Boc groups as protectors and directors for ir-catalyzed C-H borylation of heterocycles

Article abstract of DOI:10.1021/jo901822b

(Chemical Equation Presented) Ir-catalyzed C-H borylation is found to be compatible with Boc protecting groups. Thus, pyrroles, indoles, and azaindoles can be selectively functionalized at C-H positions β to N. The Boc group can be removed on thermolysis

Full text of DOI:10.1021/jo901822b

pubs.acs.org/joc
CHART 1. Borylation Regioselectivities for (a) Unprotected
Pyrrole, Indole, and (b) Pyridine
Boc Groups as Protectors and Directors for
Ir-Catalyzed C-H Borylation of Heterocycles
Venkata A. Kallepalli, Feng Shi, Sulagna Paul,
Edith N. Onyeozili, Robert E. Maleczka Jr.,* and
Milton R. Smith III*
Department of Chemistry, Michigan State University, East
Lansing, Michigan 48824-1322
unprotected pyrrole. Unfortunately, trimethylsilyl protec-
tion, the more economical alternative, was impractical as the
N-SiMe3 bond is prone to hydrolysis. For general synthetic
utility, we sought an economical, robust, yet readily remov-
able protecting group to impart regioselectivity that TIPS
protection provided. The compatibility of amides in aro-
matic borylations suggested that tert-butoxycarbonyl (Boc)
protecting groups might be inert to the borylation condi-
tions. Indeed, Gaunt and co-workers borylated N-Boc-
pyrrole under microwave conditions,3a and Shibasaki de-
scribed the borylation of an N-Boc-protected aniline.3b We
sought to establish Boc compatibility beyond these limited
examples, including in the Ir-catalyzed borylation of a Boc-
protected amino acid.4 Furthermore, we wanted to establish
Boc removal protocols that would leave the boryl intact.
N-Boc-pyrrole was a logical starting substrate for compar-
ing Boc and TIPS protecting groups. We were pleased to find
that borylation proceeded smoothly with effectively com-
plete regioselectivity for the 3-position (Table 1, entry 1).
The yields are reproducible and scale reasonably well. For
example, 100 g of the pyrrole and 1.25 equiv of pinacolbo-
rane (HBPin) afford product in 85% yield using an Ir
catalyst loading of 0.5 mol %.
N-Boc compatibility is reasonably general as indicated by
the other entries in Table 1. In addition to substituted pyrrole
(entries 2 and 3), N-Boc-indole (entry 4) and N-Boc-7-azain-
dole (entry 5) afford acceptable yields of 3-borylated pro-
ducts. The outcome for N-Boc-7-azaindole reflects a
preference for the 3-position of a five-membered nitrogen
heterocycle over sterically accessible sites in the six-mem-
bered N-heterocyclic moiety. A second borylation of N-Boc-
7-azaindole proceeds selectively at the 5-position (entry 6),
presumably because C5 is less hindered than C4.4
maleczka@chemistry.msu.edu; smithmil@msu.edu
Received August 28, 2009
Ir-catalyzed C-H borylation is found to be compatible
with Boc protecting groups. Thus, pyrroles, indoles, and
azaindoles can be selectively functionalized at C-H
positions β to N. The Boc group can be removed on
thermolysis or left intact during subsequent transforma-
tions.
Ir-catalyzed borylation of C-H bonds is emerging as a
new methodology for functionalizing aromatic and hetero-
aromatic hydrocarbons.1 For aromatic substrates, steric
effects dictate the regioselectivity, giving access to regio-
chemistry that is difficult to obtain using traditional syn-
thetic methods. While for heterocyclic substrates the origins
of regioselectivity are less apparent, it has been shown that
borylation of pyrroles and indoles occurs adjacent to the
heteroatom (Chart 1).1
We had previously shown that the borylation regioselec-
tivity for pyrrole can be shifted to the 3-position if the
nitrogen is protected with a triisopropysilyl (TIPS) group.2
Removal of the TIPS group provided a regioisomer that is
the complement to the product obtained from borylation of
The yield for N-Boc-6-azaindole was low, and the N-Boc-
imidazole reacted slowly (entry 8). In the latter case, rate
diminution from N3 coordination to Ir is compounded by
the fact that borylations adjacent to sp2-hybridized N are
difficult. For N-Boc-imidazole, extensive decomposition
occurred on workup. A stable imidazole analogue can be
isolated in good yield if the more robust dimethylsulfona-
mide protecting group is used (entry 9). Entry 10 shows that
N-Boc-pyrazole affords the 4-borylated product, whereas
borylation of N-methylpyrazole gives the 5-borylated isomer
as the major species.5
(1) (a) Cho, J. Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E. Jr.; Smith,
M. R. III. Science 2002, 295, 305–308. (b) Ishiyama, T.; Takagi, J.; Hartwig,
J. F.; Miyaura, N. Angew. Chem., Int. Ed. 2002, 41, 3056–3058. (c) Ishiyama,
T.; Takagi, J.; Yonekawa, Y.; Hartwig, J. F.; Miyaura, N. Adv. Synth. Catal.
2003, 345, 1103–1106. (d) Maleczka, R. E. Jr.; Shi, F.; Holmes, D.; Smith,
M. R. III. J. Am. Chem. Soc. 2003, 125, 7792–7793. (e) Mkhalid, I. A. I.;
Coventry, D. N.; Albesa-Jove, D.; Batsanov, A. S.; Howard, J. A. K.; Perutz,
R. N.; Marder, T. B. Angew. Chem., Int. Ed. 2006, 45, 489–491. (f) Kikuchi,
T.; Nobuta, Y.; Umeda, J.; Yamamoto, Y.; Ishiyama, T.; Miyaura, N.
Tetrahedron 2008, 64, 4967–4971.
(4) After this paper was prepared, Marder and Steel reported microwave-
accelerated iridium-catalyzed borylations of two Boc-protected heterocycles
and one example of a one-pot cross-coupling reaction: Harrisson, P.; Morris,
J.; Marder, T. B.; Steel, P. G. Org. Lett. 2009, 11, 3586–3589.
(5) Smith, M. R., III; Maleczka, R. E., Jr.; Kallepalli, V.; Onyeozili, E.
U.S. Patent Application 2008-0091027, April 17, 2008.
(2) Tse, M. K.; Cho, J. Y.; Smith, M. R. III. Org. Lett. 2001, 3, 2831–2833.
DOI: 10.1021/jo901822b
r
Published on Web 11/06/2009
J. Org. Chem. 2009, 74, 9199–9201 9199
2009 American Chemical Society

Kallepalli et al.
JOCNote
TABLE 1. Borylation of N-Boc-Protected Heterocycles and L-Tryp-
the N-Boc-protected substrates in one-pot processes, the
borylation of N-Boc-pyrrole was followed by the removal
of volatiles. Without further purification, the cross-coupling
of 1 with 3-chlorothiophene afforded biheterocycle 2 in 76%
tophana
SCHEME 1. One-Pot Borylation/C-C Cross-Coupling of
N-Boc-Pyrrole with 3-Chlorothiophene
isolated yield after 48 h (Scheme 1). Several aspects of this
sequence deserve further comment. Compound 2 was pre-
viously prepared by Buchwald and Billingsley, whose coup-
ling of purified 1 with 3-chlorothiophene afforded 2 in 51%
yield after 12 h.7 While our one-pot yield of 21% after 12 h
was considerably lower than 51%, the real advantage of the
C-H borylation/cross-coupling approach is that 1 is pre-
pared in a single step from N-Boc-pyrrole. In contrast,
synthesis of 1 from pyrrole using standard chemistry requires
multiple steps and protective group swapping.7 As such, the
total synthesis of 2 from pyrrole via conventional methods
proceeds in only 15% overall yield. On the other hand, the
one-pot C-H borylation/Suzuki-based approach allows for
the transformation of pyrrole to 2 in far fewer steps and 72%
overall yield.8
Of course a protective group must be removable. In this
regard, Boc group deprotections that leave the C-B bond of
borylated heterocycles intact are unprecedented. Thus,
deprotection of 1 was investigated. Attempts to remove the
Boc group with HCl or TFA resulted in unidentifiable
decomposition products, whereas TBAF was ineffective in
deprotection. Treatment with NaOMe yielded 42% of the
desired product on 0.5 mmol scale, but at 4.0 mmol, yields
varied significantly. In contrast, the Boc groups of 1 and all
other heterocycles (except the azaindole products) could be
cleaved thermally9 (Table 2) at scales up to 10 mmol. This
reagent-free deprotection is not only economical but also in
keeping with principles of green chemistry 1 and 8, namely,
preventing waste and avoiding solvent use.10 Significantly,
the products in Table 2 are regioisomers of the compounds
obtained by borylating the unprotected heterocycles. Of the
compounds in Table 2, those in entries 1, 4, and 5 have been
described in the primary literature, and entries 2 and 3 are
new.6
b
aSee Supporting Information for details. With 3.5 equiv of HBPin
c
used. With 3.0 mol % of [Ir(OMe)(COD)]2 and 6.0 mol % of dtbpy
used. dApproximately 90% conversion achieved, but the product
decomposed on attempted isolation. eWith 1.0 equiv of B2Pin2 used as
the borylating agent. fWith 31% of starting material recovered.
N-Boc amino acids are a very important class of Boc-
protected compounds for consideration. As shown in
Table 1, the indole nucleus of protected tryptophan can
be monoborylated. While preparation of the monobory-
lated compound (entry 11) was complicated by competing
diborylation, the pure monoborylated compound could be
obtained with no loss of stereochemistry in 43% yield
with 31% of the starting material recovered. Stereospecifi-
city was assayed by borylating D and L isomers of N-Boc
tryptophan methyl ester in separate experiments. In both
cases, the opposite enantiomer was not detected by
chiral HPLC analysis. To the best of our knowledge, this
entry represents the first example of protected R-amino acid
C-H borylation. Of the compounds in Table 1, those
in entries 1 and 6 have been described in the primary
literature, entries 4, 5, 7, 9, and 10 appear in patents, and
entries 2, 3, 8, and 11 are new.6
In summary, compatibility with Boc protecting groups
allows for manipulating the regioselectivities for Ir-cata-
lyzed borylations of nitrogen heterocycles. In addition,
(7) Billingsley, K.; Buchwald, S. L. J. Am. Chem. Soc. 2007, 129, 3358–
3366.
(8) N-Boc-pyrrole is prepared in 95% yield from pyrrole: Salman, H.;
Abraham, Y.; Tal, S.; Meltzman, S.; Kapon, M.; Tessler, N.; Speiser, S.;
Eichen, Y. Eur. J. Org. Chem. 2005, 2207–2212.
We, and others, have developed one-pot processes where
Ir-catalyzed borylations are followed by one or more chemi-
cal transformations.1a,c-f To assess the potential for using
(9) Rawal, V. H.; Cava, M. P. Tetrahedron Lett. 1985, 26, 6141–6142.
(10) http://www.epa.gov/greenchemistry/pubs/principles.html (accessed
Aug 2009).
(6) As per a search of the CAS database with SciFinder Scholar on
October 8, 2009.
9200 J. Org. Chem. Vol. 74, No. 23, 2009

Kallepalli et al.
JOCNote
TABLE 2. Thermal Deprotection of Selected N-Boc-Protected
completion of the reaction, the volatile materials were removed
on a rotary evaporator. The crude material was purified by
column chromatography or dissolved in CH2Cl2 and passed
through a plug of silica. Evaporation of solvent afforded the
product.
Borylation Products from Table 1a
N-Boc-3-(4,4,5,5-tetramethyl-1,3,2-dioxaboryl)pyrrole. White
solid in 90% yield; 1H NMR (CDCl3, 500 MHz) δ 7.61 (t, J=1.7
Hz, 1 H), 7.23 (dd, J=3.2, 2.1 Hz, 1H), 6.44 (dd, J=3.2, 1.5 Hz, 1
H), 1.56 (br s, 9 H), 1.30 (br s, 12 H); 13C NMR {1H} (CDCl3,
125 MHz) δ 148.6 (CdO), 128.8 (CH), 120.7 (CH), 116.2 (CH),
83.8 (C), 83.3 (C), 28.0 (3 CH3 of tBu), 24.8 (4 CH3 of BPin); 11
B
NMR (CDCl3, 96 MHz) δ 30.2; FT-IR (neat) νmax 3150, 2980,
2934, 1748, 1563, 1491, 1372, 1329, 1292, 1217, 1183, 1144, 1067,
976, 936, 857, 775, 691 cm-1; GC-MS (EI) m/z (% relative
intensity) Mþ 293 (13), 237 (55), 194 (39), 193 (35), 178 (76), 107
(100), 57 (14). Anal. Calcd for C15H24BNO4: C, 61.45; H, 8.25;
N, 4.78. Found: C, 61.68; H, 8.53; N, 4.70.
General Procedure for Boc Deprotection. A Schlenk flask,
equipped with a magnetic stirring bar, was charged with the
substrate and heated in air at specified temperature until bub-
bling ceased. The crude material was dissolved in CH2Cl2 and
passed through a plug of silica. Evaporation of solvent afforded
the product.
aN-Boc-protected substrates were placed in a flask and heated in air.
Ir-catalyzed borylations of protected tryptophan are shown
to be feasible for the first time, which augurs favorably for
similar functionalizations of peptides. Importantly, this
work also establishes heat as a clean agent for Boc depro-
tection of BPin-substituted heteroarenes.
3-(4,4,5,5-Tetramethyl-1,3,2-dioxaboryl)pyrrole. White solid
1
in 80% yield; H NMR (CDCl3, 500 MHz) δ 8.61 (br s, 1 H),
7.23 (ddd, J=1.5, 1.7, 2.7 Hz, 1 H), 6.82 (dd, J=1.7, 2.5 Hz, 1 H),
6.55 (ddd, J=1.5, 2.5, 2.6 Hz, 1 H), 1.31 (br s, 12 H); 13C NMR
{1H} (CDCl3, 125 MHz) δ 127.0 (CH), 118.6 (CH), 113.8 (CH),
82.9 (C), 24.8 (4 CH3 of BPin); 11B NMR (CDCl3, 96 MHz) δ
30.6; FT-IR (neat) νmax 3372, 3121, 2980, 2930, 1549, 1495,
1429, 1418, 1383, 1371, 1318, 1291, 1165, 1140, 1107, 966, 930,
860, 737, 691, 592 cm-1; GC-MS (EI) m/z (% relative intensity)
Mþ 193 (100), 178 (20), 150 (9), 107 (21). Anal. Calcd for
C10H16BNO2: C, 62.22; H, 8.35; N, 7.26. Found: C, 62.46; H,
8.35; N, 7.35.
Experimental Section
General Procedure for C-H Borylation. The reaction was set
up in a glovebox, where a Schlenk flask, equipped with a
magnetic stirring bar, was charged with the corresponding
substrate (1 mmol, 1 equiv). Two separate test tubes were
charged with [Ir(OMe)(COD)]2 (10 mg, 0.015 mmol, 3 mol %
Ir) and dtbpy (8 mg, 0.03 mmol, 3 mol %). Excess HBPin (1.1-2
equiv) was added to the [Ir(OMe)(COD)]2 containing test tube.
Solvent (1 mL) was added to the dtbpy containing test tube in
order to dissolve the dtbpy. The dtbpy solution was then mixed
with the [Ir(OMe)(COD)]2 and HBPin mixture. After mixing for
1 min, the resulting solution was transferred to the Schlenk flask.
Additional solvent (2 ꢀ 1 mL) was used to wash the test tubes,
and the washings were transferred to the Schlenk flask. The flask
was stoppered, brought out of the glovebox, and attached to a
Schlenk line in a fume hood. The Schlenk flask was placed under
N2, and the reaction was carried out at the specified tempera-
ture. The reaction was monitored by GC FID/MS. After
Acknowledgment. We thank BASF, Inc. for gifts of
HBPin and B2Pin2, and the Michigan Economic Develop-
ment Corp., the NIH (GM63188 to M.R.S.), the ACS Green
Chemistry Institute’s Pharmaceutical Roundtable, and
Pfizer for kind financial support.
Supporting Information Available: Spectral data for all new
compounds, as well as general experimental procedures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
J. Org. Chem. Vol. 74, No. 23, 2009 9201