Phosphorus Coordination Chemistry in Catalysis: Air Stable P(III)-Dications as Lewis Acid Catalysts for the Allylation of C-F Bonds

Article abstract of DOI:10.1021/acs.organomet.8b00686

Modification of C-F bonds with main-group catalysts has typically employed electron-deficient Lewis superacids in high oxidation states, and the challenges of preparing and handling such species have prevented broader adoption of metal-free reduction protocols. Here, we show that a hemilabile ligand coordinated to an easily accessed P(III) center imparts air stability without sacrificing the ability to activate C-F bonds. Catalytic C-C coupling of benzyl fluorides with allylsilanes was achieved using a P(III) complex under benchtop conditions. This application of coordination chemistry principles to main-group Lewis acids reveals a new strategy for controlling catalysis.

Full text of DOI:10.1021/acs.organomet.8b00686

Communication
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Cite This: Organometallics XXXX, XXX, XXX−XXX
Phosphorus Coordination Chemistry in Catalysis: Air Stable P(III)-
Dications as Lewis Acid Catalysts for the Allylation of C−F Bonds
Saurabh S. Chitnis,† James H. W. LaFortune,† Haley Cummings, Liu Leo Liu, Ryan Andrews,
and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
S
* Supporting Information
ABSTRACT: Modification of C−F bonds with main-group catalysts has typically
employed electron-deficient Lewis superacids in high oxidation states, and the
challenges of preparing and handling such species have prevented broader adoption of
metal-free reduction protocols. Here, we show that a hemilabile ligand coordinated to
an easily accessed P(III) center imparts air stability without sacrificing the ability to
activate C−F bonds. Catalytic C−C coupling of benzyl fluorides with allylsilanes was
achieved using a P(III) complex under benchtop conditions. This application of
coordination chemistry principles to main-group Lewis acids reveals a new strategy for
controlling catalysis.
he evolution of p-block-based catalysts has most
commonly involved variations of the substituents or
anionic substituents at P(III) with neutral donors.11 On the
basis of these developments, we envisioned families of catalysts
derived from P(III) coordination compounds. Herein, we
begin by targeting P(III)-based Lewis acid catalysts derived
from the coordination of bi- and tridentate N-based ligands to
a dicationic P(III) center. These species effect catalytic
C(sp3)−C(sp3) cross-coupling of C−F bonds with allylsilanes,
illustrating the potential of main-group coordination com-
pounds to provide reactivity complementary to transition-
metal systems.
We have recently described P(III) dications stabilized by
bipyridine and terpyridine: namely, [(bipy)PPh][B(C6F5)4]2
(1) and [(terpy)PPh][B(C6F5)4]2 (2) (Figure 1).12 In a
similar fashion, the diimino-pyridine complexes [(C5H3N)(C-
(Me)NR)2PPh][X]2 (X = O3SCF3, R = Mes (3), Dipp (4);
X = B(C6F5)4, R = Mes (5), Dipp (6)) (Figure 1) were
prepared from a one-pot reaction combining PhPCl2, Na[B-
(C6F5)4], or Me3SiOSO2CF3 and the appropriate tridentate
ligand in DCM. The cations of 3/5 and 4/6 exhibit 31P NMR
chemical shifts at 33.5 and 61.2 ppm, respectively, consistent
with the dicationic charge. X-ray structures of 3 and 5
unambiguously confirmed these formulations (Figure 2). The
P−Nimine distances in 3 were found to be 1.960(3) and
1.994(3) Å, while the P−NPy distance is 1.787(3) Å. While the
tridentate ligands are essentially planar, the phenyl ring is
approximately orthogonal to this plane, with C−P−N angles of
93.4(1), 88.7(1), and 106.3(2)°. The corresponding P−C
distance is 1.815(4) Å. Similar metrics were observed for 5,
although disorder in the aryl-N groups precludes a detailed
T
molecular charge on the main-group element. For example,
1
neutral compounds such as BF3 and AlCl3 or cationic species
such as [CPh3]+ and [SiEt3]2+ are classic main-group Lewis
acids that derive their Lewis acidity from a vacant p orbital on
the central atom. Such species have found widespread
applications in synthetic chemistry. Similarly, modifications
of the substituents on the main-group elements to alkyl or aryl
substituents have broadened the range of utility. Among the
numerous variations, the electrophilic borane B(C6F5)3 has
been widely used as a catalyst activator in olefin polymer-
izations, as a Lewis acid catalyst for hydrosilylations, and as the
quintessential Lewis acid in frustrated Lewis pair (FLP)
chemistry.3 More recently, we have focused attention on
electrophilic phosphonium cations (EPCs), which derive their
Lewis acidity from vacant σ* orbitals. Such P(V) species are
highly reactive2c,4 and have been shown to catalyze hydro-
defluorination,4d hydroarylation,5 hydrogenation,6 dehydro-
coupling, polymerization,7 hydrosilylation,7 and C(sp3)−C-
(sp3) cross-coupling.8
Targeting a broader synthetic strategy to p-block Lewis acid
catalysts, we noted the innumerable developments of
homogeneous transition-metal catalyst systems that have
emerged from the concepts of coordination chemistry. Using
judicious ligand design and ligand libraries, families of catalysts
are readily synthesized and are tunable for specific reactivity. In
contrast, these principles have had limited application in the
development of p-block-element-based catalysts. Notable
exception are catalysts derived from group 13 complexes,
where the modular design of O/N-based salen or porphyrin
ligands has been exploited for polymerization catalysts. We also
noted the seminal work of Burford9 and others,10 who have
prepared a broad array of electrophilic polycations by replacing
Received: September 19, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00686
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Organometallics
Communication
C(R)CH2SiMe3 (R = Me, H, Br, CH2Cl) furnished the
corresponding C−C coupled allylated benzyl derivatives of the
general form RC6H4CH2CH2C(R)CH2 (R = Ph, tBu, F) or
RCH2C(R)CH2 (R = Ad, Cy) and Me3SiF (Table 1 entries
1−16). In general, good to excellent conversions to the C−C
coupling product were observed for nearly all substrates at
room temperature in DCM. Although 1 does function as a
highly efficient catalyst (TON up to 500) for these C−C
coupling reactions, it does require anhydrous and O2-free
conditions, as it decomposes rapidly in air.
In marked contrast, the hypervalent P(III) terpy complex 2
(Gutmann-Beckett acceptor number 27) is a substantially
weaker electrophile in comparison to 1 (acceptor number 93)
but is more user-friendly from a practical perspective. It is air-
stable in the solid state, and solutions of 2 show only 7%
decomposition in air over 24 h. Gratifyingly, 2 also catalyzes
the C−C coupling of benzyl fluorides 4-RC6H4CH2F (R = Ph,
tBu, F), 2-PhC6H4CH2F, and 3,4-C6F2H3CH2F with the
allylsilanes CH2C(R)CH2SiMe3 (R = Me, H, Br, CH2Cl,
4-PhC6H4) to give C(sp3)−C(sp3) coupled products of the
general form RC6H4CH2CH2C(R)CH2 and Me3SiF (Table
1, entries 19−37). While these reactions are slower than those
mediated by 1 and require the elevation of temperature to 35−
100 °C, the yields are excellent even when reactions were
performed in air using wet DCM as the solvent (Table 1,
entries 21, 22, 26, 29, 32). Furthermore, the scope was
extended to effect addition of allylsilanes to 1-fluoroadaman-
tane, 1-fluoropentane, difluorotoluene, and trifluoromethylto-
luene (Scheme 1). In the last two cases, the di- and triallylated
products were detected by mass spectroscopy, but additional
decomposition, presumably via Lewis acid catalyzed polymer-
ization, was also observed. Efforts to intercept the mono- or
diallylated intermediates by adjusting the stoichiometry were
unsuccessful.
Figure 1. Phosphorus dication complexes 1−6.
Mechanistically, this catalytic activity is thought to involve
the activation of C−F bonds by the Lewis acidic phosphorus
dications. While compound 1 is coordinatively unsaturated,
dissociation of one arm of the tridentate ligand is required to
generate such Lewis acidity in compounds 2, 5, and 6. The
possibility of such hemilability was probed computationally for
2 and 5. In the case of 2, dissociation of a ligand arm yields a
species resembling 1 and is 16.6 kcal/mol uphill with an
activation barrier of 18.9 kcal/mol. Dissociation of one of the
imine arms in 5 proceeds over an activation barrier of 23.0
kcal/mol to give an intermediate that is 21.7 kcal/mol higher
in energy than the coordinatively saturated species. These
observations are consistent with the air stability of the 10-
valence-electron phosphorus centers, yet they also account for
the reactivity that is thermally accessible in solution. Further
evidence of the role of coordinative unsaturation in the
initiation of the C−F bond activation was derived from the
combination of 1 or 2 with a stoichiometric amount of p-
phenylbenzyl fluoride. This led to an immediate reaction, with
the 31P and 19F NMR spectra indicating formation of P−F
bonds and ultimately PhPF2 (Figures S76−S80), along with
poly(methylenebiphenylene) via Friedel−Crafts oligomeriza-
tion. It is noteworthy that equimolar combinations of 1 or 2
with methallyltrimethylsilane showed no reaction at room
temperature. For 2, even heating to 55 °C for 2 days showed
no reaction. 31P NMR spectroscopy showed the persistence of
the P(III) cations at the end of the catalytic reactions (Figures
S81 and S82). Silyl cation catalysis seem unlikely, as such
species have short lifetimes in DCM13 at 55 °C and are highly
Figure 2. POV-ray depiction of the dications of (a) 3 and (b) 5.
Color scheme: P, orange; C, black, N, blue. Hydrogen atoms and the
triflate anions have been omitted for clarity.
comparison; nonetheless, the metrics are comparable to those
in the molecular structures of the cations in 1 and 2.
The established ability of 1 and 2 to activate C−F bonds for
hydrodefluorination prompted an examination of the potential
for catalytic C−C coupling reactions. To this end, preliminary
experiments were performed, generating 2 mol % of species 1,
2, 5, and 6 in situ with the subsequent addition of the benzyl
fluoride and the allylsilane. These screenings were consistent
with the generation of silyl fluoride and the formation of the
C−C coupling product C6H5CH2CH2C(R)CH2, albeit in
unoptimized yields.
To elaborate these catalytic C−C bond formations, the
preparations of the dicationic species were done on a larger
scale. These syntheses of compounds 5 and 6 proved
challenging, while compounds 1 and 2 were readily prepared
and purified on a gram scale. Using 0.5−2 mol % of the species
1 as a Lewis acid catalyst in the reactions of the benzyl
fluorides 4-RC6H4CH2F (R = Ph, tBu, F) and 2-PhC6H4CH2F,
adamantyl fluoride, or C5H11F with the allylsilanes CH2
B
DOI: 10.1021/acs.organomet.8b00686
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
a
Table 1. Catalytic C(sp3)−C(sp3) Coupling of C−F Bonds with Allylsilanes by 1 or 2
fluoroalkane
R1
Me
H
Br
CH2Cl
Me
H
cat. (%)
Catalyst 1
t (h)
T (°C)
yield, %
prod.
b
1
2
3
4
5
6
7
8
4-PhC6H4CH2F
4-PhC6H4CH2F
4-PhC6H4CH2F
4-PhC6H4CH2F
4-tBuC6H4CH2F
4-tBuC6H4CH2F
4-tBuC6H4CH2F
4-tBuC6H4CH2F
4-FC6H4CH2F
4-FC6H4CH2F
4-FC6H4CH2F
4-FC6H4CH2F
2-PhC6H4CH2F
2-PhC6H4CH2F
2-PhC6H4CH2F
2-PhC6H4CH2F
Ad-F
0.8
1.0
2.4
1.5
0.4
0.7
1.5
1.2
0.3
0.2
1.3
1.9
0.2
0.4
2.0
1.7
0.3
0.2
2
2
1
1
2
1
2
1
2
1
1
1
4
4
1
1
0.5
22
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
>99
>99
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
S
b
b
98
b
86
b
72
b
47
b
b
Br
>99
b
CH2Cl
Me
H
86
9
>99
b
10
11
12
13
14
15
16
17
18
61
b
Br
67
b
CH2Cl
Me
H
Br
CH2Cl
H
70 (55)
b
>99
b
>99 (91)
b
>99
b
83
b
82
b
C5H11F
H
25
Catalyst 2
b
19
20
21
22
23
25
26
27
28
29
30
31
34
35
24
32
33
36
37
4-PhC6H4CH2F
4-PhC6H4CH2F
4-PhC6H4CH2F
4-PhC6H4CH2F
4-PhC6H4CH2F
4-tBuC6H4CH2F
4-tBuC6H4CH2F
4-FC6H4CH2F
2-PhC6H4CH2F
2-PhC6H4CH2F
2-PhC6H4CH2F
2-PhC6H4CH2F
Ad-F
Me
Me
Me
H
CH2Cl
Me
H
Me
Me
Me
H
Br
Me
H
4-FC6H4
Me
Br
Me
Me
1.4
2.9
2.9
3.3
4.8
1.5
2.9
1.2
1.2
5.1
5.3
4.1
7
20
30
30
24
30
20
24
20
20
30
40
30
0.25
24
20
20
64
24
20
35
55
55
25
55
25
55
35
35
25
55
55
25
55
55
55
55
55
100
89
A
A
A
B
D
E
c
80
d
83
d
88 (58)
b
91
c
91
d
95
F
I
b
89
c
96
M
M
N
O
R
S
T
U
V
W
X
d
88
b
95
b
85
c
86
b
C5H11F
8
83
b
4-PhC6H4CH2F
3,4-C6F2H3CH2F
3,4-C6F2H3CH2F
PhCF2H
3.7
3.7
3.4
3.7
2.7
86
d
79
67
c
b,e
b,e
PhCF3
a
b
Reactions were performed with 0.1 mmol of alkyl fluoride and 0.3 mmol of allylsilane. Performed under an N2 atmosphere using dry DCM.
c
d
e
Performed in air using dry DCM. Performed in air using undried DCM. Complete consumption of starting material was observed, but accurate
yields could not be determined due to purification issues. Isolated yields for reactions done on a 1 mmol scale are given in parentheses. Yields were
determined by NMR spectroscopy using mesitylene as internal standard.
moisture sensitive. Brønsted acid catalysis14 was discounted, as
reactions in wet DCM required higher catalyst loadings and
temperatures and longer reaction times to reach completion.
Collectively, these data suggest that 1 and 2 are catalysts rather
than stoichiometric initiators forming silyl cations from
allylsilanes. We propose a mechanism involving C−F bond
activation by coordination of the benzyl fluoride to the P(III)
Lewis acid and subsequent nucleophilic attack by the
allylsilane, affording C−C bond formation and fluoride transfer
to the silyl group, regenerating the P(III) dications (Scheme
2). It is noteworthy that the reaction of pentyl fluoride
provides a mixture of C−C coupling products; thus, a
mechanism involving carbocationic intermediates is also
possible.
A preliminary assessment of functional group tolerance of
the catalysts 1 and 2 was obtained by performing catalytic
couplings of allylsilanes CH2C(R)CH2SiMe3 (R = Me, H)
with 4-PhC6H4CH2F in the presence of 1 equiv of a
functionalized additive (see the Supporting Information).
The reactions mediated by 1 were significantly inhibited by
addition of donors such as p-FC6H4CN, OBn2, PhOH, DMF,
pyridine, and Ph2NH and were only moderately effective in the
presence of Ph2CO and Ph2O. Complex 2 exhibited better
tolerance, affording 50−84% yields of the C−C coupling
product in the presence of p-FC6H4CN, OBn2, PhOH, Ph2NH,
C
DOI: 10.1021/acs.organomet.8b00686
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Organometallics
Communication
ligand. This strategy contrasts with the previous use of highly
reactive low-coordinate main-group Lewis superacids (e.g.,
silylium, alumenium, fluorophosphonium cations). While these
findings foreshadow a more general adoption of main-group
catalysts in C−F bond functionalization, the present report
reveals the potential of easily accessible P(III) coordination
compounds in catalysis. Ongoing efforts are targeting new
catalytic applications of such main-group coordination
complexes.
Scheme 1. C−C Coupling Products Using 1 and 2 as
Catalysts
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00686.
■
S
Scheme 2. Proposed Mechanism for P(III) Dication
Experimental and crystallographic data (PDF)
Mediated C−C Coupling of C−F Bonds and Allylsilane
Accession Codes
CCDC 1869010−1869011 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.W.S.: dstephan@chem.utoronto.ca.
■
ORCID
Liu Leo Liu: 0000-0003-4934-0367
Douglas W. Stephan: 0000-0001-8140-8355
Author Contributions
†S.S.C. and J.H.W.L. contributed equally to this paper.
Notes
The authors declare no competing financial interest.
and Ph2CO, but showed no activity in the presence of DMF or
pyridine. These data illustrate that donors inhibit the reactivity
of 1, whereas for 2, the hemilability of the tridentate ligand,
which generates the active site, also provides intramolecular
competition for external donors.
ACKNOWLEDGMENTS
■
D.W.S. gratefully acknowledges the financial support of the
NSERC of Canada, the award of a Canada Research Chair, and
an Einstein Fellowship at TU Berlin.
The ability to activate C−F σ-bonds, known for their
strength and kinetic inertness, is interesting, as this transforms
a fragment generally perceived as inert into a functional group
for C−C bond formation. This aspect was furthered by the
chemoselectivity of these P(III) catalysts for C−F bonds. The
reactivity of benzyl chlorides and bromides revealed no
allylation at room temperature. However, heating the reaction
mixture of 4-PhC6H4CH2Br and CH2C(H)CH2SiMe3 to 55
°C resulted in a yield of B in 24% using 1 as the catalyst and
19% using 2 (see the Supporting Information). The high
selectivity for reactions of C−F bonds stands in contrast to
metal-mediated cross coupling reactions, where alkyl bromide
and chloride species are highly reactive whereas alkyl fluorides
are not. Thus, the present main-group catalysts provide
chemistry that is orthogonal to the plethora of transition-
metal-based C−C coupling protocols.
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DOI: 10.1021/acs.organomet.8b00686
Organometallics XXXX, XXX, XXX−XXX