Cyclic alkyl(aryl)boranes for 1,1-carboboration of monoalkynyltin compounds

Article abstract of DOI:10.5560/ZNB.2013-3018

Two cyclic alkyl(aryl)boranes, a 1-bora-indane derivative 1, and a tricyclic derivative 4, containing the boron atom in a six-membered ring, were structurally characterized by NMR techniques in solution. The solid-state structure of the 1-bora-indane 1 was determined by X-ray crystallography. The reactivity of these cyclic alkyl(aryl)boranes towards monoalkynyltin compounds, Me₃Sn-C≡C-Me and Me₃Sn-C≡C-Fc (Fc=ferrocenyl), was studied using multinuclear magnetic resonance methods (¹H, ¹¹B, ¹³C, ¹¹⁹Sn NMR). Novel alkenylboranes were formed by 1,1-carboboration reactions. This process involves an expansion of both five- and six-membered rings. Insertion into the respective B-C(aryl) bond was preferred with high selectivity. In the case of the six-membered ring in 4, the ring expansion to seven-membered rings proved to be readily reversible, and the thermodynamically stable reaction products were formed by ring contraction and concomitant transfer of the exocyclic B-ⁿPr group.

Full text of DOI:10.5560/ZNB.2013-3018

Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin
Compounds
Bernd Wrackmeyer, Peter Thoma and Wolfgang Milius
Laboratorium fu¨r Anorganische Chemie, Universita¨t Bayreuth, D-95440 Bayreuth, Germany
Reprint requests to Reprint request to Prof. Dr. B. Wrackmeyer. E-mail: b.wrack@uni-bayreuth.de
Z. Naturforsch. 2013, 68b, 493 – 502 / DOI: 10.5560/ZNB.2013-3018
Received January 19, 2013
Dedicated to Professor Heinrich No¨th on the occasion of his 85th birthday
Two cyclic alkyl(aryl)boranes, a 1-bora-indane derivative 1, and a tricyclic derivative 4, containing
the boron atom in a six-membered ring, were structurally characterized by NMR techniques in solu-
tion. The solid-state structure of the 1-bora-indane 1 was determined by X-ray crystallography. The
reactivity of these cyclic alkyl(aryl)boranes towards monoalkynyltin compounds, Me3Sn–C≡C–Me
and Me3Sn–C≡C–Fc (Fc = ferrocenyl), was studied using multinuclear magnetic resonance methods
(1H, 11B, 13C, 119Sn NMR). Novel alkenylboranes were formed by 1,1-carboboration reactions. This
process involves an expansion of both five- and six-membered rings. Insertion into the respective
B–C(aryl) bond was preferred with high selectivity. In the case of the six-membered ring in 4, the
ring expansion to seven-membered rings proved to be readily reversible, and the thermodynamically
stable reaction products were formed by ring contraction and concomitant transfer of the exocyclic
B-nPr group.
Key words: Triorganoboranes, 1,1-Carboboration, Alkynyltin Compounds, NMR Spectroscopy,
X-Ray Analysis
Introduction
Arylboron compounds have found widespread ap-
route to such compounds via pyrolysis of triorganobo-
ranes (Scheme 1) [15 – 17], as shown for A and B.
In the case of A, the structures 1 and 3 had been pro-
plications in various fields of chemistry and re- posed for the products [15 – 17]. This was confirmed in
lated sciences. Most prominent are arylboronic acids, the present work by analysis of the original mixtures,
e. g. in Suzuki coupling reactions [1], triarylboranes and in addition, compound 2 was detected. In any case,
bearing electronegative substituents as co-catalysts in 2 and 3 were found to be minor side products, and we
olefin polymerization [2, 3] and as catalysts in hy- have set out to isolate pure samples of 1 for structural
drosilylation reactions [4 – 6], triphenylborane amine characterization. In the case of B, the formation of 4
adducts as additives in antifouling agents [7 – 10], was proposed as a mixture of isomers without struc-
or various arylboranes as composites of OLEDs be- tural assignment [15 – 17]. Here, we confirm this pro-
cause of their photophysical properties [11]. Triaryl- posal by 1H and 13C NMR spectroscopy and assign the
boranes can be readily prepared by standard proce- major isomer as trans-4.
dures [12 – 14] and have attracted much attention,
Both 1 and 4 are attractive examples for study-
whereas alkyl(aryl)boranes have not been studied in ing 1,1-carboboration reactions [18 – 20], since they
great detail, owing to the limited stability of non-cyclic possess three different B–C bonds [21]. The princi-
dialkyl(aryl)- or alkyl(diaryl)boranes towards dispro- ples of 1,1-carboboration are shown in Scheme 2 for
portionation. However, cyclic alkyl(aryl)boranes are triethylborane. In the cases of 1 and 4, it is of in-
known to be fairly stable in this respect, and can be terest to find out which of the B–C bonds is pre-
prepared by various routes [12 – 14]. Ko¨ster et al. have ferred to form a new C–C bond. We have selected
reported pioneering work in this field, opening a facile alkynyl(trimethyl)tin compounds for this study, be-
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B. Wrackmeyer et al. · Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds
180 °C
B
B
B
+
+
B
3
1
3
2
A
H
H
H
180 °C
+
B
nPr2
B
B
H
B
4
cis-
tr ans-4
Scheme 1. Examples of cyclic alkyl(aryl)boranes produced by pyrolysis of triorganoboranes.
Et
provided conclusive evidence for the proposed struc-
tures of these boranes. The increased line widths of the
13C NMR signals for carbon atoms linked to boron can
be traced to partially relaxed one-bond 13C-11B spin-
spin coupling [22]. From this broadening and the 11B
nuclear spin relaxation time (1: TQ(11B) = 0.8 ms), the
coupling constants 1J(13C,11B) can be evaluated [23]:
Me
B
Et
Me
Et
BEt2
Me
MMe3
+ BEt3
Et
M
Me3M
Me3
M = Si, Ge, Sn, Pb
Scheme 2. Principles of 1,1-carboboration reactions.
cause they are easy to handle and react much more 1J(13C,11B) = 63 ± 1.5 (13C-7a), 60 ± 2 (13C-2), 55 ±
readily when compared with the corresponding silanes. 2 Hz (13C-8). These data agree well with calculated
Thus, we have used Me3Sn–C≡C–Me (5) and Me3Sn– data [24 – 27] 62.2, 59.3, 54.2 Hz, using the optimized
C≡C–Fc (6; Fc = ferrocenyl) under mild reaction con- gas-phase geometry of 1, as has also been shown pre-
ditions, hoping to distinguish eventually between ki- viously for other organoboranes [28].
netically and thermodynamically controlled products.
The molecular structure of 1 (Fig. 2) shows essen-
tially planar 1-bora-indane rings. All bond lengths are
in the expected ranges. Variations in the C–C bond
lengths for the aromatic carbon atoms of the 1-bora-
indane do not indicate BC(pp)π interactions, although
these are suggested by the 11B and 13C chemical shifts.
Results and Discussion
Cyclic alkyl(aryl)boranes
The pure crystalline 1-bora-indane derivative 1, similar to other phenylboranes [29]. The endocyclic
which is extremely sensitive towards traces of oxygen angle C1–B9–C9 (105.4(2)◦) is small, considering the
and moisture, was slowly deposited from one of sev- trigonal planar surroundings of the boron atom and
eral mixtures containing variable amounts of 1, 2, and points towards ring strain and increased reactivity of
3 (Scheme 1). 13C NMR spectra (Fig. 1, Table 1) have the B–C bonds. There appear to be weak intermolecu-
Fig. 1. 100.5 MHz 13C{1H}
NMR spectrum of the 1-bora-
indane derivative 1 showing
expansions for relevant re-
gions. Note the broadening of
13C NMR signals for C(7a),
C(2) and C(8), which is due
to partially relaxed 13C-11
spin-spin coupling [22].
B
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B. Wrackmeyer et al. · Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds
495
Table 1. 11B and 13C NMR dataa of the 1-bora-
4
3
3a
5
6
indanes 1–3.
2
7a
B
7
R
δ
13C
1
2
3
2
3
24.5 (br)
33.1
23.7 (br)
32.7
27.0 (br)
33.6
3a
4
165.6
125.8
166.6
125.6
166.4
128.4
5
133.4
133.4
133.6
6
125.8
125.9
125.9
7
132.5
133.2
134.3
7a
B-R
144.5 (br)
143.5 (br)
144.8 (br)
141.2 (br), 148.0,
125.4, 133.6, 129.9,
133.3,
30.1, 17.6, CH2CH3
77.8
24.9 (br) CH2, 32.2 35.0 (br) CH, 17.5
CH2, 145,1 (i), 127.6 (Me), 146.1 (i),
(o), 128.3 (m),
125.6 (p)
81.7
128,4 (o), 128.8
(m), 125.3 (p)
80.0
δ
11B
a
In C6D6 (10%) at 296 K; (br) denotes the broadened 13C NMR signal of carbon
atoms linked to boron [22].
etry of 1 at the B3LYP/6-311+g(d,p) [30 – 32] level of
theory. The alternative structure, with all rings in one
plane was not confirmed as a minimum in energy.
Although the 1H NMR spectrum of the borane 4, an
air-sensitive yellowish waxy solid, is expectedly fairly
complex, some of its signals are useful for assignment
1
purposes, e. g. the H NMR signal for H-2 and both
signals for H-4, which are at low and high frequency,
Table 2. 11B and 13C NMR dataa of the isomers trans-4 and
cis-4.
Fig. 2. Molecular structure of the 1-bora-indane 1 (ORTEP;
50% probability ellipsoids, hydrogen atoms omitted for
clarity). Selected bond lengths (pm), angles and di-
hedral angles (deg): C1–B9 154.7(3), C8–B9 157.1(3),
C10–B9 155.7(3), C10–C11 152.5(3), C11–C12 150.1(3);
C1–B9–C8 105.4(17), C8–B9–C10 126.96(18), C10–B9–
C1 127.64(18), B9–C10–C11 117.57(17), C10–C11–C12
114.06(16); C1–B9–C10–C11 177.63(19), C8–B9–C10–
C11 1.1(3), B9–C10–C11–C12 175.74(18), C10–C1–C12–
C13 79.7(3), C10–C11–C12–C17 97.2(2).
(d)
5
4
4a
3
(c)
(b)
6
7
4
2
8a
B
nPr
(a)
8
δ
13C
trans-4
cis-4
2
3
4
39,9 (br)
39.5
46.9
37.7 (br)
41.1
42.2
4a
5
6
7
8
8a
150.6
128.3
133.2
125.8
135.6
137.1 (br)
23.4 (br), 19.4, 18.5
27.6, 25.3, 24.8, 35.6
75.3
149.4
129.3
133.4
125.7
lar interactions (π stacking; smallest distance between
layers: 374.2 pm) between the 1-bora-indane rings. Al-
though the particular orientation (almost perpendicu-
lar to the 1-bora-indane rings) of the phenyl group in
the PhCH2CH2 substituent may be enforced by crys-
tal packing, this structure was also found as an energy
minimum for the gas phase after optimizing the geom-
135.3
136.6 (br)
24.5 (br), 19.5, 18.3
27.8, 26.9, 28.8, 31.4
75.3
B-R
a,b,c,d
δ
11B
a
In C6D6 (10%) at 296 K; (br) denotes the broadened 13C NMR
signal of carbon atoms linked to boron [22].
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496
B. Wrackmeyer et al. · Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds
Fig. 3. 100.5 MHz 13C{1H}
NMR spectrum of 4 show-
ing expansions for relevant re-
gions. Note the broadening
13C NMR signals for C(8a),
C(2) and BCH2, which is due
to partially relaxed 13C-11
spin-spin coupling [22].
B
R
B
R = Me, Fc
6
5
SnMe3
8
Me3Sn
R
B
Ph
Ph
+
hexane
B
R
-78 to +23 °C
SnMe3
1
SnMe3
R
Ph
B
(80 %)
7
9
Ph
Scheme 3. 1.1-Carboboration reactions of 1 with monoalkynyltin compounds.
respectively, of unresolved multiplets. The splitting of ganyl group, as shown for 1 in Scheme 3. It was hoped
the H-2 and H-4 signals by 1H-1H coupling is well re- that 119Sn chemical shifts [37 – 39] would be suffi-
solved, and the signals can be readily assigned to the ciently sensitive to mirror small changes in the sur-
major isomer. The 13C NMR spectrum is much more roundings of the tin atom. Indeed, three 119Sn NMR
straightforward (Fig. 3, Table 2). Together with 2D signals for 7(Me), 8(Me) and 9(Me) (Fig. 4) reflect
1H/13C shift correlations (HSQC [33], HMBC [34]), this situation when 5 was used, whereas for 6 mainly
1
and selective gradient enhanced 1D H/1H NOE dif- 7(Fc) and a small amount of 8(Fc) were the products.
ference spectra [35], all 13C NMR signals can be as- Expansion of the five-membered ring in 1 appears to
signed. Especially the NOE experiment (irradiating at be the major driving force, and for 1,1-carboboration
the 1H-2 resonance) proves unambiguously that the the B–C(7a) bond is preferred over the B–C(2) bond.
major isomer is trans-4 (ca. 80%). Calculated δ1H and The structure assignment of 7 follows from the 13C
δ
13C values [36] follow the trend of the experimen- NMR spectra which show the typical broad signals
tal data, corroborating the assignments. The calculated for boron-bonded carbon [22], and the required set for
energies for the optimized geometries of trans-4 and aryl carbon atoms, now in markedly different places
cis-4 are almost the same, trans-4 being 0.5 kcal mol−1 (Table 3) when compared with 1. For 8, the broad
more stable.
13C(2) NMR signal is missing, being replaced by the
13C(CH2) NMR signal for the methylene unit linked
to a C=C bond. Moreover, 119Sn-13C spin-spin cou-
pling constants (117/119Sn satellites), in particular those
across three bonds to the aromatic carbon atom in 7,
Reactions of the cyclic alkyl(aryl)boranes 1 and 4
with alkynyl(trimethyl)tin compounds
In both boranes 1 and 4, the reaction with alkynyltin and to the CH2 unit in 8, found in the typical range,
compounds may lead to insertion into one of the en- are conclusive. The cis position of the Me3Sn group
docyclic B–C bonds or to transfer of the exocyclic or- relative to boron is assumed by comparison with pre-
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B. Wrackmeyer et al. · Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds
497
Fig. 4. 149.1 MHz 119Sn{1H} NMR spec-
trum of the reaction mixture starting from
1 and 5, measured immediately after the
sample had reached r. t., using the refo-
cused INEPT pulse sequence [42, 43]. At
ambient temperature, the ratio of the com-
ponents present in the mixture remains
constant.
Table 3. Selected NMR parametersa of products from the reaction of 1 with 5 and 6 (Scheme 3).
3
3
4
4
4
3
3a
7a
3a
3a
2
5
6
2
5
6
5
6
2
R
B
7a B
SnMe3
Me
Ph
7a
B
7
7
7
SnMe3
7
R
8
SnMe3
9(Me)
Ph
Ph
13C
C-4,5,6,7 B-CH2CH2-Ph
δ
δ
119Sn
δ
11B
Me3Sn
Sn–C=
=C–B
R–C=
C-2,
C-3
C-3a,
C-7a
7(Me)b
(C6D6)
−6.5
147.9
[496.1]
159.8
(br)
23.2
[59.6]
30.6
(br),
31.5
128.9
[7.0],
138.5
[85.0]
129.1
[8.1],
140.5
[89.8],
125.7,
134.2,
127.4,
127.4
126.1,
133.4,
126.0,
133.0
25.8 (br), 29.2,
144.4, 127.8,
128.3, 125.4
−42.2
82.2
(710)
[331.3]
7(Fc)c
(CDCl3) [329.1]
−2.8
143.1
[520.0]
160.0
(br)
[67.5]
69.4
(Cp),
89.0
[53.2],
69.0,d
67.3
30.5
(br),
31.4
26.6 (br), 29.5,
144.4, 127.8,
128.3, 125.4
−54.7
81.5
(1320)
(4.8)
8(Me)b
(C6D6)
−6.2
n.a.
159.6
(br)
22.5
[64.5]
41.0
[69.8],
36.4
[7.8]
40.4
[67.0],
36.4
n. a.
n. a.
n. a.
n. a.
26.5 (br), n. a.
26.0 (br), n. a.
−46.4
−50.9
n. o.
n. o.
[331.0]
8(Fc)c
(CDCl3) [328.0]
−5.0
141.4
[521.0]
161.0
(br)
68.3
(Cp),
90.5
n. a.
n. a.
[62.3],
n. a.
25.9
[8.0]
9(Me)b
(C6D6)
−7.6
n.a.
158.0
(br)
27.0
(br)
n. a.
n. o.,
166.0
(br)
n. a.
n. a.
−55.2
n. o.
[321.0]
[65.4]
a
Measured at 296 K; coupling constants nJ(119Sn,13C) in Hz are given in brackets; (br) denotes the broad 13C NMR signal of carbon atoms
linked to boron; n. a. means not assigned because signals are weak, and there is partial overlap with other strong signals; n. o. means not
observed, because the signal is weak and broad; b mixture of 7(Me) with small amounts of 8(Me) and 9(Me), see Fig. 1; c mixture of 7(Fc)
with a small amount of 8(Fc); d broad because of slow rotation of the ferrocenyl group about the Fc–C= bond.
vious results [18 – 20, 40], in agreement with fairly the products are rather broad, up to 2000 Hz, differ-
narrow 119Sn NMR signals [40]. The chemical shifts ent products with similar structures cannot be distin-
δ
11B show small changes, indicating that a reaction guished by 11B NMR spectroscopy. The formation
had taken place and that three-coordinate boron atoms of side products 8 and 9 is conceivable, considering
are still present [41]. Since the 11B NMR signals of either insertion into the B–C(2) bond to give again
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498
B. Wrackmeyer et al. · Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds
Table 4. Selected NMR parametersa of the productsb from the reaction of trans-4 with 5 and 6.
5
5
4a
8a
4a
(d)
6
(d)
4
(c)
(b)
6
7
4
3
2
(c)
(b)
3
2
7
(a)
(a)
8a
B
8
8
B
SnMe3
R
Sn
Me3
R
10
(E)-11
δ
13C
C-5,6,7,8 CH2CH2CH3
δ
119Sn
δ
11B
Me3Sn
Sn–C=
=C–B
R–C=
C-2,3,4
C-4a,
C-8a
10(Me)c −9.1
132.5
[514.8]
165.8
(br)
21.1
[63.7]
49.2 (br), 141.8
127.1,
129.7,
129.7,
126.9
127.7,
129.7,
129.6,
127.5
24.5 (br), 19.8,
17.8
−42.4
−46.5
80.7
80.2
(C6D6)
[321.6]
48.0,
44.5
[4.0],
137.8
[91.5]
10(Fc)d
(CDCl3) [321.1]
−5.7
133.7
[535.3]
165.6
(br)
[71.6]
69.4
(Cp),
89.5
[60.7]
70.4,
68.3
44.2 (br), 141.5
24.6 (br), 19.1,
17.8
50.9,
44.0
[4.7],
139.5
[96.3]
11(Me)e −8.1
136.8
[536.2]
158.3
(br)
[67.4]
20.2
[68.8]
45.0 br,
41.0,
42.0
150.7,
137.2
(br)
128.5,
133.7,
125.8,
138.2
128.7,
133.7,
125.9,
137.7
34.2 [80.2],
23.2 [9.9], 15.0
−48.4
−54.3
74.4
74.8
(C6D6)
[319.0]
11(Fc)f
(CDCl3) [320.8]
−4.9
137.4
[536.1]
161.4
(br)
[65.0]
69.6
(Cp),
90.6
[72.3],
70.4,
67.9
44.1 (br), 150.1,
37.4 [83.3],
24.1
[8.5], 15.2
40.5,
42.0
137.3
(br)
a Measured at 296 K in C6D6; coupling constants nJ(119Sn,13C) in Hz are given in brackets; (br) denotes the broad 13C NMR signal of carbon
atoms linked to boron; b all signals for 10 and 11 are accompanied by weak signals representing the products formed by the reaction of cis-4
with 5 and 6; in the case of 11, only the NMR data of the major isomer are given, since those of the minor isomer are very similar; c other
13C NMR data: 28.8 (a), 29.6 (b), 27.7 (c), 38.4 (d); d other 13C NMR data: 26.7 (a), 27.5 (b), 27.7 (c), 38.4 (d); e other 13C NMR data: 28.7
(a), 27.0 (b), 29.0 (c), 36.0 (d); f other 13C NMR data: 28.7 (a), 27.0 (b), 29.1 (c), 35.9 (d).
slow, r. t.
readily and the additional condensed six-membered
ring in both isomers of 4 may be a further obstacle
as far as the reactivity of endo- and exocyclic B–C
bonds is concerned. In the beginning, the reaction of 4
with 5 affords a mixture containing mainly 10(Me) (ca.
60%) and two compounds assumed to be stereoiso-
mers of (E)-11(Me) (without assignment; together ca.
40%). After two weeks at r. t., only the stereoiso-
mers of (E)-11(Me) (≈ 2 : 1 ratio) are left, together
R = Me, Fc
5
6
Me3Sn
R
+
B
B
B
hexane
-78 to +23 °C
SnMe3
R
R
Sn
4
Me3
(E)-11
10
(two stereoisomers)
Scheme 4. 1.1-Carboboration reactions of 4 with monoalky-
nyltin compounds.
a six-membered ring 8, or transfer of the exocyclic with some impurities. These final products arise be-
CH2CH2Ph group, leaving the 1-bora-indane system cause of a transfer of the exocyclic propyl group. In
unaffected, as in 9.
Fig. 5, the comparable situation is illustrated by 119Sn
In the case of 4, 1,1-carboboration (Scheme 4) can NMR spectroscopy for the analogous reaction of 4
proceed in a similar way as for 1. However, the ex- with 6. The first product formed is 10(Fc), already
pansion of the six-membered ring may take place less accompanied by small amounts of the isomers (E)-
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B. Wrackmeyer et al. · Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds
499
Fig. 6. 400 MHz 1H/13C HMBC shift correlation [34],
taking advantage of the two-bond and three-bond 13C-
1H spin-spin coupling of the 13C(Sn–C=) and 13C(B–
C=) nuclei to 1H(=C–CH3) as indicated by encircled
B and A, respectively. The positive tilt (dashed lines)
of 119/117Sn satellite cross peaks (arrows) indicates [44]
alike signs of 3J(119Sn,1H=C−CH3), 2J(119Sn,13
C
B−C=),
and 1J(119Sn,13
C
Sn−C=), all < 0.
trans- and cis-coupling pathways. Sterical congestion
around the C=C bond in 11 is evident by exchange-
broadened 13C(SnMe3) NMR signals, and the presence
of stereoisomers is in agreement with hindered rotation
about the B–C= bond. Mainly the two stereoisomers
of (E)-11(Me) and (E)-11(Fc) are the final products
(≈ 90% after three weeks at r. t.) in an approximate
2 : 1 ratio. The results shown in Scheme 4 and in Fig. 5
indicate the reversibility of the 1,1-carboboration reac-
Fig. 5. 149.1 MHz 119Sn{1H} NMR spectra of the reaction
mixture starting from 4 and 6, measured at times after the
sample had reached r. t., using the refocused INEPT pulse
sequence [42, 43]. The smaller numbers refer to the respec-
tive isomer arising from the reaction of cis-4. No assignment
was made for the (E) isomers 11. In the case of one of the
isomers of 11, the signal for the minor isomer starting from
cis-4 was not detected with certainty.
11(Fc), which are formed via transfer of the exocyclic tions.
propyl group. Apparently, the insertion into the B–
For the detection of the fairly weak and partially
C(aryl) bond leads to the kinetically controlled prod- broadened olefinic 13C NMR signals of the two iso-
ucts 10(Me) or 10(Fc) containing a seven-membered mers of (E)-11(Me) HMBC experiments [34] proved
ring. The ring expansion is clearly evident from 13C to be particularly convenient (Fig. 6), since they also
NMR spectra (Table 4), in particular from the 13C(8a) allow to determine the signs of J(119Sn,13C) (n = 1,
n
NMR signal which, in contrast to 4 and (E)-11, is sharp 2) (2J(119S,1H=C−CH3) < 0 is known [37 – 39]). This
and accompanied by 117/119Sn satellites, typical of is of importance for n = 2, because such coupling con-
3J(119Sn,13C). Thermodynamically, the six-membered stants in vinyl- or alkenyltin compounds are generally
ring is probably more favorable, giving rise to trans- small and may be of either sign [37 – 39]; for instance,
fer of the exocyclic propyl group. In addition to the in Sn(CH=CH2)4, J(119Sn,13C) < 1 Hz. In the case
2
isomers of trans-4 and cis-4, there are two stereoiso- of (E)-11, the magnitude of 2J(119Sn,13CC=) is larger,
mers of (E)-11 for each of the starting isomers 4. In and its sign is < 0 (Fig. 6).
the course of the rearrangement of 10 into 11, the pre-
ferred stereochemistry, leading to the respective (E)- Conclusions
isomers, is retained. This can be concluded considering
the δ13C data and almost identical coupling constants,
Following the structural characterization of two
3
in particular J(119Sn,13CCH2), which are sensitive to cyclic alkyl(aryl)boranes, their reactivity towards
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B. Wrackmeyer et al. · Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds
monoalkynyl(trimethyl)tin compounds was studied us- Table 5. Crystallographic data of the 1-bora-indane 1.
ing multinuclear magnetic resonance methods. 1,1-
Carboboration took place readily, affording novel
1
Formula
C16H17B
alkenylboranes. Ring expansion of both five- and six-
membered rings was preferred, at least in the first steps.
This involved insertion into the respective B–C(aryl)
bond with high selectivity. In the case of the six-
membered ring, the ring expansion to seven-membered
rings proved to be reversible, and the thermodynami-
cally stable reaction products were formed by transfer
of the exocyclic B-nPr group.
Crystal
colorless cube
0.24 × 0.18 × 0.16
293
monoclinic
P21/c
Dimensions, mm3
Temperature, K
Crystal system
Space group
Lattice parameters
a, pm
b, pm
c, pm
β, deg
Z
935.36(19)
1302.7(3)
1079.8(2)
100.793(3)
4
Absorption coefficient µ, mm−1
Measuring range in ϑ, deg
Refl. collected / unique / Rint
Refined parameters
R1 / wR2 [I > 2σ(I)]
R1 / wR2 (all data)
Max. / min. residual electron
density, e pm−3 ×10−6
0.1
Experimental
2.22 – 26.01
8818 / 2485 / 0.085
154
0.046 / 0.101
0.1055 / 0.1200
0.115 / −0.092
General and starting materials
All preparative work as well as handling of the samples
was carried out observing precautions to exclude traces of
air and moisture. Carefully dried solvents and oven-dried
glassware were used throughout. Crude samples containing
1 were available from Ko¨ster’s work [15 – 17], from which
pure crystalline 1 (m. p. 49 – 50 ◦C) could be separated and
used for synthesis, NMR measurements, and X-ray crystal
structural analysis. The original mixture containing the iso-
mers trans-4 and cis-4 (ca. 80 : 20) was also available from
Ko¨ster’s work [15 – 17] as a yellowish waxy solid, and was
used without further purification. Trimethyl(propynyl)tin
5 [45] and ferrocenylethynyl(trimethyl)tin 6 [46 – 48] were
prepared as described.
NMR measurements in CDCl3 or C6D6 (concentra-
tion ca. 5% – 10%) with samples in 5 mm tubes at 23
± 1 ◦C: Varian Inova 400 MHz spectrometer for 1H, 11B,
13C, and 119Sn NMR; chemical shifts are given relative to
Me4Si [δ1H (CHCl3) = 7.24; δ13C (CDCl3) = 77.0; δ1H
(C6HD5) = 7.15; δ13C (C6D6) = 128.0; external Me4Sn
1H NMR data of 9-propyl-10H-1,2,3,4,4a,10a-hexa-
hydro-9-boraantracene trans-4 (see Fig. 3 or Ta-
ble 2 for numbering) in C6D6: δ1H = 0.59 (1H. “t”,
3J(1H,1H) = 11.9 Hz, H-2), 1.38 (1H, m, H-3), 2.46, 2.53
(2H, dd, 2J(1H,1H) = 15.5 Hz), 2J(1H,1H) = 3.5 Hz, H-4),
7.02 (1H, m, H-5), 7.20 (1H, m, H-6), 7.10 (1H, , m, H-7),
7.83 (1H, m, H-8), 1.13, 2.29 (2H,m, H-a), 1.70 (2H, m,
H-b), 1.76 (2H, m, H-c), 1.01, 1.63 (2H, m, H-d), 1.56, 1.37,
0.97 (7H, m, m, t, B-nPr).
1,1-Carboboration reactions (general procedure)
A stirred solution of the respective cyclic alkyl(aryl)-
[δ119Sn = 0 for Ξ(119Sn) = 37.290665 MHz]; external BF3- borane (2 mmol) in hexane (20 mL) was cooled to −78 ◦C,
OEt2 [δ11B = 0 for Ξ(11B) = 32.083971 MHz]. Chemical and an equimolar amount of the alkynyltin compound (5
shifts are given to ±0.1 ppm for 13C and 119Sn, and or 6) dissolved in hexane (20 mL) was slowly added.
±0.4 ppm for 11B; coupling constants are given ±0.4 Hz for After the mixture had reached r. t., all volatile materials
J(119Sn,13C). 119Sn NMR spectra were measured directly by were removed in a vacuum, leaving yellow oily residues.
single pulse methods or by using the refocused INEPT pulse These were completely soluble in C6D6 or CDCl3, and
sequence [42, 43] based on 2J(119Sn,1H) (55 Hz) after opti- were studied by NMR spectroscopy for a period of several
mizing the delay times in the pulse sequence. Melting points weeks.
(uncorrected) were determined using a Bu¨chi 510 melting
point apparatus. All quantum chemical calculations were car- [52.3] (9H, s, SnMe3), 2.26 [53.3] (3H, s, Me–C=), 1.70
7(Me):1H NMR (CDCl3): δ1H [nJ(119Sn,1H)] = 0.33
ried out using the GAUSSIAN 09 program package [49].
(2H, t, 5.7 Hz, H-2), 2.80 (2H, t, 5.7 Hz, H-3), 1.85 (2H, t,
1H NMR data of 1-(2-phenyl-ethyl)-1-bora-indane 1 in 8.1 Hz, B-CH2), 2.84 (2H, t, 7.4 Hz, Ph-CH2), 7.06 – 7.43
C6D6: δ1H = 1.32 (2H, t, 5.2 Hz, H-2). 2.79 (2H. t, 5.2 Hz, (9H, m, aryl, Ph).
H-3), 7.36 (1H, m, H-4), 7.23 (1H, m, H-5), 7.26 (1H, m,
H-6), 7.69 (1H, d, 7.2 Hz, H-7), 1.85 (2H, t, 8.1 Hz, BCH2), [50.6] (9H, s, SnMe3), 2.26 [53.3] (3H, s, Me−C=), 1.78
2.83 (2H, t, 8.1 Hz, Ph-CH2), 7.02 – 7.11 (5H, m) Ph). (2H, t, 4.4 Hz, H-2), 2.78 (2H, t, 4.4 Hz, H-3), 1.90 (2H,
7(Fc): 1H NMR (CDCl3): δ1H [nJ(119Sn,1H)] = 0.57
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B. Wrackmeyer et al. · Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds
501
t, 7.4 Hz, B-CH2) 2.84 (2H, t, 7.4 Hz, Ph-CH2), 3.95 (2H, Crystal structure determination of the 1-bora-indane 1
m), 4.13 (5H, s, Cp), 4.20, 4.28 (2H, 2H, m, m, C5H4),
7.13 – 7.48 (9H, m, aryl, Ph).
Details pertinent to the crystal structure determination
are listed in Table 5 [50]. Crystals of appropriate size were
selected and sealed in Lindemann capillaries. The data
collections were carried out at 293 K using a Stoe IPDS I
system (MoKα , λ = 71.073 pm, graphite monochroma-
tor). Absorption corrections did not improve the data set.
Structure solution and refinement were accomplished using
SHELXS/L-97 [51, 52].
10(Me): 1H NMR (CDCl3); δ1H [nJ(119Sn,1H)] = 0.1
[51.8] (8H, s, SnMe3), 1.85 [51.7] (3H, s, = C-Me),
6.97 – 7.04 (4H, m, aryl), 2.80 – 0.80 (12H, m, CH, CH2).
10(Fc): 1H NMR (CDCl3): δ1H [nJ(119Sn,1H)] = 0.28
[50.5] (9H, s, SnMe3), 3.96 (5H, s, Cp), 3.82, 3.95 (2H, 2H,
m, m, C5H4), 6.96 – 7.07 [4H, m, aryl), 2.93 – 0.82 (12H, m,
CH. CH2).
1
11(Me): H NMR (C6D6): δ1H [nJ(119Sn,1H)] = −0.07
[51.7] (9H, s, SnMe’3), 2.07 [52.1] (3H, s. = C-Me),
6.98 – 7.86 (4H, m, aryl), 2.55 – 0.86 (12H, m, CH, CH2).
11(Fc): 1H NMR (CDCl3) δ1H [nJ(119Sn,1H)] = 0.05
[51.5] (9H, s, SnMe3), 4.07 (5H, s, Cp), 4.05, 4,23 (2H, 2H,
m, m, C4H4), 6.99 – 7.99 (4H, m, aryl), 3.28 – 1.43 (12H, m,
CH, CH2).
Acknowledgement
Support of the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. We thank the late Professor
R. Ko¨ster for samples containing the cyclic alkyl(aryl)-
boranes 1 and 4.
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