Synthesis of Secondary and Tertiary Alkylboranes via Formal Hydroboration of Terminal and 1,1-Disubstituted Alkenes

Article abstract of DOI:10.1021/acs.orglett.6b03090

Copper-catalyzed functionalization of terminal or 1,1-disubstituted alkenes with bis(pinacolato)diboron and methanol provides formal hydroboration products with exceptional regiocontrol favoring the branched isomer. Pairing this procedure with photocataly

Full text of DOI:10.1021/acs.orglett.6b03090

Letter
pubs.acs.org/OrgLett
Synthesis of Secondary and Tertiary Alkylboranes via Formal
Hydroboration of Terminal and 1,1-Disubstituted Alkenes
Hilary A. Kerchner and John Montgomery*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055, United States
S
* Supporting Information
ABSTRACT: Copper-catalyzed functionalization of terminal
or 1,1-disubstituted alkenes with bis(pinacolato)diboron and
methanol provides formal hydroboration products with
exceptional regiocontrol favoring the branched isomer. Pairing
this procedure with photocatalytic cross-couplings using
iridium and nickel cocatalysis provides an effective, highly
regioselective procedure for the hydroarylation of terminal alkenes.
he hydroboration of simple terminal alkenes is a
transformation of fundamental importance in synthesis,
hydroboration of terminal alkynes with regioreversal compared
with traditional strategies,11 whereas styrenes afford the
terminal borane product.11b The procedure, which involves
borylmetalation followed by protonation of the resulting Cu−C
bond, provides considerable utility as a means to access the
more hindered alkenylborane products. Secondary alkylbor-
anes, such as those that potentially could be made by a branch-
selective hydroboration of terminal alkenes, are typically
accessed by methods such as addition of organolithium or
organomagnesium reagents to boron electrophiles,12 conver-
sion of alkyl halides to alkylboranes,13 hydrogenations of
vinylboranes,14 or variations of the Matteson rearrangement,
which has proven to be immensely useful in asymmetric
versions (Scheme 1).15 Impressive cascade functionalizations of
T
and the hydroboration/oxidation sequence serves as the
introduction to anti-Markovnikov regioselectivity in most
introductory organic chemistry courses.1 Despite the long
history of this process, a general approach to reversing the
regioselectivity of hydroborations of simple alkenes bearing
aliphatic substituents has remained elusive. Coupled with
oxidation of the resulting organoborane, this outcome would
provide a process equivalent to the acid-catalyzed hydration of
alkenes with accompanying advantages in scope and functional
group tolerance. Additionally, we envisioned that other
processes such as metal-catalyzed cross-coupling, when paired
with a reversal of regiochemistry in hydroborations, would
provide a useful and simple method for the synthesis of tertiary
carbon frameworks through the two-step branch-selective
reductive union of alkenes and sp2-carbon electrophiles. In
view of the tremendous recent advances in cross-couplings of
secondary organoboranes,2−5 the development of more efficient
methods for accessing the requisite branched organoboranes
could find immediate utility.
Scheme 1. Approaches to Secondary Alkylboranes
Most catalytic hydroboration methods provide the linear
alkylborane product, often with efficiencies greatly exceeding
that of the direct thermal addition of boranes to alkenes.6
Direct access to branched boranes from the hydroboration of
terminal olefins is typically accompanied by chain walking,7
which often leads to terminal or benzylic boranes. Advances in
reversing the regiochemical outcome of hydroborations are
often accomplished through the installation of directing groups,
which can override normally expected preferences based on the
alkene substitution pattern.8 More recent advances illustrated
that subtle remote electronic biases can also be used as a handle
for regiocontrol.9 Alkenes such as styrenes provide an
electronic bias for the development of catalyst−substrate
interactions that enable regioselectivity reversals, with notable
illustrations in directly accessing benzylic boranes from the
rhodium-catalyzed hydroboration of styrenes.10 The copper-
catalyzed addition of bis(pinacolato)diboron (B2Pin2) in the
presence of methanol has been utilized by Hoveyda as a formal
terminal alkenes have enabled the preparation of secondary
alkylboranes in tandem with arylation or alkenylation of the
terminal alkene carbon,16 and reports of aminoboration and
carboboration were recently disclosed.17 Furthermore, a recent
approach from Ito involves the formal hydroboration of
terminal alkenes via a process catalyzed by copper-phosphine
complexes.18 While these methods have enabled many
Received: October 13, 2016
© XXXX American Chemical Society
A
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impressive advances in the synthesis and application of
secondary alkylboranes, a more general approach to the
hydroboration of a variety of terminal and 1,1-disubstituted
alkenes to provide a regiochemical outcome opposite that of
classical thermal additions of borohydrides to alkenes would
provide an important entry to branched alkylboranes for
utilization in synthesis. The development of a highly regio- and
branch-selective hydroboration process using a copper−NHC
catalyst is described herein, and the utility of the method with
photocatalytic cross-couplings3 is demonstrated.
Efforts to access branched alkylboranes from terminal alkenes
began with exploration of hydroborations using HBPin with Ni,
Pd, Pt, Co, and Rh catalysts and the N-heterocyclic carbene
ligands IMes, SIMes, IPr, and SIPr (Table 1, entries 1−7;
complex Cu(Cl)IPr with B2Pin2/MeOH (Table 1, entry 11)
provided an optimum outcome in terms of conversion,
regioselectivity, and reproducibility across various substrate
classes, and this protocol was thus selected for further study.
A range of simple terminal olefins were examined in the
regioselective formation of branched alkylborane products.
With 4-phenyl-1-butene and 1-octene, excellent yields and
regioselectivities favoring the branched isomer were observed
(compounds 2 and 3, Scheme 2). Branching at the allylic
a
Scheme 2. Scope of Branched Alkylborane Synthesis
Table 1. Optimization of Branched Alkylborane Synthesis
b
c
entry
catalyst
Pd2dba3
ligand
reagent
solvent
1a:1b (yield, %)
1
2
IMes
SIPr
IMes
SIPr
SIPr
SIPr
SIPr
IMes
SIPr
SIPr
−
HBPin
HBPin
HBPin
HBPin
HBPin
HBPin
HBPin
B2Pin2
B2Pin2
B2Pin2
B2Pin2
B2Pin2
B2Pin2
THF
17:83 (62)
21:79 (47)
21:79 (14)
13:87 (<5)
7:93 (74)
<2:98 (62)
<2:98 (33)
67:33 (3)
79:21 (48)
95:5 (52)
Pd2dba3
Ni(cod)2
Ni(cod)2
PtCl2
THF
3
THF
4
THF
5
THF
6
CoBr2
THF
7
Rh(PPh3)3Cl
CuCl
THF
8
THF
9
CuCl
THF
10
11
12
13
CuCl
CH3CN
CH3CN
CH3CN
CH3CN
d
Cu(Cl)IPr
Cu(OAc)2
CuBr2
92:8 (81 )
SIPr
93:7 (62)
96:4 (78)
SIPr
a
All of the experiments were conducted at rt for 2 h at 0.2 M under a
nitrogen atmosphere. Experiments with B2Pin2 also used CH3OH (2
b
equiv). B2Pin2 = bis(pinacolato)diboron. Ligand HCl salts were used
with KO-t-Bu; IMes·HCl = 1,3-bis(mesityl)imidazolium chloride;
SIPr·HCl = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium
chloride. IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene.
c
With the exception of entry 11, NMR yields using 1,3,5-
trimethoxybenzene as an internal standard are shown. Regioselectivity
ratios were determined on crude reaction mixtures. Isolated yield.
a
d
B2Pin2 = bis(pinacolato)diboron; IPr = 1,3-bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene. Regioselectivity ratios were determined
on crude reaction mixtures. Isolated yields are given. Diastereomeric
ratio = 1:1. The reaction was conducted in CH2Cl2 using NaO-t-Bu at
b
c
selected data). In all cases, the terminal alkylborane product
was favored, with Co- and Rh-catalyzed processes most strongly
favoring the linear products, and no more than a 21% yield of
the desired branched product was observed in any case. We
next examined additions using B2Pin2 with methanol and Cu(I)
catalysts given the success seen in regiochemistry reversals
involving terminal alkynes by Hoveyda with similar catalyst
systems.11 The combination of CuCl with IMes provided a
nearly 2:1 ratio of regioisomers favoring the branched isomer
1a (Table 1, entry 8), whereas the use of SIPr provided
excellent regioselectivity favoring 1a, albeit in modest yield
(Table 1, entry 9). A solvent screen illustrated that CH3CN
provided superior conversions compared with THF and other
common solvents, and a variety of Cu(I) and Cu(II)
precatalysts were therefore examined in CH3CN for yield
optimization (Table 1, entries 10−13). In cases where Cu(II)
precatalysts are effective, it is likely that a Cu(I) species
generated under the reaction conditions functions as the active
catalyst.19 Among the cases explored, the use of the preformed
rt.
position was tolerated (compound 4, Scheme 2). Other
tolerated functional groups included benzyl ethers, silyl ethers,
unprotected hydroxyls, pivalate esters, and bromoarenes
(compounds 5−9, Scheme 2). Allylbenzene was a highly
effective substrate, and no isomerization to the styrene or to the
corresponding benzylic borane product was noted (compound
10, Scheme 2). Notably, compound 10 was previously prepared
in highly enantioselective fashion from prop-1-en-1-ylbenzene
via Cu−NHC-catalyzed hydroboration.20 Examination of
similar chiral NHCs in the current procedure using
allylbenzene as the substrate led to poor enantioselectivities
(see the Supporting Information for details). Therefore, the
current procedure will be most useful for non-styrenyl
substrates in comparison with this alternative method that is
highly effective for styrene hydroborations. As additional
examples, allylsilanes were tolerated in the procedure, leading
B
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Letter
to 1,2-bismetalated products (compound 11, Scheme 2).
Indoles were also tolerated to afford products borylated at
the β- or γ-position of the N-alkyl chain (compounds 12 and
13, Scheme 2). To explore selectivity among different olefin
classes, hydroboration of a substrate possessing both a
trisubstituted alkene and a monosubstituted alkene afforded
complete selectivity for the terminal alkene (compound 14,
Scheme 2). Similarly, complete selectivity was seen for
hydroboration of the terminal alkene when a 1,1-disubstituted
alkene was present (compound 15, Scheme 2). While alkenes
that possess two or more substituents were generally
unreactive, 1,1-disubstituted alkenes could be converted to
tertiary borane products under modified conditions (com-
pounds 16 and 17, Scheme 2).21
terminal alkenes to tertiary branched motifs. Alkyl(BPin)
derivatives accessed by the catalytic, regiocontrolled addition
of B2Pin2 with methanol to terminal alkenes (Scheme 4) are
Scheme 4. Photocatalytic Cross-Couplings of Branched
a
Trifluoroalkylboranes
In analogy with other reports of copper-catalyzed additions
of B2Pin2 and methanol to alkynes and styrenes,11,17 the
mechanism of the process likely involves addition of a copper
borane intermediate to the terminal alkene as the regiochem-
istry-determining step, as depicted in the conversion of 18 to
19 in Scheme 3. Formation of the linear borane is disfavored as
Scheme 3. Mechanism of Secondary Alkylborane Formation
a
Isolated yields for the photocatalytic cross-coupling step are provided
in the table. Details of the synthesis of the RBF3 salts are provided in
the Supporting Information. dFCF3ppy = 2-(2,4-difluorophenyl)-5-
(trifluoromethyl)pyridine; dme = dimethoxyethane; dtbbpy =4,4′-di-
tert-butyl-2,2′-bipyridine.
readily converted to the corresponding trifluoroborate deriva-
tives3a,23 while maintaining the versatile functional group
tolerance of the hydroboration procedure (Scheme 4). The
resulting functionalized trifluoroborate derivatives then directly
participate in nickel-catalyzed cross-couplings in the presence of
iridium photocatalysts following the Molander protocol. Both
electron-rich and electron-deficient bromoarenes participate in
the sequence (21 and 22, Scheme 4). Notably, the functional
group tolerance demonstrated in the synthesis of branched
alkylboranes is carried through the trifluoroborate synthesis/
photocatalytic cross-coupling sequence to provide the preser-
vation of sensitive functional groups such as free alcohols,
esters, and indoles (23−25, Scheme 4).
In summary, the regiodivergent hydroboration of a broad
range of simple terminal alkenes may now be readily
accomplished.24 While a range of thermal or catalyzed additions
of boranes provide access to linear alkylboranes according to
previous reports, the copper-catalyzed addition of B2Pin2 with
methanol reported herein provides highly regioselective access
to the isomeric branched alkylboranes. This procedure
complements previous methods for accessing secondary
alkylborane structures, and the utilization of the obtained
products in nickel-catalyzed photocatalytic cross-couplings
provides a branch-selective strategy for the reductive cross-
coupling of alkenes and aryl bromides. Further exploration of
asymmetric versions of the formal hydroboration is in progress,
and utilization of the racemic organoboranes will be possible
with emerging developments in stereoconvergent cross-
couplings.3
the ligand size increases (compare Table 1, entries 8 and 9)
because of the developing steric interactions experienced in
complex 20a.22 Alternatively, complex 20b, which leads to the
observed branched product, avoids steric interactions between
the ligand and the alkene substituent. Following the sterically
preferred formation of 19, protonation of the metal−copper
bond produces the desired product along with the formation of
a copper methoxide species, which is converted to the reactive
Cu−BPin complex by the reaction with B2Pin2.
Recent advances in Suzuki couplings can benefit from the
facile entry to secondary alkylboranes provided by this
procedure.2,3 For example, pairing the nickel-catalyzed photo-
catalytic cross-coupling procedure recently developed by
Molander3b with the above developments provides a
convenient and versatile method for the direct conversion of
C
DOI: 10.1021/acs.orglett.6b03090
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Letter
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b03090.
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136, 9521.
Synthetic details and spectral data (PDF)
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AUTHOR INFORMATION
■
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Corresponding Author
*E-mail: jmontg@umich.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Institutes of Health
(R35GM118133) for support of this research. The Stephenson
group (University of Michigan) is thanked for helpful
discussions on the use of photocatalysis processes.
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