Easily accessible lithium compound catalyzed mild and facile hydroboration and cyanosilylation of aldehydes and ketones

Article abstract of DOI:10.1039/c8cc02314j

Simple and readily accessible lithium compounds such as 2,6-di-tert-butyl phenolate lithium (1a), 1,1′ dilithioferrocene (1b) and nacnac lithium (1c) are found to be efficient single site catalysts for hydroboration of a range of aldehydes and ketones wit

Full text of DOI:10.1039/c8cc02314j

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Easily accessible lithium compounds catalyzed mild and facile
hydroboration and cyanosilylation of aldehydes and ketones
Milan Kumar Bisai,a Tamal Das,b Kumar Vanka,b and Sakya S. Sen*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Simple and readily accessible lithium compounds such as 2,6- selectivity or both. Hence, group 1 based complexes have
ditertbutyl phenolate lithium (1a), 1,1' dilithioferrocene (1b) and remained largely unexplored in molecular catalysis. The
nacnac lithium (1c) are found to be efficient single site catalysts paradigm has shifted with the two significant breakthroughs
for hydroboration of a range of aldehydes and ketones with HBpin that recently came from the groups of Okuda19,20 and
at room temperature. The efficacy of 1a-1c as catalysts is Mulvey.21 These breakthroughs have created a new avenue for
extended towards the cyanosilylation of aldehydes and ketones the lithium compounds to be used as single component
with Me3SiCN.
catalysts for important organic transformations.
R'
O
O
Catalyst
THF, r.t.
O
O
O
B
While observing the hundred years of the birth of lithium
chemistry,1 the growth of s-block compounds is still in the
early days finding its feet slowly and gradually from curiosity
driven explorations to important catalytic transformations.
Driven by the demand for the catalytic processes with reduced
environmental impact and low cost, numerous groundbreaking
results have been achieved in molecular catalysis derived from
the heavier alkaline earth metals complexes.2 Following the
demonstration of hydroboration of carbonyl compounds by a
magnesium alkyl complex,3 there has been a flurry of research
activity on hydroboration reactions by compounds with
alkaline earth metals4-8 as well as p-block elements.9-16 For the
heavier alkaline-earth metal catalysts, Schlenk equilibrium is
an issue, which is likely to impose a severe limitation on the
preparation as well as the activity of the catalyst. In this
context, Group 1 complexes are advantageous over their
adjacent neighbours as they are not involved in Schlenk-type
equilibrium. Moreover, as catalysts involving group 2 and p-
block elements are usually prepared from the corresponding
lithium compounds, the direct use of lithium compounds in
catalysis would reduce the need for such additional
transformations. In fact, Group 1 complexes were sporadically
reported for hydrosilylation of alkenes17 or hydrogenation of
aldimines,18 but they were not very efficient compared to their
corresponding group 2 complexes in terms of reactivity or
H
+
H
B
R
O
R
R'
R= H/alkyl/aryl; R'=alkyl/aryl
OLi—THF
THF
Li
tBu
tBu
Dipp
Li
N
Dipp
TMEDA
N
Fe
Catalyst:
Li
1c
1a
1b
Scheme 1. Hydroboration of aldehydes and ketones using 1a, 1b and 1c as catalysts.
Okuda and coworkers noted that the success of lithium
catalysts relies on the combination of the Lewis acidity of the
Li atom and the hydridicity of the borate anions. The Li
catalysts with [HB(C6F5)3] anion was reported to be inert which
was attributed to the diminished hydridicity of the borate
anion.19 Thus, there remains a scope for a study of catalytic
properties of lithium compounds with no hydridic hydrogen
present in the counter anion. In light of our interests in
developing catalytic methods for reduction of carbonyl
compounds,4,12,22,23 we attempted to use three popular and
readily accessible lithium compounds with different
electronegative substituents such as 2,6-ditertbutyl phenolate
lithium (1a), nacnac lithium (1c) and 1,1' dilithioferrocene (1b
)
(Scheme 1) for the reduction of aldehydes and ketones with
HBpin at the ambient conditions. The reason behind such
selection is to examine how the Lewis acidity of the Li center
influence the catalytic activity. For example, the Li atom in 1c is
purportedly more Lewis acidic than 1a as the Li atom in 1c is
bound to two nitrogen atoms, while the Li atom in 1a is bound
to only one oxygen atom. It is also interesting to compare the
catalytic competence of 1b with others as it possesses of two
active Li centres although they are bound to less
electronegative carbon atoms. Consistent to our hypothesis,
a.Inorganic Chemistry and Catalysis Division, CSIR-National Chemical Laboratory,
Dr. Homi Bhabha Road, Pashan, Pune 411008, India. E-mail: ss.sen@ncl.res.in
b.Physical and Material Chemistry Division, CSIR-National Chemical Laboratory, Dr.
Homi Bhabha Road, Pashan, Pune 411008, India.
Electronic Supplementary Information (ESI) available: Experimental Details, details
and
of
DOI: 10.1039/x0xx00000x
DFT
calculations,
representative
NMR
spectra.
See
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the catalytic activity (TOF) of 1c is found to be better than 1a electron-withdrawing and electron-donating functionalities
or 1b. To broaden the horizon of the lithium compounds in 3a-3h) gave good yields, demonstrating a negligible electronic
catalysis, we have successfully employed 1a 1c for the effect. Increasing the steric demands has also a little effect on
cyanosilylation of the carbonyl compounds; a transformation the yield as seen in case of hydroboration of benzophenone
thus far not known to be catalyzed by group 1 complexes. 3i). All the catalysts show good functional group tolerance.
(
-
(
The nitrile (2g), hydroxy (2e and 2f), heterocycle (2l and 2m),
amino (3g) containing substrates yielded the desired boronate
esters. Even, the bromo functionality (2k and 3e) does not
suffer from lithium–bromide exchange. In some cases, their
catalytic efficiencies vary, as 1c gives the lowest yield for 2b
and 2c among them. 1b gives the quantitative yield in the most
cases presumbaly due to the presence of two Li centres,
except 2i and 3l. Excellent chemoselectivity was observed in
competitive experiment of all three catalysts (ESI, Scheme S1).
O
B
O
B
O
B
O
B
O
B
O
B
O
O
O
O
O
O
O
O
O
O
O
O
H
H
H
H
H
H
H
H
H
H
H
H
O2
N
OH
F
HO
H3
C
2a
2c
78a/99b/60c
2d
2b
80a/98b/52c
2e
2f
71a/71b/73c
91a/94b/90c
96a/99b/97c
74a/80b/83c
O
B
O
O
O
B
O
H
O
B
B
H
B
O
O
O
O
O
O
O
O
O
H
H
MeO
MeO
H
H
H
H
H
H
NC
OMe
Br
2g
2i
82a/67b/85c
2h
2k
2j
We compared the catalytic activities of 1a-1c for
99a/>99b/>99c
98a/>99b/>99c
>99a/99b/>99c
97a/98b/99c
benzophenone and trans-cinnamaldehyde with some known
catalysts. The most active one is the lithium
hydridotriphenylborate, which showed a remarkable TOF of
O
O
O
O
B
O
B
B
B
B
O
O
O
O
O
O
O
O
O
O
H
H
H
H
H
H
H
O
1
N
66600 h− for benzophenone.19 Among rare earths, Marks'
H
H
H
1
LaNTMS has the highest TOF of ›40000 h− , while among
2pd
2m
95a/98b/99c
2n
2o
2l
94a/96b/96c
92a/82b/92c
64a/90b/65c
92a/94b/92c
transition metals,24a Mankad's copper carbene was reported to
1
the most active (TOF of 1000 h− ).24b Hill's magnesium alkyl
Scheme 2. Substrates scope for the hydroboration of aldehydes. Reaction conditions:
Catalyst: 0.1 mol%, room temperature in THF. Reaction time (except trans-
cinnamaldehyde): 1a: 60 min, 1b: 40 min, 1c: 50 min. Yields are calculated based on the
integration area of product and starting material signals in the 1H spectra using
complex was reported with
benzophenone,3 while Stasch's magnesium catalyst [(L)MgH]4
a
TOF of 500 h-1 for
mesitylene as an internal standard. Superscripts a, b and c stand for the catalysts 1a, 1b (L = Ph2PNDipp; Dipp = 2,6-iPr2C6H3) was recorded with a TOF
and 1c, respectively; dthe reaction time for trans-cinnamaldehyde reduction was 5 h.
of 1760 h-1.5 We have found 1c shows very good efficiency
with a TOF of 495 h-1. Although 1b gives the best yield, the
activities of 1a and 1b is slightly poorer (TOF: 330 h-1 and 247
h-1 respectively) than 1c. The TOF of the trans-cinnamaldehyde
reduction was calculated to be 130 h-1 for 1c, which is only
second to Okuda's lithium hydridotriphenylborate(210 h-1).19 In
comparison, 1a or 1b was recorded with a TOF of 128 and 90
h-1, respectively, but their superiority was marked over the
other reported main group catalysts for trans-cinnamaldehyde
reduction (e.g. [Mg(thf)6][HBPh3]2: 11.2 h-1,5 nacnacAl(H)OTf:
16.5 h-1,14 PhC(NtBu)2SiHCl2: 63.3 h-1,12 PhC(NiPr)2CaI: 69 h-1).4
We have investigated the hydroboration mechanism for 1a
and 1c. We found that 1a reacts with HBpin but no change in
1
the H NMR was detected in the reaction of 1a and aldehyde.
However, the NMR experiments indicate that no reaction
takes place between 1c and HBpin. Therefore, the catalytic
pathways for 1a and 1c are appeared to be different. The
reaction between 1a and HBpin in toluene-d8, shows a
resonance at δ 4.7 ppm in the 11B NMR, indicating a four-
Scheme 3. Substrates scope for the hydroboration of ketones. Reaction conditions:
Catalyst: 0.1 mol%, room temperature in THF. Reaction time: 1a: 3 h, 1b: 2 h, 1c: 2h.
coordinated sp3 boron atom.25a However, prolonged time led
Yields are calculated w.r.t. mesitylene as internal standard.
-
to formation of trialkoxyborane [2,6-tBu2-C6H3-OBpin] and BH4
anion as a singlet and a quintet started to appear at δ 21.62
and –39.83 ppm after 2-3h, respectively.25b To obtain
The hydroboration reactions for a variety of aldehydes
(Scheme 2, 2a-2p) and ketones (Scheme 3, 3a-3m) have been
mechanistic insight, full quantum chemical calculations were
done with density functional theory (DFT) at the dispersion
and solvent corrected PBE/TZVP level of theory. The pathway
is initiated with O-coordination of HBpin to the lithium atom of
1a, resulting in the exergonic (ΔE= –11.5 kcal/mol and ΔG= –
0.7 kcal/mol) formation of Int-1 having a O···Li bond length of
1.92 Å. This binding mode is in agreement with the crystal
structure of [DippnacnacCa(H2Bpin)]3 where primary bonding
between the anion and the metal cation proceeds through
evaluated using 1a/1b/1c as a catalyst (For optimization,
please see ESI, Table S1a-1c, S3a-3c). Both aliphatic and
aromatic aldehydes underwent hydroboration within an hour
with excellent TOFs (ESI, Table S2), which reflect the high
efficiency of the catalysts. Similarly, a wide range of aromatic
and aliphatic ketones were converted to the corresponding
boronate esters within 2-3 hours keeping the mol% constant
(ESI, Table S4). Acetophenone derivatives bearing both
2 | J. Name., 2012, 00, 1-3
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O···Ca interaction.26 The other possibility of coordinating and the barrier (ΔG#) corresponding to the transition state is
phenolate oxygen to boron atom of HBpin leading to a four 15.7 kcal/mol.
coordinate boron complex
(
Int-2
)
was found to be
thermodynamically unfavorable as the ΔG of reaction was 15.2
kcal/mol. In the next step, the carbonyl oxygen atom of
benzaldehyde attacks the boron centre of HBpin, with the
hydride being transferred from the boron centre to the
Scheme 5. Examples of the reported main-group catalysts for the cyanosilylation of
carbonyl compounds. This is the first report of group 1 catalyst for cyanosilylation of
aldehydes and ketones.
carbonyl carbon. This occurs through
a four-membered
transition state (TS-1), where a significant amount of the B-H
bond activation (1.29 Å) takes place which leads to the
formation of hydroboration product (Pdt) along with the
regeneration of 1a (Scheme 4). The ΔE (-27.2 kcal/mol) and ΔG
(-22.3 kcal/mol) values for this step are highly negative and the
activation energy (ΔG#) barrier corresponding to the transition
state is calculated to be 25.7 kcal/mol.
Scheme 6. The scope of cyanosilylation with aldehyde and ketone substrates. Reaction
conditions: Catalyst: 0.1 mol%, room temperature in THF. Reaction time for aldehydes:
1h and for ketones: 2h. Yields are calculated w.r.t. mesitylene as internal standard.
Scheme 4. The catalytic cycle and reaction mechanism for the aldehyde hydroboration
by catalyst 1a, ΔG and ΔG# represent the Gibbs free energy of reaction and the Gibbs
free energy of activation respectively. All values are in kcal/mol.
In contrast to a large number of publications on main
group compound catalyzed hydroboration, only a few studies
on the main group compound catalyzed cyanosilylation have
been reported (Scheme 5).12,22,23,27-32 A majority of them were
reported to catalyze only aldehydes.22,27-30 The use of alkaline
earth metal complexes in catalytic cyanosilylation is an
emerging area. Recently, we and Ma et al. independently
reported the cyanosilylation of aldehydes and ketones with a
calcium complex, PhC(NiPr)2CaI23 and a magnesium(I) complex,
In case of 1c, a weakly bound complex (Int-3) is formed
between 1c and benzaldehyde, with the oxygen atom of
benzaldehyde approaching towards the lithium atom of 1c
(ESI, Scheme S3). The ΔE and ΔG for this step are -19.5 and -5.5
kcal/mol, respectively. Subsequently, the nucleophilic attack
by the lone pair of the N atom of 1c in Int-3 to the vacant p
orbital of the boron centre of HBpin takes place. A four
coordinated B centre is thus generated (Int-4). The ΔE and ΔG
for this step are -5.3 kcal/mol and 7.2 kcal/mol, respectively.
The activation free energy for the N···B bond formation is 24.1
kcal/mol. In the next step, the hydride is transferred from the
boron centre to the electrophilic carbonyl carbon of the
benzaldehyde through a three-membered transition state (TS
{(Xylnacnac)Mg}2
(Xyl=2,6-Me2-C6H3),32
respectively.
Nevertheless, to our knowledge, the use of lithium compounds
as catalysts in cyanosilylation has not been reported so far. An
initial screening of the catalytic effect of 1a-1c revealed good
conversion in most cases at room temperature in a reasonable
amount of time (for aldehyde: 1h & for ketone: 2h) with 0.1
mol% catalyst loading (Scheme 6). The catalytic efficiencies
3), with a barrier of 21.6 kcal/mol. In this transition state, there
and selectivity of 1a-1c were found to be very similar.
is a significant amount of B-H bond activation (1.33 Å vs 1.19 Å
in the intermediate complex) occurs, which allows the hydride
transfer from the boron to the carbonyl carbon centre. In the
last step, the oxygen centre of the aldehyde attacks the boron
centre of pinacolborane and simultaneously, B···N bond
cleavage occurs along with N···Li bond formation. This takes
place through a four-membered transition state (TS-4) and
leads to the formation of hydroboration product (Pdt-1) along
with the regeneration of the catalyst. The ΔE (-14.5kcal/mol)
Benzaldehyde, acetophenone, and benzophenone (entries 4a,
4f and 4k) were smoothly converted into the corresponding
cyanohydrins. For the reduction of benzophenone, these
lithium catalysts are superior to the IV (3 mol%, 2 h, 82%
yield). No other main group catalyst is reported for
cyanosilylation of benzeophenone so far. In the case of α,β-
unsaturated cinnamaldehyde, the 1,2 addition of TMSCN
selectively took place, no Michael addition product was
formed (entry 4c). Cyanation of aliphatic aldehyde and ketone
and ΔG (-29.9 kcal/mol) values for this step are very favourable
(entries 4e
,
4m
,
4n and 4o) was found to proceed efficiently.
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-1c can include both electron
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.
Herein, we have demonstrated that the reduction of a
variety of carbonyl compounds with HBpin and Me3SiCN can
be catalyzed rapidly by very simple lithium compounds (1a-1c)
under mild and facile conditions with excellent functional
group tolerance and TOFs. We have investigated the
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studies for cyanosilylation are currently ongoing. The
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Conflicts of interest
There are no conflicts to declare.
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