Photocatalytic Hydrophosphination of Alkenes and Alkynes Using Diphenylphosphine and Triamidoamine-Supported Zirconium

Article abstract of DOI:10.1002/ejic.201801079

Reactions of alkene or alkyne with diphenylphosphine and catalytic [κ⁵-N,N,N,N,C-(Me₃SiNCH₂CH₂)₂NCH₂CH₂NSiMe₂CH₂]Zr (1) are greatly enhanced under photolysis, providing viable catalytic hydrophosphination with a broad substrate scope. Whereas diphenylphosphine had been an inaccessible substrate under thermal conditions, complete conversion of alkene substrates to tertiary phosphine is achieved in as little as four hours at ambient temperature with 1 under ultraviolet irradiation. Previously inactive alkenes are now hydrophosphination substrates with diphenylphosphine to produce tertiary phosphine ligands possessing tunable steric and electronic properties.

Full text of DOI:10.1002/ejic.201801079

A Journal of
Accepted Article
Title: Photocatalytic Hydrophosphination of Alkenes and Alkynes Using
Diphenylphosphine and Triamidoamine-Supported Zirconium
Authors: Bryan T. Novas, Christine A. Bange, and Rory Waterman
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801079
Link to VoR: http://dx.doi.org/10.1002/ejic.201801079

10.1002/ejic.201801079
European Journal of Inorganic Chemistry
FULL PAPER
Photocatalytic Hydrophosphination of Alkenes and Alkynes
Using Diphenylphosphine and Triamidoamine-Supported
Zirconium
Bryan T. Novas,[a] Christine A. Bange,[a] and Rory Waterman[a]*
This work is dedicated to Professor Koop Lammertsma on the occasion of his 70th birthday for his inspirational leadership in
phosphorus chemistry.
Carpentier, and Trifonov.[7] Despite the emergence of several
breeds of early transition-metal, rare earth, and alkaline earth
hydrophosphination catalysts since the turn of the century,
unactivated alkenes were absent in the scope of these accounts.
Abstract: Reactions of alkene or alkyne with diphenylphosphine
and catalytic [5-N,N,N,N,C-(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH-
2]Zr (1) are greatly enhanced under photolysis, providing viable
catalytic hydrophosphination with a broad substrate scope. Whereas
Triamidoamine-supported
particularly
(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (1),
zirconium
compounds,
[5-N,N,N,N,C-
have been
diphenylphosphine had been an inaccessible substrate under thermal
conditions, complete conversion of alkene substrates to tertiary
phosphine is achieved in as little as four hours at ambient temperature
with 1 under ultraviolet irradiation. Previously inactive alkenes are now
hydrophosphination substrates with diphenylphosphine to produce
tertiary phosphine ligands possessing tunable steric and electronic
properties.
effective hydrophosphination pre-catalysts with (N3N)ZrPRR’
(N3N = N(CH2CH2NSiMe3)33-, R = H or R‘, R‘ = Ph or Cy) as key
intermediates.[5, 6b, 8] While these compounds have demonstrated
reasonable activities with primary phosphine substrates, we have
recently identified substantially improved relative rates as well as
access to primary arsine substrates under photolysis.[5b, 9] The
origin of enhanced reactivity under photolysis appears to be
population of a charge-transfer state and weakening of the Zr–P
bond. The effect is greater hydrophosphination reactivity in
general, including unactivated alkene substrates.[5b]
Introduction
Sustainable management of phosphorus is becoming more
crucial than ever with high demand for phosphorus-containing
products and diminishing supply.[1] Organophosphines are
indispensable ligands for catalysis, prevalent in materials with
interesting electronics, and of course, key in many biologically
relevant molecules.[2] Selective formation of P-C bonds is an on-
going challenge.[3] While there have been terrific advances in the
scope and understanding of metal-catalyzed hydrophosphination,
distinct challenges remain.[4] One of these challenges is of the
limited accessibility of unactivated alkenes as substrates, a target
class of substrates we have had increasing success in utilizing.[5]
Hydrophosphination serves as a completely atom efficient
method of preparing organophosphines at mild conditions, in
most examples. Catalytic hydrophosphination has received
steady development in recent decades on many fronts.[4b]
Although late metal catalysts have historically been most
prominent in hydrophosphination reactions, early transition-metal,
rare earth, and alkaline earth hydrophosphination catalysts have
emerged and developed substantially.[4] These catalysts initially
received attention in the early 21st century, with work done by
Marks, Mindiola, Hill, Le Gendre, and Waterman, where substrate
tolerance was limited.[6] A resurgence of uncommon metal-based
hydrophosphination catalysts occurred nearly a decade later
exhibiting relatively faster conversions, broader substrate scope,
and greater synthetic utility with studies by Shen, Westerhausen,
Based on the increased activity of 1 in hydrophosphination
catalysis with primary phosphines under irradiation, it was
hypothesized that catalysis using 1 with diphenylphosphine could
be made viable under photolysis as well. While many catalysts
utilize diphenylphosphine—and some to great effect—none utilize
6d, 7a-c, 10]
unactivated alkenes as substrates.[6c,
Given the high
reactivity of
1 with respect to the hydrophosphination of
unactivated alkene substrates, the possibility of extending this
catalysis to secondary phosphines was intriguing. As anticipated,
irradiation does substantially improve the reactivity of 1 with
diphenylphosphine and makes unactivated alkenes viable
substrates for this kind of catalysis.
Results and Discussion
Reaction of styrene with one equivalent of Ph2PH and 5
mol % of 1 at ambient temperature under irradiation at 253.7 nm
from
a commercially available photoreactor gave nearly
quantitative conversion of styrene to the corresponding tertiary
phosphine within 10 hours. This period of hydrophosphination for
styrene with diphenylphosphine and an abundant, early-metal
catalyst is qualitatively faster than many of the examples in the
literature to date, with low catalyst loading.[6c, 7c] Prior investigation
of catalytic hydrophosphination with diphenylphosphine and 1 as
the catalyst led to negligible conversions with alkene substrates
under thermal conditions.[11] This improvement in reactivity is
consistent with hydrophosphination catalysis using primary
phosphines and 1 under irradiation.[5b]
[a]
Bryan T. Novas, Dr. Christine A. Bange, Prof. Dr. Rory Waterman
Department of Chemistry, University of Vermont, 82 University Pl.,
Burlington, VT, 05405, USA
Spectroscopic and computational analysis has indicated
that photexcitation of (N3N)ZrPHPh, the key intermediate in
E-mail: Rory.Waterman@uvm.edu
Website: http://www.uvm.edu/~waterman/
catalytic
hydrophosphination
reactions
with
1
and
phenylphosphine, populates a state that exhibits significant Zr–P
* character resulting in an elongation and weakening of the Zr–
P bond.[5b] The lowest energy transition that gives rise to this
behavior has been assigned to an n→d charge transfer band at
389.0 nm, though excitation at lower wavelengths also gives rise
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201801079
European Journal of Inorganic Chemistry
FULL PAPER
to this behavior indicating that Kasha’s Rule is obeyed. The net
result across irradiation in the UV is greater reactivity in
hydrophosphination catalysis using 1 with PhPH2 as substrate.[5b]
For similar improvements in reactivity, similar spectroscopic
behavior of (N3N)ZrPPh2 and (N3N)ZrPHPh would be anticipated.
In the UV-vis spectrum of (N3N)ZrPPh2, absorption bands at 389.0
and 301.5 nm were measured (Figure 1), bearing a strong
resemblance to the absorbance spectrum of (N3N)ZrPHPh.[5b]
From control experiments, phosphido intermediates like
(N3N)ZrPPh2 are key intermediates in these hydrophosphination
reactions.[6b] Given the spectroscopic similarity of the two
phosphido derivatives and the importance of these as
intermediates, it was hypothesized that photoexcitation would
enhance hydrophosphination catalysis of 1 with Ph2PH, which
had been modest under thermal conditions. Given the initial
success with styrene (vide supra), a greater set of substrates
were tested.
even with heating, provide trace conversions to product for
styrene substrates as well as unactivated alkenes. Michael
acceptors are converted to a limited extent, which may be the
result of uncatalyzed hydrophosphination because mixtures of
product isomers are observed.
Phenylacetylene was revisited under irradiation. Treatment
of phenylacetylene and diphenylphosphine in the presence of 5
mol % of 1 under irradiation affords the vinyl phosphine products
in 61% conversion as a mixture of E and Z isomers (Table 1, Entry
17). Prior investigation of this transformation required heating for
extended periods to provide similar conversions.[6b] Revisiting this
substrate demonstrates the enhanced reactivity under irradiation
under milder conditions.
Table 1. Intermolecular Hydrophosphination of Alkenes and
Phenylacetylene Catalyzed by 1.
Entry
1
Substrate
Product
Time
(Hours)
Percent
Conversion
98%
10
10
4
2
3
98%
80%
4
5
8
4
>99%
80%
6
7
8
4
10
6
89%
92%
Figure 1. Absorbance spectrum of (N3N)ZrPPh2 in hexanes solution.
Reaction of unsaturated substrates with one equivalent of
Ph2PH in the presence of 5 mol % of 1 at ambient temperature
under irradiation in the UV provides the corresponding tertiary
phosphine product (Table 1). Several styrene derivatives with
both electron donating and withdrawing groups were treated with
1 and diphenylphosphine to produce tertiary phosphines in great
yield exhibiting high functional group tolerance (Table 1, Entries
1-5). Unlike studies with PhPH2, there was not a significant
difference in relative rate based on the para substituent of the
styrene substrate. Vinylpyridine substrates were converted to
product in comparable times to styrene substrate with higher
conversions than previously reported (Table 1, Entries 6-7).[7c] In
our own studies, vinylpyridines are sometimes more challenging
substrates. Michael acceptors, methyl and ethyl acrylate as well
as acrylonitrile, gave excellent conversions, which was
anticipated for this class of substrate (Table 1, Entries 8-10).
Hydrophosphination of 2,3-dimethyl-1,3-butadiene was facile and
selective for the 1,4-addition product (Table 1, Entry 11). Cyclic
alkenes such as cyclohexene and norbornene were also good
substrates with high conversions with more extended reaction
times, highlighting the effectiveness of 1 as a hydrophosphination
catalyst under irradiation (Table 1, Entries 12-13).
>99%
9
8
>99%
10
11
24
10
85%
>99%
12
13
72
72
[a]83%
[a][b]53%
14
15
72
72
[a]20%
[a]23%
16
17
72
72
[a]26%
[a]61%
There are still many substrates that do not lend themselves
well to hydrophosphination.[4] Unactivated alkenes are
challenging hydrophosphination substrates, especially using
diphenylphosphine. Under these conditions, unactived alkenes
were functionalized in poor conversions over 72 hours (Table 1,
Entries 14-16). As with the examples above, light is critical.
Control reactions in ambient light or the strict exclusion of light,
[b](3.5:1.0)
Conditions 20 equiv. of alkene, 20 equiv. of Ph2PH, 1 equiv. of 1 in ca. 0.5
mL benzene-d6 at ambient temperature. Percent conversion was measured
by integration of the 1H NMR spectra. The photoreactor operates at 35 C.
Reaction times were chosen to maximize yield. [a] Percent conversion
measured via integration of 31P{1H} NMR spectra. [b] Endo/exo ration could
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10.1002/ejic.201801079
European Journal of Inorganic Chemistry
FULL PAPER
All air-sensitive manipulations were performed according to a
previously published procedure.[5b] Ph2PH was prepared according to a
modified literature procedure.[6b] Compound 1 was prepared according to
not be determined from these data. [c]
hydrophosphination product.
E to Z ratio of alkyne
a
literature procedure.[12] All other chemicals were obtained from
commercial suppliers and dried by conventional means.
NMR spectra were recorded with Bruker AXR 500 MHz
spectrophotometer in benzene-d6 solution and reported with reference to
residual solvent signal (benzene-d6,  7.16) and to Ph2PH ( -41.0) for 31
NMR spectra. Absorption spectra were recorded with a Shimadzu 2450
UV-visible spectrophotometer (Kyoto, Kyoto Prefecture, Japan) as
hexanes solutions. (N3N)ZrPPh2 was excited between 700 and 200 nm
and excitation slits were set to 2 nm.
a
Treatment of internal alkynes with two equivalents of
diphenylphosphine and 5 mol % of 1 with irradiation at 60 C
afforded vinyl phosphine products in a mixture of E and Z isomers,
with a slight preference for the E isomer determined by 31P{1H}
NMR (Table 2). These reactions under extended periods provided
exclusively the single P–H bond activation product, and none of
the double hydrophosphination product was observed. In prior
studies with 1, double hydrophosphination has been observed for
a primary phosphine substrate.[8] These observations and prior
reactivity of 1 suggest that the second hydrophosphination
reaction is disfavored due to steric constraints and these reactions
were not pursued further.
P
Hydrophosphination of alkenes and phenylacetylene were carried
out in a PTFE sealed J-Young style NMR tube charged with 0.1 mmol
diphenylphosphine in benzene-d6 solvent stock solution (1.0 M) and 0.1
mmol alkene or phenylacetylene with 5 mol % of 1 in benzene-d6 stock
solution (0.04 M). The solutions were reacted at ambient temperature for
noted period under irradiation. The consumption of substrate to product
were monitored by 1H NMR and 31P{1H} NMR spectroscopy. Reactions run
in new NMR tubes showed identical conversions as those in reused,
washed NMR tubes.
Table 2. Intermolecular Hydrophosphination of Alkynes Catalyzed by 1.
Hydrophosphination of internal alkynes were carried out in a PTFE
sealed J-Young style NMR tube charged with 0.2 mmol diphenylphosphine
in benzene-d6 solvent stock solution (1.0 M) and 0.1 mmol internal alkyne
with 5 mol % of 1 in benzene-d6 stock solution (0.04 M). The solutions were
reacted at ambient temperature for noted period under irradiation. The
consumption of substrate to product were monitored by 31P{1H} NMR
spectroscopy. Reactions run in new NMR tubes showed identical
conversions as those in reused, washed NMR tubes.
Entry
Substrate
Product
Time
Percent
(Hours)
Conversion
1
72
72
46%
[a](1.1:1.0)
2
22%
Acknowledgments
[a](3.7:1.0)
We would like to thank Dr. Andrew Roering for his initial
experiments using 1 as a hydrophosphination catalyst with
diphenylphosphine. This work was supported by the US National
Science Foundation through CHE-1565658.
Conditions 20 equiv of alkyne, 40 equiv. of Ph2PH, 1 equiv. of 1 in ca. 0.5 mL
benzene-d6 at 60 C. Percent conversion was measured by integration of the
31P{1H} NMR spectra. [a] E to Z ratio of alkyne hydrophosphination product.
Keywords: Zirconium • Hydrophosphination • Phosphorus •
Photocatalysis • Diphenylphosphine
Conclusions
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Catalytic
hydrophosphination
of
alkenes
with
diphenylphosphine using 1 is now possible under direct irradiation.
These reactions afford primarily phosphine products for styrene
substrates in similar if not improved reaction times over reported
systems. More important, unactivated alkenes are now viable
substrates for hydrophosphination with 1 under irradiation.
Photolysis also provides an improvement in reactivity with
phenylacetylene, one of the few substrates that 1 could effectively
[2]
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utilize
diphenylphosphine. Catalytic hydrophosphination of internal
alkynes to produce vinyl phosphines using and
diphenylphosphine has also been achieved. The improvement in
reactivity under irradiation is likely due to an elongation in the Zr–
P bond of (N3N)ZrPPh2 upon photoexcitation that provides more
facile insertion of unsaturated substrate.
in
thermal
hydrophosphination
catalysis
with
1
[6]
Experimental Section
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10.1002/ejic.201801079
European Journal of Inorganic Chemistry
FULL PAPER
[7]
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C. A. Bange, R. Waterman, Polyhedron 2018, Accepted.
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10.1002/ejic.201801079
European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
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Photolysis in the UV avails greater
substrate scope and qualitatively
faster hydrophosphination of
unsaturated molecules with
diphenylphosphine using a zirconium
compound, [5-N,N,N,N,C-
Hydrophosphination
catalysis
Bryan T. Novas,
Christine A. Bange,
and Rory Waterman*
(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH-
2]Zr (1), as precatalyst.
Page No. – Page No.
Photocatalytic
Hydrophosphination
of Alkenes and
Alkynes using
Diphenylphosphine
and Triamidoamine-
Supported
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