Organocatalytic trans Phosphinoboration of Internal Alkynes

Article abstract of DOI:10.1002/anie.202006096

We report the first trans phosphinoboration of internal alkynes. With an organophosphine catalyst, alkynoate esters and the phosphinoboronate Ph₂P-Bpin are efficiently converted into the corresponding trans-α-phosphino-β-boryl acrylate products in moderate to good yield with high regio- and Z-selectivity. This reaction operates under mild conditions and demonstrates good atom economy, requiring only a modest excess of the phosphinoboronate. X-ray crystallography experiments allowed structural assignment of the unprecedented and densely functionalized (Z)-α-phosphino-β-boryl acrylate products.

Full text of DOI:10.1002/anie.202006096

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Organocatalytic trans Phosphinoboration of Internal Alkynes
Authors: Russell Fritzemeier, Jan Nekvinda, Christopher Vogels, Carol
Ann Rosenblum, Carla Slebodnick, Stephen Westcott, and
Webster L Santos
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006096
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10.1002/anie.202006096
Angewandte Chemie International Edition
RESEARCH ARTICLE
Organocatalytic trans Phosphinoboration of Internal Alkynes
Russell G. Fritzemeier,a,ǂ Jan Nekvinda,a,ǂ Christopher M. Vogels,b Carol Ann Rosenblum,a Carla
Slebodnick,a Stephen A. Westcott,b,* and Webster L. Santosa,*
a
Dr. R. G. Fritzemeier, Dr. J. Nekvinda, C. A. Rosenblum, Dr. C. Slebodnick, Prof. Dr. W. L. Santos
Department of Chemistry
Virginia Tech
900 West Campus Drive, Blacksburg VA, 24061
E-mail: santosw@vt.edu
Homepage: https://www.santosgroup.chem.vt.edu
C. M. Vogels, Prof. Dr. S. A. Westcott
b
Department of Chemistry and Biochemistry
Mount Allison University
63C York Street, Sackville, New Brunswick, E4L 1G8 (Canada)
R.G.F and J.N. contributed equally to this work.
ǂ
Supporting information for this article is given via a link at the end of the document.
Abstract: We report the first trans phosphinoboration of internal
alkynes. Employing an organophosphine catalyst, alkynoate esters
and the phosphinoboronate Ph2P–Bpin are efficiently converted to the
corresponding trans α-phosphino-β-boryl acrylate products in
moderate to good yield with high regio- and Z-selectivity. This reaction
operates under mild conditions and demonstrates good atom
economy, requiring only a modest excess of the phosphinoboronate.
X-ray crystallography experiments allowed structural assignment of
the unprecedented and densely functionalized (Z)-α-phosphino-β-
boryl acrylate products.
Organoboron compounds are distinguished for their versatility in
organic synthesis and unique physical properties.[1] Notably,
organoboron species serve as substrates for the Nobel prize
winning Suzuki-Miyaura cross-coupling reaction.[2] Beyond
synthetic utility, the presence of boron in end products such as
FDA approved drugs[3] and BODIPY-derived fluorescent
materials[4] has garnered significant attention. The increasingly
diverse applications of boron in organic chemistry warrant the
continued development of efficient synthetic strategies.
Classically, organoboron species are prepared through
a
hydroboration reaction with C-C or C-heteroatom π-bonds.
However, a recent focus has been on the development of
difunctionalization reactions in which the addition of B-X reagents
(X = B,[5] N,[6] O,[7] Si,[8] Se[9] or R[6d, 10]) affords access to
substrates with two functional groups suitable for subsequent
transformations. One example that has only recently received
limited attention is the phosphinoboration reaction.
In this case, a phosphinoboronate (PB) bearing a P-B bond
reacts with a π-bond to simultaneously incorporate boron and
phosphorous functionalities in a single synthetic step. Excitingly,
this strategy has potential to supply products with applications in
ligand development for transition metal chemistry,[11] frustrated
Lewis pair (FLP) chemistry,[12] and tools for biological studies[13]
;
however, examples of phosphinoborations are scarce. This is
likely due to the relatively obscure and ligand dependent nature
of the P-B bond in PB species. Previous X-ray crystallography
studies in the 1980s indicated that the P-B bond in Ph2P–BMes2
has high double bond character as indicated by the bond angles
and the 1.859 Å P–B distance.[14] More recently, Lerner et al.
Scheme 1. Phosphinoboration reactions.
1
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10.1002/anie.202006096
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RESEARCH ARTICLE
1b). Use of halogenated solvents at room temperature favored
1,4-addition, while non-halogen solvents or colder temperatures
selected for 1,2-addition products.[16] Work by Westcott, Stephan,
and co-workers demonstrated that Ph2P–Bpin readily adds in a
1,2-fashion to carbon dioxide, carbodiimides, cyanates and
isothiocyanates.[17a] More recently, Grubba et al. reported the
synthesis of diaminophosphinoboranes and their reaction with
carbon dioxide (Scheme 1c).[18] However, while these additions
occurred at room temperature with no additives, significantly
longer reaction times were necessary, and the products were
obtained in only moderate yield. In addition to C-heteroatom π-
bonds, Stephan and co-workers reported the phosphinoboration
of diazomethane derivatives. Surprisingly, the PBs added in a 1,1-
fashion in the absence of any additives.[12c] In the case of
diazobenzene derivatives, the phosphinoboration occurs in a 1,2-
fashion to afford products with reactivity as FLPs.[12d] Additional
examples of a conjugate addition reaction with PBs have been
disclosed, demonstrating the reduction of N-heterocycles with PB
species (Scheme 1d).[16, 17b] More recently, the phosphinoboration
of acyl chlorides was disclosed.[19] Under these conditions, acyl
phosphines can be generated at room temperature or be
transformed to a diphosphine analog upon further reaction with
Ph2P–Bpin without the need for a catalyst precursor or additional
base to activate the P–B bond (Scheme 1e).
Table 1. Solvent and catalyst screening of the phosphinoboration of alkynoates.
Entry Solvent P–B (equiv) Temp (°C) Catalyst[a] Z:E[b]
Yield (%)[c]
1
2
THF
THF
1.1
1.1
1.3
1.3
1.3
1.3
1.3
1.3
1.1
1.2
2.0
1.3
1.3
1.3
1.3
1.3
1.3
1.1
rt
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
>99:1
98:2
97:3
96:4
96:4
98:2
99:1
98:2
96:4
96:4
96:4
96:4
96:4
>99:1
-
32
46
45
63
56
48
43
22
52
56
52
64
53
14
0
40
60
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
3
THF
4
DCM
5
MeCN
Toluene
EtOAc
CPME
DCM
6
7
8
9
10
11
12
13
14
15
16
17
18
DCM
DCM
Despite
these
recent
advances,
examples
of
phosphinoborations of C-C π-bonds are even more limited. In the
initial report by Westcott and co-workers, the authors noted that
uncatalyzed addition of the PB species did not occur in the case
of C-C π-bonds, such as in the cases of terminal allenes and
alkynes. However, utilizing a Rh catalyst afforded 1,2- or 1,1-
phosphinoboration products in good yield, respectively (Scheme
1f). In an effort to expand utility of phosphinoborane reagents and
access novel products containing both boron and phosphine
moieties, we sought to develop a phosphinoboration reaction of
internal alkynes. Previous work by our group and others
demonstrated that trialkylphosphines facilitate the trans addition
of boron reagents across alkyne bearing derivatives.[9c, 20] Inspired
by this work and others, we hypothesized that the required
transition metal catalysis to activate the P-B bond to add across
C-C triple bonds can be overcome by trialkylphosphine catalysts.
Herein, we disclose an unprecedented trans phosphinoboration
reaction that adds the borylpinacolyl group on the β carbon and
phosphinyl moiety on the α carbon of propiolate esters in a regio-
and trans selective manner (Scheme 1g).
We initiated our studies by employing the tributylphosphine
catalysis conditions previously developed by our group for the
corresponding hydroboration.[20e] To our delight, simply switching
the boron reagent to Ph2P–Bpin afforded the desired trans
phosphinoboration product 2a, albeit only in poor yield (entry 1).
Raising the reaction temperature resulted in only a modest
increase in yield (entry 2). Attempts to improve yield by increasing
temperature and excess Ph2P–Bpin reagent were unsuccessful
[d]
DCM
PBu3
PEt3
PCy3
PPh3
P(OEt)3
DBU
-
DCM
DCM
DCM
DCM
-
0
DCM
-
0
Toluene
-
0
[a] 0.1 equiv used. [b] Determined by GCMS. [c] Isolated yield. [d] 0.3 equiv
used.
summarized the physical properties of PB species and the
authors highlighted the ambiphilic and the strong π-bond
character of phosphaboradibenzofulvene which can undergo
Diels-Alder reactions.[15] However, this contrasts with the PB
Ph2P–Bpin characterized by Westcott and co-workers. In the case
of Ph2P–Bpin, the authors concluded that the 1.927 Å length of
the P–B bond suggests single bond character and minimal
contribution of the phosphorus lone pair to boron’s vacant p-
orbital. Consequently, coordination with a Lewis base would
activate the P-B bond and generate a highly nucleophilic
phosphorous species.[16] In contrast to other B-X additions that
often require catalysts or harsh conditions, this innate reactivity of
the P-B species has been demonstrated to afford addition
products under considerably milder conditions.
Recent developments in phosphinoborations have focused on
addition of PB species across C-heteroatom[11, 16-17] and N-N[12c, d]
π-bonds. In pioneering studies by Westcott and co-workers, it was
revealed that PBs Ph2P–Bpin and Ph2P–Bcat readily reacted with
various aldehydes, ketones, and aldimines without any additives
at room temperature to afford the corresponding 1,2-addition
products (Scheme 1a).[11, 16] In the case of an α,β-unsaturated
ketone, the authors noted that PBs react in either 1,2- or 1,4-
additions depending on temperature and solvent choice (Scheme
(entry 3). Considering solvent choice significantly influenced
f]
similar phosphine catalyzed reactions,[20c,
we screened
additional polar aprotic solvents (entries 4-8). Excitingly,
employing dichloromethane as the solvent afforded 2a in good
yield (entry 4). Similarly, acetonitrile afforded the desired product
in reasonable yield (entry 5). Toluene and ethyl acetate were
suitable solvents (entries 6 & 7), albeit not as effective as
dichloromethane. In the case of cyclopentyl methyl ether (CPME),
the product 2a was only obtained in poor yield (entry 8). We also
2
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10.1002/anie.202006096
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RESEARCH ARTICLE
sought to determine optimal stoichiometry of the P-B reagent
(entries 4, 9-11). We concluded that 1.3 equiv of Ph2P–Bpin
afforded optimal conversion and yield of 2a (entry 4). Interestingly,
two-fold excess of Ph2P–Bpin led to a slight decrease in yield
(entry 11). To further improve the yield, we attempted to increase
catalyst loading of tributylphosphine; however, this had minimal
effect on yield of 2a (entry 12). Next, we sought to determine the
optimal catalyst for this transformation (entries 13-17). Excitingly,
triethylphosphine could also serve as an effective catalyst with
only a minimal reduction in yield of 2a (entry 13). However, use of
a
catalyst
with
bulkier
substituents,
such
as
tricyclohexylphosphine, was detrimental to product yield (entry
14). Unfortunately, triphenylphosphine and triethylphosphite were
not efficient catalysts for this transformation (entries 15 & 16).
Furthermore, use of the amine DBU was also ineffective (entry
17). In the absence of catalyst, no conversion of starting material
was observed (entry 18). In all cases, the desired product was
obtained regio- and diastereoselectively with ≥96:4 preference for
the (Z) isomer (2a). Compound isolation was performed using
silica gel column chromatography.
With the optimal conditions in hand (Table 1, entry 4), we
turned our attention to the substrate scope and limitations for this
method (Scheme 2). We started by investigating the effects of
various substitutions on the aromatic ring. To our delight,
substrates bearing electron-donating groups (1b-j) were well
tolerated. Surprisingly, sterically hindered ortho-substituted
methoxy ether (2c) was isolated in higher yield than meta- and
even para-substituted ones. For these substrates, the yields
ranged from 46 to 72% with the exclusive formation of the trans
products. In the presence of a competing alkene in allylether 2h,
the phosphinoborane reagent chemoselectively reacted on the
alkynyl portion. For aryl groups bearing halogen substituents (2k-
n), the desired products were achieved in good yields (up to 67%
and excellent diastereoselectivity). A substrate with electron
withdrawing acetyl group (2o) was transformed but with
decreased yield and slight reduction in selectivity. We suspect
that a competing 1,2-addition with the ketone in 2o is occurring as
further optimization of reaction conditions (60 °C, MeCN) were
required for conversion. Likewise, biphenyl (2p) and naphthyl (2q)
were afforded in modest yield but excellent selectivity. Next, we
investigated various substrates with heteroaromatic moiety.
Fortunately, benzoxazole (1r) and thiophene (1s) afforded the
desired products (2r-2s) in moderate yields (56%) with excellent
Z selectivity.
We next investigated non-aromatic substituents on the
propiolate esters. While cyclopropyl containing substrate 2t
resulted in moderate yield and low Z:E selectivity, linear alkane
such as 2u and cyclohexyl 2v were obtained in 92:8 Z:E selectivity.
Lastly, we determined the effect of substituents on the ester group
and found that the reaction is sensitive to steric effects (Scheme
3). While methyl 3-phenylpropiolate 2a was obtained in 63% yield
(Scheme 2), the yield with the ethyl counterpart (2aa) was lower
and exacerbated in isopropyl ester 2ab with the products obtained
at 38% and 15%, respectively. Indeed, this trend continued with
phenyl ester (2ac) where no reaction was observed, but the
corresponding benzyl analog (2ad) was isolated in good yield
(65%) and diastereoselectivity (90%).
Scheme 2. Substrate scope with various alkyl, aryl and heteroaryl groups under
standard conditions mentioned in Table 1, entry 4. Isolated yields for the Z
isomer reported. Z:E ratio determined by GCMS (unless otherwise stated). [a]
Determined by quantitative 31P NMR. [b] 60°C, MeCN. [c] Yield is a combined
mixture of isomers.
ester 1 to form allenolate 3, which bears a Lewis basic oxygen
moiety. Complexation with Ph2P–Bpin activates the P-B bond for
diphenylphosphino transfer to the central sp hybridized carbon
A proposed catalytic cycle is shown in Scheme 4. Based on
e, 21]
previous work with phosphine-based catalysis,[20b,
tributylphosphine undergoes a Michael addition with alkynoate
3
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10.1002/anie.202006096
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RESEARCH ARTICLE
The trans selectivity of the reaction was confirmed by the
X-ray structure of 2f where the orientation of phosphorus (P1)
and boron (B1) is clearly in the Z configuration (Figure 1).22 In
contrast with amide counterparts,[20e, 23] internal coordination
between boron and carbonyl oxygen is negligible (2.397Å).
Scheme 3. Substrate scope of various ester groups under standard conditions
mentioned in Table 1, entry 4. Isolated yields for the Z isomer reported. Z:E ratio
determined by GCMS (unless otherwise stated). [a] Determined by quantitative
31P NMR. NR = no reaction.
via 4 to generate ylide 5a. Previous theoretical calculations from
our group suggests that resonance form 5b is energetically more
Figure 1. The molecular structure of 2f (CCDC 1998870) drawn at the 50%
probability level.
favorable
to
facilitate
the
formation
of
the
key
phosphinocyclobutane 6,[20e] which is prone for a ring expansion
to the five membered ring 7 because of the energetically
constrained cyclobutene ring and Lewis acidic boron. Expulsion
of tributylphosphine yields the desired trans phosphinoborated
product 2.
In conclusion, we have developed a novel organocatalytic
reaction that regioselectively installs the phosphorus and boron
atoms on the alpha and beta carbons of unsymmetrical alkynoic
esters, respectively, and afford trans alkenes with excellent
selectivity. Phosphinoboronates can be utilized in Suzuki-Miyaura
couplings, as valuable synthetic precursors, as ligands, and in the
study of frustrated Lewis pairs.
Acknowledgements
We acknowledge financial support by the National Science
Foundation (CHE-1414458), Natural Science and Engineering
Research Council of Canada (2016-04302) and National Science
Foundation (CHE-1726077) for crystallography experiments.
Keywords: phosphinoboration • alkynes • phosphorus • boron • organocatalysis
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or
by
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da-
ta_request@ccdc.cam.ac.uk, or by contacting The Cambridge
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fax: +44 1223 336033.)
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5
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RESEARCH ARTICLE
Entry for the Table of Contents
PBu3
O
pinB
OR
O
Ph2P Bpin
PPh2
DCM, 40 °C, 2 h
OR
• organocatalytic
• regioselective
• trans selectivity
• broad substrate scope
PB&J? No, it’s phosphinoboration across a triple bond catalyzed by tributyl phosphine. α-Phosphino-β-boryl acrylate products are
generated with excellent Z selectivity.
Institute and/or researcher Twitter usernames: @SantosLabVT
6
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