The BF3 and B(C6F5)3-catalyzed 1,3-isomerization of allylic stannanes

Article abstract of DOI:10.1016/S0022-328X(01)00642-8

A spectroscopic and chemical study of the stereospecific B(C₆F₅)₃-catalyzed 1,3-isomerization of (E,S)-1-methoxymethoxy-2-butenyl tributylstannane to (Z,S)-1-methyl-3-methoxymethoxy-2-propenyl tributylstannane is described. A pathway involving an intermediate B(C₆F₅)₃ adduct is proposed based upon ¹H- and ¹¹⁹Sn-NMR data. Analogous spectral evidence was not observed in the seemingly related BF₃·OEt₂-catalyzed isomerization. However, the similar product ratios obtained in B(C₆F₅)₃ and BF₃·OEt₂-promoted additions of the latter stannane to o- and p-methoxybenzaldehyde reveals a close similarity between the two and prompts speculation that the BF₃·OEt₂-catalyzed 1,3-isomerization proceeds by an analogous pathway. A pathway for the B(C₆F₅)₃-promoted addition of allylic stannanes to o-methoxybenzaldehyde is also presented.

Full text of DOI:10.1016/S0022-328X(01)00642-8

Journal of Organometallic Chemistry 624 (2001) 294–299
www.elsevier.nl/locate/jorganchem
The BF3 and B(C6F5)3-catalyzed 1,3-isomerization of allylic
stannanes
James A. Marshall *, Kevin Gill
Department of Chemistry, Uni6ersity of Virginia, PO Box 400319, Charlottes6ille VA 22904, USA
Received 2 November 2000; accepted 22 December 2000
Dedicated with great respect and admiration to Professor Jean F. Normant in recognition of his numerous insightful contributions to the
field of Organometallic Chemistry
Abstract
A spectroscopic and chemical study of the stereospecific B(C6F5)3-catalyzed 1,3-isomerization of (E,S)-1-methoxymethoxy-2-bu-
tenyl tributylstannane to (Z,S)-1-methyl-3-methoxymethoxy-2-propenyl tributylstannane is described. A pathway involving an
1
intermediate B(C6F5)3 adduct is proposed based upon H- and 119Sn-NMR data. Analogous spectral evidence was not observed
in the seemingly related BF3·OEt2-catalyzed isomerization. However, the similar product ratios obtained in B(C6F5)3 and
BF3·OEt2-promoted additions of the latter stannane to o- and p-methoxybenzaldehyde reveals a close similarity between the two
and prompts speculation that the BF3·OEt2-catalyzed 1,3-isomerization proceeds by an analogous pathway. A pathway for the
B(C6F5)3-promoted addition of allylic stannanes to o-methoxybenzaldehyde is also presented. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: 1,3-Isomerization; tris-(Pentafluorophenyl)borane; Tin-boron metathesis; Allylic stannanes; Chiral stannanes
1. Introduction
(1)
It has been known for some time that BF3·OEt2
catalyzes isomerizations of allylic stannanes [1]. Both
E/Z isomerization and 1,3-Sn migration have been
(2)
observed, in addition to ligand redistribution (Eqs. (1)
and (2)). However, a plausible rationale for these iso-
merizations and redistributions has proven elusive.
(3)
Based on isomer distribution in the products from
In the Denmark studies, the reaction between al-
additions of unsymmetrical allylic stannanes to alde-
lyltrimethylstannane and BF3·OEt2 ‘proved to be an
hydes, Tagliavini and co-workers suggested the inter-
enigma.’ At −80°C in CH2Cl2–CHCl3 the resonances
for C1, C3, and Me3Sn of the allylic stannane broad-
vention of allylic BF2 species as reactive intermediates
(Eq. (3)) [2]. Subsequently, Denmark et al. discounted
ened, but the signal for C2 remained sharp (Eq. (1),
this possibility because of their failure to detect the
R=Me, R1=H). At −20°C, the C1 and C3 signals
formation of Me3SnF·BF3 (l=0.7 ppm) in their low
were absent, but those for C2 and Me3Sn were visible.
temperature 13C-NMR studies on BF3-promoted addi-
tions of allyltrimethylstannane to aldehydes [3].
When the −20°C sample was cooled to −80°C the
spectrum showed signals for MenSn(CH2CHꢀCH2)4−n
indicating that methyl/allyl exchange (‘ligand redistri-
bution’) had taken place. Additional studies of a similar
nature with (E)- and (Z)-crotyltrimethylstannane
proved equally enigmatic [4]. Admixture of an 87:13
mixture of the E and Z isomers with BF3·OEt2 at
* Corresponding author. Tel.: +1-804-9247997; fax: +1-804-
9247993.
E-mail address: jam5x@virginia.edu (J.A. Marshall).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00642-8

J.A. Marshall, K. Gill / Journal of Organometallic Chemistry 624 (2001) 294–299
295
−80°C resulted in ‘an immediate’ isomerization to a
52:48 E/Z mixture. Upon warming the sample to −
40°C, the aforementioned ligand redistribution was ob-
served. The nature of the species responsible for ligand
redistribution or double bond isomerization could not
be determined.
Our interest in this issue originated with our findings
that enantioenriched a-oxygenated allylic stannanes are
stereospecifically isomerized by BF3·OEt2 to the (Z)-g-
isomers [5]. Crossover experiments revealed two impor-
tant aspects of this isomerization:
Based upon these observations we proposed the path-
way shown in Fig. 1 for these isomerizations. Though
consistent with the facts, this mechanism requires
BF3·OEt2 to act as a nucleophile toward a neutral tin
center. While such an interaction may be possible, it
lacks precedent and seems counterintuitive, given the
considerable Lewis acidity of BF3 (a reviewer called this
pathway ‘preposterous from an inorganic chemists
point of view’!). In accord with the findings of Den-
mark [4], we could detect no intermediates in the iso-
1
merization of A–D through H-, 13C-, or 119Sn-NMR
1. The reaction is intermolecular. A 1:1 mixture of
a-OMOM, a-Me3Sn and a-p-MeOC6H4CH2-
OCH2O, a-Bu3Sn allylic stannanes gave rise to all
four possible (Z)-g-OR isomers in essentially equal
amounts (Eq. (4)).
2. The reaction is bimolecular. This point was demon-
strated by dilution studies and by experiments in-
volving an enantioenriched (1) and a racemic
stannane (4) of the type described above. The
product (6) of Bu3Sn transferred to the racemic
stannane was enantioenriched to a small, but mea-
surable extent.
analysis of the isomerization reaction in CH2Cl2–
CD2Cl2 or toluene–C7D8 at −60°C.
A recent paper by Piers and co-workers rekindled
our interest in this isomerization [6]. A key finding was
the fact that (C6F5)3B forms an adduct, formulated as
8, with allyltributylstannane (7) at −60°C in toluene
(Eq. (5)). Spectroscopic evidence for this adduct in-
1
cluded H-NMR and 13C-NMR resonances at 8.0 and
190 ppm and a 119Sn resonance at 186 ppm. These
chemical shifts are consistent with a species in which
both C2 and Sn assume considerable positive character.
(5)
(4)
2. Results and discussion
2.1. NMR experiments
We repeated the Piers experiment with the a-OMOM
and g-OMOM tributylcrotylstannanes 9 and 11. When
mixed with equimolar (C6F5)3B in toluene at −60°C,
the a-OMOM stannane 9 gave rise to a signal in the
1H-NMR spectrum at 9.4 ppm which gave way to a
new signal at 9.7 ppm over time (Eq. (6)). The 119Sn
spectrum showed an initial signal at −31 ppm, which
slowly disappeared and was replaced by a signal at
+157 ppm. The appearance of this signal occurred on
1
a time scale comparable to that of the 9.7 ppm H-
NMR signal for the C2 proton. When this experiment
was repeated with 0.5 equivalents of (C6F5)3B, the
signal at −31 ppm was replaced by signals at +157
ppm, as before, and a new signal at −13 ppm. This
new signal was assumed to arise from the g-OMOM
crotylstannane 11. In support of this assumption, the
1H signal at 9.7 and −13ppm, and over time, the 157
ppm 119Sn signal were produced upon mixing equimolar
g-OMOM stannane 11 with (C6F5)3B at −60°C in
toluene. In a separate experiment we showed that
(C6F5)3B effectively catalyzes the isomerization of the
Fig. 1. Proposed pathway for BF3-catalyzed 1,3-isomerization of
a-oxygenated allylic stannanes.

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J.A. Marshall, K. Gill / Journal of Organometallic Chemistry 624 (2001) 294–299
meta and para fluorine peaks in the 19F-NMR spectrum
of the (C6F5)3B-allylic stannane complex. This differ-
ence has been correlated with the degree of anionic
character of boronate complexes [7]. Piers observed a
difference of only 4.9 ppm (versus\15 ppm in the
parent borane) in an allylstannane-(C6F5)3B complex
suggestive of considerable boronate character.
Our analysis of the data leads us to conclude that
(C6F5)3B interacts with the a-oxygenated allylic stan-
nane to produce an intermediate with cationic character
at C2. However the Bu3Sn moiety is less cationic than
the corresponding group in the Piers parent allyl system
[8]. We also conclude that the boron is less anionic in
our proposed intermediate as compared to that ob-
served by Piers (see Eq. (5)).
The foregoing findings are consistent with the path-
way shown in Fig. 2. Accordingly, the cationic interme-
diate E donates a Bu3Sn group to the a-oxygenated
stannane A to afford the bis-tributylstannylated cation
G. This transient, but pivotal, intermediate serves as the
shuttle by which the a-oxygenated stannane A is con-
verted to the g-isomer D with regeneration of cation G
in a propagation step.
Fig. 2. Proposed pathway for B(C6F5)3-catalyzed isomerization of a-
to g-oxygenated allylic stannanes.
Table 1
Additions of the g-OMOM stannane 11 to o-methoxybenzaldehyde
(13) promoted by various Lewis acids
2.2. Additions to o- and p-methoxybenzaldehyde
promoted by B(C6F5)3 and BF3·OEt2
Lewis acid
T (°C)
Yield (%)
15:16:17 a
As noted above, previous attempts to identify inter-
mediates in BF3·OEt2-catalyzed allylic stannane isomer-
izations by NMR analysis at low temperature were
unsuccessful. Conceivably, the BF3 adducts could be
present in amounts below our NMR detection
thresholds. Lacking such direct evidence, we decided to
compare addition reactions of the g-OMOM allylic
stannane 11 to aldehydes promoted by B(C6F5)3 and
BF·OEt2. In doing so, we expected to demonstrate
similarities or differences between these two Lewis acids
that might validate, or possibly invalidate, an extrapo-
lation of the pathway for B(C6F5)3-catalyzed 1,3-iso-
merization to the corresponding BF3-catalyzed
isomerization.
BF3·OEt2
B(C6F5)3
MgBr2·OEt2
−78
−78 to −20
−30 to 0
83
63
76
26:68:6
23:71:6
47:37:16
a Ratios determined by GC analysis.
a-OMOM to the g-OMOM crotylstannane in toluene
at −60°C (Eq. (7)).
(6)
For this comparison we chose o- and p-methoxyben-
zaldehyde (13 and 14). Piers had reported a \20:1 rate
difference in B(C6F5)3-promoted additions of allyl trib-
utylstannane to these aldehydes, favoring the ortho
isomer [6]. This rate difference had been previously
attributed to a boron chelation effect [9], but this
rationale was discredited by the Piers study. Our find-
ings are summarized in Table 1.
(7)
Clearly the ratio of syn and anti adducts 15 and 16
compares favorably for the two boron Lewis acids [10].
Interestingly, a small amount of the (Z)-adduct 17 is
formed in both additions. The same three adducts were
formed when MgBr2·OEt2 was employed as the Lewis
acid (Table 1). MgBr2 is known to favor a chelation
pathway in such additions [11]. Though similar product
8
We failed to detect a discrete peak for C2 in the
13C-NMR spectrum of the (C6F5)3B complex of the
foregoing stannanes. We suspect that this signal is
obscured by the toluene and C6F5 resonances. We also
saw merging (|ꢀ7 ppm) of the chemical shifts for the

J.A. Marshall, K. Gill / Journal of Organometallic Chemistry 624 (2001) 294–299
297
ratios were obtained with BF3·OEt2 and B(C6F5)3, the
reaction was faster with the former Lewis acid. An
analogous rate difference was seen in the 1,3-isomeriza-
tion studies.
Additions of the g-OMOM allylic stannane 11 to
p-methoxybenzaldehyde (14) gave nearly identical
syn:anti ratios of adducts 18 and 19 strongly favoring
the former, with both boron Lewis acids (Eq. (8)).
These ratios are in accord with the usual acyclic transi-
tion states [11].
(10)
(8)
(11)
Thus, an analogy between BF3·OEt2 and B(C6F5)3 as
Lewis acid promoters in allylic stannane additions is
established. The additions to o-methoxybenzaldehyde
bear further comment. As noted above, the initial ob-
servation of this effect was a rate enhancement in the
allylation reaction which was attributed to a chelated
intermediate H (Eq. (9)) [9]. This pathway was rejected
by Piers who proposed the intermediate J (Eq. (10)) as
one alternative based on NMR studies [6]. Piers tenta-
tively suggested that the stannyl chelate J could un-
dergo allylation by the allylic triarylboronate, as
depicted in Eq. (10), to afford the adduct K. However
he stated that a pathway involving attack on the stan-
nyloxy chelate J by the allylstannane could not be
excluded. Our findings are in accord with this latter
pathway. Were the additions to proceed through the
allylic boronate N, as in Eq. (11), the products of the
g-OMOM allylic stannane additions, O, would be re-
gioisomeric to those actually formed (L in Eq. (11)).
One further comment is in order. The isomerization
reactions are catalytic in Lewis acids while the addi-
tions require stoichiometric quantities. Thus it seems
likely that the presumed initial adduct K is converted to
boronate I which prevents turnover of the Lewis acid.
As this issue has not been studied, the suggestion,
though reasonable, is still speculative.
On a final note, Yamataka et al. have reported an
enhanced reactivity of ortho-chlorobenzaldehyde com-
pared to the para isomer in BF3·OEt2-promoted addi-
tions of crotyl tributyl tin [12]. The two aldehydes also
give different ratios of syn and anti adducts. They
propose a cyclic transition state for the ortho isomer, as
in P (Eq. (12)), to explain these differences. In princi-
pal, a similar effect could be operative in reactions of
ortho-methoxybenzaldehyde. However, if that were the
case the stereochemistry of the adducts would be enan-
tiomeric to those obtained in the present studies, as
illustrated in Eq. (12). Thus the more conventional
acyclic transition state Q best accommodates the ob-
served product stereochemistry.
(12)
3. Conclusions
(9)
In conclusion, we have obtained NMR evidence for
intermediates 10 and 12 (Eq. (6)) in the B(C6F5)3-cata-
lyzed isomerization of the a-oxygenated allylic stannane
9 to the g-isomer 11. We have also shown that both
BF3·OEt2 and B(C6F5)3 catalyze this isomerization with
virtually 100% enantio- and regioselectivity. We have

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J.A. Marshall, K. Gill / Journal of Organometallic Chemistry 624 (2001) 294–299
further demonstrated that BF3·OEt2 and B(C6F5)3 dis-
play analogous and characteristic Lewis acid properties
in the addition of the enantioenriched g-OMOM allylic
stannane 11 to o- and p-methoxybenzaldehyde. These
findings are consistent with the pathway depicted in
Fig. 2 for the 1,3-Bu3Sn shift in allylic stannanes cata-
lyzed by B(C6F5)3 and, by analogy, BF3·OEt2, as well.
with 15% EtOAc–Hex) afforded 0.049 g (72%) of alco-
hol 18 as a clear colorless oil. [h]D= −74 (c 1.2,
CHCl3); IR (neat) 3470 cm−1 1H-NMR (300 MHz,
.
CDCl3) l 7.27–7.24 (m, 2H), 6.88–6.83 (m, 2H), 5.52
(dq J=15.3, 6.0 Hz, 1H), 5.23 (dd, J=15.3, 8.1 Hz,
1H), 4.75 and 4.56 (ABq, J=6.9 Hz, 2H), 4.53 (dd,
J=7.5, 2.4 Hz, 1H), 4.03 (dd, J=7.5, 7.5 Hz, 1H),
3.80 (s, 3H), 3.33 (s, 3H), 3.08 (d, J=2.4 Hz, 1H), 1.61
(d, J=6.6 Hz, 3H). 13C-NMR (CDCl3) l 17.8, 55.2,
55.7, 76.3, 81.6, 93.7, 113.4, 126.9, 128.3, 131.7, 132.3,
159.1. Anal. Calc. for C14H20O4: C, 66.66; H, 7.99.
Found: C, 66.74; H, 8.02%.
4. Experimental
4.1. General
B(C6F5)3 was used as received from Strem Chemical.
All operations were performed under an atmosphere of
dry nitrogen or argon.
4.3.2. o-Methoxybenzaldehyde (13)
The previous procedure was followed substituting
o-methoxybenzaldehyde (0.034 g, 0.25 mmol) for the
p-isomer. Purification of the product by column chro-
matography on silica gel (elution with 15% EtOAc–
Hex) afforded 0.039 g (63%) of a 23:71:6 (by GC
analysis) mixture of alcohols 15, 16, and 17 as a clear
4.2. Tris(pentafluorophenylborane)-catalyzed
isomerization of h-OMOM stannane 9 to k-OMOM
stannane 11
1
colorless oil. 16: H-NMR (300 MHz, CDCl3) l 7.38–
To a stirred solution of (S)-a-OMOM stannane (9)
[5] (0.20 g, 0.50 mmol) in 4.5 ml of toluene at −78°C
was added B(C6F5)3 (0.051 g, 0.099 mmol) in 0.5 ml of
toluene. The resulting solution was stirred at −78°C
for 5 h. The reaction was quenched with saturated
aqueous NaHCO3, the mixture was allowed to warm to
room temperature (r.t.) and the layers were separated.
The aqueous layer was extracted with Et2O. The com-
bined extracts were dried over sodium sulfate, filtered,
and concentrated. Purification by column chromatogra-
phy on silica gel (elution with 2.5% EtOAc–Hex) af-
forded 0.13 g (66%) of (S)-g-OMOM stannane (11) [5]
as a clear colorless oil. [h]D= +136° (c 1.56, CH2Cl2).
1H-NMR (300 MHz, CDCl3) l 5.93 (d, J=6.3 Hz,
1H), 4.73 (ABq, J=6.3 Hz, 2H), 4.47 (dd, J=11.4,
6.3, 1H), 3.36 (s, 3H), 2.47 (dt, J=10.2, 5.4, 1H),
1.63–1.17 (m, 18H), 1.00–0.71 (m, 12H). The rotation
and 1H-NMR spectrum were in complete agreement
with the published values [5].
7.34 (m, 1H), 7.24–7.20 (m, 1H), 7.0–6.82 (m, 2H),
5.58 (dq, J=15.6, 6.6 Hz, 1H), 5.42 (dd, J=15.6, 7.8
Hz, 1H), 5.05 (dd, J=5.4, 5.1 Hz, 1H), 4.69 and 4.51
(ABq J=6.6 Hz, 2H), 4.3 (dd, J=9.0, 5.1 Hz, 1H),
3.85 (s, 3H), 3.18 (s, 3H), 2.77 (dd, J=5.4 Hz, 1H),
1.67 (d, J=6.6 Hz, 3H). 13C-NMR (CDCl3) l 159.8,
131.5, 128.3, 128.1, 126.5, 120.5, 110.2, 93.3, 78.6, 77.2,
72.0, 55.4, 17.9.
15: 1H-NMR (300 MHz, CDCl3) l 7.38–7.34 (m,
1H), 7.24–7.20 (m, 1H), 7.0–6.82 (m, 2H), 5.57 (dq,
J=15.6, 6.3 Hz, 1H), 5.42 (dd, J=15.6, 8.1 Hz, 1H),
5.00 (dd, J=5.4, 5.4 Hz, 1H), 4.69 and 4.47 (ABq,
J=6.6 Hz, 2H), 4.18 (dd, J=7.5, 5.7 Hz, 1H), 3.81 (s,
3H), 3.14 (s, 3H), 3.05 (d, J=5.7 Hz, 1H), 1.63 (d,
J=6.3 Hz, 3H). 13C-NMR (CDCl3) l 160.1, 130.3,
128.7, 127.7, 127.6, 120.5, 110.2, 93.6, 80.2, 77.2, 71.5,
55.0, 17.7.
4.4. BF3·OEt2-promoted additions
4.3. Tris(pentafluorophenyl)borane-promoted additions
4.4.1. p-Methoxybenzaldehyde (14)
4.3.1. p-Methoxybenzaldehyde (14)
To a stirred solution of (S)-g-OMOM stannane (11)
(0.12 g, 0.30 mmol) and p-methoxybenzaldehyde (14)
(0.034 g, 0.25 mmol) in 2.5 ml of CH2Cl2 at −78°C
was added BF3·OEt2 (0.061 g, 0.49 mmol). The result-
ing solution was stirred at that temperature for 4 h. The
reaction was quenched with saturated aqueous
NaHCO3 and the mixture was allowed to warm to r.t.
The layers were separated and the aqueous layer was
extracted with ether. The combined organic extracts
were dried over sodium sulfate, filtered, and concen-
trated. Purification by column chromatography on sil-
ica gel (elution with 15% EtOAc–Hex) afforded 0.049 g
(79%) of alcohol 18 as a clear colorless oil.
To a stirred solution of (S)-g-OMOM stannane (11)
(0.12 g, 0.30 mmol) and p-methoxybenzaldehyde (0.034
g, 0.25 mmol) in 2.5 ml of CH2Cl2 at −78°C was
added B(C6F5)3 (0.15 g, 0.29 mmol). The resulting
solution was then allowed to slowly warm to −20°C
and was stirred at that temperature for 4 h. The reac-
tion was then quenched with saturated aqueous
NaHCO3 and the mixture was allowed to warm to r.t.
The layers were separated and the aqueous layer was
extracted with ether. The combined extracts were dried
over sodium sulfate, filtered, and concentrated. Purifi-
cation by column chromatography on silica gel (elution

J.A. Marshall, K. Gill / Journal of Organometallic Chemistry 624 (2001) 294–299
299
4.4.2. o-Methoxybenzaldehyde (13)
−156.7 (s, 6F, m-F’s). 119Sn-NMR (149.2 MHz) 157
ppm.
The previous procedure was employed substituting
o-methoxybenzaldehyde (13) (0.034 g, 0.25 mmol) for
the p-isomer. Purification by column chromatography
on silica gel (elution with 15% EtOAc–Hex) afforded
0.039 g (83%) of a 26:68:6 (by GC analysis) mixture of
alcohols 15, 16, and 17 as a clear colorless oil.
Acknowledgements
These studies were supported with funding from the
National Science Foundation through research grant
CHE 99001319. We are indebted to Dr Jeff Ellena for
his assistance in securing the NMR data reported in
this study. We thank Professor Warren Piers for in-
sightful comments in his review of this manuscript.
4.5. MgBr2·OEt2-promoted addition to
o-methoxybenzaldehyde
To a stirred solution of o-methoxybenzaldehyde (13)
(0.023 g, 0.17 mmol) and (S)-g-OMOM stannane (11)
(0.12 g, 0.25 mmol) in 1.6 ml of CH2Cl2 at −30°C was
added MgBr2·OEt2 (0.052 g, 0.20 mmol). The resulting
solution was allowed to slowly warm to 0°C and stirred
at that temperature for 12 h. It was then quenched with
saturated NaHCO3 solution and allowed to warm to
r.t. The phases were separated and the aqueous layer
was extracted with Et2O. The combined organic ex-
tracts were dried over sodium sulfate, filtered, and
concentrated. Purification by column chromatography
on silica gel (elution with 15% EtOAc–Hex) afforded
0.031 g (76%) of a 47:37:16 (by GC) mixture of alcohols
15, 16, and 17 as a clear colorless oil.
References
[1] Review: Y. Yamamoto, N. Shida, in: Advances in Detailed
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The 119Sn chemical shift for Bu3Sn+ B(C6F5)4− in benzene is 263
ppm corresponding to a calculated 33% cation character. We
thank a reviewer for pointing out that the positive character of
the Sn atom in complex 12 may be diminished by coordination
with the OMOM grouping.
[9] K. Maruoka, T. Ooi, Chem. Eur. J. 5 (1999) 829.
[10] The ee and absolute stereochemistry were assigned through
1H-NMR analysis of the Mosher O-methylmandelic esters. The
relative stereochemistry was assigned by analogy and by the
characteristic chemical shifts of the OH proton of the syn (3.03
and 3.10 ppm) and anti (2.77 ppm) adducts. J.A. Marshall, A.W.
Garofalo, J. Org. Chem. 61 (1996) 8732.
4.6. NMR experiments
To a septum-sealed NMR tube (10 mm) containing
0.20 g (0.40 mmol) of B(C6F5)3 in toluene-d8 (3.5 ml)
cooled to −60°C was added 0.16 g (0.40 mmol) of
g-OMOM stannane (9) in toluene-d8 (0.25 ml) by sy-
ringe. The following NMR spectra were collected at
1
−60°C: H-NMR (500 MHz) (selected resonances): 9.4
(br, s, 1H), which over 4 h shifted to 9.7 (br, s, 1H).
19F-NMR (282.4 MHz), −128.5 (s, 6F, o-F’s), −149.3
(s, 3F, p-F’s), −156.2 (s, 6F, m-F’s) which over 4 h
shifted to −128.4 (s, 6F, o-F’s), −150.3 (s, 3F, p-F’s),
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1711.
.