Silyl alkynylphosphine-boranes: Key precursors of triazolylphosphines via tandem desilylation-Click chemistry

Article abstract of DOI:10.1039/c4ob00397g

A versatile synthesis of 1,2,3-triazolyl-4-phosphines from the borane complexes of phosphino-alkynes is reported. The efficiency of the procedure relies on the use of readily available silyl-protected alkynylphosphine-boranes, which were subjected to desilylation with TBAF followed by copper-catalyzed azide-alkyne-cycloaddition in one pot. Subsequent treatment with DABCO afforded the targeted triazolylphosphines in high yields. The reported method was applied to the synthesis of the first example of an enantioenriched P-stereogenic triazolylphosphine (98.8% ee). This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00397g

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Silyl alkynylphosphine-boranes: key precursors of
triazolylphosphines via tandem desilylation-Click
chemistry†
Cite this: Org. Biomol. Chem., 2014,
12, 3635
Romain Veillard, Elise Bernoud, Ibrahim Abdellah, Jean-François Lohier,
Carole Alayrac* and Annie-Claude Gaumont*
A versatile synthesis of 1,2,3-triazolyl-4-phosphines from the borane complexes of phosphino-alkynes is
reported. The efficiency of the procedure relies on the use of readily available silyl-protected alkynyl-
phosphine-boranes, which were subjected to desilylation with TBAF followed by copper-catalyzed
azide–alkyne-cycloaddition in one pot. Subsequent treatment with DABCO afforded the targeted
triazolylphosphines in high yields. The reported method was applied to the synthesis of the first example
of an enantioenriched P-stereogenic triazolylphosphine (98.8% ee).
Received 20th February 2014,
Accepted 25th March 2014
DOI: 10.1039/c4ob00397g
www.rsc.org/obc
lyzed synthesis of pyrrolidines.2 More recently ClickFerrophos
II ligands (Fig. 1) were developed and applied to the enantio-
Introduction
1,2,3-Triazolylphosphines are a new family of ligands recently selective rhodium-catalyzed hydrogenation of α,β-unsaturated
disclosed in the literature. The electron rich ClickPhos ligand phosphonates.3
for example (Fig. 1) was shown to display high catalytic activity
Attractive features of triazolylphosphines are the diversity of
in the Suzuki–Miyaura cross-coupling and aryl amination reac- metal coordination types involving the P-atom and/or N-atom
tions involving the poorly reactive aryl chlorides and heteroaryl from the triazolyl core,4 and the potential availability of sup-
chlorides.1 The chiral ferrocenyl ClickFerrophos I ligand ported phosphine ligands from polymer-immobilized azides.5
(Fig. 1) was able to induce excellent enantioselectivities in The major interest in triazolylphosphine ligands stems also
asymmetric rhodium- and ruthenium-catalyzed hydrogenation from their short and simple preparation based on the Huisgen
of alkenes and ketones respectively as well as in copper-cata- 1,3-dipolar cycloaddition6 between azides and alkynes. The
triazolyl core is built either through the copper(I)-catalyzed
azide/alkyne [3 + 2]-cycloaddition reaction starting from a
terminal alkyne (Click reaction)7,8 or through the [3 + 2]-cyclo-
addition reaction between azides and alkynyl Grignard
reagents9 leading regioselectively either to 1,4- or 1,5-substi-
tuted triazoles respectively (Scheme 1, eqn (1) and (2) respecti-
vely). It is noteworthy that eqn (1) and (2) are the key steps of
the synthesis of ClickFerrophos and ClickPhos respectively.
Fig. 1 Examples of triazolylphosphine ligands.
Laboratoire de Chimie Moléculaire et Thioorganique, UMR CNRS 6507, INC3M
FR3038, ENSICAEN & Université de Caen Basse-Normandie, 6 bvd Maréchal Juin,
14050 Caen, France. E-mail: carole.witulski-alayrac@ensicaen.fr,
annie-claude.gaumont@ensicaen.fr; Fax: +33 2 31 45 28 77
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, characterization data and NMR spectra for all new compounds. Chiral
HPLC chromatograms of (R)-9 and X-ray structures. CCDC 986144–986152. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ob00397g
Scheme 1 Main reported strategies towards triazolylphosphines.
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The phosphorus moiety is subsequently introduced through ling reaction between secondary diaryl-, dialkyl- or alkylaryl-
deprotonation of the triazole ring by a lithiated base followed phosphine-boranes and 1-bromoalkynes.18,19
by nucleophilic substitution at the phosphorus atom of a
chlorophosphine. It is noteworthy that the hydrolysis of the
intermediate triazolyl Grignard reagent in pathway 2 can be
Results and discussion
skipped and the reaction with chlorophosphines can be per-
formed in situ.10 No example of P-stereogenic triazolylphos-
We first examined the transformation of alkynylphosphine-
phines has been reported so far, probably as a result of the
limited access and low configurational stability of chiral
chlorophosphines.
boranes 4 into 1,4-substituted 1,2,3-triazoles 2 step by step
using (triisopropylsilyl)ethynyl-diethylphosphine-borane (4a)18
as a model substrate. The desilylation of 4a was readily
achieved at room temperature using TBAF20 as a desilylating
reagent (2.1 equiv.) in THF (Scheme 3). The resulting terminal
alkyne 3a (δP = 11.9 ppm) was isolated in 96% yield after silica
gel chromatography. It was then subjected to the classical
Click reaction conditions with benzyl azide (5a).7a,21 A copper(II)
complex, copper sulfate pentahydrate (10 mol%), was used
in the presence of sodium ascorbate (20 mol%) to reduce the
metal complex into a copper(I) complex, which is the active
species in this reaction. This catalytic system, which is aimed
to limit the oxidation of the copper(I) complex, requires a
biphasic solvent system tert-butanol–water (1 : 1) to solubilize
sodium ascorbate. Under these reaction conditions, alkyne 3a
was converted into the expected triazole 2a (δP = 7.6 ppm) in a
reasonable 65% isolated yield after overnight stirring at room
temperature.
Terminal alkyne 3a being a Michael acceptor, we were con-
cerned by its stability in the chosen aqueous solvent system.
Consequently anhydrous conditions were also tested to
perform the [3 + 2]-cycloaddition between 3a and azide 5a.
They consisted of using copper(I) thiophenecarboxylate (CuTC)
as a catalyst in toluene.22 After 3 hours at room temperature, a
full conversion of 3a into triazole 2a was observed, but the iso-
lated yield of 2a was not improved (65%). We also tested our
previously reported well-defined copper(I) complex [phen-
CuIPHPh2]23 because it is structurally related to [CuBr(PPh3)3],
a complex which was successfully used as a Click catalyst.24 In
this case, the full conversion was reached after 10 h and tri-
azole 2a was satisfactorily obtained in a higher yield of 77%.
In order to improve the procedure efficiency by skipping
the isolation of sensitive terminal alkynylphosphine-boranes 3,
we evaluated the feasibility of the one-pot transformation of
alkynylphosphine-boranes 4 into 1,4-substituted 1,2,3-triazoles
The alternative pathway based on the use of phosphino-sub-
stituted-alkynyl derivatives as Click substrates has only been
little explored despite the research interest notably in the
asymmetric series in view of the high configurational stability
of tertiary phosphines.11 Evidently phosphines are not suitable
substrates because they are converted into iminophosphor-
anes12 in the presence of azides through the famous Staudin-
ger reaction.13 Consequently the few reported triazole
syntheses mainly involve oxidized alkynylphosphorus deriva-
tives leading to oxidized derivatives, which have to be first
reduced to deliver the expected ligand.4,14,15 A valuable alterna-
tive would be to use protected phosphines such as phosphine-
borane complexes, which are known to undergo decomplexa-
tion under mild conditions.16 Only one example could be
found in the literature,17 probably because terminal alkynyl-
phosphine-boranes are sensitive molecules. They may react as
Michael acceptors undergoing nucleophilic addition to the
triple bond. Side reactions resulting from the deprotonation of
the terminal carbon atom may also occur. We reasoned that
these side reactions could be circumvented and the stability of
the starting alkynylphosphine-boranes could be enhanced by
introducing a silyl group at the terminal carbon atom. Accord-
ing to our planned retrosynthetic sequence, the targeted tri-
azolylphosphines 1 would result from the decomplexation of
their borane complexes 2, which would be available from the
click reaction between azides and terminal alkynes 3, in situ
generated through the desilylation of alkynylphosphine-
boranes 4 (Scheme 2).
Our strategy could give access to a broad range of achiral
and chiral P-stereogenic triazolylphosphines 1 notably by
taking advantage of our recently reported synthesis of alkynyl-
phosphine-boranes, based on the copper-catalyzed cross-coup-
Scheme 2 Envisioned retrosynthetic sequence towards 1,2,3-triazolyl-
4-phosphines.
Scheme 3 Stepwise synthesis of triazolylphosphine 2a from 4a.
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2 through tandem desilylation and Click chemistry. The reac-
tion between silylated alkynylphosphine borane 4a and azide
5a was chosen as a model reaction for this study. When two
equivalents of TBAF were added to the classical Click reaction
conditions (vide supra), the formation of triazole 2a was not
observed, but only a partial desilylation of 4a was observed
leading to 3a. This observation probably results from the high
solvation of the fluoride ion in water, which makes it less
nucleophilic25 and slows down the desilylation process. In
order to counterbalance this effect the amount of TBAF
was increased to three equivalents. After 3 h at 25–30 °C, a full
conversion of 4a was observed and the targeted triazole 2a was
isolated in 92% yield after silica gel chromatography, demon-
strating the superiority of the tandem reaction over the two-
step procedure. Importantly, the isolated yield of triazole 2a
was up-graded to 99% when the reaction was scaled up to 2 g
of the starting alkynylphosphine-borane 4a.
The scope of the one-pot desilylation/[3 + 2]-cycloaddition
reaction procedure was then investigated by varying the azide
substitution pattern and the phosphine reagents. The results
are displayed in Scheme 4. The reaction of 4a with phenylazide
(5b) and 4-anisylazide (5c) was not efficient at room tempera-
ture. The expected triazoles 2b and 2c were finally obtained in
65 and 55% respective yields provided that the reaction was
run at 50 °C. On the other hand, the reaction of 4a with the
electron deficient aryl azides 5d and 5e bearing a bromide or a
trifluoromethyl group in the para-position, respectively, readily
proceeded at 25–30 °C, and afforded the corresponding tri-
azoles 2d and 2e in 95 and 98% yield respectively. The struc-
tures of triazoles 2c and 2d were confirmed by X-ray diffraction
analysis (see ESI†). In agreement with the well-established
regioselectivity of the copper-catalyzed version of the Huisgen
1,3-dipolar cycloaddition, the phosphorus moiety is located at
position 4.
The scope of phosphine reagents was also evaluated. The
reaction of benzyl azide (5a) with TMS-ethynyl-di-tert-butyl-
phosphine-borane (4b) (R3 = R4 = t-Bu, R = Me) afforded the
corresponding triazole 2f in 76% yield after 3 h at 30 °C.
The reaction conditions were also successfully applied to the
synthesis of P-stereogenic triazolylphosphines 2g and 2h
through the reaction of methyl-phenyl-alkynylphosphine-
borane 4c with 5a and aryl azide 5d respectively. Triazolyl-
phosphines 2g and 2h were isolated in 88% and 76% yields
respectively. Diphenyl-alkynylphosphine 4d also underwent
the tandem desilylation/[3 + 2]-cycloaddition reaction with
various azides. The corresponding diphenyltriazolylphosphine-
boranes 2i–k were isolated in yields between 53 and 60%.
These lower yields can be ascribed to the weakness of the
phosphorus–boron bond when the phosphorus substituents
are weak electron donors like phenyl groups.
Scheme 4 Scope of the tandem desilylation/Click reaction between
alkynylphosphine-boranes and azides.
obtained in 76 and 75% yields respectively. The reaction of 4a
with azido[2.2]paracyclophane (5h) was also attempted as an
entry to [2.2]paracyclophane-derived triazolyl monophosphine
ligands, which have recently appeared in the literature.26 The
reaction was optimally performed at 40 °C for 15 h and deli-
vered the targeted triazole 2n in 77% yield. The structure of 2n
was confirmed by X-ray diffraction analysis (see ESI†). It is
worth pointing out that the access to triazoles 2d, 2h, 2j or 2m
would be difficult through pathways involving a lithium base
(Scheme 1, eqn (1) and (2)) because of the presence of sensitive
bromide or acyl functional groups.
Borane deprotection of alkyl-aryl- and diaryl-triazolyl-
phosphine-boranes 2g and 2i–k was readily achieved by treatment
with DABCO27 (2 equiv.) in THF at 60 °C. The corresponding
triazolylphosphines 1g and 1i–k were isolated in 71%, 92%,
91% and 97% respective yields. With electron rich phosphine
derivatives, such as diethyl-triazolylphosphine-boranes 2a, 2c
To further illustrate the potential of our procedure the reac-
tion of 4a with more complex structures of azides was briefly
surveyed. Thus functionalized azides 5f and 5g bearing a sulf-
anyl and a sugar function respectively were tested and shown to
readily undergo the transformation under the defined reaction
conditions. The corresponding triazoles 2l and 2m were
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Fig. 2 X-ray structure of triazolylphosphine-borane (R)-9.
Scheme 5 Application to the synthesis of enantioenriched P-stereo-
genic triazolylphosphine-borane (R)-9.
and 2e, the decomplexation was optimally performed with
a higher amount of DABCO (3 equiv.) in THF at 70 °C and
delivered the triazoles 1a, 1c and 1e in 74%, 61% and 88%
respective yields. The structures of triazoles 1e, 1i and 1j were
confirmed by X-ray diffraction analysis (see ESI†).
The synthesis of an enantioenriched P-stereogenic triazolyl-
phosphine was the next goal of this work. To the best of our
knowledge none of the chiral triazolylphosphines reported
so far are P-stereogenic.2,3,26 Chiral P-stereogenic phosphi-
nes,11a,28 by bringing chirality closer to the metal, are quite
interesting ligands for asymmetric catalysis. For this
study enantioenriched (S)-tert-butylmethylphosphine-borane
(6) (89% ee)29 was submitted to Cu(I)-catalyzed cross-coupling
with 1-bromo-2-TIPS-acetylene (7) according to our previously
described method (Scheme 5).18 The resulting alkynylphos-
phine-borane 8 was isolated in 72% yield, and then readily
converted into triazolylphosphine-borane 9 in 80% yield via
the tandem desilylation/Click procedure.
The enantiomeric excess of 9 was determined as 89% ee
according to chiral HPLC analysis demonstrating the retention
of stereopurity along the reaction sequence. Interestingly the
enantiomeric excess could be increased to 98.8% after a single
recrystallization from dichloromethane/pentane. Moreover
single crystals of 9 suitable for X-ray diffraction analysis could
be obtained allowing us to assign the (R)-configuration to 9
according to its X-ray-structure (Fig. 2). Consequently the
(R)-configuration can be also assigned to alkynylphosphine-
borane 8 demonstrating that the copper(I)-catalyzed cross-
coupling reaction between secondary phosphine-boranes and
1-bromoalkynes proceeds with retention of the configuration
at phosphorus.
Scheme 6 Synthesis of palladium(II) complex 10 (eqn (1)) and appli-
cation to the Suzuki–Miyaura cross-coupling of chlorobenzene (11a)
with arylboronic acids 12a–b (eqn (2)) and of ortho-anisyl chloride (11b)
with phenylboronic acid (12c) (eqn (3)).
10 in 93% isolated yield through reaction with [PdCl2(PhCN)2]
(Scheme 6, eqn (1)). The expected square planar trans-
[PdCl2(1f)2] structure of 10 for steric reasons was confirmed by
X-ray diffraction analysis (Fig. 3).
Gratifyingly, complex 10 (1 mol%) was shown to catalyze
the Suzuki–Miyaura cross-coupling of poorly reactive chloro-
benzene (11a) with p-tolyl boronic acid (12a) affording the
expected bis-arene 13a in 96% isolated yield (Scheme 6, eqn
(2)). The Suzuki cross-coupling of 11a with the more sterically
hindered naphthyl boronic acid (12b) afforded the corres-
ponding bis-aryl product 13b in 67% isolated yield (Scheme 6,
eqn (2)). Complex 10 (1 mol%) also efficiently catalyzed the
cross-coupling between ortho-anisyl chloride (11b) and phenyl-
boronic acid (12c) delivering the corresponding adduct 13c in
90% isolated yield (Scheme 6, eqn (3)). Importantly the yield of
bis-arene 13c was not affected when the catalyst loading was
We next performed preliminary studies on the catalytic
activity of the newly synthesized triazolylphosphines with com-
pound 2f, which has a structure close to that of the ClickPhos
ligand (Scheme 4 and Fig. 1). Borane decomplexation of 2f by
treatment with DABCO (5 equiv.) afforded triazolylphosphine
1f, which was directly converted into the palladium(II) complex
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tandem desilylation with TBAF/Click transformation and sub-
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