Diboration of 3-substituted propargylic alcohols using a bimetallic catalyst system: access to (Z)-allyl, vinyldiboronates

Article abstract of DOI:10.1039/d0cc03563g

The diboration of substituted propargylic alcohols has been achieved using a bimetallic Pd/Cu catalyst system. Thein situformation of a pentrafluoroboronic acid intermediate sufficiently activates the C-O bond towards dual catalysis affording (Z)-allyl, vinyldiboronates stereoselectively.

Full text of DOI:10.1039/d0cc03563g

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Diboration of 3-Substituted Propargylic Alcohols using a Bimetallic
Catalyst System: Access to (Z)-Allyl, Vinyldiboronates†
Cheryl L. Peck, Jan Nekvinda and Webster L. Santos*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Figure 1 Transition metal-catalyzed borylations of allene and propargylic alcohol
The diboration of substituted propargylic alcohols has been
achieved using bimetallic Pd/Cu catalyst system. In situ
formation of a pentrafluoroboronic acid intermediate sufficiently
activates the C-O bond towards dual catalysis affording (Z)-allyl,
vinyldiboronates stereoselectively.
a
R2
R3
R1
R2
Bpin
R2
R3
OCO2Me
[Pd/Cu or Ag] or
[Cu], B2pin2
R1
(a)
or
pinB
•
R1
R3
Bu
The formation of highly decorated organoboron
compounds are essential in complex molecule synthesis,
medicinal chemistry, and material science.1 Numerous
methods for the transition metal-catalyzed for diborylation of
alkynes, which are useful precursors to access high value α- or
β-vinyl boronic acid derivatives have been reported.2 However,
methods employing propargylic alcohol derived substrates are
limited.3 Several years ago, elegant work by Szabό
demonstrated that activation of propargylic alcohols with a
carbonate leaving group can alternately afford propargylic or
allenyl boronates (Figure 1a).4 This maneuver allowed for
product formation to proceed stereospecifically through an
SN2 or formal SN2’ pathway under Pd/Cu or Pd/Ag bimetallic
catalytic conditions. Unfortunately, other activating groups
were shown to be less effective, thereby limiting the
application of this method. Ito and Sawamura reported an
alternative protocol for the formation of chiral allenyl
boronates utilizing a Cu(I)-phosphine complex (Figure 1b).5
Borylation of unactivated alcohols is notoriously difficult.6
Recently, Marder7 and Ye8 disclosed a copper-catalyzed
method for the synthesis of allyl, vinyldiboronates and allenyl
boronates from propargylic alcohol derivatives (Figure 1c). In
the case of the former, activation of C‒O bond of the
propargylic alcohol substrate was successfully achieved by the
addition of Ti(OiPr)4.
OCO2Me
CuOtBu, Xantphos
B2pin2
Me
(b)
(c)
•
Me
H
Bu
R2
OH
Bpin
H
R2
Bpin
Cu(acac)2, Xantphos
B2pin2,Ti(OiPr)4
R1
R1
Bpin
(E)-geometry
R1
R2
Bpin
Bpin
R1
R2
[Pt or Pd or Au]
B2pin2
(d)
(e)
•
This Work
Pd(PPh3)2Cl2, CuI
IPr, DBU, B2pin2
R
H
Bpin
Bpin
R
PhF5B(OH)2, MeCN
rt, 24 h
OH
(Z)-geometry
derivatives.
Through a copper-catalyzed formation of an allenylboronate
intermediate, subsequent round of borylcupration generates
the (E)-allyl, vinyldiboronates. Access to the corresponding (Z)-
isomer remains elusive. Indeed, 2-boryl allylboronates have
previously only been achieved using a Pt-,2e, 2k Pd-2c, 9 or Au-
catalyzed10 diboration of allenes (Figure 1d). In addition to the
challenge of borylating unactivated systems such as
propargylic alcohols, the formation of these allyl and vinyl bis
boronated products with distinguishable chemical reactivity
inspired us to develop a method for such a transformation.
a Virginia Tech, Department of Chemistry, Blacksburg, Virginia 24061, United
States.
E-mail: santosw@vt.edu
†Electronic Supplementary Information (ESI) available: Experimental procedures
and characterization of new compounds. See DOI: 10.1039/x0xx00000x
Herein, we report
a dual palladium-copper catalyzed
diboration of unactivated propargylic alcohols utilizing
pentafluoroboronic
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DOI: 10.1039/D0CC03563G
Table 1 Optimization of Reaction Conditions.a
Pd Catalyst, Cu Catalyst,
Ligand, Base, B2pin2, Additive,
Bpin
OH
OH
Solvent, 50 °C, 24h
H
Bpin
2a
pinB
H
1a
3a
entry
1
2
3
4
5
6
7
8
Pd Catalyst
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Cu Catalyst
CuI
Base
DBU
Et3N
NaOtBu
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
Additive
none
none
none
PhF5B(OH)2
Ti(OiPr)4
Ligand
none
none
none
none
none
none
Xantphos
Dppp
RuPhos
P(OPh)3
IPr
Solvent
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
MeCN
DCM
Toluene
MeCN
MeCN
MeCN
MeCN
Ratio 2:3b
Yield (%)c
(15)
(>1)
(8)
75:25
>1:99
28:72
90:10
88:12
90:10
100:0
100:0
99:>1
97:3
98:2
98:2
97:3
100:0
100:0
93:7
96:4
98:2
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
22
11
17
29
2
12
10
37
37
<1
32
<1
49
44
16
FeCl3
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
PhF5B(OH)2
9
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
10
11d
12d
13d
14d
15d
16d
17d
18d
19e
20e,f
21
20
Pd(PPh3)2Cl2
Pd(dppf)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
none
IPr
IPr
IPr
IPr
IPr
IPr
IPr
IPr
CuCN
Cu(acac)2
CuI
CuI
CuI
CuI
CuI
CuI
none
96:4
n.r.
n.r.
n.r.
68 (36)
n.r.
n.r.
n.r.
IPr
IPr
IPr
Pd(PPh3)2Cl2
a Reaction conditions: Premix Pd catalyst (0.024 mmol), Cu catalyst (0.024 mmol), and ligand (0.059 mmol) in solvent (1.5 mL) for 30 min. 1a (0.484 mmol) and base
(0.484 mmol) were added before cannulating in a solution of B2pin2 (0.968 mmol) and additive (0.024 mmol) in MeCN (0.5 mL). b Ratio determined by GC analysis
c
of crude reaction mixture and stereochemistry was confirmed by NOESY experiments. GC yields of 2a determined (benzophenone as an internal standard).
Isolated yields shown in parenthesis. d NHC already deprotonated. e Reaction performed at rt. f pinB‒Bdan used as the borylating agent. Abbreviations: Xantphos =
4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene; Dppp = 1,3-Bis(diphenylphosphino)propane; RuPhos = dicyclohexylphosphino-2′,6′-diisopropoxybiphenyl; IPr =
1,3-Bis(2,6diidopropylphenyl)-imidazol-2-ylidine; dan = 1,8-diaminonaphtalene; n.r. = no reaction
acid as an in situ activating agent to generate (Z)-allyl, product 2a over 3a. Interestingly, pentafluorophenylboronic
vinyldiboronates (Figure 1e).
acid was more effective than FeCl3 or Ti(OiPr)4. An increase in
Preliminary studies were initiated using commercially additive amounts did not affect the yield (see SI). Bidentate
available propargylic alcohol, but-2-yn-1-ol (1), and a bimetallic and monodentate ligands with various bite and cone angles
catalyst system (Table 1, for more optimization details see were also surveyed (entries 7-11). Notably, Xantphos was
Supporting Information). To our surprise, initial conditions preferred to Dppp. Whereas monodentate phosphine ligands
afforded allyl, vinyl diboronate 2a in 15% yield with were ineffective, N-heterocyclic carbene IPr promoted the
protoboration 3a as a minor product (entry 1). The expected reaction. We further investigated different catalysts and
propargyl or allenyl boronate products were not detected. The discovered that air-stable Pd(II) catalyst, Pd(PPh3)2Cl2, was as
product stereochemistry was confirmed by nuclear Overhauser effective as Pd(PPh3)4 (entries 12-15). By screening numerous
effect NMR experiments. A survey of bases indicated that solvents, we established that aprotic polar solvents such as
more sterically hindered base, diazabicycloundecene (DBU), MeCN and DCM were more efficient than non-polar solvents
was found to favor product 2 (entries 1-3). As previous work (entries 16-18). Furthermore, conducting the reaction at room
suggested that activation of the hydroxyl group is key to temperature compared to heating was shown to be crucial in
conversion, Lewis acids and transition metal catalysts, which the formation of the diborated product 2a (entry 19). While
have shown to be successful in Friedel-Crafts reactions of B2pin2 was shown to be reactive in the borylation of buty-2-yn-
allylic alcohols, were employed (entries 4-6).7b, 11 In each case, 1-ol (1), unsymmetrical diboron reagent, pinB-Bdan, was inert
catalytic amounts of the additive increased the selectivity of (entry 20). Control experiments indicate that both Pd
2 | J. Name., 2012, 00, 1-3
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Table 2 Substrate Scope.a
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DOI: 10.1039/D0CC03563G
Pd(PPh3)2Cl2, CuI,
IPr, DBU, B2pin2
Bpin
R
H
Bpin
Bpin
R
OH
Pd(PPh3)4 (5 mol%)
CuI (5 mol%),
OH
OH
PhF5B(OH)2, MeCN,
rt, 24 h
pinB
H
H
Bpin
R
IPr (12 mol%)
1a-i
2a-i
3a-i
2j, nd
2:3b
Yield (%)c
B2pin2 (2 equiv),
DBU (1 equiv)
Major Product
Bpin
1j
PhF5B(OH)2 (5 mol%)
CH3CN, rt, 24 h
2a
96:4
68 (36)
H
Bpin
OH
C3H7
Bpin
H
H
2b
2c
2d
94:6
94:6
76 (19)
41 (35)
29 (14)
4, 41% yield
H
Bpin
Bpin
Scheme 1. Transformation of 1j to 4.
C7H15
conversion of starting material indicates that the cyclohexyl
group may be inhibiting product formation. A series of ether
linkages were next tested. Although the reaction with the
terminal methyl ether (1e) resulted in a lower yield, the tert-
butyldiphenylsilyl (1f) protected counterpart was more
effective affording a 54% yield. Additionally, the propargylic
alcohol bearing the benzyl ether (1g) was well tolerated
affording a 59% yield (2g). We next tested 3-phenylpropargylic
H
Bpin
Bpin
Bpin
87:13
H
MeO
Bpin
Bpin
2e
2f
91:9
95:5
28 (14)
54 (28)
59 (24)
H
TBDPSO
BnO
alcohol 1h and, despite obtaining
a low yield, the
Bpin
Bpin
Bpin
regioselectivity was significantly higher compared to previous
substrates. To further explore the functional group tolerance
of the reaction, but-2-yne-2,4-diol (1i) was subjected to the
reaction conditions. Instead of forming the desired product 2i,
product 2a was isolated in a low yield. Finally, we investigated
sterically encumbered tertiary propargylic alcohol 1j.
Surprisingly, instead of affording 2j, 1j gave the cis reduced
product 4 in 41% yield (Scheme 1).
We also performed the reaction with phenyl allene 5 under
the same reaction conditions and found that 2h is formed in
minor amounts (Scheme 2). In contrast, the reaction with
propargylic boronate 6 with B2pin2 under identical conditions
afforded 2h in 20% yield. Based on the results above and the
fact that an allene intermediate is not detected during the
reaction, our studies that propargylic boronate could be an
intermediate in the Pd/Cu dual catalysis.
H
2g
90:10
H
Bpin
Bpin
a
Ph
2h
2i
99:1
11 (7)
n.d.
H
Bpin
HO
Bpin
‒
H
Bpin
a
Reaction conditions: Premix Pd(PPh3)2Cl2 (0.024 mmol), CuI (0.024 mmol), and
IPr (0.059 mmol) in MeCN (1.5 mL) for 30 min. Propargylic alcohol 1 (0.484 mmol)
and DBU (0.484 mmol) were added before cannulating in a solution of B2pin2
(0.968 mmol) and PhF5B(OH)2 (0.024 mmol) in MeCN (0.5 mL). b Ratio determined
by GC analysis of crude reaction mixture and stereochemistry was confirmed by
c
NOESY experiments. GC yields determined (benzophenone as an internal
standard). Isolated yields shown in parenthesis. n.d. = not detected.
In summary, we developed
a
simple and mild
palladium/copper catalyzed method for the diboration of
propargylic alcohols to afford (Z)-allyl, vinyldiboronates. Key to
and Cu are required in the reaction (entries 21-22). The low
yields may, at least in part, be due to the fact that copper
alone catalyzes the formation of the side product 3 (see SI,
Table 4, entry 15). The optimal reaction conditions afforded a
68% yield of product 2a within 24 h at room temperature
(entry 19); the instability of products with silica required fast
column chromagraphy.
With optimized conditions in hand (Table 1, entry 19), we
investigated the scope and limitations of the reaction. As
shown in Table 2, aliphatic propargylic alcohols containing
methyl (1a) and propyl (1b) substituents were efficiently
diborated in good yields with excellent Z selectivity. The longer
heptyl (1c) substituent resulted in a slightly decreased yield
(41% yield). Unfortunately, a 29% yield was observed with the
larger propargyl cyclohexyl (1d) group along with a marginal
decrease in the product ratios (2d:3d = 87:13). The incomplete
Bpin
Pd(PPh3)2Cl2, CuI, IPr,
DBU, B2pin2, PhF5B(OH)2,
MeCN, rt, 24 h
•
H
Bpin
5
6
2h
, 4% yield
Bpin
Bpin
Pd(PPh3)2Cl2, CuI, IPr,
DBU, B2pin2, PhF5B(OH)2,
MeCN, rt, 24 h
H
Bpin
2h
, 20% yield
Scheme 2
Borylation of potential intermediates 5 and 6.
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4. (a) Y. Yang and K. J. Szabó, J. Org. Chem., 2016, 81, 250-255; (b)
the success of the reaction is the in situ activation of the C-O
bond by pentrafluoroboronic acid, which under Pd/Cu catalysis
provides a propargyl boronic acid derivatives.
This work was supported by the Virginia Tech Chemistry
Department, National Science Foundation (CHE-1414458) and
AllyChem (B2pin2 donation).
T. S. N. Zhao, Y. Yang, T. Lessing and K. J. Szabó,ViJe.wAAmrtic.leCOhnelmine.
DOI: 10.1039/D0CC03563G
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5. H. Ito, Y. Sasaki and M. Sawamura, J. Am. Chem. Soc., 2008, 130,
15774-15775.
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Conflicts of Interest
There are no conflicts to declare.
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Pd/Cu
, NHC ligand
5B(OH)2
R
H
B
Bpin
Bpin
2pin2
PhF
, DBU
OH
MeCN, rt, 24 h
R
Z
( )-geometry
A Pd/Cu catalyst system facilitates the diboration of unactivated propargylic alcohols with
pentafluoroboronic acid and diboron to generate (Z)-allyl, vinyldiboronates.