Synthesis and characterization of tricarbastannatranes and their reactivity in B(C6F5)3-promoted conjugate additions

Article abstract of DOI:10.1002/anie.201500983

The synthesis and characterization of a series of tricarbastannatranes, in the solid state and in solution, are described. The structures of the complexes [N(CH₂CH₂CH₂)₃Sn](BF₄), [N(CH₂CH

Full text of DOI:10.1002/anie.201500983

Angewandte
Chemie
DOI: 10.1002/anie.201500983
Alkylation
Synthesis and Characterization of Tricarbastannatranes and Their
Reactivity in B(C6F5)3-Promoted Conjugate Additions**
Azadeh Kavoosi and Eric Fillion*
Abstract: The synthesis and characterization of a series of
tricarbastannatranes, in the solid state and in solution, are
spectroscopy, mass spectrometry (MS), and X-ray crystallog-
raphy.
described.
The
structures
of
the
complexes
The
formation
of
the
tricarbastannatrane
[N(CH2CH2CH2)3Sn](BF4),
[N(CH2CH2CH2)3Sn](SbF6),
[N(CH2CH2CH2)3Sn](BF4) (2) in THF was reported by
[N(CH2CH2CH2)3Sn]4[(SbF6)3Cl], and [(N(CH2CH2CH2)3-
Sn)2OH][MeB(C6F5)3] were determined by X-ray crystallog-
raphy. Furthermore, the B(C6F5)3-promoted conjugate addi-
tion of alkyl-tricarbastannatranes to benzylidene derivatives of
Meldrumꢀs acid was investigated, and detailed mechanistic
studies are presented.
Tzschach and Jurkschat, and it has a characteristic
119Sn NMR shift at d = 103 ppm (deshielded; see
Scheme 1).[7] It was suspected that the chemical shift might
not be indicative of free [N(CH2CH2CH2)3Sn]+ (3) in solution,
as 3 could potentially interact with THF.
The formation of 2 was reinvestigated in the absence of
a Lewis-basic solvent by the addition of AgBF4 to a solution
of 1 in 1,2-dichloroethane (Scheme 1). A 119Sn NMR chemical
A
lkyl-tricarbastannatranes are compounds with three fused
five-membered rings, in which the transannular N–Sn inter-
À
action makes the apical Sn C bond longer, and consequently
more reactive.[1,2] These reagents are air- and moisture-stable,
and readily prepared from chloro-tricarbastannatrane (1) and
the corresponding Grignard,[3] organolithium,[4,5] and dialkyl-
zinc reagents.[6d] It has been shown that alkyl-tricarbastanna-
tranes efficiently and selectively transfer the apical alkyl
group to a palladium(II) center.[3,4,6] The transfer generates
a Lewis acidic tricarbastannatrane which is stabilized by
delocalization of the positive charge to the nitrogen atom
Scheme 1. Preparation of the tricarbastannatrane 2.
shift
of
d = 145.8 ppm,
corresponding
to
[N(CH2CH2CH2)3Sn]+ (3) in complex
2
was observed
À
through formation of a transannular N Sn bond.
(Table 1, entry 2). NMR experiments also revealed that 2
was stable at room temperature for more than one week and
remained unchanged for more than 2 hours at 708C. Crystal-
lization of 2 from a n-pentane/1,2-dichloroethane mixture
yielded crystals that were analyzed by X-ray crystallography.
As depicted in Figure 1, the salient feature of the structure is
Utilizing alkyl-tricarbastannatranes as nucleophilic alky-
À
lating agents in C C bond-forming reactions is of great
synthetic interest. To the best of our knowledge, the direct
transfer of the apical alkyl group of alkyl-tricarbastannatranes
to an electrophilic carbon center has not yet been reported.
Herein, we present the B(C6F5)3-promoted conjugate addi-
tion of alkyl-tricarbastannatranes to benzylidene derivatives
of Meldrumꢀs acid. Furthermore, the structure and Lewis
acidity of tricarbastannatranes were established using NMR
[8,9]
À
its exceptionally short Sn N bond (2.22 ꢁ).
In addition,
the counterion [BF4]À interacts with the positively charged 3
(Sn F 2.37 ꢁ),[10] and HRMS (ESI) supported the formation
À
of 2 with an ion peak at m/z 260.04512, which corresponds to
3.
Additional information about the structure of tricarbas-
tannatranes was obtained by preparing [N(CH2CH2CH2)3Sn]
(SbF6) (4a) through the reaction of AgSbF6 with 1 (Fig-
ure 2a). The formation of 4a in solution was supported by
a deshielded 119Sn NMR signal at d = 197.8 ppm (Table 1,
[*] A. Kavoosi, Prof. E. Fillion
Department of Chemistry, University of Waterloo
200 University Ave W, Waterloo, ON N2L 3G1 (Canada)
E-mail: efillion@uwaterloo.ca
[**] This work was supported by the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canadian Foundation for
Innovation (CFI), the Ontario Innovation Trust (OIT), and the
University of Waterloo. Dr. J. Assoud, University of Waterloo, is
gratefully acknowledged for X-ray structure determination. Further
details on the crystal structure investigation may be obtained from
the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leo-
poldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysda-
ta@fiz-karlsruhe.de), on quoting the depository numbers CSD-
429137 (2), CSD-429136 (4a), CSD-429134 (4b) and CSD-429135
(9).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500983.
Figure 1. X-ray Structure of [N(CH2CH2CH2)3Sn](BF4) (2).
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Table 1: NMR studies on tricarbastannatranes.
Entry
Tricarbastannatranes
NMR chemical shifts [ppm]
1H
13C
119Sn
11B
1.13(t)
1.78 (m)
2.42(t)
1.47(t)
1.96 (m)
2.57(t)
1.61(t)
2.04 (m)
2.64(t)
13.0
22.9
54.3
12.6
23.6
55.0
14.2
24.4
55.4
1
2
3
17.6
n.a.
145.8
197.8
À2.1
n.a.
1.64(t)
2.04 (m)
2.63(t)
16.5
24.7
55.5
4
198.1
À7.2
1.37 (brs)
1.94 (m)
2.56 (t)[a]
11.5
5
6
7
23.3
131.8
61.4
À1.6
À1.7
À1.6
54.7[b]
13.0
broad
22.8
54.3[c]
1.45 (t)
12.5
1.95 (m)
23.6
142.5
2.57 (t)[d]
54.9[d]
Figure 2. X-ray Structure of a) [N(CH2CH2CH2)3Sn](SbF6) (4a) and
b) [N(CH2CH2CH2)3Sn]4[(SbF6)3Cl] (4b).
1.46 (t)
12.5
8
1.95 (m)
23.6
144.2
À1.6
2.56 (t)[e]
54.9[f]
[a] One broad signal observed for THF at d=1.82 ppm and the other
THF signal overlaps with the 1,2-dichloroethane signal. [b] Two signals at
d=25.01 and 68.12 ppm belong to THF. The carbon chemical shifts of
free THF in 1,2-dichloethane are d=25.2 and 66.9 ppm. [c] Two signals
at d=44.6 and 46.9 ppm belong to DABCO. Chemical shift of free
DABCO in 1,2-dichloroethane is d=47.09 ppm. [d] CH3CN signals in 1H
and 13C NMR spectra were d=2.16 and 1.5 ppm, respectively.
[e] Diphenylacetylene proton chemical shifts are d=7.35 and 7.51 ppm.
[f] 13C NMR chemical shifts of diphenyl acetylene are d=88.5, 122.3,
127.9, 127.9, and 131.0 ppm. n.a.=not applicable. DABCO=1,4-
diazabicyclo[2.2.2]octane.
[B[3,5-(CF3)2C6H3]4][11] was synthesized from Ag[B-
[3,5-(CF3)2C6H3]4].
A
deshielded 119Sn NMR signal was
observed at d = 198.1 ppm (Table 1, entry 4). The coordina-
tion of various Lewis bases to 2 was then studied (Table 1,
entries 5–8). While the addition of one equivalent of DABCO
showed a significant change of the 119Sn NMR chemical shift
from d = 145.8 to 61.4 ppm (Dppm = 84.4), adding one
equivalent of CH3CN (Dppm = 3.3) or diphenylacetylene
(Dppm = 1.6) showed negligible changes.
A
119Sn NMR
chemical shift of d = 131.8 ppm was observed after one
equivalent of THF was added to 3 (Dppm = 14.0), thus
indicating its moderate coordinating ability toward 3.[12] This
data reflects the exceptional stability and moderate Lewis
acidity of 3, thus resulting in the transannular Lewis acid/
Lewis base Sn–N interaction.
entry 3). In this complex, a longer Sn–F interaction (Sn-F
2.48 ꢁ and 2.52 ꢁ) and a more deshielded Sn center depict
a looser interaction between 3 and [SbF6]À compared to its
interaction with [BF4]À in 2. In addition, the Sn N bond
À
length is 2.21 ꢁ, thus suggesting a stronger transannular
Lewis-acid–base interaction than in 2. Of note, the complex
4a was stable for more than a week in 1,2-dichloroethane at
room temperature. A solution of 4a containing traces of
chloride ion crystallized to yield [N(CH2CH2CH2)3Sn]4
[(SbF6)3Cl] (4b). The crystal lattice of this complex is defined
by the space group I23, in which one chlorine atom is
surrounded by four tricarbastannatranes and the [SbF6]À
counter ions are shared along the edge of the unit cell
The ability of N(CH2CH2CH2)3SnMe (6) to transfer its
apical methyl group was then examined by using B(C6F5)3
(7).[13] The complex [N(CH2CH2CH2)3Sn][MeB(C6F5)3] (8)
was formed upon addition of 7 to a solution of 6 in 1,2-
dichloroethane. The generation of 8 was monitored by
119Sn NMR spectroscopy, and a remarkable change in the
119Sn NMR chemical shift from d = À16.3 to 253.0 ppm
(Dppm = 269.3 ppm) was observed (Table 2, entry 2). Fur-
thermore, the presence of [MeB(C6F5)3]À was detected by
HRMS (ESI), thus showing an ion peak at m/z 527.00751.
After the addition of one equivalent of DABCO to 8, the
119Sn NMR signal at d = 61.9 ppm suggested formation of
a strong Lewis base/Lewis complex with 3 (Table 2, entry 3),
as previously observed for 2 (Table 1, entry 6). In addition to
NMR data, the formation of the DABCO·tricarbastannatrane
was supported by HRMS (ESI) analysis, which showed an ion
peak at m/z 372.14609.
À
(Figure 2b). The Sn N bond length in 4b is 2.22 ꢁ and the
distance between chlorine and tin atoms is 2.92 ꢁ, which is
[8]
À
significantly longer than the Sn Cl bond of 2.61 ꢁ in 1.
According to the X-ray structure, there is no interaction
between the chlorine and tin atoms in 4b, thus establishing
the formation and stability of 3.
Then, complex [N(CH2CH2CH2)3Sn][B[3,5-(CF3)2C6H3]4]
(5), containing the bulky and noncoordinating counter ion
2
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Table 2: NMR studies on 8 and 9
Entry Tricarbastannatranes
NMR chemical shifts (ppm)
1H
13C
119Sn
11B
À0.39 (s) À5.3
0.59 (t)
1.59 (m)
2.33 (t)
0.43 (brs)
1.76 (m)
2.12 (m)
2.71 (t)
0.42 (brs)
1.20 (t)
7.5
22.9
54.2
1
2
3
À16.3
n.a.
17.6
25.3
253.0 À15.5[a]
61.9 À15.5
55.9[b]
7.5
22.5
1.87 (m)
53.9[d]
2.48 (t)[c]
0.50 (brs)
1.05 (m)
1.84 (m)
2.46 (t)
11.3
23.2
54.7
4
43.1 À14.9[e]
Figure 3. X-ray structure of the compound 9.
[a] The 11B NMR chemical shift of B(C6F5)3 in 1,2-dichloroethane is
d=57.3 ppm. [b] No signal for a methyl group bonded to boron is
observed because of the quadrupolar relaxation of the boron. [c] Two
signals at d=2.61 and 2.84 ppm belong to DABCO. [d] Two signals at
d=45.1 and 45.9 ppm belong to DABCO. [d] NMR studies on 9 were
carried out in CDCl3.
Table 3: B(C6F5)3-Promoted reaction of 6 with 10a.
Entry Equiv of 6 Equiv of 7 Equiv of 10a Conv. [%][a] Yield [%][b]
Although 8 was stable at room temperature for more than
24 hours, decomposition to unidentified products was
observed upon warming the solution to 358C in a sealed
NMR tube. The complex 8, an oil, could not be characterized
by X-ray crystallography. However, when one equivalent of
water was reacted with two equivalents of 6 and one
1
2
3
4
5
1
1.2
2
2
2
1
1
0
1
1
1
1
1
1
0
<20
0
>95
<20
n.d.
n.d.
n.d.
92
0.2
n.d.
[a] Determined by analysis of the 1H NMR spectra of the crude reaction
mixtures. [b] Yield of isolated product. n.d.= not determined.
equivalent of
7
in 1,2-dichloroethane, the complex
[(N(CH2CH2CH2)3Sn)2OH][MeB(C6F5)3] (9) was obtained
as colorless crystals (Scheme 2, Figure 3). In solution, NMR
(Table 2, entry 4) and HRMS (ESI) data supported the
formation of 9, and the ion peak at m/z 527.09664 was
attributed to [(N(CH2CH2CH2)3115Sn)2OH]+.
compatible with a series of functional groups and afforded
good to excellent yields (78–92%) of methylated products
11a–l (Table 4).
We then sought to gain more insight into the mechanism
by which the methyl group is delivered from 6 to 10a, as
complex [N(CH2CH2CH2)3Sn][MeB(C6F5)3] (8) was shown to
be inert (Table 3). As illustrated in Scheme 3, [CD3]-6 and
10a were added to 8 to provide [CD3]-11. This result indicates
that 6 is the sole methyl donor in this transformation.
Therefore explaining the need for two equivalents of 6, and
that [MeB(C6F5)3]À only serves as a bystander (Scheme 3).
The tricarbastannatrane 3 likely acts as a Lewis acid and binds
to 10a.
Scheme 2. Synthesis of compound 9.
The reactivity of 8 in the conjugate addition reaction to
the Meldrumꢀs acid 10a, was then investigated.[14] Unexpect-
edly, in the presence of one equivalent of 6 and one equivalent
of 7, no reactivity was observed (Table 3, entry 1), while the
reaction displayed full conversion into the product 11a with
two equivalents of 6 and one equivalent of 7 (Table 3,
entry 4). By using 0.2 equivalents of 7 less than 20%
conversion into product 11a resulted (Table 3, entry 5).
With an optimized procedure in hand, we investigated the
scope of the electrophile. The reaction was found to be
According to the above observations, we propose that the
first step in the B(C6F5)3-promoted conjugate reaction is the
formation of 8 (Scheme 4). Then, 10a is activated through
coordination of one of its carbonyl groups to 3 to form the
complex 12. The latter was detected by a 119Sn NMR spectra
showing a signal at d = 129.6 ppm (Dppm = 123.4). Subse-
quently, methyl delivery from the second equivalent of 6
yields the tin enolate 13, in addition to 8. The tricarbastanna-
trane 8 is then scavenged by the Lewis basic 13, thus yielding
14, and rationalizing the lack of turnover and the need for two
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Table 4: B(C6F5)3-Promoted reaction of 6 with benzylidene derivatives of
Meldrum’s acid (10a–l).
one week at room temperature and it was trapped in situ with
iodomethane to form product 15.
As shown in Table 5, the substrate scope was explored by
adding the alkyl-tricarbastannatranes 16a, 16c, and 16 f to
Table 5: B(C6F5)3-promoted reaction of 16 with 10a.
Entry
Ar
Product
Yield [%][a]
1
2
3
4
5
6
7
8
4-ClC6H4 (10a)
11a
11b
11c
11d
11e
11 f
11g
11h
11i
92
90
78
83
88
91
81
92
90
82
85
79
3-(MeO)C6H4 (10b)
2-Naphthyl (10c)
4-(CN)C6H4 (10d)
4-BrC6H4 (10e)
3-[B(O2C6H12)]C6H4 (10 f)
4-[B(O2C6H12)]C6H4 (10g)
3-FC6H4 (10h)
3-BrC6H4 (10i)
4-(CO2CH3)C6H4 (10j)
4-FC6H4 (10k)
Entry
R
R’
Product
Yield [%][a]
1
2
3
4
5
6
nBu (16a)
iPr (16b)
allyl (16c)
benzyl (16d)
vinyl (16e)
nBu
H
allyl
benzyl
vinyl
17a
17b
17c
17d
17e
17 f
89
74[b]
96
49
34
86
9
10
11
12
11j
11k
11l
ꢀ
=
CH2C CMe (16 f)
H2C C=CMe
4-(NO2)C6H4 (10l)
[a] Yield of isolated product. [b] See the Supporting Information for NMR
studies on the reactivity of 16b.
[a] Yield of isolated product.
10a, thus yielding products 17a, 17c, and 17 f, respectively, in
high yields. Moderate yields were obtained with derivatives
16d and 16e (Table 5, entries 4 and 5). Interestingly, addition
of iPr-tricarbastannatrane (16b) to 10a led to 17b, the
reduced alkene product (Table 5, entry 2). When the reaction
was monitored by 1H NMR spectroscopy, the presence of
propene gas[15] and the complex [N(CH2CH2CH2)3Sn][HB-
(C6F5)3] (18) was detected. In addition, [HB(C6F5)3]À was
identified by HRMS, thereby showing an ion peak at m/z
512.99267.[16] The product 17b was obtained in 72% yield by
the reaction of only one equivalent of 10a with one equivalent
of 7 and 16b (Scheme 5a). Therefore, [HB(C6F5)3]À is likely
Scheme 3. Reaction of [CD3]-6 and 8 with 10a.
Scheme 5. Reactions of 18 with 10a.
Scheme 4. Proposed mechanism. DMF=N,N-dimethylformamide.
the hydride source in this transformation. In addition, 17b
was obtained in 83% yield as the only product in the reaction
involving one equivalent of 18 and one equivalent of 6 with
10a (Scheme 5b).
In conclusion, the structures of a series of tricarbastanna-
tranes in solution and in the solid state have been determined.
The formation of the stable tricarbastannatrane 3 and its
moderate Lewis acidity was confirmed by 119Sn NMR spec-
troscopy. In addition, the structures of the tricarbastanna-
equivalents of 6 for the reaction to proceed. Monitoring the
reaction by NMR spectroscopy showed a single 119Sn NMR
signal at d = 47.7 ppm, which is consistent with symmetrical
14. In addition, the formation of 14 was further confirmed by
HRMS (ESI), which displays an ion peak at m/z 801.15004
with an isotope distribution pattern attributed to the ion
[C32H50O4N2ClSn2]+. The intermediate 14 was stable for about
4
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[5] K. Jurkschat, A. Tzschach, J. Organomet. Chem. 1984, 272, C13 –
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C16.
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À
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Moreover, the conjugate addition of alkyl-tricarbastanna-
tranes to benzylidene derivatives of Meldrumꢀs acid was
carried out in the presence of B(C6F5)3 under mild reaction
conditions. The mechanism of the addition has been inves-
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[7] A. Tzschach, K. Jurkschat, Pure Appl. Chem. 1986, 58, 639 – 646.
À
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À
[9] The Sn N bond length in 6 is 2.62 ꢁ. A. Tzschach, K. Jurkschat,
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Received: February 2, 2015
Published online: && &&, &&&&
À
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Angewandte
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Communications
Alkylation
Trane of thought: Spectroscopic investi-
gation on the structure of tricarbastan-
natranes has been carried out. The B-
(C6F5)3-promoted conjugate addition of
alkyl-tricarbastannatranes to benzylidene
derivatives of Meldrum’s acid and
detailed mechanistic studies are pre-
sented.
A. Kavoosi, E. Fillion*
&&&& — &&&&
Synthesis and Characterization of
Tricarbastannatranes and Their Reactivity
in B(C6F5)3-Promoted Conjugate
Additions
6
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Angew. Chem. Int. Ed. 2015, 54, 1 – 6
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