Essential role of phosphines in organocatalytic β-boration reaction

Article abstract of DOI:10.1039/c2ob26899j

The use of phosphines to assist the organocatalytic β-boration reaction of α,β-unsaturated carbonyl compounds has been demonstrated with a selected number of substrates. The new method eludes the use of Broensted bases to promote the catalytic active spec

Full text of DOI:10.1039/c2ob26899j

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Organic &
Biomolecular
Chemistry
PAPER
Essential role of phosphines in organocatalytic
β-boration reaction†
Cite this: Org. Biomol. Chem., 2012, 10,
9677
a,b
a
a
a,b
Cristina Pubill-Ulldemolins, Amadeu Bonet, Henrik Gulyás, Carles Bo* and
a
Elena Fernández*
The use of phosphines to assist the organocatalytic β-boration reaction of α,β-unsaturated carbonyl com-
pounds has been demonstrated with a selected number of substrates. The new method eludes the use
Received 27th September 2012,
Accepted 24th October 2012
3
of Brönsted bases to promote the catalytic active species and PR becomes essential to interact with the
substrate resulting in the formation of a zwitterionic phosphonium enolate. This species can further
DOI: 10.1039/c2ob26899j
+
deprotonate MeOH when B
2
pin
2
is present forming eventually the ion pair [α-(H),β-(PR
3
)-ketone] -
www.rsc.org/obc
[B pin
2
2
·MeO]− that is responsible for the catalysis.
Introduction
Catalytic β-boration of activated olefins has gained an impor-
tant recognition in synthetic chemistry since Norman, Marder
and co-workers discovered the first Pt mediated 1,4-diboration
1
of α,β-unsaturated ketones. Further development of the reac-
tion has been characterised by the use of alternative metal
complexes to catalyse this selective transformation with the
common feature that the addition of a base seems to be
2
–9
required for efficiency.
The role of the base has been dis-
cussed to some extent to accelerate the reaction assisting trans-
metallation between the metal species and the diboron
reagents (Scheme 1). But the most recent approaches based on
1
0
organocatalytic β-boration reaction have postulated that the
base could directly be involved in the activation of the diboron active “Cu-Bpin” species for β-boration.
Scheme 1 Representative examples of the role of the base in promoting the
1
1
reagent (Scheme 2), replacing the catalytic role of the metals
in the activation of the B–B bond.
1
2
The enhanced nucleophilicity of the Bpin moiety, in the
Cu-catalysed and organocatalysed versions, is responsible for
the selective attack at the β-position of α,β-unsaturated ester,
ketone, amide and imine substrates. The resulting borylated
carbanionic intermediate is readily protonated in the protic
medium (MeOH) to form the expected β-borated product
1
3
2 2
Scheme 2 Activation of diboron reagent B pin by a base.
(Scheme 3). Since base seems to be the key point in the for-
mation of the nucleophilic Bpin moiety, we present here a
a
Dpt. Química Física i Iorgànica, Universitat Rovira i Virgili, C/Marcel.lí
Domingo s/n, 43007 Tarragona, Spain. E-mail: mariaelena.fernandez@urv.cat;
http://www.quimica.urv.net/tecat/catalytic_organoborane_chemistry.php;
Fax: +34 977559563
b
Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans, 17,
43007 Tarragona, Spain
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 3 Nucleophilic attack of the Bpin moiety via Cu-catalysed and organo-
c2ob26899j
catalysed reactions followed by protonation.
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Table 1 Phosphine assisted organocatalytic β-boration of α,β-unsaturated car-
a
bonyl compounds
b
b
Entry
1
Substrate
Product
Conv. (%)
I.Y. (%)
99
87
27
43
86
80
—
34
2
3
4
Scheme 4 Phosphine assisted β-boration reaction of α,β-unsaturated carbonyl
compounds in MeOH.
novel method where base is replaced by PR to assist the orga-
3
nocatalytic β-boration (Scheme 4).
5
6
99
99
88
89
Results and discussion
In order to prove the viability of the new phosphine assisted
base-free β-boration reaction, we chose a series of α,β-unsatu-
rated substrates and B pin as the diboron source. Table 1
2 2
shows that within 6 h, at 70 °C, all the substrates tested were
7
8
81
99
69
86
quantitatively converted, except those with α-substituted and
β-disubstituted methyl groups using 5 mol% of PCy
3
and
methanol as the unique solvent.
9
99
93
99
90
85
87
Aimed at unravelling the role of the phosphine, the first
target was to establish unequivocally whether phosphines
could activate the diboron reagent B pin , like alkoxides
1
1
0
1
2
2
do. Characterization of the hypothesized adduct [B
R = Me, Cy, Ph) was not possible either theoretically or by
NMR experiments. No change in the B pin signal (30.5 ppm, 12
2 2 3
pin ·PR ]
1
4
(
2
2
99
91
1
1
3
s) in B{H} NMR could be observed when PR was added to a
solution of the diboron compound in MeOH. This simple
experiment clearly showed that, unlike the methoxide anion,
a
2 2 3
Standard conditions: substrate (0.5 mmol), B pin (0.55 mmol), PCy
b
phosphines do not form any stable adduct with B
2
pin
2
, in- (0.02 mmol), MeOH (2 mL), 70 °C, 6 h. Conversion calculated using
1
H NMR spectroscopy and GC.
dependently of the nucleophilic character of the phosphine.
This is in good agreement with Marder and co-workers’ obser-
1
5
vations since a more acidic diboron compound, such as
B(1,2-S , is required to afford mono and bis-phos-
phine adducts, with both PMe Ph and PEt . A subtle balance
[
2 6 4 2 2
C H ) ]
2
3
dictates these weak B–P interactions: bis(catecholato)diboron-
cat ) does not form any adduct and B pin either. In a more
(
B
2
2
2
2
1
6
recent example, acidity on the boron was achieved by intro-
ducing halogen atoms directly bonded to one boron moiety.
Among the possible interactions between the phosphine
Scheme 5 Ion pair formation at stoichiometric level.
and MeOH, the phosphine acting as a Brönsted base is un- stoichiometric experiments were conducted to explore the for-
likely. Previous experimental and theoretical studies demon- mation of new species upon mixing stoichiometrically PMe
strated that strongly basic trialkyl phosphines do not (E)-hex-4-en-3-one and B pin in MeOH (Scheme 5, Fig. 1).
deprotonate MeOH. We showed that the presence of the Quantitative formation of a new species was observed: com-
diboron reagent increases the Brönsted acidity of MeOH, plete disappearance of the PMe signal at −62.0 ppm in the
P{ H}-NMR spectrum, and appearance of a new signal at
3
,
2
2
1
7
3
−
31
1
because the conjugate base MeO is stabilized by forming
−
11a
11
1
the [B pin ·MeO] adduct.
Nevertheless, deprotonation of 30.4 ppm. In the B{ H}-NMR, the signal of B
2
pin
(see NMR 30.5 ppm) completely disappeared, and two new broad peaks
at +35 ppm and +1.3 ppm were detected, as is shown in Fig. 1
2
(broad s,
2
2
2 2
MeOH does not occur even in the presence of B pin
studies in ESI†).
Next we focused on possible interactions between the phos- and Table 2. These two new boron signals correspond to the
2
3
−
phine and the substrate, since it is known that trivalent phos- sp and sp boron atoms of the well known [B
2
2
pin ·MeO]
1
1
phines react as nucleophiles with some α,β-unsaturated anionic adduct. NMR data thus suggest the formation of a
1
7–19
−
compounds to form phosphonium enolates.
A series of new ion-pair species, in which the anion is the [B pin ·MeO]
2 2
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Fig. 2 DFT derived molecular structure of ion-pair: ([α-H,β-PMe
2 2 2 2
hexanone] [B pin ·MeO] . Methyl groups of the B pin are omitted for clarity.
3
-3-
+
−
11
1
31
1
Fig. 1
3
B–{ H}-NMR and P–{ H} for: (red) the interaction of PMe and
(
[
E)-hex-4-en-3-one in MeOH, in the presence of B
(E)-hex-4-en-3-one] = 0.5 M); (black) same mixture of PMe
] = 0.5 M, [B pin ] = 0.5 M).
2
pin
2
(1 mL, [PMe ] = 0.5 M,
3
3
, B pin in MeOH
2
2
without the substrate (1 mL, [PMe
3
2
2
11
31
Table 2 Computed and experimental
in ppm) for several species
B
and
P NMR chemical shifts
(
NMR chemical shifts (ppm)
Computed Experimental
1
1
31
11
31
Species
B
P
B
P
B
PMe
2
pin
3
2
30.5
30.5
−63.0
31.4
−62.0
30.4
3
3
1.5(sp )–
1.3(sp )–
35.0(sp )
2
2
33.0(sp )
Fig. 3 Reaction energy profile for the formation of ([α-H,β-PR
3
-
+
·MeO]−2. Purple: PMe
propionaldehyde] [B
2
pin
2
3
; green: PCy
3
; red: PPh
3
. Elec-
tronic and Gibbs free energies (in parentheses) computed at BP86 are given
in kcal mol−
1
.
adduct, and the cationic counterpart is the [α-(H),β-(PR )-
hexanone] . Further ESI-MS experiments confirmed the mass
3
+
for the cation (see values in ESI†). Consequently, the new peak that enabled quantitative formation of I2 in the NMR experi-
3
1
1
at 30.4 ppm in the P{ H}-NMR spectrum corresponds to a ments. For PCy , the reaction is thermoneutral whereas for
3
phosphonium salt, which should derive from the nucleophilic PPh it is slightly endothermic. These values reflect well the
3
attack of the phosphine at the β-carbon of the substrate, fol- equilibrium between free phosphine and I2 observed in NMR.
lowed by the protonation of the zwitterionic phosphonium Using
a metahybrid exchange–correlation potential that
enolate by MeOH. It is important to remark that quantitative includes dispersion corrections, such as M06 (see details in
formation of this phosphonium species requires the presence ESI†), the general picture does not change although the reac-
of the diboron reagent.
tion is exothermic for all the three phosphines. We must point
3
-3- out that the assembly of four molecular entities implies an
The molecular structure of the ion-pair ([α-H,β-PR
+
−
hexanone] [B pin ·MeO] ) is shown in Fig. 2. This structure entropic cost, which is apparently huge as ΔG values reflect.
2
2
was fully characterized computationally as a global minimum Note the exaggerated accumulated value for TS2, for instance.
in the potential energy surface, taking into account its confor- Solvent effects introduced through continuous solvent models
mational flexibility. The computed NMR chemical shifts for do not take into account the entropy gain/loss due to solvent
this structure precisely coincide with the measured NMR shifts reorganization, a component that can partly compensate the
1
1
31
in both B and P NMR (Table 2).
entropy loss of merging two species. We evaluated an average
The study of the formation of ion-pair species has revealed value of 12 units in Gibbs free energy per encounter, so
interesting features. Fig. 3 shows the computed reaction roughly 36 kcal mol− in excess are accumulated in TS2 and I2.
energy profile for the formation of I2 from acrolein. Values for We will show below that formation of the final β-borated
energy activation barriers are rather reasonable and reflect dis- product, in the next reaction steps, largely overcomes the
tinct behaviour for each phosphine. Actually, formation of I2 cost of forming I2. Reaction energy profiles exhibit a clear
1
is exothermic for PMe
3
only, which is precisely the phosphine trend (PMe < PCy < PPh ) that does not coincide either with
3
3
3
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2
0
21
Brönsted basicity or with the Tolman angle. The most
basic PCy phosphine also displays the largest bulkiness, so a
3
combination of stereo-electronic effects determines this
reactivity.
The structure of the I1 intermediate in Fig. 3 corresponds
to the most stable conformation of the zwitterionic phos-
phonium enolate species, in which a dative bond between the
oxygen and the formally positively charged pentavalent phos- Fig. 4 Molecular structure and plot of the HOMO of the zwitterionic phos-
phonium enolate, I1.
phorus compound is formed. Topological analysis of elec-
tronic charge density revealed the existence of a P–O bond
2
2
critical point. The Laplacian of the charge density (see values
in ESI†) indicated that the P–O bond is mainly dative.
A similar system was studied previously with MP2 based
Table 3 Computed pK
a
values in dimethylsulfoxide (DMSO) and bond dis-
tances for some I1 species. All distances are given in Å
2
3
methods in the context of the phosphine-catalyzed hydro-
Species
pKaDMSO
d(P–O)
d(P–Cax)
d(P–C
1
)
2
d(P–C )
1
8
alkoxylation of enones and α,β-unsaturated substrates,
I1–PCy
I1–PMe
3
29.0
27.6
25.8
2.07
2.02
1.98
1.96
1.90
1.92
1.91
1.84
1.85
1.89
1.84
1.83
although the cyclic most stable form was not considered in
that study. The pentavalent phosphorus atom adopts a trigonal
bipyramid coordination, thus favouring the localization of
3
I1–PPh
3
23.4 (exp. value: 26.8)
α α
the negative charge in C . The highest coefficient of C in
the HOMO can be clearly visualized in Fig. 4. The P–C
bond length between the phosphorus atom and the carbon in
the axial position is slightly longer than the other two (Table 3,
P–Cax = 1.90 Å with respect to P–C = 1.84 and P–C = 1.84). We
1
2
also observed that the P–O bond length slightly changes with CG-MS led to new discoveries. As we mentioned above, when
the type of substituent in the phosphorus centre. The shortest mixing 1 equiv. of PMe with 1 equiv. of B pin and 1 equiv. of
3
2
2
P–O bond length for PPh can be rationalized by the fact that (E)-hex-4-en-3-one in 1 mL of MeOH ([reactants] = 0.25 M),
3
1
the phenyl groups have higher electron density than the alkyl we observed complete formation of an I2 ion-pair by H NMR,
3
1
1
31
11
groups. Since the two tested trialkyl phosphines present
almost equal P–O distances, electronic effects rather than ing the temperature to 70 °C, no conversion to the desired
sterics might control the establishment of this P–O bond.
β-borated product was observed even after heating overnight.
P NMR, H– P correlation NMR and B NMR. Upon increas-
At this point we wondered how basic I1 species are, and However, addition of an excess of substrate facilitated the for-
how is their reactivity compared to that of a very strong organic mation of the β-borated product. After 1 h at 70 °C, with
base, such as Verkade’s. In the alcohol–base system, Verkade’s 3 equiv. of substrate, 1 equiv. of PMe
and 1 equiv. of B pin
2 2
3
superbase showed the best performance for deprotonating in 1 mL of MeOH ([reactants] = 0.25 M, except for [(E)-hex-4-
1
1a
methanol.
The C position is by far the most basic spot in en-3-one] = 0.75 M) complete conversion was observed by
α
1
1
these zwitterionic species, and hence will be the atom that
becomes protonated. By using the computational protocol
B NMR and GC (see ESI† for more details).
Experiments above demonstrated that: (i) I2 species does
described in the Computational details section, we computed not evolve to the β-borated product directly and (ii) the reaction
pK
values for some I1 species as well as for the Verkade’s base proceeds from I2 with an additional substrate. All these experi-
Table 3). According to the pK
values, the zwitterionic species ments might indicate that 1 equiv. of substrate with respect to
a
(
a
I1 are very strong bases, as strong or even stronger than the amount of phosphine is sacrificed to promote formation
2
Verkade’s base. The trend clearly identifies the most basic of the nucleophilic sp B and consequently the nucleophilic
phosphine and the highest pK
a
value.
boron attack. This idea derives in a new methodology for
Data collected up to this point clearly showed that the phos- β-borating α,β-unsaturated carbonyl compounds replacing
phine could interact with the substrate through a nucleophilic the required Brönsted bases, thus using only phosphine/
attack at the β-carbon of the activated olefin. Then, the very methanol.
strong base formed in situ, i.e., the zwitterionic phosphonium
With all this information in hand, following the energy
enolate species I1, deprotonates MeOH assisted by B pin ,
2
2
profile from Fig. 3, we envisioned a mechanism starting with
forming the ion-pair I2. This conclusion is crucial, since it the attack of a phosphine on the β-position of the α,β-unsatu-
suggests that catalytic amounts of phosphine are enough to rated carbonyl compound (TS1, Fig. 5).
guarantee the formation of methoxy ions, which eventually
Attack of the trivalent phosphorus nucleophile on the most
activate the diboron reagent. This is clearly important as we electrophilic carbon of the α,β-unsaturated compound results
found that phosphines can replace the standard bases used in in the formation of the phosphonium enolate (I1). This
previous work (Cs
2
CO
3
, NaOtBu, NaOMe, Verkade, etc.).
species can further deprotonate MeOH. On the basis of our
Stoichiometric experiments in Schlenk and in NMR NMR studies described above, this process is favoured by the
1
1
31
−
tube, and monitoring the reaction by B NMR, P NMR, and presence of B pin that stabilizes the MeO anion forming the
2 2
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another molecule of B
2
pin
2
that stabilizes the generated
−
MeO . Note that the overall process is strongly exothermic
ΔE = −47.9 kcal mol− ) and exergonic (ΔG = −6.5 kcal mol ),
1
−1
(
TS3 being the most energetically demanding transition state
in the catalytic cycle (22.1 kcal mol− with respect to I2). The
mechanism in Fig. 5 resembles that proposed by Toste et al.
1
1
8
for the phosphine-assisted alkoxylation of activated olefins.
We also consider two alternative reaction pathways: the
2
intramolecular attack of the nucleophilic sp boryl unit of the
2 2 β
pin of the phosphonium salt
·MeO− towards the C
adduct B
by an SN type of reaction (TS-SN ), and the direct addition of
2
2
the nucleophilic boryl unit to the alkene, assisted by the phos-
phine (TS-PR ) (Fig. 6). The computed energy barriers for the
alternative mechanisms (see ESI† for details) demonstrated
that the reaction pathways through TS-SN or TS-PR are more
3
2
3
energetically demanding than the intermolecular attack via
TS3.
Conclusions
Novel reaction conditions for the organocatalytic β-boration
reaction of α,β-unsaturated carbonyl compounds using only
phosphine and alcohol are presented here for the first time.
The sole use of catalytic amounts of phosphine catalyses the
β-boration reaction. No Brönsted base is required to activate
2 2
the diboron reagent bis(pinacolato)diboron, B pin . The scope
of the reaction is demonstrated, and includes esters, acyclic
and cyclic ketones.
Fig. 5 Suggested catalytic cycle for the new base-free organocatalytic β-bora-
−1
−1
tion reaction. Electronic energy (kcal mol ) and Gibbs free energy (kcal mol
;
in parentheses) computed at the BP86 level relative to two molecules of acryl-
aldehyde, MeOH, B
clarity.
2
pin
2
plus PMe
3
. Methyl groups of the B
2
pin
2
are omitted for
The development of this novel transformation is based on
convergent spectroscopic, stoichiometric and theoretical
evidence of in situ formed reactive species. The phosphine
directly attacks the most electrophilic carbon of the α,β-unsatu-
rated carbonyl compound resulting in the formation of a
strongly basic zwitterionic phosphonium enolate species. This
intermediate is further protonated by the excess of MeOH, a
process that is particularly favoured by the presence of
−
bis(pinacolato)diboron that stabilizes the MeO anion, thus
−
forming the Lewis acid–base [B
2
pin
2
·MeO] adduct.
A considerable interest in this organocatalytic transfor-
mation can be expected due to the simplicity of the method-
ology. The results presented demonstrate that β-boration can
be performed by the unique presence of catalytic amounts of
phosphine in MeOH. The simplicity of the system and the
Fig. 6 Transition state structures for alternative C–B bond forming steps.
Selected interatomic distances in Å.
−
[
B pin ·MeO] adduct. Therefore, the second transition state understanding of the role of the phosphine open now an
2
2
(
TS2) of our mechanistic proposal involves the protonation of unlimited palette of possibilities and could have wider impli-
−
I1 in a concerted manner with the interaction of the MeO
cations to other organocatalytic β-boration approaches.
with the B pin to provide a phosphonium intermediate type
2
2
−
with the nucleophilic [B
I2 ion-pair). In the next step (TS3), the sp boryl unit of the
2
pin
2
·MeO] adduct as a counter-ion,
2
(
Acknowledgements
activated diboron reagent in the in situ formed ion-pair (I2)
attacks the C
β
of another molecule of the substrate. From TS3, Research supported by the Spanish Ministerio de Economia y
it releases the “(pin)BOMe” byproduct and leads directly to the Competitividad (MINECO) through project CTQ2010-16226
formation of an enolate ion-pair I3. Protonation of the enolate and CTQ2011-29054-C02-02, the Generalitat de Catalunya
partner in I3 results in the formation of the β-borated product (2009SGR-00462) and the ICIQ Foundation. We thank
and regeneration of I2, since the protonation is assisted by AllyChem for the gift of bis(pinacolato)diboron.
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7
For Ni-catalysed β-boration, see: K. Hirano, H. Yorimitsu
and K. Oshima, Org. Lett., 2007, 9, 5031.
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1
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(
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3
4
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