Efficient synthesis of diborylalkenes from alkenes and diboron by a new PSiP-pincer palladium-catalyzed dehydrogenative borylation

Article abstract of DOI:10.1021/ja205186k

The efficient synthesis of various diborylalkenes such as 1,1-, trans-1,2-, and cyclic 1,2-diborylalkenes from alkenes and diboron was achieved for the first time. Selective preparation of di- and monoborylalkenes was also realized by the appropriate choi

Full text of DOI:10.1021/ja205186k

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Efficient Synthesis of Diborylalkenes from Alkenes and Diboron by a
New PSiP-Pincer Palladium-Catalyzed Dehydrogenative Borylation
Jun Takaya, Naohiro Kirai, and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology and CREST-JST, O-okayama, Meguro-ku, Tokyo 152-8551, Japan
S Supporting Information
b
Table 1. Selective Synthesis of 1,1-Diboryl- and
Monoborylalkenes
ABSTRACT: The efficient synthesis of various diborylalk-
enes such as 1,1-, trans-1,2-, and cyclic 1,2-diborylalkenes
from alkenes and diboron was achieved for the first time.
Selective preparation of di- and monoborylalkenes was also
realized by the appropriate choice of reaction conditions.
The reaction was found to proceed via a new mechanism of
dehydrogenative borylation through a monoborylpalladium
complex bearing an anionic PSiP-pincer ligand as a key
intermediate, which realized the efficient borylation without
sacrificial hydroboration or hydrogenation of the alkene.
iborylalkenes have attracted much attention as versatile
D
building blocks in organic synthesis because these com-
pounds enable further multiple carbonÀcarbon bond formations
via Pd-catalyzed cross-coupling reactions, leading to facile stereo-
selective construction of π-conjugated materials and biologically
active compounds such as tamoxifen and polyene natural
products.1,2 Several methods for the preparation of these classes
of compounds have been reported, but most of them necessitate
multistep procedures along with the use of specific stoichio-
metric reagents.3À6 Probably the most general method is the Pt-
catalyzed diboration of alkynes, but only cis-1,2-diborylalkenes
are available by this method.7 Thus, it is highly desirable to
develop a concise method for the preparation of 1,1- and trans-
1,2-diborylalkenes starting from simple alkenes and diboron to
expand the utility of diborylalkenes.8 Herein we report a
highly efficient protocol for the synthesis of such molecules
through PSiP-pincer palladium-catalyzed double dehydrogena-
tive borylation of alkenes with diboron, by which a variety
of diborylalkenes can be prepared without sacrificial hydroboration
or hydrogenation of the alkene. The reaction involves a
monoborylpalladium(II) complex as the key catalytic species.
It was found that treatment of a 1:2 mixture of styrene and
B2pin2 with a 2.0 mol % loading of a PSiP-pincer palladium catalyst
with 3,5-bis(trifluoromethyl)phenyl groups on phosphorus (1a)
and AlEt3 in toluene at room temperature gave the 1,1-diboryla-
tion product, β,β-diborylstyrene 3a, in 90% yield (Table 1, entry
1).9À11 The same reaction with 1.0 equiv of B2pin2 selectively gave
the monoborylation product, (E)-styrylboronic ester 4a, in 92%
yield stereoselectively (entry 2). It should be noted that analysis of
the crude mixture by GCÀMS showed the formation of HBpin,
but neither hydroboration nor hydrogenation products were
observed at all. This selective synthesis of mono- or 1,1-dibor-
ylalkenes depending on the amount of the diboron was also
applicable to other electronically activated alkenes. Vinylferrocene
(2b) and N-vinylphthalimide (2c) gave the corresponding 1,
a Isolated yields are shown. b The monoborylation product 4c was
obtained in 3% yield.
1-diborylalkenes 3b and 3c in high yield with the use of 2 or 3
equiv of B2pin2 with a 5 mol % loading of the more active electron-
withdrawing PSiP-pincer catalyst 1b at 60 °C (entries 3 and 5),
whereas the reaction using 1.0 equiv of B2pin2 with catalyst 1a at
the same temperature afforded monoborylation products 4b and
4c in high yield with excellent E selectivity (entries 4 and 6).
Another novel aspect of this reaction was disclosed when
nonelectronically activated, sterically less demanding terminal
alkenes were employed as substrates. Thus, the reaction of
allyl(triphenyl)silane (2d) under diborylation conditions using
2 mol % catalyst 1b exhibited another mode of diborylation,
trans-1,2-diborylation, to give (Z)-1,2-diborylalkene 5d selec-
tively in 80% yield (Table 2, entry 1). The structure and Z
geometry of 5d were confirmed by X-ray analysis [see the
Supporting Information (SI)]. It should be noted that this
product is a highly useful module for multiple carbonÀcarbon
bond formations, and there has been no general method for the
synthesis of such trisubstituted trans-diborylated alkenes.
Furthermore, the monoborylation protocol using 1.0 equiv of
Received: June 9, 2011
Published: July 24, 2011
r
2011 American Chemical Society
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|

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Table 2. Selective Synthesis of trans-1,2-Diboryl- and
Scheme 1. 1,2-Diborylation of Cyclooctenea
Monoborylalkenes
a The monoborylated alkene (30%) was also obtained.
Scheme 2. Proposed Mechanism
are the first example of an efficient synthesis of trans-1,2-diboryl-
and cyclic 1,2-diborylalkenes from alkenes, and a selective route
to synthetically useful diboryl- and monoborylalkenes has also
been developed. Therefore, this new borylation reaction should
find abundant use in organic synthesis.
The reaction is thought to proceed through double dehydro-
genative borylation of the alkene in which the alkene reacts with a
monoborylpalladium(II) complex bearing an anionic PSiP-pin-
cer ligand as the key catalytic species, resulting in complete
suppression of sacrificial hydroboration or hydrogenation of the
alkene (Scheme 2). First, the triflate complex 1 undergoes
transmetalation with AlEt3 followed by β-hydride elimination
to generate monohydridopalladium complex 6,14 which reacts
with B2pin2 to give HBpin and monoborylpalladium complex 7
bearing the PSiP-pincer ligand. Borylpalladium 7 undergoes
alkene insertion and β-hydride elimination to give the mono-
borylation product 4 with regeneration of palladium hydride 6,
which immediately reacts with B2pin2 to regenerate the key
catalytic species, borylpalladium 7. A second borylation of 4 in
the presence of excess B2pin2 gives the diborylation product 3 or
5 with high regioselectivity depending on the kind of substituent
on the alkene. The predominant trans selectivity in 1,2-diboryla-
tion is reasonably explained by considering a syn-insertion/syn-
elimination mechanism via alkylpalladium(II) intermediate 10.15
Most of the previously reported dehydrogenative borylations of
alkenes with diboron or borane have been limited to monobor-
ylation, and sacrificial hydroboration and/or hydrogenation of
the alkene are inevitable problems caused by coordinatively
unsaturated hydrido(boryl)- or dihydridometal species (A or
B, respectively) generated by the reaction of the metal catalyst
with borane or dihydrogen produced during the reaction.16,17
Therefore, an excess amount of alkene is usually required.18,19
Szꢀabo recently reported dehydrogenative borylation of alkenes
under oxidative conditions involving a monoborylpalladium(IV)
complex as a proposed intermediate, but the reaction requires a
stoichiometric oxidant such as PhI(TFA)2 for the Pd(II)/Pd(IV)
catalytic cycle and excess alkene is still required.20 On the
a Isolated yields are shown, unless otherwise noted. b Determined by 1H
NMR analysis. c (Z)-4d was obtained as a mixture with a small amount of
1-propenyl(triphenyl)silane and some isomers. d Obtained as a mixture
with 5e (13%). e 5f (13%) was also obtained. f 5g (15%) was also
observed (NMR yield). g Obtained as a mixture with 4h (7%) and
dechlorinated product (1%). h 5h (3%) was also obtained. i NMR yield.
j In THF. k The reaction was carried out in the presence of 3.0 equiv of
alkene with respect to B2pin2, and the yield of 4j was based on B2pin2.
B2pin2 was also successful in this case, affording monoborylated
allylsilane 4d in good yield (entry 2). Various other terminal
alkenes also turned out to undergo the trans-1,2-diborylation.
Allyl(trimethyl)silane (2e) and allylcyclopentane (2f) afforded
(Z)-1,2-diborylalkenes 5e and 5f, respectively, in good yield with
good stereoselectivity when >2 equiv of B2pin2 was used (entries
3 and 5), whereas the use of 1.0 equiv of B2pin2 gave the
corresponding monoborylation products 4e and 4f in moderate
yield (entries 4 and 6). This reaction demonstrated good
compatibility with functional groups. Thus, silyl ether, chloro,
and acid anhydride moieties were not affected under the reaction
conditions, and functionalized (Z)-1,2-diborylalkenes 5gÀi and
alkenylboronic esters 4gÀi were obtained in moderate to good
yields under the appropriate conditions (entries 7À12). The 1,2-
diborylation of simple alkenes such as 1-octene also proceeded
without problem when the reaction was carried out in THF
(entry 13), although the monoborylation required the use of an
excess amount of the alkene to obtain alkenylboronic ester 4j in
high yield (entry 14).12 Furthermore, this 1,2-diborylation pro-
tocol was applicable to cyclooctene, giving cis-1,2-diborylcy-
clooctene 5k, which is not available by the Pt-catalyzed
diboration protocol, in good yield (Scheme 1).13 These results
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Scheme 3. Synthesis of a Monoborylpalladium Complex
Bearing an Anionic PSiP-Pincer Ligand and Its Reaction with
Styrene
palladium-catalyzed double dehydrogenative borylation. The
unprecedented mechanism involving a monoborylpalladium
complex bearing the PSiP-pincer ligand, leading to selective
reactions without sacrificial hydroboration nor hydrogenation,
has also been disclosed. This protocol provides the new possi-
bility of utilizing alkenylboronic esters as a module for multiple
carbonÀcarbon bond formation in synthetic organic chemistry.
’ ASSOCIATED CONTENT
S
Supporting Information. Preparative methods, spectral
b
and analytical data for compounds 1À5 and 7, and crystal-
lographic data (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
niwasawa@chem.titech.ac.jp
’ ACKNOWLEDGMENT
This research was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” (22105006) from the Minis-
try of Education, Culture, Sports, Science, and Technology
of Japan.
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Figure 1. ORTEP diagram of monoborylpalladium complex 7c bearing
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have been omitted for clarity). Selected bond lengths (Å) and angles
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