Palladium-Catalyzed Oxidative Borylation of Allylic C-H Bonds in Alkenes

Article abstract of DOI:10.1021/acs.orglett.7b03296

This communication describes an efficient palladium pincer complex-catalyzed allylic C-H borylation of alkenes. The transformation exhibits high regio- and stereoselectivity with a variety of linear alkenes. A synthetically useful feature of this allylic C-H borylation method is that all allyl-Bpin products can be isolated in usually high yields. Preliminary mechanistic studies indicate that this C-H borylation reaction proceeds via Pd(IV) pincer complex intermediates.

Full text of DOI:10.1021/acs.orglett.7b03296

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium-Catalyzed Oxidative Borylation of Allylic C−H Bonds in
Alkenes
Lujia Mao,† Rudiger Bertermann,† Simon G. Rachor,† Kalman J. Szabo,*,‡ and Todd B. Marder*,†
́
́
́
̈
†Institut fur Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universitat
̈
̈
Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
‡Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden
S
* Supporting Information
ABSTRACT: This communication describes an efficient
palladium pincer complex-catalyzed allylic C−H borylation of
alkenes. The transformation exhibits high regio- and stereo-
selectivity with a variety of linear alkenes. A synthetically useful
feature of this allylic C−H borylation method is that all allyl-
Bpin products can be isolated in usually high yields. Preliminary
mechanistic studies indicate that this C−H borylation reaction
proceeds via Pd(IV) pincer complex intermediates.
llyl boronates are important synthetic intermediates that
Scheme 1. Transition-Metal-Catalyzed Borylation of Allylic
A
react with aldehydes to afford stereodefined homoallylic
C−H Bonds
alcohols.1 Therefore, there is a great demand for efficient
methodologies to synthesize allyl boronates from readily
available substrates. Although borylations of allylic halides,2a−c
carbonates,2b,d−h ethers,2i and alcohols3 have been successfully
developed, they all require extra steps to prepare the
́
prefunctionalized starting materials. Szabo and co-workers
developed an iridium-catalyzed allylic borylation of cyclo-
alkenes, in which the selectivity was controlled by the addition
of 1,8-diazabicyclo[5.4.0]undecane (DBU) or methylimidazo-
le,4a,b and a Pd(TFA)2-catalyzed allylic borylation of exocyclic
alkenes via allyl-Pd(II) intermediates4c (Scheme 1). Recently,
Gong and co-workers reported a Pd-catalyzed allylation of
simple alkenes via oxidative C−H borylation, which mainly
focused on allylbenzene and its derivatives.5 A general problem
with these allylic C−H borylation methods is that the catalysts
react with the allyl-Bpin products, and therefore, they must be
trapped within the reaction system. Accordingly, several
methods for one-pot C−H borylation−allylboration reactions
have been reported. However, a more general allylic C−H
borylation process that would allow the isolation of the allyl-
Bpin products is still highly desirable.
Pd(IV) species as the key intermediate, in which the selectivity
is controlled by conformational factors.8a Marder and co-
workers reported catalytic borylations of both C−H and C−X
bonds,11 including the synthesis of allyl boronates.11j,k Herein
we present a Pd-catalyzed highly regio- and stereoselective
allylic C−H borylation of alkenes, the isolation of the allyl-Bpin
products, and also a one-pot route to homoallyl alcohols
directly from alkenes.
Palladium-catalyzed C−H bond functionalization reactions
have emerged as powerful strategies in organic synthesis,6 and
oxidative C−H borylation reactions have been reported by
several research groups,4c,5,7 in which Pd(II) species were
proposed to be generated in situ and act as the active catalysts.
Alternatively, C−H functionalization mediated by high-valent
palladium species with unique reactivity and selectivity8 was
also shown to be an efficient methodology to transform allylic
C−H bonds into C−O9 and C−Si10 bonds. To the best of our
knowledge, only the allylic C−H borylation of cycloheptene has
Received: October 23, 2017
́
been demonstrated by Szabo and co-workers, involving a
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03296
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b c
, ,
As palladium pincer complexes have proved to be highly
selective catalysts for many C−H functionalization reactions,6c
we applied these catalysts in the C−H borylation reaction. We
found that in the presence of F-TEDA-BF4 (Selectfluor) as an
oxidant, NCN pincer complex A is an effective catalyst
precursor for allylic C−H borylation, giving the desired product
2a in 83% isolated yield with excellent regio- and stereo-
chemistry (Table 1, entry 1). The reaction was scaled up to 1
Scheme 2. Substrate Scope I
a
Table 1. Optimization of Conditions
a
Standard conditions: 1 (0.2 mmol, 1 equiv), Pd−NCN complex A
(10 mol %), F-TEDA-BF4 (0.2 mmol, 1 equiv), K2CO3 (0.4 mmol, 2
equiv), B2pin2 (0.3 mmol, 1.5 equiv), MeNO2 (1 mL), 60 °C.
b
c
Isolated yields are shown. E/Z isomer ratios were determined by 1H
NMR analyses of the crude products.
variation from the standard
conditions
yield of 2a
conv.
(%)
b
c
entry
(%)
E:Z
d
for the allylic C−H borylation, and the corresponding products
2b and 2c were obtained in good yields of 84% and 79%,
respectively. A particular merit of this reaction is that the allyl
boronate products 2b and 2c (as well as the rest of the allyl-
Bpin products) could be isolated.15 As mentioned above, the
previously reported palladium (and iridium)-catalyzed C−H
borylation methods usually did not allow isolation of the allyl
boronate products.4,5 Exocyclic allyl boronates 2d and 2e were
obtained from the allylic C−H borylation in yields of 86% and
78%, respectively. With disubstituted alkenes 1f and 1g, the
desired allyl boronates 2f and 2g were obtained in yields of 83%
and 68%, respectively. The nonconjugated diene 1h was
borylated smoothly, and 2h was produced in 72% yield. With
allylbenzene 1i as the starting material, product 2i was formed
in a moderate yield of 69%. Allyl boronate 2j was obtained in
61% yield when allyl phthalimide 1j was employed as the
starting material. Cyclohexene 1k was also borylated smoothly
and provided the allylic product 2k in 71% yield.
1
2
no change
89 (83 )
50:1
42:1
−
−
43:1
100
49
B instead of A
C instead of A
D instead of A
42
24
15
82
e
3
100
100
100
f
4
5
F-TEDA-PF6 instead of
F-TEDA-BF4
g
6
without A
N.D.
−
−
−
48:1
35:1
41:1
39:1
6
77
17
84
59
65
46
h
g
7
without F-TEDA-BF4
without K2CO3
N.D.
8
9
14
79
54
59
39
Na2CO3 instead of K2CO3
KF instead of K2CO3
THF instead of MeNO2
CH2Cl2 instead of MeNO2
10
11
12
a
Standard conditions: Pd−NCN complex A (10 mol %), 1a (1 equiv),
K2CO3 (2 equiv), B2pin2 (1.5 equiv), F-TEDA-BF4 (1 equiv), MeNO2
(1 mL), 60 °C. All of the reactions were carried out on a 0.2 mmol
scale. Yields were determined by GC−MS analyses vs a calibrated
internal standard and are averages of two experiments. E/Z isomer
ratios were determined by H NMR analyses of the crude products.
Isolated yield. Yield of 3a = 67%. Yield of 3a = 79%. N.D. = not
b
c
1
α-Methyl allylbenzene 1l gave allyl boronate 2l in 71% yield
with the Z geometry at the CC bond (Scheme 3). With the
phenyl ring bearing Me, OMe, or F substituents (1m−q), the
d
e
f
g
h
detected. Yield of 3a = 73%.
mmol, and 2a was isolated in 79% yield. A much lower yield of
2a (42%) was obtained at ca. 49% conversion12 when we
replaced A with its bromide analogue B (Table 1, entry 2). The
yield of 2a dropped to 24% and 15%, and isomerization
product 3a was formed in yields of 67% and 79% using Se
pincer complexes C and D, respectively (Table 1, entries 3 and
4), possibly because they are less stable.13 F-TEDA-PF6 was
a b c
, ,
Scheme 3. Substrate Scope II
−
similar to the BF4 salt as an oxidant, and the desired product
2a was obtained in 82% yield (Table 1, entry 5).14 No desired
product 2a was obtained in the absence of a Pd catalyst or F-
TEDA-BF4 (Table 1, entries 6 and 7). Product 2a was formed
in only 14% yield without K2CO3 (Table 1, entry 8), but the
use of Na2CO3 as an alternative was successful (Table 1, entry
9), whereas KF was significantly less effective (Table 1, entry
10). THF and CH2Cl2 proved less effective as solvents than
MeNO2 (Table 1, entries 11 and 12). Additional screening
experiments are described in the Supporting Information.14
With the optimized conditions in hand, we next started to
investigate the scope of the reaction (Scheme 2). Simple
alkenes, such as 1-hexene (1b) and 1-pentene (1c), are suitable
a
Standard conditions: 1 (0.2 mmol, 1 equiv), Pd−NCN complex A
(10 mol %), F-TEDA-BF4 (0.2 mmol, 1 equiv), K2CO3 (0.4 mmol, 2
equiv), B2pin2 (0.3 mmol, 1.5 equiv), MeNO2 (1 mL), 60 °C.
b
c
Isolated yields are shown. E/Z isomer ratios were determined by 1H
NMR analyses of the crude products, and the stereochemistry at the
CC bond was determined by H,1H NOESY.
1
B
DOI: 10.1021/acs.orglett.7b03296
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
corresponding allyl boronates (2m−q) were obtained in good
yields of 70−79% with good stereoselectivity. When the phenyl
group was replaced by a naphthalenyl group, the desired
product 2r was isolated in 64% yield.
Scheme 5. Plausible Mechanism of the Allylic C−H
Borylation
As allyl boronates have significant applications in the
synthesis of homoallyl alcohols,1 we investigated the use of
our allylic C−H borylation of alkenes in a one-pot carbonyl
allylation reaction (see Schemes 4 and S1). Homoallyl alcohols
can thus be prepared directly from alkenes with high efficiency
and in high overall yields.
Scheme 4. Application in One-Pot Carbonyl Allylation
Reactions
We briefly studied the mechanistic features of the above Pd-
catalyzed allylic C−H borylation reactions. F-TEDA-BF4 is
reported to be capable of oxidizing Pd(II) to form a Pd(IV)−F
species.16 The observation of an 19F signal at −324 ppm is
consistent with the presence of a fluoride ligand coordinated to
Pd(IV). For example, Sanford and co-workers reported an 19F
shift at −324 ppm for a Pd(IV)-F species.17 In addition, NCN
pincer complexes are known to form Pd(IV) complexes. For
C−H borylation reaction is that the allyl-Bpin products can be
isolated in high yields. This is probably a consequence of the
application of pincer complex A as the catalyst, which
selectively catalyzes C−B bond formation while avoiding
subsequent C−B bond cleavage-based side reactions. In
previous Pd-catalyzed C−H borylation reactions with non-
pincer complex catalyst sources (e.g., Pd(OAc)2), the reactions
stopped because of product inhibition, i.e., reaction of the allyl-
Bpin product with the Pd catalyst.4c Another interesting and
fortunate feature of our catalyst system is that F-TEDA-BF4 is
able to oxidize Pd(II) to Pd(IV) in the pincer complex but does
not oxidize boron in either the diboron reagent or the allyl
boronate product.
example, the groups of Canty18 and Szabo8a
́
reported oxidation
of Pd(II)−NCN complexes to their Pd(IV) analogues using
strong oxidants, such as hypervalent iodine reagents. Indeed,
1
when we monitored the change of the H NMR spectrum of
NCN pincer complex A upon addition of F-TEDA-BF4, we
1
observed systematic changes in the H NMR signals character-
istic of the increase in coordination number from square-planar
Pd(II) to a Pd(IV) species with a square-pyramidal or
octahedral geometry (p S22 in the Supporting Information).
Therefore, we suggest that in the initial step, NCN pincer
complex A is oxidized by F-TEDA-BF4 to generate Pd(IV)−F
species I containing a vacant coordination site (Scheme 5).
Then alkene 1 undergoes C−H activation to form η1-allyl
complex II. The C−H activation step probably proceeds by a
concerted metalation−deprotonation-type mechanism19 in
which the coordinated acetate assists in the deprotonation of
1. Subsequently, B2pin2 undergoes transmetalation with
Pd(IV)−F to give III, facilitated by the formation of F-Bpin.
Finally, C−B reductive elimination gives product 2 and
regenerates the catalyst. A possible explanation of the surprising
Z selectivity of the product formation is that the reaction
involves η1-allyl palladium complexes (II and III), which are
unable to undergo isomerization to give the thermodynamically
more stable E product.20
In conclusion, we have developed an efficient method to
synthesize and isolate allyl boronates that exhibits high regio-
and stereoselectivity with a variety of alkenes and has been
extended to a one-pot carbonyl allylation reaction that proceeds
in high overall yields. An interesting mechanistic feature is that
the reaction proceeds via a Pd(II)/Pd(IV) catalytic cycle.
Formation of the Pd(IV) intermediate occurs by a unique
combination of NCN incer complex A and application of F-
TEDA-BF4 as the oxidant. An important novelty of the present
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.7b03296.
Experimental procedures and compound characterization
data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
́
*E-mail: kalman@organ.su.se (K. J. Szabo).
*E-mail: todd.marder@uni-wuerzburg.de (T. B. Marder).
ORCID
Todd B. Marder: 0000-0002-9990-0169
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
L.M. thanks the China Scholarship Council for a Ph.D.
fellowship. K.J.S. thanks the Swedish Research Council and the
Knut and Alice Wallenberg Foundation for financial support.
We thank AllylChem Co. Ltd. for a generous gift of B2pin2.
C
DOI: 10.1021/acs.orglett.7b03296
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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́
́
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