Addition of a Cyclophosphine to Nitriles: An Inorganic Click Reaction Featuring Protio, Organo, and Main-Group Catalysis

Article abstract of DOI:10.1002/anie.201704991

The addition of a cyclotriphosphine to a broad range of nitriles gives access to the first examples of free 1-aza-2,3,4-triphospholenes in a rapid, ambient temperature, one-pot, high-yield protocol. The reaction produces electron-rich heterocycles (four lone pairs) and features homoatomic σ-bond heterolysis, thereby combining the key features of the 1,3-dipolar cycloaddition chemistry of azides and cyclopropanes. Also reported is the first catalytic addition of P?P bonds to the C≡N bond. The coordination chemistry of the new heterocycles is explored.

Full text of DOI:10.1002/anie.201704991

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Angewandte
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Accepted Article
Title: Addition of a Cyclophosphine to Nitriles: An Inorganic "Click"
Reaction Featuring Protio-, Organo-, and Main Group Catalysis
Authors: Saurabh Sunil Chitnis, Hazel A. Sparkes, Vincent T. Annibale,
Natalie E. Pridmore, Alex M. Oliver, and Ian Manners
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201704991
Angew. Chem. 10.1002/ange.201704991
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10.1002/anie.201704991
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Addition of a Cyclophosphine to Nitriles: An Inorganic “Click”
Reaction Featuring Protio-, Organo-, and Main Group Catalysis
Saurabh S. Chitnis,[a] Hazel A. Sparkes,[a] Vincent T. Annibale,[a] Natalie E. Pridmore,[a] Alex M.
Oliver,[a] and Ian Manners*[a]
Abstract: We report the addition of a cyclotriphosphine to a broad
range of nitriles giving access to the first examples of free 1-aza-2,3,4-
triphospholenes in a rapid, ambient temperature, one-pot, high-yield
protocol. The reaction produces electron-rich heterocycles (four lone
pairs) and features homoatomic -bond heterolysis, thereby
combining the key features of the 1,3-dipolar cycloaddition chemistry
of azides and cyclopropanes. We also report the first catalytic addition
of P-P bonds to the CN triple bond. The coordination chemistry of
the new heterocycles is explored.
Translating the highly-evolved methods of modern organic
synthesis to inorganic substrates is interesting from
a
fundamental perspective and can also reveal atom-efficient
protocols for accessing inorganic molecules and materials with
unique properties. For example, the transformative role of
transition-metal catalysis in organic chemistry has inspired metal
mediated homo- and hetero-couplings of other p-block
elements,[1a-c] yielding
a
wealth of interesting inorganic
Scheme 1. Dipolar additions of N3, C3, and P3 frameworks with CX triple bonds.
molecules[1d-f] and polymers[1g-j] having properties that are
complementary to those accessible with organic analogues.[1k-m]
Owing to such examples there is significant interest in the
development of new high-yielding, and, in particular, catalytic
reactions for assembling complex inorganic frameworks from
simple synthons in a modular fashion, as defined by the modern
state-of-the-art in organic synthesis.
first catalytic addition of P-P bonds to nitriles. These novel P3/CN
additions combine the challenging homoatomic -bond
heterolysis observed in the cycloaddition between the C3/CN
pair, with the rich coordination potential of the electron-rich
heterocycles obtained from N3/CN cycloadditions.
There is renewed interest[4a-c] in the fundamental chemistry
of phosphorus heterocycles due to their potential for applications
as optoelectronic materials,[4d] anion sensors,[4e] linkers in
supramolecular chemistry,[4f] and metal-free catalysts.[4g,h] As a
result, the stoichiometric and catalytic additions of P-P  bonds to
alkenes, alkynes, and ketones is an area of intensive research.[5]
Previous work has established that P-P bonds that are polarized
by electrophile attachment can also add to nitrile CN bonds, but
the products in all cases were only isolated in the coordination
sphere of a Lewis acid, restricting their further utility.[6] In contrast,
the protocols revealed here yields free heterocycles, enabling us
to debut a systematic study of their coordination chemistry
towards hard and soft, metal and non-metal electrophiles.
Density functional theory (DFT) calculations suggested that
In this context, the 1,3-cycloaddition of azides with
alkynes,[2a] nitriles,[2b] phosphaalkynes,[2c] and arsaalkynes[2d] is a
rapid and atom-economic means of assembling heterocycles in a
single step (Scheme 1a). The versatility and reliability of such
reactions has been recognized as defining the gold-standard for
“click” chemistry.[2e] The concept of “click” dipolar cycloaddition
translates smoothly from N3 synthons such as azides to C3
synthons such as donor-acceptor (DA) cyclopropanes, which add
to unsaturated dipoles (ketones, nitriles, aldehydes, etc.) under
Lewis acid catalysis to give five-membered organic heterocycles
(Scheme 1b).[3] Inspired by the success of the N3 to C3 translation,
we have now developed a rapid (< 5 mins), ambient temperature,
one-pot protocol that enables the click-like addition of a P3
t
synthon (P3 Bu3) to the CN triple bonds in a broad range of
t
the addition of the prototypical cyclotriphosphine P3 Bu3 to MeCN
nitriles (Scheme 1c), giving access to a hitherto unisolated class
of phosphorus heterocycles in excellent yields. A catalytic variant
of the reaction has also been developed, representing the
is exothermic by 50 kJ mol-1 (see Supporting Information), but no
t
reaction occurred when P3 Bu3 was refluxed in neat MeCN for 24
h (Scheme 2a). When one equivalent of HOTf was added as an
t
electrophilic activator to equimolar mixtures of P3 Bu3 and MeCN
in toluene, complete conversion to [(tBuP)3P(tBu)H]+ was
[a]
Dr. S. S. Chitnis, Dr. H. A. Sparkes, Dr. V. T. Annibale, Dr. N. E.
Pridmore, A. M. Oliver, Prof. I. Manners
School of Chemistry
University of Bristol
Cantock’s Close, Bristol, BS8 1TS, United Kingdom
E-mail: ian.manners@bristol.ac.uk
observed (Scheme 2b), consistent with Weigand’s report of
t
t
Me3SiOTf-catalyzed conversion of P3 Bu3 to P4 Bu4,[7a,13] and
protonation of the latter cyclotetraphosphine with HOTf.[7b]
However, when one equivalent of HOTf was added to a solution
t
of P3 Bu3 in either neat RCN or in a 1:1 (by volume) RCN/toluene
Supporting information for this article is given via a link at the end of
the document.
solution, an immediate reaction took place yielding the N-
protonated P3CN frameworks [1R]+ in essentially quantitative
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10.1002/anie.201704991
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random copolymer where 40% of the nitrile groups were
converted to the corresponding azatriphospholene (Scheme 2d,
Figure S7-10, Supporting Information).
Compounds [1Me][OTf], [1tBu][OTf], 2Me, 2tBu and 2Ph
have been isolated and characterized by elemental analysis,
NMR spectroscopy, and vibrational spectroscopy. The molecular
structures of [1tBu][OTf] and 2Me (Figure 1) were further
established by X-ray diffraction and unambiguously show the
proposed P3CN framework. To the best of our knowledge, free
azatriphospholenes have not been isolated before, although one
example was trapped in 17 % yield between two W(CO)5
fragments. [6c]
Figure 1. Molecular structure of [1tBu][OTf] (left) and 2Me (right) in the solid
state. Non-essential hydrogen atoms have been omitted for clarity.
To develop an atom-efficient alternative to the above
stoichiometric protonation-addition-deprotonation sequence, we
hypothesized that transient activation by a catalytic amount of
electrophile might also effect the same transformation. Indeed,
reactions of azides and DA-cyclopropanes with nitriles are usually
t
catalyzed by added electrophiles.[3] When solutions of P3 Bu3 in
1:1 (v/v) MeCN/PhMe were heated to 90 oC with a variety of
t
electrophiles (15 mol% relative to P3 Bu3), we observed 80-100 %
t
t
t
conversion of P3 Bu3 to either P4 Bu4, or a mixture of P4 Bu4 and
the desired nitrile-cyclophosphine addition product, 2Me (Table 1)
The observation that Brønsted acids are catalytically active
(entries 1 and 2) is particularly interesting considering Baudler’s
t
previous report of stoichiometric reactivity between P3 Bu3 and
t
Scheme 2. Reactions of P3 Bu3 with nitriles. Isolated yields are in parentheses.
HCl in THF to form the ring-opened linear species
All other yields were determined by 31P NMR spectroscopy.
t
H(tBu)P−P(tBu)−P(tBu)Cl.[8] The ratio of 2Me to P4 Bu4 varied
significantly between the electrophiles tested. For example, only
t
P4 Bu4 was detected in the case of B(C6F5)3 (entry 3). Drawing
spectroscopic yield for most derivatives within the time of mixing
(Scheme 2c). In situ deprotonation of [1R]+ by Et3N yielded neutral
1-aza-2,3,4-triphospholenes, 2R, in excellent spectroscopic yield
within minutes (Figure S1, Table S1) and >80 % isolated yield for
select derivatives following extraction into pentane and
recrystallization (Scheme 2c). The material remaining in the
upon Gabbaï’s work on stibonium ion catalysis,[9] Ph3SbCl(OTf)
was also tested and, unexpectedly, provided the best ratio of
t
desired 2Me to P4 Bu4 (entry 6, Figure S2 for crude 31P NMR
spectra). The structure of this novel Sb(V) complex was
determined by X-ray crystallography (Figure S3).
Screening a series of nitriles with Ph3SbCl(OTf) showed that
t
mother liquor in each case was P4 Bu4. The reaction scales well,
t
the product distribution shifts towards P4 Bu4 formation with
as demonstrated by the successful isolation of 2Me on a multiple-
gram scale. Both aliphatic and aromatic nitriles are suitable
substrates. Within aliphatic derivatives, very hindered nitriles are
well-tolerated (e.g. tBuCN) but within aromatic derivatives, yields
were lowered marginally for the bulky naphthalene-1-carbonitrile
(2Nap, 70%) and significantly for the electron-deficient 4-
bromobenzonitrile (2pBrPh, 33%). While liquid nitriles were better
substrates, being used as co-solvents to create the required nitrile
excess, solid nitriles could also be used in saturated solutions. For
example, poly(4-cyanostyrene) could be functionalized to yield a
increasing steric bulk at the carbon (Scheme 3a). Given the high
yields obtained from stoichiometric routes (Scheme 2c), we
conclude that kinetic rather than thermodynamic factors govern
the product ratios. Control experiments confirmed that
t
Ph3SbCl(OTf) (15 mol%) effects the catalytic conversion of P3 Bu3
t
to P4 Bu4 in the absence of MeCN in 1:1 v/v MeNO2/PhMe in < 5
minutes at 90 oC (Scheme 3b). The choice of MeNO2 as a
substitute for MeCN was based on the similar dielectric constants
for the two solvents (ε = 37.5 for MeCN and 35.9 for MeNO2).
t
When only one equivalent of MeCN was used relative to P3 Bu3
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determined by 31P NMR spectroscopy.
t
Table 1. Catalyst screening for the addition of P3 Bu3 to MeCN. Yields
t
Entry
Catalyst
% Conversion
% 2Me
% P4 Bu4
Scheme 4. Formation of [3][OTf] and [4][OTf].
1
2
3
4
5
6
7
[H(OEt2)2][B(C6F5)4]
HOTf
93
32
72
0
68
17
100
44
85
16
20
100
84
To clarify the role of the electrophile in the above reactions,
we prepared isolable triflate salts of [(tBuP)2P(tBu)Me]+ and
[MeCNMe]+ as models for the unstable[7a,10] protonated ions
[(tBuP)2P(tBu)H]+ and [MeCNH]+. The molecular structure of
[MeCNMe][OTf][11] was determined (Figure S11) as symmetric
alkylnitrilium cations have not previously been structurally
characterized.[12]
B(C6F5)3
[Ph3C][B(C6F5)4]
TMSOTf
100
84
56
15
84
80
Ph3SbCl(OTf)
Ph3Sb(OTf)2
96
Heating a solution of [(tBuP)2P(tBu)Me][OTf] to 90 oC in
MeCN for 2 h yielded the P-methylated cationic heterocycle [3]+,
which was identified spectroscopically (Scheme 4a).[6e]
100
t
Combining [MeNCMe][OTf] and P3 Bu3 in MeCN at 20 oC resulted
t
in 1:1 v/v MeNO2/PhMe as solvent, P4 Bu4 was once again the
sole product observed, consistent with the formation of P4 Bu4 in
reaction of P3 Bu3 with HOTf in the presence of one equivalent of
MeCN (cf. Scheme 3c and Scheme 2b).
in rapid conversion (< 5 minutes) to the N-methylated [4]+, which
was isolated and characterized crystallographically as its triflate
salt (Scheme 4b, Figure S11). These results indicate that the
ability to undergo dipolar addition can be unlocked by electrophilic
activation of either the cyclophosphine or the nitrile. Based on the
above experiments, tentative mechanisms for the electrophile-
t
t
These experiments suggest that i) formation of 2Me and
t
t
P4 Bu4 occurs via separate catalytic cycles, ii) the P4 Bu4 forming
cycle is more rapid than the 2Me forming cycle, and iii) a large
excess of MeCN (e.g. when MeCN is the cosolvent) kinetically
t
catalyzed formation of 2R from P3 Bu3 and RCN are proposed in
Scheme S2 (Supporting Information).
t
outcompetes P4 Bu4 formation to yield 2Me at the expense of
With a facile route to the new P3CN heterocycles 2R in hand,
we assessed their ability to coordinate classical metallic and non-
metallic acceptors. The HOMO of 2Me (Figure 2, left) shows
prominent lobes at all pnictogen atoms, foreshadowing a
coordinative ambiguity involving these donor sites. The equimolar
reaction of 2Me and 2Ph with HOTf in DCM yielded [1Me][OTf] or
[1Ph][OTf] quantitatively, indicating protonation of the nitrogen
atoms. In contrast, DCM solutions containing 2R and [Rh(cod)Cl]2
t
P4 Bu4. When only one equivalent of MeCN is used, or when bulky
nitriles such as tBuCN are utilized, the catalytic cycle involving P-
P/C≡N addition is sufficiently retarded that the nitrile-free and
comparatively faster P4 Bu4 forming catalytic cycle dominates the
product distribution.
t
(2:1
stoichiometry)
exclusively
yielded
complexes
[Rh(2R)(cod)Cl] (R = Me, Ph), featuring Rh-P interactions (1JPRh
≈
150 Hz). The structures of [1Me][OTf] and both rhodium
complexes were established crystallographically (Figure 2 right,
and Figure S5).
Scheme 3. Substrate scope and control reactions for the catalytic addition of
t
nitriles to P3 Bu3. Yields were determined by 31P NMR spectroscopy.
Figure 2. Calculated HOMO of 2Me (left) and molecular structure of
[Rh(2Ph)(cod)Cl] (right). Hydrogen atoms have been omitted for clarity.
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In summary, we have disclosed the first stoichiometric and
catalytic additions of P-P bonds to the C≡N triple bond giving
access to P-C=N-P frameworks free of electrophile stabilization.
Using these reactions, the first examples of free
azatriphospholenes, 2R, have been prepared and structurally
characterized in good yields with a broad substrate scope in
molecular or polymeric nitriles. The reaction presents the P3/C≡N
analogue of the well-established N3/C≡N and C3/C≡N dipolar
cycloadditions. Due to the simultaneous presence of multiple hard
and soft donor sites, the electron-rich heterocycles 2R exhibit a
rich coordination chemistry and may find applications as ligands
in heterobimetallic catalysis.
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Acknowledgements
S. S. C. and V. T. A. acknowledge the Natural Sciences and
Engineering Research Council of Canada for postdoctoral
fellowships.
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Keywords: phosphorus heterocycles • main group elements •
catalysis • click chemistry • cycloaddition
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The catalytic cycloaddition of nitriles to
a cyclotriphosphine gives the first
example of free 1-aza-2,3,4-
Saurabh S. Chitnis, Hazel A. Sparkes,
Vincent T. Annibale, Natalie E.
Pridmore, Alex M. Oliver and Ian
Manners*
triphospholenes. This electrophile-
mediated reaction is rapid (< 5 mins),
and proceeds at ambient temperature
in a one-pot, high-yield protocol with a
broad scope in nitriles (11 examples),
representing a phosphorus analogue
of the azide-nitrile and cyclopropane-
nitrile “click” [3+2] cycloadditions.
Page No. – Page No.
Addition of a Cyclophosphine to
Nitriles: An Inorganic “Click”
Reaction Featuring Protio-, Organo-,
and Main Group Catalysis
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