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FULL PAPER
Highly Diastereoselective and Enantioselective Preparation of Homoallylic
Amines: Application for the Synthesis of b-Amino Acids and g-Lactams
P. Veeraraghavan Ramachandran* and Thomas E. Burghardt
[a]
Dedicated to Professor Ei-ichi Negishi on the occasion of his 70th birthday
Abstract: Reactions of N-silyl- and N-
aluminoimines with B-allyldiisopino-
campheylborane in the presence of
methanol, followed by oxidative
workup furnished homoallylic amines
in good yields and high
ee
.A
11
B NMR
spectroscopy study revealed that the
reactions do not proceed, even at room
temperature, unless a molar equivalent
of water or methanol is added. The
first reagent-controlled asymmetric
crotylboration and alkoxyallylboration
of aldimines furnishing b-methyl or b-
alkoxy homoallylic amines in very high
diastereoselectivity and enantioselec-
tivity are reported herein. Crotylbora-
tion and alkoxyallylboration of imines
proceed only with the “allyl”-boron
“ate” complexes, instead of the “allyl”-
dialkylboron reagents used with alde-
hydes. The addition of methanol is nec-
essary for these reactions as well. Ap-
plication of this methodology for the
conversion of representative nitriles to
b-amino acids in two steps has been de-
scribed. Additionally, a procedure for
the preparation of chiral d-amino alco-
hols and g-lactams from nitriles is also
reported.
Keywords: allylic compounds ·
amines · asymmetric synthesis ·
boranes · imines
Introduction
sulfiniyl,
[4]
N-sulfonyl,
[5]
N-trialkylsilyl,
[6]
oximes,
[7]
and vari-
ous N-metalloimines.
[8]
N-silylimines have been first pre-
pared by Rochow and co-workers by reacting aromatic alde-
hyde with lithium bis(trimethylsilyl)amide.
[9]
Reactions of
such imines with organometallic reagents for the prepara-
tion of homoallylic amines was first reported by Hart and
co-workers.
[10]
Enolizable N-silylimines are not stable, but
were characterized at very low temperatures (
1008C).
[11]
Their preparation and in situ trapping at low temperatures
furnished only low yields of the synthesized b-lactam prod-
ucts.
[12]
This inefficiency of N-silylimines, however, could be
alleviated by the use of N-aluminoimines, which were first
prepared by Cainelli and co-workers by partial reduction of
nitriles with diisobutylaluminum hydride (DIBAL-H).
[13]
Among the advantages of N-aluminoimines are the ready
availability or ease of preparation of the starting nitriles,
ease of the imine formation, and relatively high stability of
both aromatic and aliphatic imines, even at room tempera-
ture. Due to the above advantages of N-aluminoimines and
our ongoing interest in organoaluminum chemistry,
[14]
they
became a very interesting substrate for our investigation.
In spite of aldimines being very attractive starting materi-
als for syntheses, the inherent instability of unsubstituted al-
dimines and the lack of reactivity of the more stable N-sub-
stituted ones significantly limit their use.
[15]
Furthermore, at-
Preparation of optically pure homoallylic amines is an im-
portant task in organic synthesis. Such amines are excellent
building blocks for the synthesis of a plethora of nitrogen-
containing natural products.
[1]
Owing to the importance of
homoallylic amines, there are abundant literature reports
for their preparation using a wide variety of methods.
[1d,2]
Diastereoselective addition of allyl organometallic com-
pounds to N-substituted imines is a widely recognized proce-
dure for the preparation of homoallylic amines.
[3]
While the
use of substrate-controlled asymmetric additions is a
common procedure, the use of reagent control still remains
undeveloped.
[3]
Numerous N-substituted imines have been
developed and applied for the syntheses of a variety of mol-
ecules, such as amino acids, b-lactams, heterocycles, aziri-
dines, alkaloids, and amines.
[3]
These aldimines include N-
[a] Prof. P. V. Ramachandran, T. E. Burghardt
Herbert C. Brown Center for Borane Research
Department of Chemistry, Purdue University
560 Oval Dr., West Lafayette, IN 47907-2084 (USA)
Fax: (+1)765-494-0239
E-mail: chandran@purdue.edu
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J.
2005,
11
, 4387 –4395
DOI: 10.1002/chem.200401295
2005 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim
4387
tempted 1,2-additions to aldimines quite often fail due to
the tendency of enolizable compounds to undergo deproto-
nation instead of addition.
[3e]
Among the developments in
allylations of aldimines, organoboron compounds have
found synthetic utility due to the Lewis acidity of boron,
which coordinates to nitrogen and improves the electrophi-
licity of the aldimine. Allylborations and crotylborations of
a-optically pure N-substituted aldimines in high diastereose-
lectivity due to the sterically influenced chirality according
to the Cram rules, using B-“allyl”-9-BBN, was described by
Yamamoto and co-workers.
[16]
Reactions of arylimines,
oximes and oxime ethers with allyl and crotylboronates
were reported to proceed in moderate to good diastereose-
lectivity.
[7]
However, use of N-borylimine containing a chiral
auxiliary, followed by the addition of alkyllithium reagents
gave only poor enantioselectivity.
[17]
Asymmetric allylboration, crotylboration, and alkoxyallyl-
boration of aldehydes with reagents I–IV derived from a-
pinene
[18]
(Figure 1) are well documented and widely used in
organic syntheses.
[19]
In spite of a few reports regarding
chiral allylboration of N-masked imines,
[20]
reactions of
these and other boron-based chiral reagents with imines to
form homoallylic amines remain significantly underdevel-
oped. Itsuno and co-workers had reported the preparation
of assorted homoallylic amines in moderate to good enantio-
selectivities from various N-protected imines, such as N-
oxime ethers, N-sulfenimines, and N-trimethylsilylimines, by
using chiral allylboranes derived from tartrate esters, diols,
a-amino alcohols, and a-pinene.
[20]
They concluded that N-
trimethylsilylimines are the most reactive species for such
allylations. During our work on the applications of a-
pinene-based “allyl”-borating reagents we always obtained
homoallylic alcohols in very high
ee
during the allylboration
of aldehydes.
[19]
However, the enantioselectivity reported for
the allylboration of N-trimethylsilylbenzaldimine (1a) with
I, complete within 3 h, was only a moderate 73%.
[20a]
Since
the rate of allylboration of aldehydes with I was established
as exceptionally fast at
788C, and fast even at
1008C,
[21]
it appeared desirable to obtain comparable information
about the allylboration of imines. The low
ee
value and the
slow rate of the reaction prompted us to undertake a project
involving “allyl”-boration of N-substituted aldimines. The
preliminary results were reported earlier.
[22]
Crotylboration and alkoxyallylboration of N-silyl- or N-
aluminoimines for the preparation of densely functionalized
homoallylic amines have never been reported. In this paper,
we describe the results of our investigations on the “allyl”-
boration of N-silyl- and N-aluminoimines.
Results and Discussion
Allylboration of N-silylimines: N-Trimethylsilylbenzaldimine
(1a) was mixed with (
)-B-allyldiisopinocampheylborane
(I)at
788C in THF and the reaction was monitored with
11
B NMR spectroscopy. The spectrum of an aliquot revealed
only unchanged starting materials, even after several hours
at room temperature. This was a surprise since allylboration
of aldehydes with I was previously established as an ex-
tremely fast reaction
[21]
and the previous report
[20a]
claimed
that the reaction was complete within 3 h. However, the de-
sired amine 2a was obtained in good yield after aqueous
workup, which was very exothermic. We rationalized that
the reaction must have taken place during the workup when
the addition of water resulted in the liberation of the
“naked” aldimine intermediate, which rapidly reacted with
I. We were not able to identify this intermediate spectro-
scopically, presumably due to the extremely fast rate of the
allylboration.
[21]
Consequently, 1a was mixed with I at
788C in THF and upon dropwise addition of 1 equivof
water dissolved in THF to the reaction mixture and workup
provided 2ain 90% yield and 92%
ee
, which was considera-
bly better than 70% yield and 73%
ee
previously reported
(Scheme 1).
[20a]
Lowering the reaction temperature to
1008C increased the chiral induction to 94%
ee
.
Scheme 1. Allylboration of N-silylimines: a) I, THF;
100 or
788C;
b) 1a–e;c)H
2
O; 1h,
100 or
788C; d) NaOH, H
2
O
2
;
788C
!
RT.
Thp = thiophene.
After we had standardized the reaction conditions, we ex-
amined the allylboration of other N-silylimines prepared
from aromatic aldehydes, namely 2-thienyltrimethylsilyli-
mine (1b), 4-methoxybenzaldimine (1c), 2-chlorobenzaldi-
mine (1d), and 2-furfuraldimine (1e) (Scheme 1). The
above N-silylaldimines upon reaction with I in the presence
of 1 equivof water, followed by workup furnished the ex-
pected homoallylic amines 2b–e in good yields and high
enantioselectivity (Table 1). The yields and enantioselectivi-
ties of 2band c were considerably better than previously re-
ported.
[20a]
We thus established that N-silylimines were not
reactive toward I unless a molar equivalent of water was
added to the reaction mixture.
[22]
Later studies revealed that
methanol could replace water.
[23]
Figure 1. a-Pinene-based reagents for asymmetric allylboration.
Crotylboration of N-silylimines: Crotylboration of aldehydes
with II and III
[18c]
is a well-known and highly utilized proce-
dure for the syntheses of numerous complex natural prod-
ucts.
[19,24]
However, crotylboration of N-substituted imines
4388
2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim
Chem. Eur. J.
2005,
11
, 4387 – 4395
Homoallylic Amines
FULL PAPER
Table 1. Allylboration of N-silylimines.
Entry Imine
T
[8C] Homoallylic amine
R Yield [%]
[a]
ee
[%]
[b]
1 1a Ph
78 2a 90 92
1 1a Ph
100 2a 87 94
2 1b 2-Thp
78 2b 72 81
3 1c 4-MeO-(C
6
H
4
)
78 2c 74 92
4 1d 2-Cl-(C
6
H
4
)
78 2d 69 82
5 1e 2-furyl
78 2e 86 86
[a] All yields are of pure isolated products. [b] Enantiomeric excess was determined with HPLC using Chiracel
OD-H column and hexanes/isopropanol as the mobile phase.
amine 3ain 78% yield, 92%
ee
and 96%
de
. The reaction of VI
with 1a, after the addition of
methanol was also complete
within 3 h and alkaline oxida-
tive workup, followed by purifi-
cation provided the amine 4ain
76% yield, 90%
ee
, and 98%
de
(Scheme 2).
with these reagents has never been reported. Unlike I,
which can be prepared and stored for short periods of time,
the reagents II and III are made fresh and used immediate-
ly.
[18c]
The treatment of
cis
-or
trans
-butene with Schlossers
base (equimolar mixture of butyllithium and potassium
tert
-
butoxide) at
558C furnishes the crotyl anion.
[25]
Reaction
of this anion with
B
-methoxydiisopinocampheylborane fur-
nishes the corresponding boron “ate” complex (V or VI). In
the crotylboration of aldehydes, this “ate” complex is
broken by the addition of 1.3 equivBF
3
·OEt
2
to generate
trialkylborane reagents II or III (Figure 2).
[18c,26]
However,
we found that addition of BF
3
·OEt
2
had detrimental effect
on crotylboration of N-silylaldimines and multiple unidenti-
fied products were obtained. Indeed, a blank experiment, in
which 1a was mixed with methanol and BF
3
·OEt
2
showed
immediate degradation of 1a (based on a
1
H NMR analysis
of an aliquot). Therefore, we resorted to crotylboration of
the aldimines with the “ate” complexes V and VI (Figure 2).
Scheme 2. Crotylboration of N-silylimines.
Alkoxyallylboration of N-silylimines: Alkoxyallylboration of
aldehydes with IV
[18d]
has found many applications in organ-
ic syntheses.
[19,28]
However, there are no literature reports
regarding alkoxyallylboration of imines for the preparation
of b-alkoxy homoallylic amines. Similar to the reagents II
and III, alkoxyallylboration reagent IV is prepared freshly
before reaction. Treatment of allyl ether with
sec
-butyllithi-
um furnishes the (
Z
)-allylic anion due to the coordination
between the oxygen and lithium.
[18d]
The allylic anion reacts
with
B
-methoxydiisopinocampheylborane to give the corre-
sponding boron “ate” complex VII, which upon the addition
of BF
3
·OEt
2
gives the reagent IV (Figure 3).
[18d]
However,
again, as in the case of crotylboration of N-silylimines, the
reactions were not compatible with BF
3
·OEt
2
and the “ate”
complex VII was used for alkoxyallylboration.
Figure 2. Preparation of reagents for asymmetric crotylboration. Ipc =
isopinocampheyl.
To the best of our knowledge, this is the first report of
crotylboration with “ate” complexes V or VI. We had re-
cently reported similar alkoxyallylboration of fluoral with
“ate” complex VII.
[27]
We do not know the exact mechanism
of the crotylboration with the “ate” complex. The
11
B NMR
spectrum of the reaction mixture containing 1a and V
showed a peak at d 4, corresponding to the “ate” com-
plex
[18c]
and in
1
H NMR spectrum there was a peak at d 9.0,
corresponding to the unreacted N-silylimine. Upon addition
of methanol, we observed an exothermic reaction and
11
B NMR spectrum of an aliquot showed the formation of a
broad peak at d 48 and disappearance of the peak at d 4.
Additionally, we observed the disappearance of the peak at
d 9.0 in the crude
1
H NMR spectrum. We are currently in-
vestigating the mechanism in detail.
The reaction was complete within 3 h and alkaline oxida-
tive workup provided the expected b-methyl homoallylic
Figure 3. Preparation of alkoxyallylborane reagents.
Our experience in alkoxyallylboration revealed that the
best results were obtained with 3-[(2-methoxyethoxy)me-
thoxy]prop-1-ene (allyl-OMEM; R = CH
2
OCH
2
CH
2
OCH
3
)
due to the ease of hydrolysis of the MEM group.
[28a]
Accord-
ingly, this reagent was evaluated for the preparation of the
b-alkoxy homoallylic amine. Thus, when 1a and VII were
mixed in THF at
788C, followed by the addition of 1 equiv
methanol, upon completion of the reaction (3 h, monitored
with
11
B NMR spectroscopy), oxidative workup furnished
Chem. Eur. J.
2005,
11
, 4387 –4395
2005 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim
4389
P. V. Ramachandran and T. E. Burghardt
at
788C (Scheme 4; Table 2, entry 1).
[29]
Thus, we con-
firmed that methanol is a critical additive in the allylbora-
tion of N-aluminoimines as well. This finding and the spec-
trometric data support the postulated mechanism (Figure 4).
Scheme 3. Alkoxyallylboration of N-silylimines.
the expected amine 5a in 65% yield, 95%
ee
, and 98%
de
(Scheme 3).
Given the ease of preparation of N-aluminoimines and
the possibility of the use of enolizable imines as the starting
materials, we chose to use the N-aluminoimines rather than
N-silylimines for an expanded study of crotylboration and
alkoxyallylboration (see below).
Scheme 4. Allylboration of N-aluminoimines: a) DIBAL-H; Et
2
O, 1 h,
08C. b) 1) 4a–c, f–i added to I,Et
2
O,
100
!
558C; 2) MeOH; 3 h,
100
!
558C; 3) NaOH, H
2
O
2
;
788C
!
RT.
Allylboration of N-aluminoimines: Although we obtained
good results from the reaction of the N-silylaldimines with I
after the addition of water, we
could not extend the reaction
to the preparation of aliphatic
homoallylic amines. We envis-
aged that the use of N-alumi-
noimines could alleviate this
issue as the stability of the N-
aluminoimines is considerably
higher.
[13]
Itsuno and co-work-
ers have reported the allylbora-
tion of N-aluminoimines.
[29]
However, the importance of
methanol or water is not recog-
nized in their account. More-
over, the yields of the amine
products were low and the %
ee
were inconsistent with the
reaction temperature. For ex-
ample, N-aluminobenzaldimine 7a provided the homoallylic
amine 2a in 59% yield and 33%
ee
at
788C and 67%
yield and 69%
ee
at 258C.
[29]
This suggested that the reac-
tion could be taking place during the workup. We speculated
that the addition of methanol or water might be equally im-
portant in the allylboration of N-aluminoimines as well.
Partial reduction of benzonitrile (6a) with DIBAL-H in
Et
2
Oat08C furnished the corresponding N-aluminoimine
7a, which was added to a solution of I in Et
2
O/pentane at
1008C. After several hours, the
11
B NMR spectrum of an
aliquot revealed a peak at d 78 corresponding to unchanged
I and
1
H NMR showed the presence of unreacted N-alumi-
noimine (d 9.0), supporting our intuition. Again, addition of
methanol initiated an exothermic reaction and within mi-
nutes the
11
B NMR spectrum of an aliquot showed a peak at
d 47, corresponding to an amino dialkylborane species,
[30]
while the d 9.0 peak observed in
1
H NMR disappeared.
Upon completion of the reaction (within 3 h, based on
11
B NMR analysis), oxidative workup furnished the expected
amine 2a in 90% yield and 88%
ee
, which is considerably
better than the 59% yield and 33%
ee
reported previously
Table 2. Allylboration of N-aluminoimines.
Entry
N-Aluminoimine
T
[8C]
Homoallylic amine
R
Yield [%]
[a]
ee
[%]
[b]
1
7a
Ph
100
2a
90 (87)
[c]
88 (94)
[c]
2
7b
2-Thp
78
2b
74 (72)
[c]
81 (81)
[c]
91 (92)
[c]
4 7f 4-NO
2
-(C
6
H
4
)
100 2f 85 86
4 7g C
6
F
5
78 2g 61 79
6 7h
n
Bu
100 2h 12 78
[d]
7 7h
n
Bu
55 2h 65 60
[d]
8 7i Chx
100 2i 16 89
[d]
9 7i Chx
55 2i 62 68
[d]
[a] All yields are of pure isolated products. [b] Enantiomeric excess was determined with HPLC using Chiracel
OD-H column and hexanes/isopropanol as the mobile phase. [c] The values in parentheses are for the amines
obtained from N-silylimines 1a–c. [d] Enantiomeric excess determined by
19
F NMR spectroscopy after conver-
sion to Mosher amides; results were confirmed using HPLC analysis.
7c
4-MeO-(C
6
H
4
)
100
2c
89 (74)
[c]
Figure 4. Tentative mechanism for the allylboration of N-aluminoimines.
We believe that the allylboration of imines proceeds anal-
ogous to the allylboration of aldehydes, via a six-membered
chair-like transition state with the stereochemical outcome
determined by the isopinocampheyl auxiliary (Fig-
ure 5).
[18a,31]
In the cases of the crotylboration, there are
eight possible transition states, four of which will be pre-
dominant, depending on the geometry of the isopinocam-
pheyl group and the butene used.
[32]
Comparing the optical
rotation values of the obtained amine 2a with those report-
4390
2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim
Chem. Eur. J.
2005,
11
, 4387 – 4395
3
Homoallylic Amines
FULL PAPER
Figure 5. Transition states in allylboration of aldimines.
Imine 7a, obtained from benzonitrile (6a), was thus treat-
ed with V or VI in THF at
788C and after the addition of
1 equivof methanol, followed by the workup provided the
expected amines 3a and 4a, respectively, in good yields and
very high
de
and
ee
, similar to the results established from
the reaction of 1a with V and VI (Scheme 5; Table 4, en-
tries 1 and 5). Crotylboration of another aromatic substrate,
N-alumino-2-thiophenecarbaldimine (7b) proceeded
smoothly in THF furnishing the desired products 3b and 4b
(Scheme 5; Table 4, entries 2 and 6). Unfortunately, aliphatic
imines 7h–i did not react so readily at
788C in THF and
gave only marginal yields of the desired amines along with
several unidentified degradation products. Unlike the reac-
tions of 7h–i with I, increasing the reaction temperature to
558C did not improve the yields. Attempted use of catalyt-
ic In(OTf)
3
or use of a bulky proton source or water as the
additive did not augment the yield, either. Propitiously, ex-
amination of various solvents revealed that the reaction pro-
ceeded satisfactorily at
788C when pentane was used as
the solvent for both the preparation of imine 7iand the cro-
tylboration and the homoallylic amine 3i was obtained in
74% yield and
>
98% diastereoselectivity (Table 3).
ed in the literature,
[33]
we assigned its configuration as
S
.We
assigned the configuration of other homoallylic amines
based on this analogy as well as on the basis of the configu-
ration of several amino acids prepared from these amines
(see below). Configuration of the obtained homoallylic
amine supports the mechanism and the postulated transition
states.
After we had obtained the homoallylic amine 2a in high
ee
from benzonitrile (6a) via N-aluminoaldimine 7a,we
turned our attention to other classes of nitriles: heteroaro-
matic (2-thiophenecarbonitrile: 6b), electron-donating (4-
methoxybenzonitrile: 6c), electron-withdrawing (4-nitroben-
zonitrile: 6f), and perfluorinated (pentafluorobenzonitrile:
6g). Additionally, we extended the reaction to aliphatic ni-
triles (valeronitrile: 6h and cyclohexanecarbonitrile: 6i)
(Scheme 4). Thus, treatment of 6a–c and 6f–i with 1 equiv
of DIBAL-H furnished the desired N-aluminoimines 7a–c
and 7f–i, which were transferred to a cooled solution of
1.2 equivof I, followed by the addition of 1 equivof metha-
nol (Scheme 4). Allylborations of the aromatic substrates
7a–c and 7f–g were facile and upon oxidative workup, the
corresponding homoallylic amines 2a–c and 2f–g were ob-
tained in 61–90% yield and 86–91%
ee
(as determined by
HPLC analysis; Table 2; entries 1–5). To our dismay, ali-
phatic substrates 7h–i did not provide such good results and
amines 2h–i were obtained only in very poor yields (12–
16%) at
1008C, while the enantioselectivity remained
good (Table 1, entries 6, 8). Fortunately, reactions run at
higher temperatures afforded better yields and we found
that at
558C aliphatic homoallylic amines 2h–i were ob-
tained in 62–65% yield and 60–79%
ee
(Scheme 4; Table 2,
entries 7 and 9). Comparison of the yields, enantioselectivi-
ties, and diastereoselectivities of the homoallylic amines ob-
tained by allylboration of N-silyl- and N-aluminoimines re-
vealed marginal advantage of the N-aluminoimines in terms
of yields, while the N-silylimines furnished amines in slightly
higher
ee
(Table 2). However, because of the ease of prepa-
ration of the N-aluminoimines from the nitriles, we recom-
mend utilization of this protocol for the preparation of ho-
moallylic amines via allylboration.
Table 3. Effects of temperature, Lewis acids, and solvents on crotylbora-
tion of aliphatic imine 7iwith V.
Solvent
Lewis acid Additive
T
[8C] Yield
[%]
THF
none
MeOH
78
21
[a]
THF
none
MeOH
55
32
[a]
THF
none
H
2
O
78
38
[a]
THF
none
t
BuOH
78
!
25
27
[a]
THF
In(OTf)
3
(0.1 equiv)
MeOH
78
22
[a]
Et
2
O none MeOH
78 30
[a]
Et
2
O + pentane
[b]
none MeOH
78 35
[a]
pentane + THF
[c]
none MeOH
78 38
[a]
pentane none H
2
O
78 74
[a] Pure material was not obtained (50–90% purity by
1
H NMR; the im-
purities were not identified). [b] A 1:1 mixture was used for the prepara-
tion of 7i and crotylboration. [c] 7i was prepared in THF and pentane
was used as the solvent for crotylboration.
Hence, crotylboration of 7h–i with V and VI was carried
out in pentane. After the addition of 1 equivof methanol
and completion of the reaction (within 3 h, monitored by
11
B NMR spectroscopy), alkaline oxidative workup yielded
b-methyl homoallylic amines 3h–i and 4h–i, respectively, in
good yields (61–78%), excellent diastereoselectivities
(
>
98%), and high enantioselectivities (79–89%)
(Scheme 5; Table 4, entries 3–4, 7–8).
Having understood that the alkoxyallylboration of N-sily-
limine 1a required the “ate” complex VII to proceed; we
evaluated this reaction on N-aluminoimines. Accordingly, 7a
was mixed with VII, followed by the addition of 1 equivof
methanol and the reaction was monitored with
11
B NMR
spectroscopy. Upon completion (within 3 h, based on
Crotylboration and alkoxyallylboration of N-aluminoimines:
Once we had achieved satisfactory crotylboration and alkox-
yallylboration of N-silylimine 1a with the “ate” complexes
V–VII, we initiated a project involving the crotylboration of
N-aluminoimines. Similar to the N-silylimines, the reactions
proceeded only with V or VI. Repeating the blank experi-
ment of mixing 7a with BF
3
·OEt
2
and methanol furnished
the same results as observed in the case of N-silylimine 1a.
Chem. Eur. J.
2005,
11
, 4387 –4395
2005 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim
4391
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