N-Heterocyclic Carbene Non-Innocence in the Catalytic Hydrophosphination of Alkynes

Article abstract of DOI:10.1002/cctc.201900220

Studies on alkyne hydrophosphination employing nickel-NHC catalysts (NHC=N-heterocyclic carbene) revealed that the free N-alkyl substituted NHCs themselves were catalytically active. DFT calculations showed the mechanism involves the NHC acting as a Br?ns

Full text of DOI:10.1002/cctc.201900220

Accepted Article
Title: N-Heterocyclic Carbene Non-Innocence in the Catalytic
Hydrophosphination of Alkynes
Authors: William Blackaby, Samuel Neale, Connie Isaac, Sara Sabater,
Stuart Macgregor, and Michael Whittlesey
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10.1002/cctc.201900220
N-Heterocyclic Carbene Non-Innocence in the Catalytic Hydrophosphination of
Alkynes
William J. M. Blackaby, Samuel E. Neale, Connie J. Isaac, [a] Sara Sabater, Stuart A.
[
a]
[b]
[a]
[
b]
[a]
Macgregor* and Michael K. Whittlesey*
[
a]
W. J. M. Blackaby, C. J. Isaac, Dr. S. Sabater, Prof. M. K. Whittlesey
Department of Chemistry
University of Bath
Claverton Down, Bath BA2 7AY, UK
E-mail: m.k.whittlesey@bath.ac.uk
[
b]
S. E. Neale, Prof. S. A. Macgregor
Institute of Chemical Sciences
Heriot-Watt University, Edinburgh, EH14 4AS, UK.
E-mail: S.A.Macgregor@hw.ac.uk
Corresponding author: Professor M. K. Whittlesey
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Abstract
Studies on alkyne hydrophosphination employing nickel-NHC catalysts (NHC = N-
heterocyclic carbene) revealed that the free N-alkyl substituted NHCs themselves were
catalytically active. DFT calculations showed the mechanism involves the NHC acting as a
Brønsted base to form an imidazolium phosphide species which then undergoes rate-limiting
nucleophilic attack at the terminal alkyne carbon. This mechanism explains the preference
seen experimentally for reactions with aryl substituted phosphines and alkynes, while the
rearrangements of the alkenyl anion formed upon P–C bond formation account for the
observation of both Z- and E-regioisomers of the products.
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Introduction
N-heterocyclic carbenes (NHCs) have proven to be highly effective ligands for the
transition metal catalyzed addition of E–H groups (E = H, group 13-15 element) across
[
1]
unsaturated C–C bonds. In such processes it is usually assumed that the NHC plays the role
of a strongly bound ‘spectator’ ligand that remains coordinated to the metal center throughout
the catalytic cycle. Moreover, even if dissociation of the NHC ligand were to take place, the
lack of precedent for reactions of NHCs with EH bonds, even under stoichiometric
conditions,[2-5] would suggest the free carbenes would remain innocent and thus not
[
6]
contribute to catalysis. Herein, we report an example that demonstrates this is not the case.
Results and Discussion
During studies of PH bond addition to alkynes (hydrophosphination) catalyzed by N-
[
7]
alkyl substituted NHC complexes of nickel (Scheme 1a), control experiments demonstrated
that catalysis also proceeded when only the NHC was present in the absence of any nickel.
Thus, a 10 mol% loading of IMe (for the structures of all of the carbenes used in this study,
4
[
8]
see Chart 1) gave 81%% conversion of phosphine to a 3.3:1 molar ratio of the Z- and E-
anti-Markovnikov products 1a and 1b after 24 h at room temperature in THF (Scheme 1b).[9]
The Z-product 1a was also favoured in the Ni catalyzed reaction, although this also resulted
in significant amounts of double hydrophosphination.
Other N-alkyl substituted NHCs proved to be equally adept at catalysis (Table 1, entries
1-5; ESI), yielding the Z-anti-Markovnikov product as the major isomer in the majority of
cases. Neither unsaturated (entries 6, 7) nor saturated (entry 8) N-aryl substituted NHCs gave
good catalytic activity. Both the diamidocarbene 6MesDAC (entry 9) and cyclic alkyl amino
carbene Et2CAAC (entry 10) proved totally ineffective.
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Scheme 1. Comparison of (a) Ni-NHC and (b) NHC-alone catalysed hydrophosphination of
PhCCH with PPh H.
2
Chart 1. Structures and abbreviations of the carbenes used in this study.
t
The complete absence of side products seen with I Bu (entry 4) led us to conduct more
detailed investigations of the hydrophosphination reaction with this NHC. No reaction was
[
10]
observed with just PhCCH (10 equiv), whereas treatment of the carbene with PPh H (10
2
t
,[11] and the product of dehydrocoupling,[12]
H and PhCCH with 10 mol%
equiv) resulted in formation of the aminal, I BuH
2
Ph
2
P-PPh
2
. NMR monitoring of a catalytic reaction of PPh
2
ItBu in THF as a function of time revealed that hydrophosphination was in fact very rapid,
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ChemCatChem
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t
with 78% consumption of the phosphine within 20 min. NMR spectroscopy showed that I Bu
was still intact at the end of the catalytic run. Upon addition of further PPh
2
H and PhCCH,
70% of the phosphine was consumed within 1.5 h. Catalysis proved possible even at just 1
t
mol% I Bu, with 95% conversion of PPh H taking place over ca. 2.5 days.
2
Table 1. NHC catalysed hydrophosphination of PhCCH with PPh
H.[a]
2
Entry
Carbene
Conversion
%) of PPh
Hydrophosphination products
Z:E ratio
(
2
H (%) as percentage of all products
1
2
3
4
5
6
7
8
9
IMe4
8
9
8
8
9
1
1
2
5
9
2
4
90
74
93
100
100
71
-
2.8:1
0.9:1
2.0:1
3.5:1
IEt Me
2
2
i
I Pr Me
2
2
ItBu
ICy
2.6:1
IMes[c]
IPr[c]
-[d]
0
-
6-Mes[c]
6MesDAC[c]
Et2CAAC
-[d]
9
0
100
-
-
-
1
0
6[e]
0
[a]Conditions: Carbene (0.04 mmol), alkyne (0.40 mmol), PPh
H (0.44 mmol), THF (0.5 mL),
room temperature. Based on phosphine consumption as quantified by inverse-gated
2
[
b]
3
1
1
[c]
[d]
P{ H} NMR spectra. 18 h reaction time. Only the Z-isomer of PhCH=CH(PPh ) was
2
[
e]
-H addition to CAAC formed.[3b]
formed. Product of Ph
2
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To ascertain further details about the catalysis, the electronic properties of the alkyne
were varied and shown to impact on the product Z:E ratios (see Scheme 2 and ESI). Both
para-F and para-OMe substituents increased significantly the Z:E ratio (9:3:1 and 35:1
respectively), whereas the ortho-2-pyridyl-substituted substrate led to preferential generation
of the E-isomer (Z:E = 0.01:1). The internal alkynes PhCCPh and PhCCMe proved to be
reactive, but afforded significant amounts of dehydrocoupling. Only dehydrocoupling was
observed with 1-hexyne.
t
Scheme 2. I Bu catalyzed hydrophosphination of alkynes with PPh
2
H. Numbers shown on the
right give the ratio of Z:E isomers and (in parentheses) percentage conversion of PPh H.
2
t
The mechanism of the I Bu catalyzed hydrophosphination of PhCCH with PPh
2
H was
probed using DFT calculations, with optimizations performed in THF solvent with the TPSS
functional and final energies computed with M05-2X.[13] Reaction of I Bu with PPh
t
H
2
involves an initial adduct, I, at +6.0 kcal/mol from which phosphine deprotonation occurs
t
through TS(I-II) at +18.1 kcal/mol. This forms a contact ion-pair, [I BuH][PPh
2
] (II, +4.9
kcal/mol), which accesses PC bond formation via TS(II-III) at +17.9 kcal/mol to give III,
the product of net carbene insertion into the PH bond, at +9.1 kcal/mol. No direct pathway
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for this insertion process could be characterised and, at least for this combination of
substrates, the insertion product is thermodynamically less favoured than the ion-pair II. Both
t
the ion-pair and insertion product are thermodynamically uphill with respect to free I Bu and
PPh H and this is consistent with their non-observation experimentally. They are, however,
2
kinetically accessible.
t
Figure 1. Computed free energy reaction profile for P-H activation of PPh H with I Bu
2
(kcal/mol, computed at the M052X-D3(THF)/def2-TZVP//TPSS(THF)DZP level).
The computed pathways for the onward reaction with phenylacetylene to form the Z-
and E-isomers of PhCH=CH(PPh ) are shown in Figure 2 with details of key transition states
2
in Figure 3. Initial nucleophilic attack of the phosphide ion within ion-pair II at the terminal
alkyne carbon occurs via TS(II-IV) at +23.2 kcal/mol; the alternative attack at the internal
carbon (that would lead to the Markovnikov product) involves a much less accessible
transition state at 29.4 kcal/mol (see ESI). The new contact ion-pair formed, IV (+9.3
kcal/mol), comprises a Z-alkenyl anion and the imidazolium cation.
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Figure 2. Computed free energy reaction profiles for formation of Z- and E-PhCH=CH(PPh )
2
via reaction of contact ion-pair II with PhCCH (kcal/mol computed at the M052X-
D3(THF)/def2-TZVP//TPSS(THF)DZP level)
Formation of 1a then requires movement of the imidazolium to place the C2–H bond
adjacent to the alkenyl anion lone pair; this is most simply modelled by dissociation to a
solvent-separated ion-pair at +15.7 kcal/mol which then recombines to give V at +4.6
a
t
kcal/mol. Proton transfer within V
a
readily occurs to form 1a and free I Bu at -25.5 kcal/mol.
Alternatively, isomerization of IV can occur via an allenyl transition state structure TS(IV-
V
b
) at 15.3 kcal/mol (Figure 3). This generates contact ion-pair V
b
at +4.6 kcal/mol with
t
[14]
facile proton transfer then forming the E-isomer 1b and I Bu at -28.1 kcal/mol.
Combining Figures 1 and 2 shows hydrophosphination to be strongly exergonic (G = -
5.5 and -28.1 kcal/mol for 1a and 1b respectively) and to have a reasonable overall barrier
2
of 23.2 kcal/mol in which phosphide nucleophilic attack is rate-limiting. The geometry of
TS(II-IV) (Figure 3, in which the imidazolium cation is omitted for clarity) shows
2
developing sp -character at both alkyne carbons and during this process (and ultimately in
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intermediate IV), a formal negative charge developing on the -carbon (with respect to PPh )
2
can be stabilised via delocalization around the adjacent phenyl ring. This accounts for the
lack of hydrophosphination seen with 1-hexyne; indeed, the transition state computed using
propyne as a model aliphatic alkyne is much higher in energy at +31.5 kcal/mol. With
PhCCMe, an overall barrier of 29.0 kcal/mol is computed, consistent with the much lower
reactivity of this substrate compared to PhCCH and presumably reflecting the greater steric
encumbrance associated with attack at an internal alkyne carbon. Details of all additional
pathways are provided in the ESI.
Based on these computed profiles, the ratio of the hydrophosphination products, 1a:1b,
will be determined by the rates of the different rearrangements of intermediate IV. While
formation of V
b
is well-defined via TS(IV-V
b
) at 15.3 kcal/mol, it is less straightforward to
assess the formation of V
a
. In Figure 2 this is assumed to proceed via a solvent-separated ion-
pair at 15.7 kcal/mol, but other possibilities can be considered, including rearrangement
within the contact ion-pair, or addition of a second, external imidazolium cation directly to
the alkenyl carbon. As such, 15.7 kcal/mol may be considered as an upper limit for this
rearrangement barrier. The two processes leading to 1a and 1b are therefore likely to be
competitive, and indeed experimentally both isomers are observed in most cases (Scheme 2).
Recomputing these rearrangements with p-XC
ion-pair becomes more accessible than the isomerization via TS(IV-V
pair is 1.1 kcal/mol more stable (+15.3 kcal/mol cf. +16.4 kcal/mol), while for X = OMe the
difference increases to 2.0 kcal/mol (ion-pair: +19.2 kcal/mol; TS(IV-V ): +21.2 kcal/mol).
Formation of V (and hence the Z-isomer 1a) therefore becomes more favored with these
substituted alkynes, reflecting the higher Z:E-ratios observed experimentally. NBO charge
calculations on TS(IV-V ) indicate an increase in negative charge in the aryl substituent that
6
H
4
CCH (X = F, OMe) shows the separated
b
): for X = F the ion-
b
a
b
appears to be poorly accommodated by +M substituents. In contrast, +I substituents would be
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expected to facilitate isomerization and lead to lower Z:E ratios. This is confirmed with
calculations on p-CF CCH for which the isomerization transition state was computed
3
C
6
H
4
to lie 5.1 kcal/mol below the solvent-separated ion-pair (see ESI) indicating the E-isomer
should be preferred.[15] This was subsequently borne out experimentally with the I Bu-
catalyzed hydrophosphination of this substrate giving exclusively the E-isomer of p-
t
CF
3
C
6
H
4
CH=CH(PPh ) (ESI).
2
Figure 3. Computed geometries for key transition states TS(II-IV) and TS(IV-V
b
). Selected
distances are given in Å and spectator imidazolium cations have been omitted for clarity.
Hydrophosphination with a simple dialkylphosphine, PMe H, with PhCCH was also
2
t
modeled. As for PPh
2
H, insertion of I Bu into the PH bond involves a two-step process via a
contact ion-pair; however in this case the neutral carbene insertion product (IIIMe, +7.0
kcal/mol) is now significantly more stable than the preceding ion-pair (IIMe, +22.2 kcal/mol).
Onward reaction of IIMe with PhCCH entails a transition state at +35.1 kcal/mol. This
t
reflects the lack of reactivity observed experimentally upon replacing PPh
ESI) and correlates with the much higher pK
In conclusion, we have shown that N-alkyl substituted NHCs can promote the catalytic
2
H with P Bu
2
H
(
a
of dialkyl- versus diaryl-phosphines.[16]
hydrophosphination of alkynes. The proposed mechanism (Figures 1 and 2) indicates that
initial P–H activation proceeds via deprotonation with the NHC acting as a Brønsted base. P–
C bond formation involves rate-limiting nucleophilic attack of the resultant phosphide at the
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more accessible alkyne carbon, with Z:E selectivity being determined by the rearrangements
of the resultant alkenyl anion. This mechanism accounts for the scope of the reaction which
favors relatively acidic arylphosphines, as well as aryl-substituted alkynes in which charge
delocalization around the aryl ring provides additional stabilization of the alkenyl anion
intermediate. This catalytic hydrophosphination has parallels to NHC-catalyzed conjugate
addition reactions in which the NHC acts as a general base to activate alcohols, amines and
ketones.[17]
While examples of base-catalyzed hydrophosphination have previously been
reported,[18,19] these typically employ more explicit bases such as KO Bu or KHMDS. In
contrast, in catalysis with transition metal-NHC complexes, the role of the NHC is usually
assumed to be as a ligand, but this study shows that the capability of any dissociated, free
NHC to act as a base and perform catalysis in its own right should not necessarily be
neglected. Finally, it is interesting that the catalytic alkyne hydrophosphination reported
herein does not rely on E–H bond ‘oxidative addition’ (i.e. carbene insertion) of the type
often targeted in putative main group catalysis.[21] While we compute such species to be
kinetically accessible, in the present case they lie off-cycle and thus do not contribute directly
to catalysis.
t
[20]
Acknowledgements
We thank the EPSRC (Doctoral Training Award for WJMB), Heriot-Watt University
(James Watt Scholarship for SEN) and Royal Society (Newton Fellowship to SS) for
financial support. Drs. Ruth Webster, James Taylor and David Carbery (all University of
Bath) are thanked for valuable discussions, and Dr. Rodolphe Jazzar (UC San Diego) is
acknowledged for the gift of Et2CAAC.
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[6]
[7]
[8]
2
1
3
1
1
by inverse-gated P{ H} NMR spectroscopy, following the work by Oro and co-
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[
[
9]
The Z:E ratio was unchanged when catalytic reactions with IMe were performed in
4
toluene, benzene or cyclohexane, although conversions (72, 46 and 55% respectively)
were lower than in THF.
10] In the case of IMe , addition of PhCCH (10 equiv, THF) led to the instantaneous
4
1
appearance of a H NMR spectrum (ESI) suggestive of the formation of the addition
[
3a,5g]
product (IMe )(CCPh)H ( = 3.76 ppm (1H), 2.44 ppm (6H), 1.63 ppm (6H)).
4
However, this species decomposed in solution overnight. Additional characterisation
is the subject of ongoing studies.
[
[
[
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frequency calculations and these also provided thermal and entropic corrections.
Energies were then re-computed with the M05-2X functional with a def2TZVP basis
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set and including corrections for dispersion (D3) and THF solvent. Final energies
quoted in the text are free energies. See ESI for full details.
[
[
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5
5
H N-CCH (for which the E-isomer is
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[
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t
PhCCH with PPh
2
H using KHMDS and KO Bu as catalysts was performed under
the same conditions employed in Table 1. In both cases, complete consumption of the
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10.1002/cctc.201900220
phosphine took place in ca. 15 min, to afford a 1.2:1 and 1.7:1 ratio of E:Z isomers
respectively. Over longer times (ca. 4 h), this ratio changed to 0.1:1 (KHMDS) and
t
0
.94:1 (KO Bu).
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Table of Contents Entry
Free N-alkyl substituted N-heterocycle carbenes (NHCs) promote the catalytic
hydrophosphination of aryl-substituted alkynes through a mechanism involving nucleophilic
attack of an imidazolium phosphide species at the terminal alkyne carbon. These results
highlight the potential non-innocence of NHCs in their use as ligands in metal catalysed
transformations.
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