꢀdefined chiral binding site,
as well as render catalyst synthesis modular. Furꢀ
thermore, as a design from our previous studies with
CDC pincer ligands,6b we hypothesized that in situ
metalation of Rh(I) onto the pyrazolium 6 would furꢀ
nish a (CDC)ꢀRh(III)–H (A) that could undergo miꢀ
gratory insertion to generate an electrophilic Rh(III)ꢀ
(
η3ꢀallyl) intermediate (B) (Scheme 1c).8 Nucleophilic
addition of indole to the Rh(III)ꢀ(
3ꢀallyl) followed by
η
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sentative bidentate phosphines also afforded <2%
conversion to 10 (entries 10–12).
oxidative protonation at Rh (C→D) regenerates the
Rh(III)–H. Subsequent ligand substitution by diene 4
affords product 7 and reꢀgenerates (CDC)ꢀRh(III)
complex A. Reports by a number of groups have
demonstrated enantioselective Pdꢀ and Rhꢀcatalyzed
hydroalkylations,9 hydroaminations,10 and hydroarylaꢀ
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,
tions1f 11 of 1,3ꢀdienes through the intermediacy of an
electrophilic metalꢀallyl species formed by metal–
hydride migratory insertion.
In seeking to understand what effect the isomeric E/Z
purity of the 1,3ꢀdiene has on the enantioselectivity,
we prepared and examined the hydroarylation reacꢀ
tion with diene 8 formed as an E/Z mixture favoring
the Zꢀisomer (75:25, Z/E) (Eq 1). With 5.0 mol %
(L2)ꢀRh under standard conditions (see Entry 4, Table
1), indole 10 is formed with similar efficiency (64%
yield) and in the same enantioselectivity, 94:6 er (vs
94:6 er Table 1). Thus, the E/Z stereoisomeric purity
of the diene does not noticeably affect er.
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aReactions performed under N2 atmosphere. bConversion to 10 deterꢀ
mined by analysis of 400 or 600 MHz 1H NMR spectra of crude reactions
with DMF as internal standard. cDetermined by HPLC analysis; see the
Supporting Information for details.
We began our catalytic studies by examining the abilꢀ
ity of chiral pyrazolium salts L1–4 to promote enantiꢀ
oselective hydroarylations. Addition of NꢀBnꢀindole
to 8 (Table 1) to afford 10 served as the representative
reaction. As shown in entries 1–2 of Table 1, hyꢀ
aReactions performed under N2 atmosphere. bYield represents isolated
yield of purified material and is an average of two experiments. cDeterꢀ
mined by HPLC of SFC analysis; see the Supporting Information for
droarylation in the presence of 2.5 mol
%
[Rh(C2H4)2Cl]2 and L1 in CH2Cl2 at 35 °C affords
<2% conversion to 10. When 10 mol % NaBArF is
4
1
details. dDetermined by analysis of 400 or 600 MHz H NMR spectra of
crude reactions. e3.75 mol % [Rh(C2H4)2Cl]2 at 60 °C.
used, reaction proceeds to 83% conversion and 90:10
er (entry 3). The result in entry 4 shows that if the
PPh2 is replaced by P(pꢀtol)2, conversion and er inꢀ
creases; treatment of 8 and 9 with 5 mol % Rh, 6 mol
% L2 affords 10 in >98% conversion and 94:6 er.12
Changing the phosphine to P(pꢀMeOC6H4)2 results in
a decrease to 90:10 er (entry 5). The importance of a
tridentate scaffold is highlighted by the inefficient
hydroarylation promoted by bidentate CDC L4; 10 is
generated in 15% conversion, 3:1 rr, and 55:45 er (enꢀ
try 6). As the data in entries 7 and 8 indicate, 5 mol
% LiBArF and KBArF as an additive, deliver 10 in
Data shown in Table 2 illustrates a brief scope of the
(CDC)ꢀRhꢀcatalyzed reaction with various terminal
dienes and indoles. As shown in entries 1–8, treatꢀ
ment of 8 with a variety of substituted indoles and 5.0
mol % Rh, 6 mol % L2, and 10 mol % NaBArF for
4
24 h at 35 °C in CH2Cl2 furnished hydroarylation
products 11a-h efficiently and in high enantioselectivꢀ
ity (91:9 to 95:5 er). In addition, we found transforꢀ
mation with the more sterically hindered 2ꢀmethylꢀNꢀ
Meꢀindole also proceeded effectively generating 11i
in 51% yield and in slightly diminished 90.5:9.5 er.
Accordingly, we also varied the aryl diene compoꢀ
nent. Hydroarylation of 1,3ꢀdienes containing differꢀ
ent aryl groups (halogens, CF3, OMe, Me) was also
4
4
58% and 25% conversion but in diminished 78:22 er
and 87:13 er, respectively. Use of NaBF4, which conꢀ
tains a less dissociating counter anion, delivers <2%
conv (entry 9). Control reactions employing repreꢀ
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efficient and enantioselective (entries 11–16). The
catalytic method is also compatible with thiophene
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groups; for example, the formation of 11p (entry 16)
in 85% yield and 85:15 er, is representative. Catalytic
hydroarylation also proceeds with alkylꢀsubstituted
dienes efficiently but with lower enantioselectivity;
11q was furnished in 85% yield and 72:28 er. The
reaction is also tolerant of pyrrole nucleophiles but
results in lower er with L2 compared indoles.13
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which leads to a (CDC)ꢀRh(III)ꢀallyl, we studied the
catalyst formation. Following the catalytic method,
treatment of [Rh(C2H4)2Cl]2, and L1 with NaBArF in
4
a–dSee Table 2.
CD2Cl2 at 22 °C for 1 h results in the formation of
three new Rh species by 31P NMR; however, no Rh-
hydride is observed. Performing the same reaction
but with the addition of diene 8 (10 equiv), affords a
dark red/purple solution that contains a single species
by 31P NMR. Again, no Rhꢀhydride is observed.
Next, we set out to examine the ability of (CDC)ꢀRh
complexes to promote catalytic C–H arylation with
internal 1,4ꢀdisubstituted 1,3ꢀdienes; in general, interꢀ
nal olefins represent significantly more challenging
classes of substrates in catalytic hydrofunctionalizaꢀ
tions methods. With the (CDC)ꢀRh catalyst derived
from L2, reactions proceed efficiently with isomeriꢀ
cally impure dienes, with some substrates requiring
slightly elevated temperatures (60 °C), affording
products in high er (>96:4 er). For example, treatꢀ
ment of 1ꢀphenylꢀ4ꢀmethylꢀbutadiene and 9 with 5.0
mol % (L2)ꢀRh at 50 °C for 24 h furnished 12 in 66%
isolated yield and 98:2 er (Scheme 2). Additionally,
internal dienes bearing alkyl, alkyl halide, ester, keꢀ
tone, and silyl ether functional groups are tolerated
furnishing products 13–17 in 50–90% yield, 97:3–
98:2 er and good siteꢀselectivity. Notably, reaction of
an internal diene bearing unprotected alcohol results
in similar efficiency but lower er; the formation of 18
in 65% yield and 87:13 er, is representative.
aTwo BArF – ions omitted for clarity. Selected bond lengths for
4
19 (Å): Rh1–C1, 2.081(5); Rh1–P1, 2.3948(14); Rh1–P2,
2.3454(16); Rh1–C27, 2.181(5); Rh1–C28, 2.166(5); Rh1–C29,
2.281(5); C27–C28, 1.404(8); C28–C29, 1.390(8); Rh1–C7, 3.108(6).
Deuterium labeling experiments to probe the fidelity
of proton transfer from the Cꢀ3 position on indole to
the product were undertaken (Scheme 3a). Reaction
of d1-21 with 8 in the presence of (L2)ꢀRh affords d1ꢀ
11b with 67% deuterium in the methyl group. Furꢀ
thermore, analysis of the reaction mixture revealed
the presence of d1/d2-8; 36% deuterium incorporation
into the terminal diene. This data indicates Rh(III)–
hydride insertion is rapid and reversible.
After careful experimentation, the Xꢀray crystal strucꢀ
ture of dicationic [(L1)ꢀRh(III)ꢀη
3ꢀallyl]2BArF comꢀ
4
plex 19 was obtained, and is depicted in Figure 1.14 As
indicated by the ORTEP diagram, the pincer (L1)ꢀ
Rh(III) complex has a pseudoꢀtrigonal bipyramidal
geometry. Of note, the C7 methine proton resides
~2.5 Å from the Rh center, potentially providing addiꢀ
tional stabilization for the observed dicationic (CDC)ꢀ
Rh(III) complex. Analysis of the πꢀallyl fragment
To gain insight into the structure of the metalated ligꢀ
and verify that a (CDC)ꢀRh(III)–H is generated,
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shows the Rh–C distance to the Phꢀsubstituted allylic
1
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2865. (l) Zhang, C.; Santiago, C. B.; Crawford, J. M.; Sigman, M. S.
J. Am. Chem. Soc. 2015, 137, 15668–15671. (m) Friis, S. D.; Pirnot,
M. T.; Buchwald, S. L. J. Am. Chem. Soc. 2016, 138, 8372–8375.
terminus (2.281(5)) is 0.10 Å longer than the correꢀ
sponding distance to the Meꢀsubstituted terminus
(2.181(5)). These values are inconsistent with similar
allyl C27–C28/C28–C29 bond lengths, suggesting the
distortion is caused by sterics of the phenyl group.
Also, the resulting observed major (S)ꢀenantiomer
formed in the catalytic reactions must occur via addiꢀ
tion of indole to the least hindered allyl terminus.
(2) (a) Thalji, R. K.; Ellman, J. A.; Bergman, R. G. J. Am. Chem.
Soc. 2004, 126, 7192–7193. (b) O’Malley, S. J.; Tan, K. L.; Watzke,
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Control reactions confirmed catalytic diene hydroaryꢀ
lation in the presence of 5 mol % 19 and 5 mol %
NaBArF results in >98% conv to 10 in 8:1 rr and
4
89:11 er. Notably, without NaBArF 10 is formed in
4
6:1 rr, and 85:15 er, demonstrating the additive effect
of Na salts in obtaining high rr and er. Furthermore,
the stoichiometric reaction between 19 and 9 at 35 °C
in CH2Cl2 results in <2% conv to 10.15
(
3
) Roberts, C. C.; Matías, D. M.; Goldfogel, M. J.; Meek, S. J. J.
Am. Chem. Soc. 2015, 137, 6488–6491.
(4) For a related Rhꢀcatalyzed hydroarylation promoted by αꢀ
dicationic bidentate phosphines, see: Gu, L.; Wolf, L. M.; Zieliński,
A.; Thiel, W.; Alcarazo, M. J. Am. Chem. Soc. 2017, 139, 4948–4953.
In summary, we have developed the first chiral CDC
ligand that promotes enantioselective Rhꢀcatalyzed
hydroarylation of terminal and internal 1,3ꢀdienes.
Further studies of other (CDC)ꢀRhꢀcatalyzed enantiꢀ
oselective hydrofunctionalizations are in progress.
(
(
5
6
) Cyclic bentꢀallenes are a subclass of carbodicarbenes.
) For a review of carbon(0) ligands, see: (a) Melaimi, M.; Soleilꢀ
havoup, M.; Bertrand, G. Angew. Chem. Int. Ed. 2010, 49, 8810–
8849. For examples of transition metal catalysis, see: (b) Goldfogel,
M. J.; Roberts, C. C.; Meek, S. J. J. Am. Chem. Soc. 2014, 136, 6227–
6230. Hsu, Y.ꢀC.; Shen, J.ꢀS.; Lin, B.ꢀC.; Chen, W.ꢀC.; Chan, Y.ꢀT.;
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Int. Ed. 2015, 54, 2420–2424. (d) Pranckevicius, C.; Fan, L.; Stephan,
D. W. J. Am. Chem. Soc. 2015, 137, 5582–5589. (e) Goldfogel, M. J.;
Meek, S. J. Chem. Sci. 2016, 7, 4079–4084. (f) Goldfogel, M. J.;
Roberts, C. C.; Manan, R. S.; Meek, S. J. Org. Lett. 2017, 19, 90–93.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and spectral
and analytical data for all products. This material is available free
of charge via the Internet at http://pubs.acs.org.
(7) (a) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. An-
AUTHOR INFORMATION
gew. Chem. Int. Ed. 2008, 47, 3206–3209. (b) Lavallo, V.; Dyker, C.
A.; Donnadieu, B.; Bertrand, G. Angew. Chem. Int. Ed. 2008, 47,
5411–5414. (c) Melaimi, M.; Parameswaran, P.; Donnadieu, B.;
Frenking, G.; Bertrand, G. Angew. Chem. Int. Ed. 2009, 48, 4792–
4795. (d) Fernández, I.; Dyker, C. A.; DeHope, A.; Donnadieu, B.;
Frenking, G.; Bertrand, G. J. Am. Chem. Soc. 2009, 131, 11875–
11881. (e) Chen, W.ꢀC.; Hsu, Y.ꢀC.; Lee, C.ꢀY.; Yap, G. P.; Ong, T.ꢀ
G. Organometallics 2013, 32, 2435–2442.
Corresponding Author
sjmeek@unc.edu
Author Contributions
‡J.S.M. and C.C.R. contributed equally.
(8
) For examples of electrophilic metalꢀ(η3ꢀallyl) species generated
Notes
Authors declare no competing financial interests.
by M–H additions to 1,3ꢀdienes, see: (a) Takahashi, K.; Miyake, A.;
Hata, G. Bull. Chem. Soc. Jpn. 1972, 45, 1183−1191. (b) Andell, O.
S.; Bäckvall, J. E.; Moberg, C. Acta Chem. Scand. 1986, B 40,
184−189. (c) Mignani, G.; Morel, D.; Colleuille, Y. Tet. Lett. 1985,
26, 6337–6340. For an overview of Rhꢀcatalyzed allylations, see: (d)
Koschker, P.; Breit, B. Acc. Chem. Res. 2016, 49, 1524–1536.
ACKNOWLEDGMENT
Financial support was provided by the NSF (CHEꢀ1665125) and
University of North Carolina. C.C.R. is an NSF graduate research
fellow. We are grateful to M. Brookhart for helpful discussions
and R. Sommer for assistance obtaining the Xꢀray structure of 19.
(
9
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Chem. 2004, 69, 7552–7557.
10) Pd: (a) Löber, O.; Kawatsura, M.; Hartwig, J. F. J. Am. Chem.
(
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(
(
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