Synthesis of Vinyl-, Allyl-, and 2-Boryl Allylboronates via a Highly Selective Copper-Catalyzed Borylation of Propargylic Alcohols

Article abstract of DOI:10.1021/acs.orglett.7b03294

An efficient methodology for the synthesis of vinyl-, allyl-, and (E)-2-boryl allylboronates from propargylic alcohols via Cu-catalyzed borylation under mild conditions is reported. In the presence of commercially available Cu(OAc)₂ or Cu(acac)₂ and Xantphos, the reaction affords the desired products in up to 92% yield with a broad substrate scope (43 examples). Isolation of an allenyl boronate as the reaction intermediate suggests that an insertion-elimination-type reaction, followed by borylcupration, is involved in the borylation of propargylic alcohols.

Full text of DOI:10.1021/acs.orglett.7b03294

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Vinyl‑, Allyl‑, and 2‑Boryl Allylboronates via a Highly
Selective Copper-Catalyzed Borylation of Propargylic Alcohols
Lujia Mao,† Rudiger Bertermann,† Katharina Emmert,† Kalman J. Szabo,*,‡ and Todd B. Marder*,†
́
́
́
̈
†Institut fur Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universitat
̈
̈
Wurzburg, 97074 Wurzburg, Germany
̈
̈
‡Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden
S
* Supporting Information
ABSTRACT: An efficient methodology for the synthesis of
vinyl-, allyl-, and (E)-2-boryl allylboronates from propargylic
alcohols via Cu-catalyzed borylation under mild conditions is
reported. In the presence of commercially available Cu(OAc)2
or Cu(acac)2 and Xantphos, the reaction affords the desired
products in up to 92% yield with a broad substrate scope (43
examples). Isolation of an allenyl boronate as the reaction
intermediate suggests that an insertion−elimination-type
reaction, followed by borylcupration, is involved in the borylation of propargylic alcohols.
lcohols constitute a highly attractive class of starting
catalyzed by Pd(dba)2 and organic iodides, which afforded (Z)-2-
Amaterials as they are inexpensive and often easily derived boryl allylboronates with CC bonds at an internal position as
major products.13b Subsequently, Morken developed enantiose-
lective diborations of allenes employing Pd2(dba)3 as a catalyst
precursor to provide 2-boryl allylboronates with CC bonds at
the terminal position.11a,c,13c,d Recently, borylative opening of
strained rings was reported (e.g., cyclopropanes and epoxides)
with an allenyl moiety affording boryl allylboronates.14 A general
protocolfortheselectivesynthesisofvinyl-, allyl-, and(E)-2-boryl
allylboronates from more readily available propargylic alcohols
via Cu catalysis has not been established. Marder and others have
developed Cu-15a−e or Zn-catalyzed15f−i borylations of organic
halides, as Cu and Zn have relatively low toxicity and are
inexpensive. Herein, we used catalytic borylation of propargylic
alcohols to synthesize vinyl-, allyl-, and (E)-2-boryl allylboronates
(Scheme 1).
from natural sources.1 In particular, propargylic alcohols are
useful building blocks in organic synthesis,2 as they possess
inherent alkynyl and hydroxyl functional groups and can be easily
accessed from terminal alkynes and aldehydes or ketones.
Meyer−Schuster rearrangement of propargylic alcohols to α,β-
unsaturatedaldehydesorketonespromotedbyBrønstedorLewis
acids has been extensively applied to convert readily available
materials into versatile enone products.2a Recently, significant
progress has been made to synthesize allene derivatives via
trapping of allenyl intermediates, generated from propargylic
alcohols and their derivatives, with various reagents, such as
B2pin2.3 Cu(I)-catalyzed synthesis of allenyl boronates via
borylation of propargylic carbonates was reported.3a A Pd/Cu
bimetallicsystemwasalsodevelopedforthecatalyzedsynthesisof
allenyl boronates from propargylic carbonates.3b The first Cu-
catalyzed borylation of benzyl-, allyl-, and propargyl alcohols
providing the corresponding organic boronates using a
commercially available Cu catalyst under mild condition was
recently reported.3c
Vinyl boronates are widely used in Suzuki−Miyaura coupling
reactions,4a andallylboronatesarevaluablebuildingblocksforthe
synthesis of homoallyl alcohols.4b−d Among other approaches,5
borylcupration of allenes,6 which can be synthesized from
propargylic alcohols,7 has been established8−10 as an efficient
means to synthesize vinyl- and allylboronates. Note that
bis(boronate) esters are useful synthetic intermediates in a
numberof one-pot C−C bond-forming sequences.11 Alkenes and
alkynes are well-known feedstocks for metal-catalyzed dibora-
tions,12 and diboration of allenes has emerged as a powerful tool
for the preparation of bis(boronate) esters.13 Miyaura developed
thefirstPt-catalyzeddiborationofalleneswithB2pin2 providing2-
boryl allylboronates,13a and Cheng reported diboration of allenes
Previously, allenylboronates were prepared from multisub-
stituted propargylic alcohols catalyzed by 10 mol % of
[Cu(CH3CN)4]2+[BF4 ]2 with 1.2 equiv of B2pin2 at 40−60
−
°C.3c However, when the reaction was carried out with 1a as the
starting material, 30% of diboration product 2a was obtained
instead of the expected allenylboronate (Table S1, entry 2).
Therefore, we explored the borylation of propargylic alcohols
using other Cu salts as catalyst precursors and Xantphos as the
ligand (Table 1). (E)-2-Boryl allylboronate 2a was obtained in
66% yield when we employed 10 mol % of Cu(acac)2 as catalyst
precursor, 13 mol % of Xantphos as ligand, and 1 equiv of
Ti(OiPr)4 as a Lewis acid (Table 1, entry 1). However, the yield of
2a dropped to 57% when [Cu(CH3CN)4]+[BF4]− was used
instead of Cu(acac)2 (Table 1, entry 2). Control experiments
revealed that the diboration reaction is catalyzed by Cu, with
Received: October 23, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03294
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b c
, ,
Scheme 1. Transition-Metal-Catalyzed Borylation of
Propargylic Alcohols
Scheme 2. Synthesis of (E)-2-Boryl Allylboronates
a
Standard conditions: 1 (0.20 mmol), Cu(acac)2 (10 mol %),
Xantphos (13 mol %), B2pin2 (0.44 mmol), Ti(OiPr)4 (0.20 mmol),
b
c
CH2Cl2 (1 mL), 60−80 °C, 18−24 h. Isolated yield. Geometry of
CC bonds determined by H−1H NOESY NMR.
1
Table 1. Condition Screeninga
functional groups, such as isopropyl or cyclohexyl, were present.
Diboration products 2h−j were obtained in good to excellent
yields of 81−92% upon replacement of nBu by ethyl or phenethyl
groups. Propargylic alcohols bearing aromatic substituents were
also good substrates for diboration, giving 2k−n in good yields of
69−78%. With a phenyl ring bearing an OMe group at the meta
position, 2o was formed 71% yield. Yields of 2p−r dropped to 60,
54, and 57%, respectively, when OMe, Me, and F were introduced
at the ortho position. Naphthyl-containing substrate 1s gave 2s in
72% yield. Again, the nBu group can be replaced by a phenethyl
group, and corresponding products 2t and 2u were formed in 76
and 72% yields, respectively. The regio- and stereoselectivity of
our Cu-catalyzed diborations are different from those of Pd-
catalyzed allene diborations,13b−d which afforded (E)-2-boryl
allylboronates. Pd-catalyzed allene diborations give either (Z)-2-
boryl allylboronates13b or 2-boryl allylboronates with CC
bonds at the terminal position.13c,d The regioselectivity of our
diborations also suggests that a cascade involving the insertion/
elimination16b,c and a borylcupration of the resulting allenyl
boronates might be taking place.8−10
d
entry
variation from reaction conditions
no change
% of 2a
% SM 1a
1
2
3
4
5
6
66
25
34
94
93
67
[Cu(CH3CN)4]+[BF4]− instead of Cu(acac)2 57
e
e
without catalyst
ND
without Xantphos
without Ti(OiPr)4
ND
20
f
CH2Cl2 was used instead of MTBE
87 (81)
a
All reactions were carried out on a 0.2 mmol scale. Reaction
conditions: Cu(acac)2 (10 mol %), Xantphos (13 mol %), 1a (1
equiv), B2pin2 (2.2 equiv), Ti(OiPr)4 (1 equiv), MTBE (1 mL), 60 °C,
b
18 h. Yields determined by GC-MS analysis vs a calibrated internal
c
standard and are averages of two experiments. Geometry of CC
d
bonds determined by 1H−1H NOESY NMR. Unreacted starting
When 1,1-disubstituted propargyl alcohols as substrates and
e
f
i
material. Not detected. Isolated yield.
1.5 equiv of PrOH were added, allyl boronates were obtained
fromtheborylationreaction(Scheme3). With2-phenylbut-3-yn-
2-ol (3a) as the starting material, desired allyl boronate 4a was
formed in 74% yield. Allyl boronates 4b−f were obtained in
moderate to good yields of 68−82% with the phenyl ring bearing
different substituents (3b−f). 3g, with a fused ring system,
provided 4g in 83% yield. The geometry of the CC bonds was
determined by 1H−1H NOESY, and NMR data are identical to
those in the literature.17 When the phenyl ring system was
replaced by aliphatic substituents, such as 3h−j, allyl boronates
4h−j were also afforded in 44−61% yields. Because alkyl-
substituted allyl boronates are unstable on silica gel and are
Xantphos as the ligand (Table 1, entries 3 and 4). Yield of 2a
dropped to 20% in the absence of Ti(OiPr)4, which turns the OH
moiety into a good leaving group (Table 1, entry 5).16a When we
replaced MTBE with CH2Cl2 as the solvent, 1a was fully
converted and 2a was obtained in 87% yield (Table 1, entry 6; for
details see SI).
With optimized conditions in hand, we investigated the scope
of the diboration reaction (Scheme 2). Desired products 2b−e
wereobtainedin86−91%yieldfromdiborationof1b−e.Yieldsof
2f and 2g dropped to 74 and 63%, respectively, when bulkier
B
DOI: 10.1021/acs.orglett.7b03294
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b c
, ,
heterocyclic compound 5lgave corresponding products 6kand6l
in 71 and 78% yields, respectively. Regioselectivity of the
borylation reaction agrees with a mechanism reported for the
borylcupration of alkynes (for a detailed mechanism, see SI).18b,c
Based on preliminary results (see SI), a plausible mechanism is
shown in Scheme 5.
Scheme 3. Synthesis of Allyl Boronates
Scheme 5. Plausible Mechanism for the Diboration of
Propargylic Alcohols
a
Standard conditions: 3 (0.2 mmol), Cu(OAc)2 (10 mol %), Xantphos
(13 mol %), B2pin2 (0.4 mmol), Ti(OiPr)4 (0.20 mmol), PrOH (0.3
i
b
c
mmol), MTBE (1 mL), 60 °C, 18 h. Isolated yield. Geometry of
d
CC bonds determined by 1H−1H NOESY NMR. Z/E isomer
ratios were determined by 1H NMR spectroscopy of the crude
products, and isolated yields of (Z)-4a−g are given in the table.
volatile compounds, flash chromatography and rotary evapo-
ration of solvents contribute to yield losses to varying extents.
Regio- and stereoselectivity of the above borylation reaction
suggest mechanistic similarities to the borylcupration of
allenylsilanes8c (for a detailed mechanism, see SI).
Monosubstituted propargyl alcohols with 1.5 equiv of iPrOH
afforded vinyl boronates from the borylation reactions (Scheme
4).
a,b
Scheme 4. Synthesis of Vinyl Boronates
Propargylic alcohols 1 can be activated by Ti(OiPr)4 to
generate I and iPrOH (eq 1).16a We hypothesize that the active
catalyst is a Cu(I) species.19a Active Cu(I) species A could be
generated via reduction by an electron-rich anionic adduct of
B2pin2 or a boryl anion nucleophile.19b Transmetalation between
A and B2pin2 affords Cu-Bpin species B.3c,5h Via intermediates C
and D, an insertion/elimination occurs between B and I to
provide allenylboronates E,16b,c and catalytic species A is
regenerated. Transmetalation between A and B2pin2 regenerates
B. Borylcupration of E affords F.8a,b,9a Because the E-geometry is
thermodynamicallymorestablethantheZ-geometry, (E)-2-boryl
allylboronates 2 were afforded via γ-alcoholysis of F.8a,b,9a
In summary, we developed methodology for the synthesis of
vinyl-, allyl-, and 2-boryl allylboronates from propargyl alcohols
under mild conditions, broadening the utility of alcohols in
synthetic methodology. An insertion/elimination followed by a
borylcupration of the allenyl boronate intermediate is apparently
involved in the overall mechanism.
a
Standard conditions: 5 (0.20 mmol), Cu(OAc)2 (10 mol %),
Xantphos (13 mol %), B2pin2 (0.21 mmol), Ti(OiPr)4 (0.20 mmol),
b
iPrOH (0.3 mmol), toluene (1 mL), 60 °C, 18 h. Isolated yield.
c
Reaction carried out on a 1 mmol scale.
When we employed 1-phenylprop-2-yn-1-ol (5a) as the
starting material, vinyl boronate 6a was obtained in 79% yield.18a
Vinyl boronates 6b and 6c were formed in 83 and 80% yields,
respectively, with the phenyl ring bearing electron-donating
groups. 6d and 6e were formed in 75 and 69% yields, respectively,
whentheparasubstituentswereForCl, and6fwasformedin84%
yield with an OMe group at the meta position. With an OMe
group at the ortho position, 6g was formed in 71% yield. With
other groups at the ortho position, 6h−j were obtained in yields of
50−64%. Compound 5k, bearing a fused ring system, and
C
DOI: 10.1021/acs.orglett.7b03294
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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ASSOCIATED CONTENT
* Supporting Information
■
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Experimental procedures and compound characterization
data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: kalman@organ.su.se.
*E-mail: todd.marder@uni-wuerzburg.de.
ORCID
Todd B. Marder: 0000-0002-9990-0169
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.B.M. thanks AllylChem Co. Ltd. for a generous gift of B2pin2,
and the DFG for funding. L.M. thanks the China Scholarship
Council for a Ph.D. fellowship. We thank Profs. S.K. Bose (Jain
University) and W. Santos (Virginia Tech) for helpful
discussions. K.J.S. thanks the Swedish Research Council and the
Knut and Alice Wallenberg Foundation for financial support.
́
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