Spiroborate catalyzed reductions with N,N-diethylaniline borane

Article abstract of DOI:10.1016/j.tetlet.2010.09.022

Reduction of esters, amides, and ketones by N,N-diethylaniline borane is accelerated by catalysts derived from spiroborate complexes. Esters are reduced at ambient temperature in less than 4 h with this amine borane and 5 mol % spiroborate 6. Functional group selectivity shows ketone and tertiary amide reduction is faster than ester or nitrile reduction.

Full text of DOI:10.1016/j.tetlet.2010.09.022

Tetrahedron Letters 51 (2010) 5973–5976
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Spiroborate catalyzed reductions with N,N-diethylaniline borane
Brian M. Coleridge, Thomas P. Angert, Lucas R. Marks, Patrick N. Hamilton, Christopher P. Sutton,
⇑
Karl Matos, Elizabeth R. Burkhardt
BASF Corporation, 1424 Mars-Evans City Road, Evans City, PA 16033, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2010
Revised 2 September 2010
Accepted 7 September 2010
Available online 17 September 2010
Reduction of esters, amides, and ketones by N,N-diethylaniline borane is accelerated by catalysts derived
from spiroborate complexes. Esters are reduced at ambient temperature in less than 4 h with this amine
borane and 5 mol % spiroborate 6. Functional group selectivity shows ketone and tertiary amide reduc-
tion is faster than ester or nitrile reduction.
Ó 2010 Elsevier Ltd. All rights reserved.
A number of alumino- and borohydride reducing agents1 are
available for ester reduction but some reducing agents present sig-
nificant limitations, either in selectivity or handling. Singaram has
elegantly demonstrated the abilities of lithium aminoborohydrides
in selective ester and amide reductions.2 Late stage reduction of
functional groups during the synthesis of drug candidates presents
significant challenges due to the need for highly selective reagents.
For example, in attempting the reduction of only the methyl ester
in a multi-functional molecule during the synthesis of a viral RNA
replication inhibitor, Agbodjan et al.3 tried a number of reducing
agents with limited success. In addition, substrates can be prone
whereas when catalyzed by MeCBS (1) the reduction is complete
within 15 min at ambient temperature.
Oxazaborolidine catalysts are envisioned to increase ketone
1
0
reaction rates by two convergent mechanisms, (1) carbonyl coor-
dination of the substrate to a Lewis acidic boron to increase the
electrophilic character of the carbon, coupled with (2) dynamic
equilibrium of borane coordination to the Lewis basic nitrogen
center of the catalyst to facilitate proximal interaction of the sub-
strate with borane and to increase hydride nucleophilicity. Since
oxazaborolidines have not been studied for accelerated reduction
1
1
of ester or amide functional groups, we embarked on a plan to
investigate ester reduction. (R)-MeCBS (1, 5 mol %) was found to
dramatically increase the rate of ester reduction by DEANB at
ambient temperature when compared to long reaction times at
elevated temperature required with DEANB alone, (Scheme 1,
Table 1, entries 1 and 2). Screening of compounds with both Lewis
acidic and Lewis basic sites, Figure 1, allowed catalyst structure
optimization, see Table 1. Altering the oxazaborolidine structure
by removal of the phenyl groups or methyl on boron decreased cat-
alytic activity. Dialkylaminoborane or aminoborate derivatives, 4
and 5, respectively, were ineffective.
4
to epimerization; therefore a non-basic reducing agent is desir-
able to minimize undesired by-product formation.
Borane-tetrahydrofuran complex (BTHF), borane-dimethyl sul-
fide (DMSB), and N,N-diethylaniline borane (DEANB) have all been
widely used for selective reduction operating via an electrophilic
5
mechanism. An example of selective ester reduction using DMSB
successfully produced (S)-3,4-dihydroxybutyric acid methyl ester
from
equiv of DMSB in THF at reflux reduced a lactam to the amine
49% yield) in the presence of an ethyl ester, while applying 6 equiv
L
-malic acid dimethylester.6 In a multi-functional substrate,
2
(
7
reduced both the ester and the amide. Reduction using boranes in
THF at reflux can lead to ring opening of THF giving butyl borate,
thus requiring an excess of borane for complete substrate
reduction.
Spiroborates with diverse structural and electronic environ-
mentswerepreparedbyreactionofdialkylboraneordialkoxyborane
and aminoalcohols. Compound 6 derived from N-methyl-2-amino-
ethanol and catecholborane showed excellent results compared to
catalysts 7 and 8 prepared from 9-BBN and pinacolborane. The lower
activity of 7 and 8 is possibly due to steric bulk around boron. On the
other hand, the simplest spiroborate structure, 9, was also ineffec-
tive. Changing the secondary amine to pyridine, primary amine or
tertiary amine in the catalyst structure dramatically decreased
activity (10, 11, and 12 of Fig. 1). The chiral spiroborate of Ortiz-
Development of selective reducing agents that are easily han-
dled, stable, yet reactive enough to perform the desired reduction
under mild conditions, that is, at ambient temperature, is a highly
desirable goal to expand the organic chemist’s toolbox.
DEANB exhibits low reactivity toward many substrates, however,
the reactivity of DEANB toward ketone reduction is dramatically
accelerated by utilizing oxazaborolidine catalysts.8 For example,
9
DEANB reduction of acetophenone required 6–8 h at reflux,
DEANB,
Additive, THF
O
OH
+ EtOH
O
⇑
Corresponding author. Tel.: +1 724 538 1236; fax: +1 724 538 1260.
E-mail address: elizabeth.burkhardt@basf.com (E.R. Burkhardt).
Scheme 1. Ethyl butyrate reduction.
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.022

5
974
B. M. Coleridge et al. / Tetrahedron Letters 51 (2010) 5973–5976
Table 1
Ethyl butyrate reduction by DEANB: catalyst screening
Probing the structure of the ester demonstrated a relationship
a
between reduction rate and ester structure and electronics, Table
2, entries 4–10. Ethyl n-butyrate and ethyl i-butyrate were reduced
rapidly while ethyl benzoate reduction was slowly reduced. t-Butyl
esters are reduced much more slowly compared to i-propyl esters,
entries 6 and 9 versus entries 7 and 8. Therefore, alkyl esters can be
reduced selectivity in the presence of these less-reactive esters. For
example, reduction of a 1:1 mixture of ethyl butyrate and ethyl
benzoate resulted in 90% reduction of ethyl butyrate but only 1%
reduction of the benzoate at 20 °C. In contrast to lithium amino-
borohydride, a stronger reducing agent, which rapidly reduced
both aliphatic and aromatic esters at 0 °C, DEANB/5 mol % Spiro-
CAT selectively reduced the aliphatic ester.
Entry
Additive
Temp (°C)
Time (h)
9, 48c
7
5
1
2
3
4
5
None
5 mol % 1
10 mol % 6, 13
10 mol % 2, 3 , 7–9
10 mol % 4, 5, 10–12
85, 50
20
20
20
20
b
10–24
>24d
a
b
c
1
:1 ratio of ester: DEANB, borane added to ester and catalyst in THF.
Catalyst formed in situ.
7% complete reduction.
8
d
Incomplete reduction.
Reduction of ethyl benzoates can be pushed to completion by
extended reaction time at 50 °C, entries 5 and 10. Reduction of
ethyl 3-bromobenzoate at 50 °C delivered 3-bromobenzyl alcohol
with no loss of the halogen, entry 10. The reduction of methyl
Ph
H
H
H
N
Ph
O
O
Me
O
B
Me
O
OH
H
B
N
B N
N
1
N
B
2
Me
3
4
5
Me
(S)-mandelate was slower than expected but the chiral center of
O
B
O
H
O
N
O
O
O
O
O
O
O
the diol product was unaffected by the reduction conditions.
Next, we explored the scope of functional group reductions
with the DEANB/5 mol % SpiroCAT formulation. Amides are com-
B
B
B
N
N
N
Me
H
Me
O
H
Me
O
H
Me
6
7
8
9
1
3
Ph
monly reduced with DMSB or BTHF; for example, a peptide
amide was successfully reduced selectively in the presence of an
H
O
B
O
O
N
O
O
O
Ph
O
B
B
O
14
O
N
H
ester using BTHF at À20 °C. However, borane derivatives of dial-
N
N
B
O
H
H
Me Me
kylanilines and sterically hindered amines are significantly less
reactive than BTHF and require prolonged heating at elevated tem-
peratures to drive a tert-amide reduction to completion.15
With DEANB/5 mol % SpiroCAT, several tertiary amides were
successfully reduced, see Table 3. While aliphatic amides were re-
duced easily at ambient temperature, the benzamide reduction
benefited from higher reaction temperature (50 °C, entry 6). The
amine product from reduction of Weinreb amide retained the N-
methoxy group.
1
0
11
12
13
Figure 1. Compounds tested in ester reduction with DEANB.
Mercales12 13, similar in steric bulk to (R)-MeCBS, also gave excel-
lent results. In view of the fact that an ester reduction does not gen-
erate a chiral center, the chiral catalysts, 1, 2, 3, and 13, were not
selected for further study due to their chirality adding superfluous
cost and complexity.
Unfortunately, butyramide (2.33 equiv of DEANB) and N-meth-
ylproprionamide (2 equiv of DEANB) were not rapidly reduced un-
der these reaction conditions. In fact, the secondary amide appears
to inhibit the catalytic activity of 6. For example, addition of
10 mol % of N-methylpropionamide to an ethyl butyrate reduction
using DEANB with 5 mol % 6 showed only 92% ester reduction in
21 h. Likewise, dimethylacetamide reduction was significantly
slower in the presence of 10 mol % of N-methylpropionamide. In
addition, ester reduction at rt required 24 h in the presence of
1 equiv of nitrile. Understanding this inhibition phenomenon is
the subject of further work.
Further evaluation with a formulation of DEANB with 10 mol %
(dubbed SpiroCAT) added to ethyl butyrate in THF showed even
6
higher catalytic activity, Table 2 entry 1, compared to addition of
DEANB to the ester/catalyst mixture. By adding the ester into the
DEANB/SpiroCAT formulation, reduction was quite exothermic
and complete within 30 min. Catalyst loading at 5 mol % SpiroCAT
in DEANB required 4 h for ester reduction regardless of formulation
age, entry 2. Reduction in methyl t-butyl ether was complete in
3
.5 h, while toluene required 6 h compared to 4 h in THF, entry 3.
Further investigation of the activity and selectivity of the
DEANB/5% SpiroCAT system showed ketones are rapidly reduced
in less than 1 h at 20 °C. On the other hand, the nitrile reduction
rate with the formulation was only slightly increased over DEANB
alone. In contrast, Singaram has shown diisopropylaminoborane in
Table 2
Catalyzed ester reduction by DEANB and SpiroCAT at rt
Entry
Substratea
Mol % 6
Time (h)
2.5
Conversionb (yield)c
1
2
3
4
5
6
7
8
9
Ethyl butyrate
Ethyl butyrate
Ethyl butyrate
Ethyl isobutyrate
Ethyl benzoate
10
5
5
10
10
10
10
5
100 (64)
100
100
d
4, 4
3.5 , 6
4
the presence of LiBH catalysis as an effective reducing agent for
e
f
nitriles at rt, however lithium aminoborohydrides do not reduce
2.5
>72
>72
4.8
3.3
48
100
1
6
g
nitriles at rt.
90 (83)
h
t-Butyl acetate
95
i-Propyl acetate
i-Propyl butyrate
t-Butyl propionate
Ethyl 3-Br-benzoate
Methyl (S)-mandelate
100
100 (41)
86
90 (90)
85 (83)
Table 3
5
5
5
Amide reduction by DEANB and 5 mol % SpiroCAT, 6
i
1
1
0
1
24
24
Substratea
Timeb (h);
isolated yield)
j
Entry
Temp (°C)
(
a
b
c
d
e
f
1
:1 ratio of ester: DEANB, DEANB/SpiroCAT added to ester in THF.
50c
1
2
3
4
5
6
N,N-Dimethylacetamide
N,N-Dimethylacetamide
N,N-Dimethylisobutylamide
N-Methoxy-N-methylacetamide
N-Methylcaprolactam
6
2
3.3
8
4 (82)
3.5 (76)
Conversion monitored by ReactIR.
Unoptimized isolated yield.
Using formulation aged one year.
Reaction in t-butyl methyl ether.
Reaction in toluene.
20
20
20
20
50
N,N-Dimethylbenzamide
g
h
i
2
6 h at 50 °C with 5 mol % SpiroCAT.
Reaction 50% conversion in 9 h by ReactIR, and 95% in 72 h by GC.
4 h at 50 °C, yield adjusted for 8% ester recovered.
Plus 4 h at 50 °C, yield adjusted for 15% ester recovered.
a
b
c
1:1.67 ratio of amide: DEANB, formulation added to amide in THF.
Time to 100% conversion by ReactIR.
2
j
Without SpiroCAT.

B. M. Coleridge et al. / Tetrahedron Letters 51 (2010) 5973–5976
5975
Based on Brown’s work, amides were reduced by borane com-
plexes in preference to ester, nitrile and nitro groups.17 Reductions
O
O
O
N
O
O
Me
O
O
"
BH
3
"
-H2
Me
B
H
B
O
N
B
B
O
N
H
BH
to probe the selectivity of functional groups with this new reagent
formulation were conducted with a deficit (relative to total sub-
strate amount) of DEANB/SpiroCAT at rt or slightly elevated tem-
peratures, Table 4. The results show the selective reduction of
dialkylamides in the presence of nitriles or esters and aliphatic es-
ters in the presence of aliphatic nitrile or benzoate functionalities.
Carboxylic acids are rapidly reduced by DEANB without addi-
tives but SpiroCAT did not appear to increase the reduction rate.
When a mixture of ethyl butyrate and benzoic acid were reduced
with a slight excess of borane, the acid was reduced preferentially,
with only a minor amount of ester reduction due to the presence of
excess borane. Thus, the activity of SpiroCAT was not affected by
the presence of carboxylic acid.
14
H B
3
15
2
6
Me
O
O
Me
B
O
O
"BH3"
Me
B
O
N
O
N
H
H
H
H
H
B
B
1
6
H
15
H
Scheme 2. Proposed active catalyst in SpiroCAT reductions.
O
O
O
O
B
O
B
H
+
N
B
O
B
H
3
H C
H
N
H
15
17
18
CH
3
Further investigation of the DEANB/SpiroCAT formulation has
revealed that 1 equiv of hydrogen evolves from reaction of borane
and SpiroCAT. Neither aminoborane (15) nor bis(dialkylami-
no)diborane with bridging amine groups was seen in the region
Scheme 3. Possible oxazaboroline catalyst.
1
1
of the B NMR spectrum between 0–2 and 3–6 ppm, respec-
1
8
tively. Subsequent interaction of the intermediate with another
ducts with different substituents has been reported at 3.2 by Corey
(no coupling given) and at 6 and 1.5 ppm by Contreras. The ab-
1
1
25
borane generates an aminodiborane, 16, observed in the B NMR
spectrum as a broadened triplet at À18 ppm, Scheme 2. In one
case, the peak was resolved as a dt; J = 130, 30 Hz. It is not known
at this time if 16 is the active catalyst or the resting state of the cat-
sence of resonances in the region between 1–6 ppm suggests an
absence of oxazaborolidine–borane adduct in the DEANB-SpiroCAT
formulation.
N-Methylaminoethanol (5 mol %) and excess DEANB were
heated together in an attempt to generate an oxazaborolidine cat-
alyst in situ. The B NMR spectral resonances were at 28 ppm (d,
J = 158 Hz), 12 ppm (s, spiroborate) and as an unresolved shoulder
on DEANB at À18 ppm. The doublet coupling constant of the peak
at 28 ppm is closer to that of a dialkoxyborane than oxazaboroli-
dine. Although this in situ catalyst showed peaks in the 11B NMR
spectrum similar to but not identical to those observed in the
DEANB/SpiroCAT formulation, the catalytic activity of the in situ
prepared catalyst was considerably less than observed using Spiro-
CAT in the ethyl butyrate reduction, 9.25 h versus 4 h. The exact
nature of the active catalyst warrants further study.
Spiroborate 6, used in a formulation with DEANB, was found to
effectively accelerate the reduction of esters, tertiary amides, and
several other functional groups. Esters, which are typically difficult
to reduce with borane complexes, can be reduced at room temper-
ature in less than 4 h. Probing the selectivity of the catalyzed
reductions has shown that tertiary amide reduction preferentially
occurs in the presence of esters or nitriles. These results demon-
strate the mild and selective nature of this catalyst system and
indicate utility in reduction of selected functional groups of a
multi-functional structure.
19
alyst. Dimethylaminodiborane and morpholinodiborane, inde-
pendently prepared for comparison of the 11B NMR spectra, were
ineffective catalysts for ester reduction by DEANB.
1
1
Further experiments, with BTHF and SpiroCAT to explore the
intermediates also show formation of amine borane 14 followed
by conversion to aminodiborane 16 in the presence of additional
borane. The catalyst prepared from SpiroCAT and BTHF reduces
ethyl butyrate in 4 h at rt with DEANB as the reducing agent.
Another mechanistic possibility is the formation of oxazaboroli-
dine 18 and catecholborane (17) from the aminoalcohol moiety 15,
1
1
Scheme 3. The B NMR spectrum of the DEANB-SpiroCAT formula-
2
0
tion also displays a small doublet at 27.5 ppm (J = 190 Hz) in les-
ser amount relative to the aminodiborane triplet at À18 ppm.
Oxazaborolidine compounds with a B–H have been observed by
1
1
21
B NMR spectroscopy at 25–28 ppm (monomer) and 7.6 (di-
2
2
mer) while catechol borane is at 28 ppm (J = 190 Hz). Addition
2
6
of catechol borane to the formulation increased the doublet at
2
7.5 ppm and the decoupled spectrum showed one resonance in
2
3
this area. In the DEANB-SpiroCAT formulation an oxazaborolidine
would likely exist as the BH adduct which is usually observed at
À12 to À14 ppm as a quartet or as a complex with coordinated
N,N-diethylaniline. The ring-boron of oxazaborolidine-borane ad-
3
24
Table 4
Catalyzed selective reduction by DEANB and 5 mol % SpiroCAT, 6
Entry Substrate A; % reduceda
Substrate B; % reduceda Substrate C; % reduceda DEANB: total substrate ratiob Reaction Temp (°C) Reaction time
1
2
3
4
5
6
7
8
9
0
Ethyl butyrate; 90%
N,N-Dimethyl-acetamide; 99+% Benzonitrile; 0%
N,N-Dimethyl-acetamide; 99+% Heptane nitrile; 0%
N,N-Dimethyl-acetamide; 80%
N,N-Dimethyl-acetamide; 88%d
Ethyl butyrate; 99+%
Ethylbenzoate; 1%
—
—
—
—
0.8:2
1.66:2
1.66:2
1.68:2
1.66:3
1.33:2
0.8:2
1:2
20
20+
20
20
40–45
50
20
20
20
20
8
2
6
3
20
4
20
6
17
4
c
Ethyl butyrate; 5%
Benzonitrile: 0%
Benzonitrile: 12%
Benzonitrile: 34%
Heptane nitrile; 0%
Benzonitrile: 0%
Benzoic acid; 99%
Ethyl benzoate; 0%
—
—
—
Ethyl benzoate; 0%
—
Ethyl butyrate; 80%
Ethyl butyrate; 90%
e
Ethyl butyrate; 80%
Ethyl butyrate; 12%
1:3
1.35:2
1
a
b
c
GC yield.
DEANB/SpiroCAT added to substrate mixture.
Reaction exothermed and gelled.
Amide reduction 77% complete in 30 min.
d
e
1
00% at 24 h.

5
976
B. M. Coleridge et al. / Tetrahedron Letters 51 (2010) 5973–5976
1
2. Stepanenko, V.; Ortiz-Marcailes, M.; Correa, W.; De Jesus, M.; Espinosa, S.;
Acknowledgments
Ortiz, L. Tetrahedron: Asymmetry 2006, 17, 112–115; Huang, K.; Merced, F. G.;
Ortiz-Marcailes, M.; Melendez, H. J.; Correa, W.; De Jesús, M. J. Org. Chem. 2008,
73, 4017–4026; Stepanenko, V.; De Jesús, M.; Correa, W.; Bermúdez, L.;
Vázquez, C.; Guzmán, I.; Ortiz-Marciales, M. Tetrahedron: Asymmetry 2009, 23,
We gratefully acknowledge Mr. Charles S. Bello for GC analysis
of products and Dr. Christopher D. Hewitt and Dr. Lauren R. Cafiero
for helpful discussions.
2659–2665; Huang, K.; Ortiz-Marcailes, M.; Correa, W.; Pomales, E.; Lopez, X.
Y. J. Org. Chem. 2009, 74, 4195–4202.
13. Fu, R.; Zhao, B.; Shi, Y. J. Org. Chem. 2009, 74, 7577–7580; Bannister, R. M.;
Brooks, M. H.; Evans, G. R.; Katz, R. B.; Tyrrell, N. D. Org. Process Res. Dev. 2000,
Supplementary data
4
, 467.
1
4. Roeske, R. W.; Weitl, F. L.; Prasad, K. U.; Thompson, R. M. J. Org. Chem. 1976, 41,
1260–1261.
5. Brown, H. C.; Kanth, J. V. B.; Zaidlewicz, M. J. Org. Chem. 1998, 63, 5154–5163;
Brown, H. C.; Kanth, J. V. B.; Dalvi, P. V.; Zaidlewicz, M. J. Org. Chem. 1999, 64,
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.09.022.
1
6
263–6274; Kanth, J. V. B. Aldrichim. Acta 2002, 35, 51–58; Salunkhe, A. M.;
References and notes
Burkhardt, E. R. Tetrahedron Lett. 1997, 38, 1519–1522.
6. Haddenham, D.; Pasumansky, L.; DeSoto, J.; Eagon, S.; Singaram, B. J. Org. Chem.
2009, 74, 1964–1970.
1
1.
2.
3.
Seyden-Penne, J. Reduction by the Alumino- and Borohydrides in Organic
Synthesis, 2nd ed.; Wiley-VCH: New York, 1997.
Pasumansky, L.; Goralski, C. T.; Singaram, B. Org. Process Res. Dev. 2006, 10,
17. Brown, H. C.; Krishnamurthy, S. Tetrahedron 1979, 35, 567–607.
11
18.
B NMR resonances expected based on the corresponding resonances of
9
59–970.
morpholinoborane at 0.7 ppm (t, J = 113 Hz) and bis(morpholino)diborane at
3 ppm (t, J = 115 Hz). Bis(dimethylamino)borane is at 32 ppm (d, J = 130 Hz)
and dimer is at 1.6 (t, J = 67 Hz).
Agbodjan, A. A.; Cooley, B. E.; Copley, R. C. B.; Corfield, J. A.; Flanagan, R. C.;
Glover, B. N.; Guidetti, R.; Haigh, D.; Howes, P. D.; Jackson, M. M.; Matsuoka, R.
T.; Medhurst, K. J.; Millar, A.; Sharp, M. J.; Slater, M. J.; Toczko, J. F.; Xie, S. J. Org.
Chem. 2008, 73, 3094–3102; Flanagan, R. C.; Xie, S.; Millar, A. Org. Process Res.
Dev. 2008, 12, 1307–1312.
19. Burg, A. B.; Randolph, C. L., Jr. J. Am. Chem. Soc. 1949, 71, 3451–3455; Balulescu,
C. R.; Keller, P. C. Inorg. Chem. 1978, 17, 3707–3708; BASF data:
Morpholinodiborane À18.3 (br t, J = 142 Hz), Schwartz, L. D.; Keller, P. C. J.
Am. Chem. Soc. 1972, 94, 3015–3019. dialkylaminodiborane at À18 to À21
(J = 130, 30 Hz).
4
.
Hida, T.; Mitsumori, S.; Homa, T.; Hiramatsu, Y.; Hashizume, H.; Okada, T.;
Kakinuma, M.; Kawata, K.; Oda, K.; Hasegawa, A.; Masui, T.; Nogusa, H. Org.
Process Res. Dev. 2009, 13, 1413–1418.
20. Catechol borane 11B NMR resonance is 28 ppm (d, J = 190 Hz), BASF internal
data.
5
6
.
.
Brown, H. C. Hydroboration; W.A. Benjamin: New York, 1962.
Saito, S.; Hasegawa, T.; Inaba, M.; Nishida, R.; Fuji, T.; Nomizu, S.; Moriwake, T.
Chem. Lett. 1984, 13, 1389–1392.
21. Joshi, N. N.; Srebnik, M.; Brown, H. C. Tetrahedron Lett. 1989, 30, 5551–5554.
22. Corey, E. J. J. Am. Chem. Soc. 1987, 109, 5551–5553; Corey assigned
oxazaborolidine monomer at 28.3 ppm as broad singlet and dimer at
7.6 ppm as doublet (J = 130 Hz) but this coupling constant is inconsistant for
a boron bridged hydrogen (J = 30–50 Hz), see: Noeth, H.; Wrackmeyer, B.
Nuclear Magnetic Resonance Spectroscopy of Boron Compounds; Springer: Berlin,
1978.
7
.
.
Bryans, J. S.; Large, J. M.; Parsons, A. F. J. Chem. Soc., Perkin Trans. 1 1999, 2905–
2
910.
8
Salunkhe, A. M.; Burkhardt, E. R. Tetrahedron Lett. 1997, 38, 1523–1526; Cho, B.
T.; Shun, Y. S. Bull. Korean Chem. Soc. 1999, 20, 397–399; Chung, J. Y. L.;
Cvetovich, R.; Amato, J.; McWilliams, J. C.; Reamer, R.; DiMicheke, L. J. Org.
Chem. 2005, 70, 3592–3601; Bertrand, B.; Durrassier, S.; Frein, S.; Burgos, A.
Tetrahedron Lett. 2007, 48, 2123–2125.
23. Presence of catecholborane was confirmed by B–H stretching band in the IR
À1
spectrum at 2656 cm
.
9
.
Salunkhe, A. M.; Burkhardt, E. R. Tetrahedron Lett. 1997, 38, 1519–1522.
24. Brunel, J. M.; Maffei, M.; Buono, G. Tetrahedron: Asymmetry 1993, 4, 2255–
2260. The oxazaborolidine–borane adduct 11B NMR peak corresponding to the
amine borane would be hidden by resonance of DEANB.
25. Tlahuext, H.; Contreras, R. Tetrahedron: Asymmetry 1992, 3, 1145–1148.
26. Use of SpiroCAT with DMSB or BTHF also dramatically accelerates ester
reduction at rt.
1
0. Linney, L. P.; Self, C. R.; Williams, I. H. J. Chem. Soc., Chem. Commun. 1994, 14,
651–1652; Jones, D. K.; Liotta, D. C.; Shinkai, I.; Mathre, D. J. J. Org. Chem. 1993,
8, 799–801.
1
5
1
1. 2-Aminoethanol with 2 equiv of borane was shown to rapidly reduce ketones
but not esters or amides. Istuno, S.; Wakasugi, T.; Ito, K.; Hirao, A.; Nakahama,
S. Bull. Chem. Soc. Jpn. 1985, 58, 1669–1673.