Reductive openings of acetals: Explanation of regioselectivity in borane reductions by mechanistic studies

Article abstract of DOI:10.1021/jo800396g

(Graph Presented) The mechanisms of regioselective reductive openings of acetals were investigated in several model systems by a combination of Hammett plots, kinetic experiments, density functional calculations, and 11B NMR. The regioselectivity of borane reductions of cyclic acetals can be controlled by the choice of borane. Lewis acid activation of BH₃·NMe ₃ increases the reaction rate and renders the borane the most electrophilic species, which associates to the more electron-rich oxygen of the acetal. In contrary, without activation, the regioselectivity is instead directed by the Lewis acid, as exemplified by the reaction with BH ₃·THF.

Full text of DOI:10.1021/jo800396g

Reductive Openings of Acetals: Explanation of Regioselectivity in
Borane Reductions by Mechanistic Studies
Richard Johnsson, Dan Olsson, and Ulf Ellervik*
Organic Chemistry, Lund UniVersity, P.O. Box 124, SE-221 00 Lund, Sweden
ulf.ellerVik@organic.lu.se
ReceiVed February 18, 2008
The mechanisms of regioselective reductive openings of acetals were investigated in several model systems
by a combination of Hammett plots, kinetic experiments, density functional calculations, and 11B NMR.
The regioselectivity of borane reductions of cyclic acetals can be controlled by the choice of borane.
Lewis acid activation of BH3 ·NMe3 increases the reaction rate and renders the borane the most electrophilic
species, which associates to the more electron-rich oxygen of the acetal. In contrary, without activation,
the regioselectivity is instead directed by the Lewis acid, as exemplified by the reaction with BH3 · THF.
Introduction
carbohydrate chemistry, where reductive openings of acetals
facilitated protective group manipulation.3
In 1951, Doukas and Fontaine opened the acetal diosgenin
using LiAlH4 and HCl, to give dihydrodiosgenin.1 Later
investigations established the active reagent in the reduction as
LiAlH4-AlCl3 and the chemo-, stereo-, and regioselectivity of
reductive openings of cyclic acetals were intensively explored.2
The methodology turned out to be especially fruitful in
The harsh conditions, that made the use of many function-
alities impossible, turned out a major problem. Thus, Garegg
and Hultberg introduced the reagent combination NaCN-
BH3-HCl-THF which, interestingly, gave the opposite regi-
oselectivity compared to the original method.4 In order to get
back to the original regioselectivity the Garegg group turned
(1) Doukas, H. M.; Fontaine, T. D. J. Am. Chem. Soc. 1951, 73, 5917–5918.
(2) (a) Eliel, E. L.; Rerick, M. J. Org. Chem. 1958, 23, 1088. (b) Eliel, E. L.;
Badding, V. G.; Rerick, M. N. J. Am. Chem. Soc. 1962, 84, 2371–2377.
(3) (a) Bhattacharjee, S. S.; Gorin, P. A. J. Can. J. Chem. 1969, 47, 1195–
1206. (b) Lipta´k, A.; Joda´l, I.; Na´na´si, P. Carbohydr. Res. 1975, 44, 1–11.
5226 J. Org. Chem. 2008, 73, 5226–5232
10.1021/jo800396g CCC: $40.75 2008 American Chemical Society
Published on Web 04/30/2008

ReductiVe Openings of Acetals
SCHEME 1. Examples of Regioselective Openings of
Benzylidene Acetals
gives the 4-O-benzyl ether in combination with BH3 ·THF and
the 6-O-benzyl ether with BH3 ·NMe3.9
Results
In our investigations of reductive openings of compound 1
we found that, irrespective of the Lewis acid used (AlCl3,
BF3 ·OEt2, In(OTf)3, Cu(OTf)2, AgOTf), the 4-O-benzyl ether
2 was formed using BH3 ·THF, and the 6-O-benzyl ether 3 was
isolated using BH3 ·NMe3 as reducing agent, in THF as solvent.
To our surprise, reductions using BH3 ·NMe3 were generally
faster compared to reactions using BH3 ·THF, despite the general
belief that BH3 ·THF is the more reactive reagent.10
A. Lewis Acid Activation of BH3 ·NMe3. We have previ-
ously introduced benzylic-phenolic acetals for the synthesis
of fluorescently labeled probes.11 These acetals were opened
under reductive conditions to yield double benzylic ethers, and
we realized that these compounds form an excellent model
system to investigate regioselective reductive openings of cyclic
acetals. Compounds 4a-i (Scheme 3) were thus synthesized
from the corresponding dimethyl acetals and opened using
BH3 ·NMe3-AlCl3 in THF at 0 °C.12 In all cases, double
benzylic ethers (compounds 5a-i) were isolated as the single
products.
their attention to boranes. They found that the reagent combina-
tion BH3 ·NMe3-AlCl3 gave different regioselectivity in dif-
ferent solvents, i.e., 6-O-benzyl ethers in THF and 4-O-benzyl
ethers in toluene or dichloromethane/ether mixtures. Due to
degradation, reactions in toluene usually gave low yields. The
methods are summarized in Scheme 1.
The proposed mechanism for reductive opening of acetals
presumes coordination of the Lewis acid to one of the oxygen
atoms, according to path A in Scheme 4, followed by reduction
of either the unopened acetal or the oxocarbenium ion. Irrespec-
tive of mechanistic pathway, the Lewis acid must associate with
the oxygen that will become a free hydroxyl group. Since only
compounds 5a-i were isolated, the Lewis acid must have
coordinated to the phenolic oxygen.
Despite the numerous reports on acid-mediated nucleophilic
addition to acetals,6 and the immense importance of regio-
selective reductive openings of acetals for organic chemistry
in general and for carbohydrate chemistry in particular, the
underlying principles for the regioselectivity are still not
understood.7 However, one mechanism, proposed by Garegg
(Scheme 2),8 explain the regioselective outcome by the differ-
ence in steric bulk between AlCl3 and a proton. This means
that in the original method, AlCl3 is supposedly first coordinated
to the sterically less demanding oxygen at position 6 (path A).
In contrary, in the NaCNBH3-HCl method the proton, which
is not restricted by steric effects, coordinates to O-4 to give the
more stable oxocarbenium ion at C-6 (path B).
The electrostatic potentials of the oxygen atoms in compounds
4a-i were calculated using density functional theory at the
B3LYP/6-31G* level.13 Throughout the series, the benzylic
oxygen showed the highest electrostatic potential. Interestingly,
the benzylic oxygen is thus both the more basic and also the
less sterically hindered oxygen atom, but not the one chosen
by the Lewis acid. With respect to these calculations, there are
two alternative explanations: (i) the relative energies of the two
oxocarbenium ions direct the regiochemistry or (ii) the outcome
is instead directed by initial coordination of the borane, followed
by coordination of the Lewis acid and subsequent reduction
(Scheme 4, path B).
There are several problems associated with this mechanistic
explanation. The major issue is the problem to explain the
regiochemical outcome of the reaction with BH3 ·NMe3-AlCl3
in THF, which follows path B, despite the obvious conclusion
that the strongly solvated AlCl3 ·THF would preferably associate
with the less sterically hindered O-6 to give the 4-O-benzyl
ether.5 This mechanism also fails to rationalize why Cu(OTf)2
To investigate these two options, we calculated the energy
of BH3 associated to the oxygen atoms of compound 4a,
generating structures 7a and 7b, as well as the relative energies
of two analogous oxocarbenium ions, i.e., structures 8a and 8b
(Chart 1).14 From the energy differences, the ratios were
calculated using the Boltzmann distribution and showed exclu-
sive preference for 7a and 8a. This means that the most stable
oxocarbenium ion and the most stable boron complex would
lead to the same product (i.e., compound 5a). As a control, we
calculated the energy of AlCl3 associated to the two oxygen
(4) (a) Garegg, P. J.; Hultberg, H. Carbohydr. Res. 1981, 93, C10–C11. (b)
Garegg, P. J.; Hultberg, H.; Wallin, S. Carbohydr. Res. 1982, 108, 97–101.
(5) (a) Ek, M.; Garegg, P. J.; Hultberg, H.; Oscarson, S. J. Carbohydr. Chem.
1983, 2, 305–311. (b) Fu¨gedi, P.; Garegg, P. J.; Kvarnström, I.; Svansson, L.
J. Carbohydr. Chem. 1988, 7, 389–397. (c) Fu¨gedi, P.; Birberg, W.; Garegg,
P. J.; Pilotti, Å Carbohydr. Res. 1987, 164, 297–312.
(6) (a) Sammakia, T.; Smith, R. S. J. Am. Chem. Soc. 1992, 114, 10998–
10999. (b) Bartels, B.; Hunter, R. J. Org. Chem. 1993, 58, 6756–6765. (c)
Denmark, S. E.; Almstead, N. G. J. Am. Chem. Soc. 1991, 113, 8089–8110. (d)
Mori, I.; Ishihara, K.; Flippin, L. A.; Nozaki, K.; Yamamoto, H.; Bartlett, P. A.;
Heathcock, C. H. J. Org. Chem. 1990, 55, 6107–6115.
(9) Shie, C.-R.; Tzeng, Z.-H.; Kulkarni, S. S.; Uang, B.-J.; Hsu, C.-Y.; Hung,
S.-C. Angew. Chem., Int. Ed. 2005, 44, 1665–1668.
(10) Smith, K.; Pelter, A. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Ed.; Pergamon Press: Oxford, 1991; Vol. 8, p 708
(11) Johnsson, R.; Mani, K.; Cheng, F.; Ellervik, U. J. Org. Chem. 2006,
71, 3444–3451.
(7) Stick, R. V. Carbohydrates: The Sweet Molecules of Life; Academic Press:
London, 2001, p 53.
(12) Experimental details can be found in the Supporting Information.
(13) Spartan ‘02 for Macintosh, Wavefunction, Inc., Irvine, CA.
(14) Jaguar, version 7.0, Schro¨dinger, LLC, New York, NY, 2007.
(8) Garegg, P. J. In PreparatiVe Carbohydrate Chemistry; Hanessian, S., Ed.;
Marcel Dekker: New York, 1996; pp 53-67..
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Johnsson et al.
SCHEME 2. Mechanistic Pathways Proposed for Regioselective Reductive Opening of Cyclic Acetals
SCHEME 3. Reductive Opening of Phenolic-Benzylic
Acetals
charge of the phenolic oxygen, which in turn implies a
diminished importance of the oxocarbenium ion pathway.
To gain further insight in the reaction mechanism, the
concentrations of 4a, BH3 ·NMe3, and AlCl3 were systematically
varied and the initial-rate kinetics determined (Figure 2).
From these data, we conclude that the reaction is first order
with respect to 4a. Interestingly, the kinetic data for variation
of [AlCl3]0 show that the reaction is of higher order (Figure
2b, dotted line indicates first order kinetics). One possible
explanation is activation of the borane by the Lewis acid, a
concept suggested, but not further investigated by Brown.15 To
investigate this option, we estimated the energies for formation
of different complexes as shown in Scheme 5. The dissociation
energies for known complexes (Table 1), in combination with
calculated data for formation of complexes of 4a with the
electrophiles (i.e., BH3 and AlCl3), indicate that reactions a and
b are thermodynamically unfavorable (by 32 and 45 kJ/mol,
respectively), whereas reaction c is thermoneutral.12,14 The
driving force is the formation of the highly stabilized
AlCl3 ·NMe3.
Activation of the borane by AlCl3 can also explain the
complex kinetics shown by variation of [BH3 ·NMe3]0. At low
amounts of borane, the relative amount of AlCl3 is high (7:1),
resulting in unusually high reaction rates.
atoms of compound 4a. Not surprisingly, we found a 100:0
distribution of the Lewis acid associated with the benzylic
oxygen.
To distinguish between the two mechanistic possibilities, we
turned to Hammett plots. Compounds 4a-i were subjected to
BH3 ·NMe3-AlCl3 in THF, and the rate constants were deter-
mined by initial-rate analysis over the linear part of the plots
(Figure 1).
The Hammett correlation for the A ring was best fitted to
normal σpara-parameters, while the correlation for the B ring
best fitted to σ+-parameter. Both Hammett plots resulted in
negative F values with a more pronounced effect for the B ring
compared to the A ring. This indicates that the buildup of a
positive charge of the acetal carbon is more important than the
Finally, to examine the fate of the borane in the reaction we
performed a 11B NMR study. Compound 4a was dissolved in
THF-d8 and subjected to the reductive conditions. At the
beginning of the reaction, only BH3 · NMe3 (quartet at -7.5
ppm) can be seen (Figure 3). Over the course of the reaction, a
new peak (triplet at 8 ppm) appears, which corresponds to a
BH2 group, probably associated with oxygen.17 The species
Me3NBH2 would have appeared at 38 ppm, where no peak was
observed. This clearly shows that the borane is cleaved from
the amine, possibly by interaction with the Lewis acid.
SCHEME 4. Possible Mechanistic Pathways for Regioselective Reductive Opening of Phenolic-Benzylic Acetals
5228 J. Org. Chem. Vol. 73, No. 14, 2008

ReductiVe Openings of Acetals
CHART 1. Energies for Compounds 7 and 8 Calculated
Using Density Functional Theory at the B3LYP/6-31G**
Level
B. Regioselectivity by Choice of Borane. Due to unfavor-
able thermodynamics, it is reasonable to assume that BH3 ·THF
is not activated by the Lewis acid. Thus, in order to evaluate
the differences between BH3 ·NMe3 and BH3 ·THF, we turned
to catechols, where it is possible to fine tune the stereoelectronics
without interference of steric effects. We therefore synthesized
catechol acetals and subjected these to the reductive conditions
(Scheme 6). Generally, these acetals were more difficult to open,
compared to the six-membered rings, and the analogous nitro
compound was inert under these conditions.
Compound 9a was opened using BH3 ·NMe3 to give a mixture
of 10a:11a in a 34:66 ratio. A similar opening of 9b, carrying
a less electron-donating methyl group, gave a 47:53 distribution
of 10b:11b (duplicate reactions). Interestingly, opening of 9a
and 9b using BH3 ·THF gave a 65:35 mixture of 10a:11a and
a 53:47 mixture of 10b:11b, i.e., the opposite results compared
to BH3 ·NMe3.
FIGURE 2. Initial rate analysis of the formation of product (P).
Variation of (a) [4a]0, (b) [AlCl3]0, (c) [BH3 ·NMe3]0. When constant
[4a] ) 0.033 mol L-1, [AlCl3] ) 0.23 mol L-1, [BH3 ·NMe3] ) 0.20
mol L-1. Error bars show standard deviation.
SCHEME 5. Possible Reactions between Reactive Species in
the Mixturea
a While reactions a and b are thermodynamically unfavorable, reaction
c is thermoneutral.
The energies of possible reactive intermediates (Chart 2) were
calculated using density functional theory at the B3LYP/6-
31G** level, and ratios (leading to 10 and 11) were estimated
using the Boltzmann distribution. The data are summarized in
Table 2.14
These data show a remarkable agreement with initial coor-
dination of the borane in the case of BH3 ·NMe3 as reducing
agent (i.e., 12:13) and association of the Lewis acid in the case
FIGURE 1. Hammett plots of (a) substituent variation in the A ring
and (b) substituent variation in the B ring. Error bars show standard
deviation.
J. Org. Chem. Vol. 73, No. 14, 2008 5229

Johnsson et al.
SCHEME 6. Reductive Opening of Catechol Acetals
TABLE 1. Dissociation Energies for Lewis Acid-base
Complexes16
complex
dissociation energy (kJ/mol)
BH3 ·NMe3
AlCl3 ·NMe3
AlCl3 ·THF
AlCl3 ·2THF
160
199
90
132
of BH3 ·THF (i.e., 15:14). It seems less likely that the outcome
is directed by the oxocarbenium ions (i.e., 17:16).
The results can be explained by BH3 ·NMe3 being activated
by AlCl3, and that this activation results in both higher reaction
rates and also, somewhat surprising, that the borane is the most
electrophilic species and thus directs the reactivity. It is also
reasonable to assume that no such activation would occur in
the reaction with BH3 ·THF and that the selectivity instead
depends on the Lewis acid, similar to the earlier proposed
mechanism.
CHART 2. Possible Reactive Intermediates
To shed further light on the mechanism of reductive opening
of carbohydrate benzylidene acetals, we investigated the reaction
kinetics with respect to [AlCl3]0, in the openings of compound
1, using BH3 ·NMe3 (to give 3) and BH3 ·THF (to give 2). The
reactions were monitored by quantitative TLC, and the initial
rate kinetics were determined (Figure 4).18
We conclude that reductive opening of compound 1 follows
first order kinetics using BH3 ·THF, but higher order kinetics
using BH3 ·NMe3. This is in full agreement with our mechanistic
proposal, where the activation of BH3 ·NMe3 requires a second
equivalent of Lewis acid compared to the reaction using
BH3 ·THF. The regioselectivity is directed by association of the
most electrophilic species (i.e., BH3 or AlCl3 depending on
activation) to the most electron-rich oxygen.
TABLE 2. Experimental and Calculated Ratios for Reductive
Opening of Compounds 12-17
acetal 9a
acetal 9b
experimental ratioa (BH3 ·NMe3)
experimental ratioa (BH3 ·THF)
calculated ratio (12:13)b
34:66
65:35
35:65
70:30
2:98
47:53
53:47
46:54
52:48
22:78
calculated ratio (15:14)b
calculated ratio (17:16)b
a 10:11. b The ratios are calculated to indicate the formation of 10:11.
Discussion
With these observations, we propose a mechanism for
reductive openings of acetals using BH3 ·NMe3 in THF, where
the Lewis acid activates the borane complex and the more
electron-rich oxygen of the acetal associate with the borane
(Scheme 7). The reduction takes place by activation of the other
oxygen by a second molecule of AlCl3, which explains the
higher reaction order.
Since the introduction of reductive openings of benzylidene
acetals, a plethora of reagent combinations have been developed.
The methods are summarized in Table 3.
From the data in Table 3 we conclude that BH3 ·THF
generally gives reductive openings to the free 6-hydroxyl (i.e.,
(15) Brown, H. C.; Murray, L. T. Inorg. Chem. 1984, 23, 2746–2753.
(16) (a) Jonas, V.; Frenking, G.; Reetz, M. T. J. Am. Chem. Soc. 1994, 116,
8741–8753. (b) Richards, R. L.; Thompson, A. J. Chem. Soc. A 1967, 1244–
1248. (c) Rablen, P. R. J. Am. Chem. Soc. 1997, 119, 8350–8360.
FIGURE 3. Stacked 11B NMR spectra of the reductive opening of
compound 4a (0-45 min).
5230 J. Org. Chem. Vol. 73, No. 14, 2008

ReductiVe Openings of Acetals
TABLE 3. Examples of Reagents for Reductive Opening of 1
conditions
yield 2:3 (%)
ref
BH3 ·THF, Bu2BOTf, THF/CH2Cl2, 0 °C, 1 h
BH3 ·THF, Cu(OTf)2, THF/CH2Cl2, rt, 45 min
BH3 ·THF, CoCl2, THF, rt, 10 min
87:0
94:0
19
9
20
21
9
22
9
23
23
9
100:0
87-94:0
78:3
62:20
0:40
73:3
55:30
40:0
BH3 ·THF, M(OTf)n,a CH2Cl2, rt, 3-5 h
BH3 ·Me2S, Cu(OTf)2, THF, rt, 10 h
BH3 ·Me2S, BF3 ·Et2O, CH2Cl2, 0 °C
BH3 ·Me3N, Cu(OTf)2, THF/CH2Cl2, rt, 25 h
BH3 ·Me2NH, BF3 ·Et2O, CH2Cl2, 0 °C
BH3 ·Me2NH, BF3 ·Et2O, CH3CN, 0 °C
9-BBN, Cu(OTf)2, THF, rt, 27
Me2EtSiH, Cu(OTf)2, CH3CN, 0 °C, 30 min
PMHS,b AlCl3, Et2O/CH2Cl2, rt, 12 h
DIBAL, CH2Cl2, 0 °C, 6 h
0:84
80:0
75:8
9
24
25
a M ) V(O), Sc, Pr, Nd, Sm, Eu, Gd. b Polymethylhydrosiloxane.
way HCl would activate NaCNBH3. Interestingly, LiAlH4 reacts
with AlCl3 in ether solvents to give AlH3, which reacts similarly
to BH3, to explain the selectivity in the original method for
reductive openings. Finally, DIBAL can act as both Lewis acid
and reducing agent in these reactions.
FIGURE 4. Initial rate analysis for the formation of compound 2 using
BH3 ·THF (diamonds) or 3 using BH3 ·NMe3 (circles). [1]0 ) 0.054
mol L-1, [BH3 ·NMe3]0 ) [BH3 ·THF] 0 ) 0.217 mol L-1. All reactions
were performed in duplicate.
Conclusions
To summarize, Lewis acid activation of the borane complex
renders the borane the most electrophilic species, which
associates to the more electron-rich oxygen of the acetal
(Scheme 8, path B). In contrary, without activation, the
regioselectivity is instead directed by the Lewis acid (path A).
SCHEME 7. Proposed Mechanism for Lewis Acid
Activation of Borane Acetal Openings Using BH3 ·NMe3
Experimental Section
For general experimental details, synthesis of all starting materi-
als, and kinetic data see the Supporting Information.
General Procedure for Hammett Plots. Compounds 4a-i
(0.146 mmol) and BH3 ·NMe3 (0.874 mmol) were dissolved in THF
(3 mL), and an ice-cold solution of AlCl3 (1.019 mmol) in THF
(1.5 mL) was then added at 0 °C. Samples (0.450 mL) were taken
at different times and quenched by addition of NaHCO3 (aq satd,
0.500 mL). The samples were then extracted twice with Et2O (0.450
mL) and concentrated in vacuum. The residues were dissolved in
CDCl3 and analyzed by 1H NMR. All reactions were run in
triplicate. The molar concentration of product was determined from
NMR peak ratios and plotted versus time. Linear regression over
the linear part of the curves (determined by f values) gave the
reaction rates.
General Procedure for Rate Determination of Reductive
Opening of 4a. The amounts of AlCl3, BH3 ·NMe3, and 4a were
varied according to Figure 2. Compound 4a and BH3 ·NMe3 were
dissolved in THF (3 mL), and an ice-cold solution of AlCl3 in THF
(1.5 mL) was then added at 0 °C. Samples (0.450 mL) were taken
at different times and quenched by addition of NaHCO3 (aq satd,
0.500 mL). The samples were then extracted twice with Et2O (0.450
mL) and concentrated in vacuum. The residues were dissolved in
CDCl3 and analyzed by 1H NMR. All reactions were run in
duplicate. The molar concentration of product was determined from
NMR peak ratios and plotted versus time. Linear regression over
the linear part of the curves (determined by f values) gave the
reaction rates.
compound 2). BH3 ·Me2S seems to be a borderline case, while
BH3 ·NMe3 (Table 3 and our data) gives the free 4-hydroxyl
(3) in THF. When the reactions are performed in nonpolar
solvents, such as toluene or CH2Cl2, the selectivities are probably
directed exclusively by the Lewis acid, but the higher reactivity
of the unsolvated electrophiles results in degradation and
unselective reactions.5
It is reasonable to assume that the active borane in other
methods actually is a borane-solvent complex. For example, a
solution of BH3 ·THF can be formed by reaction of NaBH4 with
AlCl3 in THF or by addition of BF3 ·OEt2 to LiAlH4. In a similar
(17) No¨th, H.; Wrackmeyer, B. In Nuclear Magnetic Resonance Spectroscopy
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Berlin, 1978.
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General Procedure for Rate Determination of Reductive
Opening of 1. The amounts of AlCl3 were varied according to
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Johnsson et al.
SCHEME 8. Regiochemical Outcome Is Directed by the Relative Nucleophilicity of the Acetal Oxygens toward the Lewis Acid
(Path A) or the Activated Borane (Path B)
Figure 4. To a solution of 1 in THF (0.250 mL, 0.216 M) were
added BH3 ·THF (0.215 mL, 1 M in THF) or BH3 ·NMe3 (0.250
mL, 0.088 M in THF), THF (0.100-0.435 mL), and finally AlCl3
(0.100-0.400 mL, 1.09 M in THF). Samples (0.050 mL) were taken
at different times, quenched by addition of NaHCO3 (0.250 mL,
satd aq), and extracted with ether (0.100 mL). All reactions were
run in duplicate. The molar concentration of product was determined
from quantitative TLC ratios and plotted versus time. Linear
regression over the linear part of the curves (determined by f values)
gave the reaction rates.
AlCl3 (18 mg, 0.13 mmol) was added, and the reaction was followed
by 11B NMR.
Acknowledgment. The Swedish Research Council, The
Crafoord Foundation, and The Royal Physiographic Society in
Lund supported this work.
Supporting Information Available: Complete experimental
procedures, product characterization, spectral data, kinetic data,
and computational data. This material is available free of charge
via the Internet at http://pubs.acs.org
11B NMR Experiments. Compound 4a (4 mg, 0.02 mmol) and
BH3 ·NMe3 (8 mg, 0.11 mmol) were dissolved in THF-d8(0.75 mL).
JO800396G
5232 J. Org. Chem. Vol. 73, No. 14, 2008