Iridium-catalyzed borylation of secondary C-H bonds in cyclic ethers

Article abstract of DOI:10.1021/ja305596v

The borylation of secondary C-H bonds, specifically secondary C-H bonds of cyclic ethers, with a catalyst generated from tetramethylphenanthroline and an iridium precursor is reported. This borylation occurs with unique selectivity for the C-H bonds located β to the oxygen atoms over the weaker C-H bonds located α to oxygen atoms. Mechanistic studies imply that the C-H bond cleavage occurs directly at the β position rather than at the α position followed by isomerization of a reaction intermediate.

Full text of DOI:10.1021/ja305596v

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Communication
Iridium-Catalyzed Borylation of Secondary C–H Bonds in Cyclic Ethers
Carl W Liskey, and John F. Hartwig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja305596v • Publication Date (Web): 17 Jul 2012
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Iridium-Catalyzed Borylation of Secondary C–H Bonds in
Cyclic Ethers
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Carl W. Liskey and John F. Hartwig*
Department of Chemistry, University of Illinois, 600 South Mathews Avenue, Urbana, Illinois 61801, United States
Department of Chemistry, University of California, Berkeley, California 94720, United States
KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords, provide
significant keywords to aid the reader in literature retrieval.
Supporting Information Placeholder
lectivity of this catalyst for reactions of primary C-H bonds
has precluded the borylation of secondary C-H bonds with this
ABSTRACT: The borylation of secondary C-H bonds, specif-
ically secondary C-H bonds of cyclic ethers, with a catalyst
system, and no subsequent catalyst has been reported that
generated from tetramethylphenanthroline and an iridium pre-
functionalizes secondary C-H bond with boron reagents.
cursor is reported. This borylation occurs with unique selectiv-
Here, we describe the development of an iridium-
phenanthroline catalyst that functionalizes secondary C-H
bonds for the first time in good yield. In particular, cyclic
ethers undergo borylation with a unique selectivity. Prior C-H
activation10,11 and functionalization4 of cyclic ethers has oc-
curred at the weaker C-H bond α to oxygen. This C-H boryla-
tion occurs at the C-H bond β to oxygen.
ity for the C-H bonds located β to the oxygen atoms over the
weaker C-H bonds located α to oxygen atoms. Mechanistic
studies imply that the C-H bond cleavage occurs directly at the
β position rather than at the α position followed by isomeriza-
tion of a reaction intermediate.
Initially, we sought to identify conditions for the iridium-
catalyzed borylation of alkyl C-H bonds with
bis(pinacolato)diboron (B2pin2) by evaluating the reactivity of
several catalysts for the reaction of n-octane. Recent studies on
the reactivity of Ir-trisboryl complexes toward aromatic C-H
bonds12,13 showed that more electron-rich iridium-trisboryl
complexes react faster with aromatic C-H bonds than do less
electron-rich complexes, while Ir-trisboryl complexes contain-
ing bulky phosphine ligands react much more slowly with
aromatic C-H bonds than do complexes containing bipyridine
ligands. On the basis of these observations with arene sub-
strates, we proposed that Ir complexes containing planar,
strongly electron-donating bidentate ligands could catalyze the
borylation of aliphatic C-H bonds.
The selective cleavage and functionalization of C-H bonds by
transition-metal complexes offers new strategies for the for-
mation of fine chemicals and complex molecules from rela-
tively simple starting materials. One of the greatest challenges
associated with C-H functionalization reactions is to achieve
high selectivity for a single C-H bond among the many related
types of C-H bonds. Thus, new catalytic reactions that func-
tionalize C-H bonds with novel selectivities are being actively
sought.
Many of the current methods to functionalize aliphatic C-H
bonds selectively occur with a group that chelates to a metal
center and directs the C-H functionalization1,2 or with C-H
bonds that are inherently more reactive based on electronic
effects.3,4 For instance, insertion reactions with carbenes typi-
cally occur at the most electron-rich C-H bond. Although these
strategies for C-H functionalization lead to selective transfor-
mations, the scope of such transformations is limited to sub-
strates that contain a chelating group in close proximity to the
C-H bond that becomes functionalized or a C-H bond that is
inherently more reactive based on the electronic properties of
the substrate.
Although the combination of [Ir(COD)OMe]2 (COD = cy-
clooctadiene) and 4,4’-di-tert-butylbipyridine (dtbpy) catalyz-
es the borylation of arenes at low temperatures with high turn-
over numbers, no alkylborane products were observed from
the reaction of n-octane with B2pin2 in the presence 5 mol %
[Ir(COD)OMe]2 and 10 mol % dtbpy. A series of reactions of
n-octane with B2pin2 in the presence of [Ir(COD)OMe]2 and
various ligands containing imine, pyridine, and N-
heterocycliccarbene moieties14 showed that the combination of
[Ir(COD)OMe]2 and phenanthroline ligands gave the product
of alkane borylation. The borylation of n-octane in the pres-
ence of 10 mol % of a combination of [Ir(COD)OMe]2 and
1,10-phenanthroline (phen) gave the 1-octylboronate ester
product in 45% yield. However, the borylation of octane with
a catalyst ligated by the more strongly electron-donating lig-
and 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen) at 120
°C gave a high 88% yield of the 1-octylboronate ester product
(eq 1), consistent with the hypothesis that more electron-rich
complexes effect C-H bond functionalization more readily
Our laboratory previously reported the functionalization of
aliphatic C-H bonds with metal-boryl catalysts and discrete
complexes in which high selectivity for terminal C-H bonds
was observed.5 Rhodium and ruthenium catalysts that contain
pentamethylcyclopentyldienyl ligands (Cp*) catalyzed the
formation of 1-alkylboronate esters from alkanes and diboron
or borane reagents.6-8 The selectivity of these aliphatic C-H
borylation processes is controlled by the steric properties of
the substrate and we recently delineated the origin of this ste-
ric control.9 Although the borylation of aliphatic C-H bonds
with Cp*Rh catalysts occurs in high yields, the exquisite se-
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than less electron-rich complexes.15,16 The borylation of n-
octane under these conditions occurs with the same high selec-
tivity for primary C-H bond functionalization that is observed
with rhodium and ruthenium catalysts.
The C-H borylation also occurred with substituted cyclic
Table 2. Borylation of Secondary C-H Bonds
1
2
3
4
5
6
7
[Ir(COD)OMe]2 (5 mol %)
Me4phen (10 mol %)
C6H13
CH3
+
C6H13
CH2Bpin + HBpin (1)
O
O
neat, 120 ºC, 24 h
B
B
O
O
B2pin2
8
1 equiv
9
Entry
1
Reactant
Product
Yield (%)a
83
Catalyst
(mol %)
Most striking, reactions of octane in tetrahydrofuran (THF)
catalyzed by Me4phen and [Ir(COD)OMe]2 showed that this
system also catalyzes the borylation of secondary C-H bonds.
The reaction of THF with B2pin2 in the presence 10 mol % of
this catalyst combination at 120 °C formed 3-boryl tetrahydro-
furan in 70% yield (Table 1, Entry 1). Again, the borylated
product was formed in lower yield from reactions with phen as
ligand (Entry 2) than from those with Me4phen as ligand. A
significantly higher yield was observed from reactions con-
ducted with (η6-mes)IrBpin3 (mes = mesitylene) as the source
of iridium, presumably due to the formation of the active cata-
lyst in a higher yield with a pre-formed trisboryl complex than
with a species that must convert to a boryl complex before
catalysis occurs (Entry 3). The reaction also occurred with 5
equiv of THF in the presence of an inert solvent, but a lower
yield of the boronate ester was observed (Entry 4).
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4
2
3
4
6
90
72b
4
5
6
4
6
4
61
63c,d
64e
Table 1. Borylation of Tetrahydrofuran
O
O
O
O
[Ir], Ligand
120 ºC
+
B
B
O
O
7
8
41e,f
Bpin
B2pin2
Entry
Source of Ir
Ligand
Catalyst
Yield (%)a
40e
39g
1
2
3
[Ir(COD)OMe]2 Me4phen
10 mol %
10 mol %
4 mol %
4 mol %
70
48
8
9
6
[Ir(COD)OMe]2
(η6-mes)IrBpin3
(η6-mes)IrBpin3
phen
Me4phen
Me4phen
104
72b
10
4
a Yield of isolated secondary boronate ester product follow-
ing purification by silica-gel chromatography. 10:1 ratio of
a
Yield of boronate ester based on B2pin2 as determined by gas
chromatographic analysis. b Reaction was conducted in isododec-
ane as solvent with 5 equiv THF.
b
c
products were observed by GC. Isolated as the protected
alcohol. d 10:1 ratio of products by GC. Isolated as the tri-
e
f
The scope of this C-H borylation of secondary C-H bonds in
cyclic ethers catalyzed by Me4phen and (η6-mes)IrBpin3 is
shown in Table 2. The borylation of 5-, 6-, and 7-membered
cyclic ethers all proceeded in good yield (Entries 1-3). In all
cases, the C-H functionalization occurred with high selectivity
at the position beta to oxygen. The functionalization of tetra-
hydropyran at the β-position implies that the selectivity for β-
borylation arises from an enhanced reactivity of the catalyst
for this type of C-H bond, rather than a decreased reactivity of
the catalyst toward the α C-H bonds. Even reaction of the
seven-membered ether, hexamethylene oxide, occurred with
almost exclusive selectivity at a C-H bond β to oxygen, further
demonstrating the enhanced reactivity of this position for the
iridium catalyst. Further work is needed to achieve the boryla-
tion of smaller-ring oxygen heterocycles. The borylation of
oxetanes occurred, but the products were unstable to the reac-
tion conditions leading to low yield; the borylation of epoxides
did not occur under the conditions for the borylation of the
substrates in Table 2.
fluoroborate salt. Reaction conducted with 5 equiv substrate
in Cy-H. g Reaction conducted at 140 ºC; yield determined by
gas chromatography.
ethers. C-H borylation occurred with both 2,2- and 2,5-
disubstituted cyclic ethers in good yield (Entries 4-5). The
reaction occurred diasteroselectively to form predominantly
the endo product of the oxabicyclonorbornane. Cyclic ethers
containing a heteroatom at the 4-position also underwent the
C-H borylation reaction (Entries 6-7). The reaction of 1,4-
dioxane occurred in high yield, indicating that the reaction can
occur α to oxygen when the substrate lacks C-H bonds β to
oxygen. (In this case, it was necessary to convert the pinacol
boronate ester resulting from this processes to the correspond-
ing trifluoroborate salt, vide infra, to facilitate isolation under
the conditions described above.) The borylation of N-pivaloyl
morpholine also occurred alpha to the oxygen atom. Because
the analogous N-pivaloylpiperidine lacking the oxygen of the
morpholine derivative reacted in low yield (ca 15%) and gave
multiple products, we presume that the selectivity of the reac-
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tion of the N-pivaloyl morpholine is due to the steric bulk of
the B-C bond by reductive elimination from
tetrahydrofuranyliridium species.
a
3-
1
2
3
4
5
6
7
8
the pivalate substituent on the nitrogen atom suppressing reac-
tion at the position β to oxygen. The examples in Table 2 also
show that the borylation reaction is compatible with cyclic
ethers containing protected alcohol and amino functionalities
(Entries 7-8). Finally, the borylation of cyclohexane also oc-
curred for the first time, but this reaction required higher tem-
peratures and catalyst loadings and occurred in lower yield
(Entry 9).
To distinguish between these two pathways, a deuterium label-
ing experiment was performed. The borylation of 2,2,5,5-
tetradeuterotetrahydrofuran (THF-d4) was conducted in the
presence of (η6-mes)IrBpin3 and Me4phen (Scheme 2). No loss
of deuterium in the borylated product was detected by mass
spectrometry, and no incorporation of a proton into the 2-
1
position was detected by H NMR spectroscopy. Moreover,
A one-pot conversion of secondary C-H bonds to further de-
rivatives can be important for the isolation of stable products
and utility of the products. Thus, we developed conditions for
the conversion of the tetrahydrofuranylboronate ester to the
corresponding trifluoroborate, boronic acid, and alcohol prod-
ucts (Scheme 1). The reaction of THF and B2pin2 in the pres-
ence of (η6-mes)IrBpin3 (4 mol %) and Me4phen (4 mol %) to
generate the 3-boryltetrahydrofuran product, followed by addi-
tion of an aqueous solution of KHF2 in methanol solvent to the
crude boronate ester led to the trifluoborate salt in 63% yield.
The 3-tetrahydrofuryl boronic ester product was also convert-
ed in situ to the corresponding boronic acid. 3-
Tetrahydrofuranylboronic acid was isolated in 76% yield over
two steps by treating the boronate ester with NaIO4 and HCl
(aq).
HBpin, not DBpin, was observed as the side product by
11BNMR spectroscopy. These observations are consistent with
a direct C-H activation at the 3-position, not by initial C-H
activation at the 2-position followed by isomerization to a 3-
tetrahydrofuranyliridium species.
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Scheme 2. Potential pathways for C-H functionalization.
D
D
O
D
N
O
D
D
D
H
via
Ir
+
DBpin
N
Bpin
Bpin
Bpin
D
not observed
D
O
O
D
D
D
D
cat
C-H Activation at 2-position
D
D
N
B2pin2
via
Ir
Bpin
These products were also converted to alcohols under standard
conditions. 3-Hydroxytetrahydrofuran was generated in a
straightforward fashion by treating the crude boronate ester
with aqueous NaBO3. Because the corresponding 3-
hydroxytetrahydrofuran is volatile, the alcohol was isolated as
the benzoate ester in 73% overall yield. Efforts to identify
catalysts for the coupling of these secondary boronate deriva-
tives are ongoing.
N
Bpin
O
D
D
D
D
+
HBpin
cat = (η6-mes)Ir(Bpin)3/Me4phen
Bpin
C-H Activation at 3-position
The rate constants for the Ir-catalyzed C-H borylation of THF
were measured independently, and a primary kinetic isotope
effect of 2.6 was observed for the reaction of THF-d8 vs THF
(Scheme 3). However, a kinetic isotope effect was not ob-
served for the borylation of 2,2,5,5-THF-d4 vs THF. These
data are further inconsistent with an initial C-H cleavage at the
2-position followed by isomerization.
Scheme 1. Functionalizations of 3-boryltetrahydrofuran
O
a, b
Scheme 3. Measurement of the kinetic isotope effect.
BF3K
O
O
O
O
O
O
O
a, c
a, d
+
B
B
(η6-mes)Ir(Bpin)3 (4 mol %)
Me4phen (4 mol %)
OBz
H
O
O
B2pin2
1 equiv
+
Dn
B2pin2
neat, 100 ºC
Dn
Bpin
B(OH)2
O
O
O
D
D
D
D
Conditions: a. B2pin2 (0.50 mmol), (η6-mes)IrBpin3/Me4phen
(4 mol %), neat, 120 ºC, 14 h; b. KHF2 (aq, 4.5 M, 4.5 equiv),
MeOH, 63% yield; c. NaBO3•4H2O, THF: H2O; BzCl, Et3N,
DMAP, DCM, 74% yield; d. NaIO4 (1.5 equiv), HCl (aq, 1M, 1
equiv) 76% yield.
D8
2.6
1.0
2.5
Relative Rate
Scheme 4. Competition experiments for the borylation of
cyclic ethers
The high β-selectivity observed in the iridium-catalyzed
borylation of cyclic ethers could result from two general
pathways (Scheme 2). One pathway involves by direct func-
O
O
O
O
Ir cat, B2pin2
+
+
neat, 120 ºC, 14 h
( )
( )
n
n
Bpin
H
Bpin
H
tionalization of the
β
C-H bond through
a
3-
tetrahydrofuranyliridium boryl complex and one by the more
common activation of the α C-H bond, followed by isomeriza-
tion of the 2-tetrahydrofuranyl complex to the same 3-
tetrahydrofuranyl complex. The direct functionalization path-
way is shown as at the bottom of Scheme 2, and the indirect
pathway is shown at the top of Scheme 2. Both pathways form
O
O
O
O
O
Me
Me
Bpin
Bpin
Bpin
Bpin
ratio of productsa
1:4.6
1.3:1
2.2:1
2.4:1
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a
The ratio of product yields for the borylation of THF com-
AUTHOR INFORMATION
1
pared to other cyclic ethers was determined by GC analysis.
Corresponding Author
2
3
4
5
6
7
8
9
To determine the relative rates for borylation of cyclic ethers
with varying structures, we conducted competition experi-
ments. The relative ratios of products from the borylation of
tetrahydrofuran versus cyclic ethers containing different ring
sizes and substitution patterns are shown in Scheme 4. The
borylation of the five-membered cyclic ether occurs preferen-
tially over the borylation of six- or seven-membered cyclic
ethers. This trend correlates with the slightly higher Lewis
basicity of the oxygen atom of the five-membered cyclic ether,
compared to that of the oxygen in the six- and seven-
membered cyclic ethers.17,18 Also, the borylation of THF oc-
jhartwig@berkeley.edu
ACKNOWLEDGMENT
We thank the NSF (CHE-1156496) for support of this work,
Johnson Matthey for [Ir(COD)OMe]2, and [Ir(COD)Cl]2, and Al-
lyChem and BASF for B2pin2. C.W.L. thanks Abbott Laboratories
and the NSF graduate research fellowship program for predoctoral
fellowships.
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curred
faster
than
the
borylation
of
2,2-
dimethyltetrahydrofuran, but by only a 2.4:1 ratio. This ratio is
again consistent with the greater Lewis basicity of the oxygen
atom in THF towards Lewis acids compared to 2,2-
dimethyltetrahydrofuran. Finally, the borylation occurs prefer-
entially with dioxane over THF by a factor of 4.6:1. Although
dioxane is less Lewis basic than THF, the C-H bonds in diox-
ane are weaker than the β C-H bonds in THF. Based on these
data for the labeling and competition experiments, we tenta-
tively propose the high β-selectivity of secondary C-H boryla-
tion arises from the coordination of the Lewis basic oxygen
atom of the cyclic ether to the Lewis acidic boryl ligand to
form a 6-membered transition state (Figure 1).
7) Lawrence, J. D.; Takahashi, M.; Bae, C.; Hartwig, J. F. J.
Am. Chem. Soc. 2004, 126, 15334.
8) Murphy, J. M.; Lawrence, J. D.; Kawamura, K.; Incarvito,
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Carmona, E. Acc. Chem. Res. 2010, 43, 572.
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13) (a) Liskey, C. W.; Wei, C. S.; Pahls, D. R.; Hartwig, J. F.
Chem. Commun. 2009, 5603. For studies on the role of the
basicity of a boryl group, see: (b) Chotana, G. A.; Vanchura,
B. A.; Tse, M. K.; Staples, R. J.; Maleczka, R. E.; Smith, M.
R. Chem. Commun. 2009, 5731. (c) Vanchura, B. A.;
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R. J.; Maleczka, R. E.; Singleton, D. A.; Smith, M. R. Chem.
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14) See Supporting Information for details of the ligands
examined.
15) Clauti, G.; Zassinovich, G.; Mestroni, G. Inorg. Chim. Acta
1986, 112, 103.
H
O
O
H
Bpin
Bpin
Bpin
Bpin
N
N
N
N
Ir
Ir
Bpin
Bpin
Figure 1. Proposed origin of selectivity in secondary C-H boryla-
tion.
In summary, we have identified an Ir complex that catalyzes
the borylation of secondary C-H bonds for the first time in
good yield. Moreover, this C-H borylation reaction occurs
with unique selectivity for the 3-position of cyclic ethers. A
series of isotopic labeling experiments are consistent with high
selectivity for direct C-H cleavage at the 3-position, rather
than initial cleavage of the weaker C2 C-H bond, followed by
isomerization. Computational studies are currently underway
to help elucidate the details of the mechanism and origin of
selectivity of this reaction.
16) This combination of ligand and iridium precursor also led to
an increase in the scope of the silylation of C-H bonds, such
that it encompassed the directed silylation of unactivated
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Linkage; Patai, S., Ed.; Wiley and Sons: 1967, 243.
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1998, 925.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and spectra for all new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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TOC Graphic
1
2
3
4
5
6
(η6-mes)Ir(Bpin)3
Me4phen
X
X
O
O
O
O
+
R
B B
R
( )
n Y
( )
n Y
H
Bpin
B2pin2
1 equiv
n = 0, 1, 2
X = O, CH2
Y = CH2, O, NPiv
7
8
9
10
11
12
13
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ACS Paragon Plus Environment