Expanding the limits of catalysts with low-valent main-group elements for the hydroboration of aldehydes and ketones using [L?Sn(ii)][OTf] (L? = aminotroponate; OTf = triflate)

Article abstract of DOI:10.1039/c8dt02857e

A triflatostannylene [L^?Sn(ii)][OTf] (2) is reported here as an efficient catalyst with low-valent main-group element for the hydroboration of aldehydes and ketones (L^? = aminotroponate). Using 0.025-0.25 mol% of compound 2, hydroboration of various aldehydes and ketones is accomplished in 0.13-1.25 h at room temperature; the aliphatic aldehydes show an impressive TOF of around 30?000 h^?1. DFT calculations are performed to explore the mechanistic aspects of this reaction suggesting that the reaction proceeds via a stepwise pathway with hydridostannylene [L^?Sn(ii)H] (2a) as the active catalyst and the H atom transfer from the Sn-H bond to the carbonyl carbon being the rate determining step.

Full text of DOI:10.1039/c8dt02857e

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PAPER
Expanding the limits of catalysts with low-valent
main-group elements for the hydroboration of
aldehydes and ketones using [L†Sn(II)][OTf]
(L† = aminotroponate; OTf = triflate)†
Cite this: Dalton Trans., 2019, 48,
664
Mahendra Kumar Sharma, a Mursaleem Ansari,b Pritam Mahawar,a
a
Gopalan Rajaraman b and Selvarajan Nagendran
*
A triflatostannylene [L†Sn(II)][OTf] (2) is reported here as an efficient catalyst with low-valent main-group
element for the hydroboration of aldehydes and ketones (L† = aminotroponate). Using 0.025–0.25 mol%
of compound 2, hydroboration of various aldehydes and ketones is accomplished in 0.13–1.25 h at room
temperature; the aliphatic aldehydes show an impressive TOF of around 30 000 h−1. DFT calculations are
performed to explore the mechanistic aspects of this reaction suggesting that the reaction proceeds via a
stepwise pathway with hydridostannylene [L†Sn(II)H] (2a) as the active catalyst and the H atom transfer
from the Sn–H bond to the carbonyl carbon being the rate determining step.
Received 12th July 2018,
Accepted 12th November 2018
DOI: 10.1039/c8dt02857e
rsc.li/dalton
minium atoms, such as [L′AlH2] (I) and [{(2,4,6-Me3-C6H2)NC
(Me)}2(Me)(H)]AlH·(NMe2Et) (J), were exploited by Roesky and
Introduction
In organic chemistry, the reduction of aldehydes and ketones Nembenna, respectively.20,21 Jones and co-workers used group
is one of the most important transformations due to the 14 element compounds, namely germylene L#GeH (K) and
requirement of the resultant alcohols for fine chemical pro- stannylene L#SnH (L) hydrides, as potential catalysts [(L# = −N
duction and natural product synthesis.1 In this respect, the (Ar′)(SiPri3), Ar′ = C6H2{C(H)Ph2}2Pri-2,6,4)],22 and germylene
catalytic hydroboration of aldehydes and ketones using HBpin L*Ge: (M) was introduced by Zhao and co-workers {L* = CH
is a very significant reaction. This reaction in recent years has [(CvCH2)CMe][N(Ar)]2} (Chart 1).23 Wesemann et al. reported
witnessed a surge in the use of main-group element com- that intramolecularly stabilized germylene and stannylene
pounds as catalysts.2–25 Alkali metal hydridotriphenylborates PhCH(PPh2)M(Ar*) [M = Ge (N), Sn (O)] can also be used as
[(L)M][HBPh3] (M = Li (A), Na (B), K (C); L = tris{2-(dimethyl- catalysts [Ar* = 2,6-(2,4,6-iPr3C6H2)2C6H3] (Chart 1).24 An ami-
amino)ethyl}amine) and magnesium hydridotriphenylborates dinate ligand stabilized dichlorosilane PhC(NtBu)2SiHCl2 (P)
[Mg(thf)6][HBPh3]2 (D) were used as catalysts by the group of was found to be catalyst by the group of Sen.25 1,3,2-
Okuda.14,15 A benzamidinatocalcium iodide, [PhC(NiPr)2CaI] Diazaphospholene (HCNtBu)2PH (Q), a group 15 element
(E), isolated by Sen and co-workers was demonstrated as a compound, was shown to be useful by Kinjo and co-
useful catalyst.16 Mulvey and co-workers employed lithium di- workers.26 As we have a long-standing interest in developing
amidodihydridoaluminate (HMDS)2AlH(μ-H)Li(THF)3 (F) as a the applications of compounds with low-valent main-group
catalyst.17 The groups of Hill and Stasch utilized magnesium elements, we researched on the use of base-stabilized stanny-
based catalysts [L′MgnBu] (G) and [(L″MgH)4] (H) (L′ = HC
(CMeNAr)2, L″ = ArNHPPh2; Ar = 2,6-iPr2C6H3), respect-
ively.18,19 Group 13 element compounds that contain alu-
aDepartment of Chemistry, Indian Institute of Technology Delhi, Hauz Khas,
New Delhi 110 016, India. E-mail: sisn@chemistry.iitd.ac.in
bDepartment of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai 400 076, India
†Electronic supplementary information (ESI) available: Characterization data of
hydroboration products, copies of NMR spectra, and computational details.
CCDC 1848467 and 1848468. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c8dt02857e
Chart 1 Catalytic hydroboration of aldehydes and ketones using com-
pounds with low-valent main-group elements.
664 | Dalton Trans., 2019, 48, 664–672
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Scheme 1 Synthesis of triflatostannylene [L†Sn(II)][OTf] (2).
lenes as catalysts for the transformation under discussion.
This exercise has resulted in identifying [L†Sn(II)][OTf] (2) as
an efficient low-valent main-group catalyst for the hydrobora-
tion of aldehydes and ketones (L†
(Chart 1).
= aminotroponate)
Fig. 2 Molecular structure of triflatostannylene 2. Thermal ellipsoids
are drawn at the 40% probability level. All hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (°): Sn1–O2 2.305(2),
Sn1–O1 2.096(4), Sn1–N1 2.216(1), Sn1–O1* 2.754(2); O1–Sn1–N1 73.2(4),
O1–Sn1–O2 88.6(3), N1–Sn1–O2 89.3(4). Data collection temperature:
100 K.
Results and discussion
The reaction of SnCl2 with a stoichiometric amount of L†Li
gave yellow colored monochlorostannylene 1 in 85% yield (L† =
aminotroponate) (Scheme 1).27 The treatment of 1 with silver
triflate in dichloromethane yielded triflatostannylene 2 quanti-
tatively as an off-white solid (Scheme 1). Stannylenes 1 and 2 are
soluble in benzene, toluene, diethyl ether, tetrahydrofuran, di-
chloromethane, and chloroform. They are stable at room temp-
erature under an inert atmosphere and are characterized by
NMR spectroscopic studies. In the 19F NMR spectrum of com-
pound 2, a signal at −78 ppm due to the triflate moiety was
seen. In the 119Sn NMR spectra of compounds 1 and 2, singlet
resonances at −158 and −379 ppm were observed, respectively.
Single crystal X-ray diffraction studies confirm the mole-
cular structures of stannylenes 1 and 2 (see the ESI† for
details); they crystallized in the monoclinic and triclinic space
(2.096(4) Å) bond in it, and reveals the weakly coordinating
nature of OTf with the tin atom.
To study the efficiency of the well-defined stannylenes 1
and
2 in the hydroboration of aldehydes and ketones
using pinacolborane (HBpin), optimization experiments were
carried out by using benzaldehyde as the substrate. When
benzaldehyde reacted with 1.2 equiv. of HBpin in the absence
of a catalyst at room temperature, no conversion was witnessed
(entry 1, Table 1). The reactions between benzaldehyde and 1.2
equiv. of HBpin using 0.1 mol% of 1 and 2 in toluene took 40
and <5 min at room temperature to afford 97 and >99% con-
ˉ 28
groups P21/c and P1. Both are dimeric due to intermolecular
Sn⋯X interactions (X = Cl 1, O 2) (Fig. 1 and 2). The Sn⋯Cl
(3.415 Å) and Sn⋯O (2.754 Å) bond lengths are longer than the
Sn–Cl (2.531(2) Å) and Sn–O(2)OTf (2.305(2) Å) bonds in them,
Table 1 Optimization of the reaction conditions for the hydroboration
of benzaldehyde using HBpin in the presence of stannylene 1/2 as a
catalyst
†
respectively. The latter bond in 2 is longer than the Sn(1)–O(1)L
Entry
Cat. (mol%)
Solvent
Time (h)
% conversion
1
2
3
4
5
6
7
—
1 (0.1)
2 (0.1)
2 (0.025)
2 (0.025)
2 (0.025)
2 (0.025)
Toluene
Toluene
Toluene
Toluene
Benzene
THF
12
Trace
97
>99
>99
92
0.66
<0.09
0.33
0.33
0.33
0.33
84
70
CH2Cl2
Conditions: Benzaldehyde (1 mmol), HBpin (1.2 mmol), room
temperature. % conversions were obtained by 1H NMR spectroscopy;
(a) for aldehydes, the integration of RCHO was compared with that of
RCH2(OBpin), (b) for methyl ketones, the integration of RC(O)CH3 was
compared with that of RCH(OBpin)(CH3) or RCH(OBpin)(CH3), and (c)
for benzophenone, the integration of (C6H5)2CO (only the ortho
protons of phenyl rings) was compared with that of (C6H5)2CH(OBpin)
or (C6H5)2CH(OBpin) (only the ortho protons of phenyl rings).
Fig. 1 Molecular structure of compound 1. Thermal ellipsoids are
drawn at the 40% probability level. All hydrogen atoms are deleted for
clarity. Selected bond lengths (Å) and angles (°): Sn1–Cl1 2.531(2), Sn1–
O1 2.127(4), Sn1–N1 2.183(5), Sn1–Cl1# 3.415(2); O1–Sn1–N1 74.12(2),
O1–Sn1–Cl1 90.19(1), N1–Sn1–Cl1 91.86(14). Data collection tempera-
ture: 150 K.
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version (entries 2 and 3, Table 1). This shows that stannylene 2 functional group tolerability of catalyst 2 (entry 3, Table 2).
is far better than its chloro analogue 1; for this reason only 2 Aromatic aldehydes with one or more electron donating
was studied further. As the rate of hydroboration with groups (OMe, Me) on the aryl ring were converted (% conver-
0.1 mol% of 2 was very high, to accurately monitor the reaction sion above 98) into the corresponding boronate esters within
completion, the catalytic loading was reduced to 0.025 mol% 0.5 h with a TOF of around 7800 h−1 (entries 4–7, Table 2). The
in all the further studies with aldehydes. Under this catalytic substrates with electron withdrawing groups (Cl, Br, NO2)
loading, benzaldehyde took 0.33 h for >99% conversion (entry underwent smooth hydroboration reactions to afford the
4, Table 1). To choose the right solvent, 1.2 equiv. of HBpin corresponding boronate esters faster (0.3–0.41 h) than the sub-
was reacted with 1 equiv. of benzaldehyde for 0.33 h using strates with electron donating groups on the aryl ring (entries
0.025 mol% of 2 in various common solvents other than 8–10, Table 2). This can be realized from the TOF values also
toluene. The data obtained [benzene, 92% conversion (entry 5, (9600–13 200 h−1). Heterocyclic aldehydes, such as 2-thiophene-
Table 1); tetrahydrofuran, 84% conversion (entry 6, Table 1); carboxaldehyde and 2-furaldehyde, were rapidly hydrobory-
and dichloromethane 70% conversion (entry 7, Table 1)] lated with a % conversion of 99 in 0.25 h with a TOF of
showed that toluene is the best solvent for this reaction. The 15 840 h−1 without the de-aromatization of the heterocyclic
TOF calculated for benzaldehyde under the optimized reaction rings (entries 11 and 12, Table 2).
conditions is 12 000 h−1 (entry 4, Table 1).
After aldehydes, the catalytic hydroboration of ketones was
The substrate scope for catalyst 2 is discussed in Table 2. examined. Under 0.025 mol% loading of 2, acetone took 1.6 h
Aliphatic aldehydes, such as propionaldehyde and n-butyralde- for 99% conversion indicating that the hydroboration of ali-
hyde, took only 0.13 h to yield the corresponding boronate phatic ketones is relatively slower than that of aliphatic alde-
esters providing a TOF of 30 461 h−1 (entries 1 and 2, Table 2). hydes. Therefore, for short reaction times, hydroboration of
The hydroboration of trans-cinnamaldehyde afforded the ketones was studied with 0.25 mol% of 2. With this catalytic
hydroborylated product after 0.25 h with 99% conversion and loading of 2, near-quantitative conversion of acetone into the
showed a TOF of 15 840 h−1. Furthermore, this occurred corresponding boronate ester was achieved in 0.16 h with a
without affecting the CvC double bond and stands for the TOF of 2475 h−1 (entry 13, Table 2). Other aliphatic ketones,
Table 2 Hydroboration of aldehydes and ketones catalyzed by compound 2a
Entry
Substrate
Catalyst (mol%)
Time (h)
% conversion
TOF (h−1
)
1
2
3
4
5
6
7
8
Propionaldehyde
n-Butyraldehyde
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.13
0.13
0.25
0.50
0.50
0.50
0.50
0.30
0.30
0.41
0.25
0.25
0.16
0.16
0.16
0.16
0.41
0.41
0.41
0.33
0.33
0.33
0.33
0.33
0.50
1.25
>99
>99
99
99
98
99
98
99
99
99
99
99
99
99
99
99
99
99
98
99
99
99
99
98
98
99
30 461
30 461
15 840
7920
7840
7920
7840
13 200
13 200
9658
15 840
15 840
2475
2475
2475
2475
965
trans-Cinnamaldehyde
4-Methoxylbenzaldehyde
2-Methylbenzaldehyde
4-Methylbenzaldehyde
2,4,6-Trimethylbenzaldehyde
4-Chlorobenzaldehyde
4-Bromobenzaldehyde
3-Nitrobenzaldehyde
2-Thiophenecarboxaldehyde
2-Furaldehyde
Acetone
4-Phenyl-2-butanone
2-Pentanone
Methylisopropylketone
Acetophenone
4-Methylacetophenone
4-Methoxyacetophenone
4-Fluroacetophenone
4-Chloroacetophenone
4-Bromoacetophenone
4-Nitroacetophenone
2-Chloroacetophenone
3-Bromoacetophenone
Benzophenone
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
965
956
1200
1200
1200
1200
1187
784
0.25
0.25
0.25
0.25
0.25
316
a Conditions: Aldehyde/ketone (1 mmol), HBpin (1.2 mmol). % conversions were obtained according to the procedure mentioned in the footnote
of Table 1.
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such as 4-phenyl-2-butanone, 2-pentanone, and methyliso-
propylketone, could also be converted into the corresponding
hydroborylated products within 0.16 h to afford a % con-
version of 99 with a TOF of 2475 h−1 (entries 14–16, Table 2).
Then, ketones with one aromatic group on the carbonyl
carbon were studied. It was found that they require longer
reaction times than the aforementioned aliphatic ketones.
Thus, a reaction of acetophenone with HBpin took 0.41 h for
almost quantitative conversion with a TOF of 965 h−1 (entry
17, Table 2). Aromatic ketones with electron donating substitu-
ents on the phenyl ring, such as 4-methylacetophenone and
4-methoxyacetophenone, underwent hydroborylation similar
to acetophenone and produced the corresponding hydrobory-
lated products in 0.41 h with a % conversion of 98 (entries 18
and 19, Table 2). Aromatic ketones with electron withdrawing
substituents (such as 4-fluroacetophenone, 4-chloroacetophe-
none, 4-bromoacetophenone, 4-nitroacetophenone, and
2-chloroacetophenone) on the aryl ring gave a % conversion
around 98 after 0.33 h with a TOF of 1200 h−1 (entries 20–24,
Table 2). In contrast, 3-bromoacetophenone required 0.50 h to
produce 98% conversion into the desired boronate ester with a
TOF of 784 h−1 (entry 25, Table 2). Ketones with two aromatic
groups on the carbonyl carbon, namely, benzophenone, took
1.25 h for 99% conversion (entry 26, Table 2) with a TOF of
316 h−1. The TOF for this substrate is lower than those of the
ketones with one aromatic group on the carbonyl carbon.
The efficiency of catalyst 2 is compared with other main
group catalysts A, E, K, L, and Q for which the TOF values are
reported. For this purpose, the TOF values obtained for a few of
the standard substrates, such as propionaldehyde, n-butyr-
aldehyde, benzaldehyde, acetophenone, benzophenone, and
cinnamaldehyde, are considered (Table 3). To the best of our
knowledge, the current leading main group catalytic system for
the hydroboration of aldehydes/ketones is A.14 As shown in
Table 3, with the exception of cinnamaldehyde, the efficiency of
A with respect to all the other substrates is better than that of 2.
Chemoselective hydroboration using compound 2 was also
screened (Scheme 2). The reaction of equimolar amounts of
benzaldehyde, acetophenone, and HBpin with 0.025 mol% of 2
(eqn (1), Scheme 2) resulted in the exclusive hydroboration of
benzaldehyde only with 99% conversion after 0.33 h. Repetition
of this reaction with activated ketones, such as 4-bromoaceto-
phenone instead of acetophenone (eqn (1), Scheme 2), also
Scheme 2 Inter- and intramolecular chemoselective hydroboration of
aldehydes in the presence of other functional groups using
2
(0.025 mol%). % conversions given below the structures of the hydro-
borylated products were obtained according to the procedure men-
tioned in the footnote of Table 1.
afforded the same result. These reactions offer support to the
intermolecular chemoselectivity achievable using catalyst 2. The
intramolecular chemoselective hydroboration of aldehydes in
the presence of other functional groups, namely ketone, ester,
and amide, was also investigated (eqn (2)–(4), Scheme 2). In all
these reactions, only the aldehydic groups of 4-acetylbenzalde-
hyde, methyl 4-formylbenzoate, and 4-acetamidobenzaldehyde
were hydroborylated within 0.33 h without affecting the keto,
ester, and amide functional groups in them, respectively.
To understand the mechanism of hydroboration of alde-
hydes and ketones using 2, DFT calculations were carried out
using benzaldehyde as the substrate. Two mechanistic path-
ways were studied: (a) concerted (see Scheme S1 in the ESI;†
Fig. 3) and (b) stepwise (see Scheme S2 in the ESI;† Fig. 4). In
the concerted pathway, catalyst 2 initially interacts with benz-
aldehyde to give 2a′ (Sn–OC = 2.980 Å) and its formation is
computed to be endothermic by 16.6 kcal mol−1 (weak
Table 3 Comparison of the TOF values obtained (for a few of the standard substrates) using 2 with other main group catalysts A, E, K, L, and Q
Catalysta
Substrate
2
A
E
K
L
Q
Propionaldehyde
n-Butyraldehyde
Benzaldehyde
Acetophenone
Benzophenone
Cinnamaldehyde
30 461 (>99)
30 461 (>99)
12 000 (99)
965 (99)
316 (99)
15 840 (99)
—
—
—
249 (83)
9.5 (95)
8.6 (86)
69 (91)
2000 (>99)
—
67 (>99)
—
1.7 (>99)
—
≥13 300 (>99)
—
—
≥60 000 (≥99)
≥60 000 (≥99)
≥60 000 (≥99)
66 600 (≥99)
210 (≥99)
—
800 (>99)
—
80 (>95)
—
404 (>99)
—
1.8 (>99)
—
a % conversions/yields are given in brackets.
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Fig. 3 Computed (B3LYP-D2/def2-TZVP) free energy profile for the concerted mechanism proposed for the catalytic hydroboration of benz-
aldehyde using HBpin’ in the presence of catalyst 2 (HBpin’ = a model compound with all the methyl groups of HBpin replaced by hydrogen atoms).
Free energy (kcal mol−1) and selected bond lengths (Å) are also shown. All hydrogen atoms (except B–H) are omitted for clarity.
complex formation) (OC represents the oxygen of the alde- computed to be exothermic by −18.9 kcal mol−1. Due to the
hyde). Then, the nucleophilic carbonyl oxygen of benzaldehyde prohibitively high barrier height for the formation of TS6 from
coordinates with the electrophilic tin(II) atom to form 2b′ (Sn– 2, the concerted pathway can be ruled out.
OC = 2.579 Å); this leads to stabilization in comparison with
In the stepwise pathway, the formation of a hydridostanny-
2a′. In the next step, 2b′ reacts with HBpin′ to generate 2c′; the lene 2a as an active catalyst from the reaction of precatalyst 2
formation of this species is endothermic. 2c′ then gets trans- with HBpin′ was considered (Scheme S2 in the ESI;† Fig. 4) and
formed to 2d′ (which is an adduct between catalyst 2 and the the formation of 2a is endothermic by 13.7 kcal mol−1 from
hydroborylated product 3) through a transition state TS6 where compound 2. Interaction between the nucleophilic carbonyl
σ-bond metathesis occurs. The activation energy of the TS6 is oxygen of benzaldehyde and the electrophilic tin(II) atom of 2a
62.2 kcal mol−1 and it is characterized by a single imaginary leads to a weak complex 2b followed by the coordination of the
frequency of −528 cm−1. TS6 involves a four-membered bora- oxygen atom of benzaldehyde to the tin atom of 2b to result in
cycle weakly attached to the tin atom through the carbonyl 2c. This is indicated by the reduction in the Sn⋯OC bond
oxygen atom. The Sn–O, C–O, and B–H bonds in TS6 are length from 2.953 Å (in 2b) to 2.691 Å (in 2c). The formations
elongated to 2.744, 1.306, and 1.288 Å from the corresponding of 2b and 2c are endothermic by 11.2 and 9.3 kcal mol−1 from
values of 2.522, 1.236, and 1.186 Å seen in 2c′, respectively. But catalyst 2a, respectively; this endothermicity is correlated with
the O–B and C–H bonds in 2c′ (4.725 Å and 3.406 Å) are shor- the energy required to form the Sn⋯OC bond. Then, the hydro-
tened to 1.596 and 1.835 Å in TS6, as anticipated, respectively. gen atom transfer from 2c to benzaldehyde leads to an alkoxys-
These changes in the bond lengths indicate that the breaking tannylene intermediate 2d through a transition state TS1. The
of the π-bond of the CvO group and the formation of the O–B formation of TS1 is the rate limiting step with a small energy
bond take place simultaneously. Furthermore, as one would barrier of 26.9 kcal mol−1 from 2a and it is characterized by an
expect for the bond formation between the B and O atoms, the imaginary frequency of −499 cm−1. The TS1 involves a four-
Mulliken charges on these atoms are found to be 0.85 and membered stannacycle, where the hydrogen atom interacts
−0.33 (in the TS6), respectively. In the next step, 2d′ affords the with the electrophilic carbonyl carbon atom and the nucleo-
hydroborylated product 3 and catalyst 2. The formation of 3 is philic carbonyl oxygen atom makes bonding interaction with
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Fig. 4 Computed (B3LYP-D2/def2-TZVP) free energy profile for the plausible stepwise mechanism proposed for the catalytic hydroboration of benz-
aldehyde using HBpin’ in the presence of active catalyst 2a. The formation of compound 2a is visualized through the reaction of precatalyst 2 with
HBpin’. Free energy (kcal mol−1) and selected bond lengths (Å) are also shown. All hydrogen atoms (except Sn–H and B–H) are omitted for clarity.
the electrophilic tin atom of 2c. Accordingly in TS1, the Sn–H atom transfer takes place through TS3 and results in 2h.
(1.867 Å) and C–O (1.274 Å) bonds are elongated in compari- The energy of TS3 is nearly the same as that of 2f suggesting a
son with the former (1.787 Å) and latter (1.227 Å) bonds in 2c. barrierless process. In the second possibility, hydride transfer
At the same time, the Sn⋯OC bond is shortened to 2.490 Å from the B–H bond of HBpin′ to the tin atom of 2e proceeds
from the corresponding value of 2.691 Å in 2c. These indicate through TS4 to afford an intermediate 2g. TS4 has a prohibi-
the weak interaction of the nucleophilic hydrogen atom (com- tively high energy barrier of 68.0 kcal mol−1; and it clearly indi-
puted Mullikan charge QH = −0.24 e) with the carbonyl carbon cates partial B–H bond breakage and Sn⋯H bond formation.
atom (QC = 0.11 e). The formation of alkoxystannylene 2d is Thus, the B–H bond in TS4 is elongated (1.686 Å) in compari-
slightly exothermic with respect to 2a. The interaction of 2d son with that in 2e (1.186 Å), and the Sn⋯H bond is strength-
with HBpin′ then leads to a weak complex 2e, and the for- ened (1.842 Å) relative to that in 2e (4.756 Å). But our attempts
mation of 2h (an adduct between active catalyst 2a and the to obtain intermediate 2g were unsuccessful, and the exact
hydroborylated product 3) from 2e was visualized through two barrier height for this process could not be estimated. In the
different possibilities. The first possibility is through TS2, next step, 2g should lead to 2h through the transition state
where the B atom of HBpin′ interacts with the alkoxide oxygen TS5. In TS5, interaction between the boron atom of HBpin′ and
atom of 2e. The calculated barrier height for the formation of the alkoxide oxygen atom occurs and this step also has a prohibi-
TS2 is 2.5 kcal mol−1 from 2e. TS2 is a four membered stanna- tive kinetic barrier of 55.4 kcal mol−1. Thus, our calculations
cycle and its structure clearly indicates the partial formation of clearly indicate that (from 2e) the initial hydrogen atom transfer
the O⋯B bond (2.190 Å against 5.924 Å in 2e). For the for- to the tin(II) centre followed by the O–B bond formation as seen
mation of the bond between B and O atoms, the charges on in the second possibility is highly unfavourable. Instead, the cal-
them are also conductive (0.80 and −0.61 respectively). culations clearly reveal that (from 2e) the initial O–B bond for-
Strengthening of this interaction results in intermediate 2f, mation followed by the hydrogen atom transfer to the tin(II)
where the B–O bond is almost fully formed. From 2f, hydrogen atom as in the case of the first possibility is highly feasible.
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Therefore, the formation of 2h from 2e should take the path as resonances. For 119Sn NMR spectroscopic studies, Me4Sn was
described in the first possibility. Finally, 2h affords the free used as an external reference.
catalyst 2a and the hydroborylated product 3. The formation of
Synthesis of chlorostannylene 1
the hydroborylated product 3 from 2h is exothermic by
−18.9 kcal mol−1, and therefore its formation serves as a L†Li (2.00 g, 10.91 mmol) and toluene (30 mL) were taken in a
thermodynamic sink for the overall reaction to take place. The Schlenk flask. This flask was kept inside the refrigerator (at
presence of a hydride on the tin(II) atom in 2a eases out the −40 °C) in the glove box for 1 h. Then, it was taken out and
energetics of the process and makes the stepwise pathway a SnCl2 (2.07 g, 10.91 mmol) was added to it and the mixture
probable one. Jones and co-workers proposed a similar step- stirred for 12 h at room temperature. The reaction mixture was
wise pathway for the catalytic hydroboration of aldehydes and then filtered using a G4 frit containing Celite. Removal of the
ketones using a two-coordinate stannylene L#SnH (L).22
solvent from the filtrate under reduced pressure gave a yellow
solid. It was washed with hexane (10 mL) and dried under
vacuum to afford an analytically pure sample of compound 1.
Single crystals of compound 1 suitable for X-ray diffraction
studies were obtained from its saturated toluene solution at
Conclusions
To conclude, a base stabilized triflatostannylene 2 is demon- −28 °C. Yield: 3.05 g, 85%. Mp: 121 °C (decom.). Anal. calcd
strated as an efficient catalyst for the hydroboration of various for C11H14ClNOSn (M = 330.40): C, 39.99; H, 4.27; N, 4.24.
1
aldehydes and ketones with a very good functional group toler- Found: C, 39.83; H, 4.09; N, 4.19. H NMR (300 MHz, CDCl3):
3
ance. Catalyst 2 is also promising for the chemoselective δ 1.08 (d, JHH = 6 Hz, 6H, CH(CH3)2), 2.17–2.26 (m, 1H,
3
3
hydroboration of aldehydes in the presence of keto, ester, and CH(CH3)2), 3.45 (d, JHH = 6 Hz, 2H, CH2), 6.90 (t, JHH = 9.9
3
3
amide groups. DFT calculations suggest that a stepwise Hz, 1H, CH), 7.02 (d, JHH = 12.0 Hz, 1H, CH), 7.28 (t, JHH
=
pathway that involves hydridostannylene 2a as an active cata- 10.2 Hz, 2H, CH), 7.34–7.45 (m, 1H, CH). 13C{1H} NMR
lyst is more preferred over a concerted pathway that uses com- (75 MHz, CDCl3): δ 21.49 (CH(CH3)2), 29.19 (CH(CH3)2), 55.68
pound 2 as the catalyst.
(CH2), 120.61 (CAr), 123.99 (CAr), 126.41 (CAr), 137.38 (CAr),
Encouraged by the efficiency of compound 2 for the hydro- 137.83 (CAr), 164.31 (CAr), 176.13 (CAr). 119Sn{1H} NMR
boration of aldehydes and ketones, we are currently checking (111.88 MHz, CDCl3): δ −158.42 ppm.
its potential to cyanosilylate aldehydes and ketones using tri-
Synthesis of triflatostannylene 2
methylsilyl cyanide (TMSCN). This should be interesting, as to
the best of our knowledge there is no example of a tin com- A solution of compound 1 (3.00 g, 9.07 mmol) in dichloro-
pound that can catalyze the cyanosilylation of aldehydes and methane (30 mL) was prepared in a Schlenk flask using the di-
ketones. The initial results obtained are highly encouraging; chloromethane taken out from the refrigerator of the glove
further details will be published elsewhere soon.
box. AgOTf (2.33 g, 9.07 mmol) was added to it and the reac-
tion mixture was stirred for 10 min at room temperature. The
reaction mixture was then filtered using a G4 frit containing
Celite. Removal of the solvent under reduced pressure from
the filtrate gave an off-white solid. It was washed with hexane
(15 mL) and dried under vacuum to give an analytically pure
Experimental section
General information
All the experiments were performed under an atmosphere of sample of compound 2. Yield: 3.95 g, 98%. Mp: 117 °C
dry N2 using glove box [GP(Concept)-T2, Jacomex work station] (decom.). Anal. calcd for C12H14F3NO4SSn (M = 444.01): C,
techniques. The temperature inside the glove box (room temp- 32.46; H, 3.18; N, 3.15. Found: C, 32.29; H, 3.27; N, 3.25. 1H
3
erature) was around 43 °C. Hexane, THF, benzene, and toluene NMR (400 MHz, CDCl3): δ 1.09 (d, JHH = 6.4 Hz, 6H, CH
3
were dried using conventional procedures. The lithium salt of (CH3)2), 2.19–2.28 (m, 1H, CH(CH3)2), 3.60 (s, JHH = 7.2 Hz,
3
aminotroponate (L†Li; L† = aminotroponate) was synthesized 2H, CH2), 7.34 (t, JHH = 9.2 Hz, 2H, CH), 7.62–7.67 (m, 2H,
3
according to the literature procedure.27 SnCl2 and AgOTf were CH), 7.77 (t, JHH = 9.6 Hz, 1H, CH). 13C{1H} NMR (100 MHz,
purchased from Sigma Aldrich and used as received without CDCl3): δ 21.14 (CH(CH3)2), 28.40 (CH(CH3)2), 53.95 (CH2),
any further purification. CDCl3 for NMR spectroscopic analysis 121.27 (CAr), 124.81 (CAr), 130.76 (CAr), 139.06 (CAr), 140.32
was obtained from Sigma Aldrich and dried over molecular (CAr), 162.31 (CAr), 173.98 (CAr). 19F{1H} NMR (376.46 MHz,
sieves (4 Å). The melting points of compounds 1 and 2 were CDCl3):
δ
−78.04. 119Sn{1H} NMR (149 MHz, CDCl3):
recorded using a Unitech Sales digital melting point apparatus δ −379.26 ppm.
by sealing the samples in glass capillaries and the reported
values are uncorrected. Elemental analyses were carried out
using a PerkinElmer CHN analyzer. 1H, 13C, and 119Sn NMR
General procedure for the hydroboration of aldehydes and
ketones
spectra were recorded on a 300 MHz Bruker Topspin/400 MHz To a solution of aldehyde/ketone (1 mmol) and HBpin
Bruker Topspin/400 MHz JEOL JNM-ECA NMR spectrometer. (1.2 mmol) in toluene (1 mL), catalyst 2 (0.025–0.25 mol%) was
The chemical shifts δ are reported in ppm and are referenced added. The resulting solution was stirred for an appropriate
internally to residual solvent (1H NMR) and solvent (13C NMR) time period at room temperature. The solvent was then
670 | Dalton Trans., 2019, 48, 664–672
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removed under reduced pressure, and the resultant product The data were collected under a steady flow of cold dinitrogen
1
was characterized by H and 13C NMR spectroscopy. As men- using Mo Kα radiation (λ = 0.71073 Å). The data were inte-
tioned in the footnote of Table 1, % conversions were obtained grated using SAINT, and an empirical absorption correction
1
by H NMR spectroscopy; (a) for aldehydes, the integration of was applied using SADABS.30 The structures were solved by
RCHO was compared with that of RCH2(OBpin), (b) for methyl direct methods and refined by full matrix least-squares on F2
ketones, the integration of RC(O)CH3 was compared with that using the SHELXTL software.31,32 All non-hydrogen atoms were
of RCH(OBpin)(CH3) or RCH(OBpin)(CH3), and (c) for benzo- refined anisotropically. The positions of the hydrogen atoms
phenone, the integration of (C6H5)2CO (only the ortho protons were fixed according to a riding model and were refined
of phenyl rings) was compared with that of (C6H5)2CH(OBpin) isotropically.
or (C6H5)2CH(OBpin) (only the ortho protons of phenyl rings).
A sample calculation of % conversion is shown in Fig. S72.†
The chemical shift (δ) values (in ppm) of RCHO, RC(O)CH3,
and (C6H5)2CO (only the ortho protons of phenyl rings) in the
substrates (used in the present study) are presented in
Table S3.† Note: The substrates propionaldehyde, n-butyralde-
hyde, acetone, 2-pentanone, and methylisopropylketone have
portion of the manuscript. P. M. assisted M. K. S. to carry out
volatilities equivalent to or greater than that of toluene. To
Author contributions
All the experimental studies were performed by M. K. S., who
also wrote the experimental part of the manuscript. M. A.
carried out the theoretical studies, and drafted the theoretical
some of the experiments. The experimental and theoretical
write-ups of the manuscript were corrected by S. N. and G. R.,
respectively.
find out the % conversions in the reactions involving these
1
substrates, the H NMR spectra of the reaction mixtures were
recorded without the removal of any volatiles; 0.1 mL of the
reaction mixtures was taken in NMR tubes with 0.4 mL of
CDCl3 and the 1H NMR spectra were recorded. The resultant
NMR spectra (that contain toluene signals) are provided in the
ESI (Fig. S11, S14, S40, S45, and S48†). Even for the substrates
having volatilities lower than that of toluene, their loss (if any)
Conflicts of interest
There are no conflicts to declare.
during the removal of the volatiles under reduced pressure can
make the reported % conversions erroneous. To show that this
is not happening, five such substrates having high boiling
Acknowledgements
points (162–210 °C) (such as benzaldehyde, 4-methyl-
benzaldehyde, 2-thiophenecarboxaldehyde, 2-furaldehyde, and
Technology Delhi (IITD), New Delhi, and Indian Institute of
4-fluroacetophenone) (see Table S2† for the boiling point data
M. K. S., P. M., and M. A. thank the Indian Institute of
Technology Bombay (IITB), Mumbai, India, for research fellow-
ships, respectively. S. N. thanks the SERB, Department of Science
of the substrates) were chosen and their catalytic hydrobora-
tion reactions were carried out. After the completion of the
and Technology (DST), New Delhi, India, for funding (SB/S1/
reactions, as done for the volatile substrates (vide supra), the
IC-46/2013) and the DST-FIST for establishing single crystal X-ray
diffraction (SR/FST/CSII-027/2014) and HRMS (SR/FST/CS-1-195/
2008) facilities in the Department of Chemistry, IIT Delhi. G. R.
1H NMR spectra of the reaction mixtures were recorded
without the removal of any volatiles. The obtained spectra
along with toluene signals (see Fig. S8, S23, S34, S37, and
thanks the INSA (Indian National Science Academy) and the
S57†) reveal the absence of substrate peaks. Furthermore,
these spectra are comparable to the corresponding spectra
recorded on the crude products obtained after the removal of
all the volatiles. On the basis of these data, it can safely be con-
SERB (EMR/2014/000247) for funding. Dedicated to Prof. Mitsuo
Kira on the occasion of his 75th birthday.
cluded that there is no loss of substrates during evaporation.
Although all the reported spectra are for the crude hydrobory-
Notes and references
lated products, impurities (such as residual HBPin, catalyst,
and starting material) are not seen in them due to the follow-
ing reasons: (i) the evaporation process helped in removing
the excess HBpin, (ii) the catalyst was used in very small quan-
tities in comparison with the substrates, and (iii) the % conver-
sions with respect to most of the substrates are high,
respectively.
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