Alkoxide activation of aminoboranes towards selective amination

Article abstract of DOI:10.1002/anie.201305098

Piece of the (inter)action: The interaction of alkoxides with the sp ² Bpin (pin=pinacol) moiety in aminoboranes forms the in situ Lewis acid-base adduct [RO^-→B(OR)₂-N(R′)₂] which enables the amino moiety to react as a strong nucleophile with several electrophiles, thus providing amino alcohols, β-enamino esters, and β-hydroxy amides in a direct and remarkably selective way (see scheme). Copyright

Full text of DOI:10.1002/anie.201305098

Angewandte
Chemie
DOI: 10.1002/anie.201305098
Organocatalysis
Alkoxide Activation of Aminoboranes towards Selective Amination**
Cristina Solꢀ and Elena Fernꢁndez*
Dedicated to Professor Mukaiyama
ꢀ
ꢀ
Recent advances in the activation of B B and B Si interele-
ment bonds have led to them increasingly being used to
functionalize unsaturated compounds by boryl or silyl addi-
ꢀ
ꢀ
tion. Although it is well known that B B and B Si bonds can
be activated in the presence of transition-metal complexes by
oxidative addition or transmetallation pathways,[1] current
metal-free approaches make intermolecular boryl[2] and silyl[3]
transfer possible by the sole addition of catalytic amounts of
donor reagents. Simple alkoxides (ꢀOMe, ꢀOtBu) favor the
ꢀ
ꢀ
formation of the Lewis acid-base adducts [RO !B(OR)2
B(OR)2][4] and [RO !B(OR)2 SiMe2Ph],[3c] which facilitate
the release of a boryl or silyl moiety with enhanced
nucleophilic character (Figure 1). In this context, we
ꢀ
ꢀ
Scheme 1. a,b) Amination reactions that proceed through iminium ion
formation. Refs. [10,11] c) Amination reaction developed in this study.
DMF=N,N-dimethylformamide, NMP=N-methylpyrrolidone,
THF=tetrahydrofuran.
a series of b-amino ketones and esters were synthesized by
reacting bis(dialkylamino)boron enolates with aldehydes
(Scheme 1a), or alternatively silyl ketene acetals with alde-
hydes and aminoboranes (Scheme 1b). The function of the
amino-substituted boron compounds was attributed to an
iminium-ion generator from carbonyl compounds. Herein, we
have developed a new method for preparing b-amino
carbonyl compounds by the simple Lewis acid–base inter-
Figure 1. Illustrative pictures of the Lewis acid-base adducts [MeOꢀ!
ꢀ
ꢀ
ꢀ
ꢀ
B(OR)2-B(OR)2], [tBuO !B(OR)2 SiMe2Ph], and [RO !B(OR)2
N(R’)2]. E=electrophilic reagent.
became intrigued with elucidating the possible Lewis acid–
base interaction between aminoboranes, having the formula
(RO)2B-NR’2, and alkoxides so that the nucleophilic charac-
ter of the amine group towards organic electrophiles could be
increased.
Aminoboranes have been previously synthesized from
B(NR2)3[5] or borylnitrenes[6] and applied in organic synthesis
to generate organoboron compounds.[7] Suginome and co-
workers efficiently demonstrated the use of aminoborane
derivatives in amination reactions[8] such as as Strecker-type
aminative cyanation,[9] reductive amination,[10] and a Man-
nich-type reaction.[11] By using this latter methodology,
action of aminoboranes with alkoxydes, thus forming the
ꢀ
ꢀ
adduct [RO !B(OR)2 N(R’)2] in situ, and enhancing the
nucleophilic character of the amino group so that it can react
selectively with a,b-unsaturated carbonyl compounds
(Scheme 1c).
With this idea in mind, we first attempted to find the
optimal reaction conditions for the amination of 4-hexen-3-
ꢀ
one (1) with Bpin NMe2 (2a; pin = pinacol; Table 1). When
the reaction was carried out in MeOH as the solvent at 708C,
no amination product was observed (Table 1, entry 1). The
sole addition of 5 mol% of NaOtBu favored the formation of
5-dimethylamino-hexan-3-one (3a) as the only product, with
a conversion of 28% (Table 1, entry 2). This result is in
agreement with the fact that the base reacts with MeOH to
generate the alkoxide[4] which might interact with the amino-
borane to form the Lewis acid-base adduct. The use of
a phosphine as an additive (10 mol% of PCy3) was beneficial
and increased the formation of the desired product to 95%
(Table 1, entries 3–7). The role of the phosphine might
involve an interaction with the a,b-unsaturated carbonyl
substrate, thus resulting in the formation of a strongly basic
zwitterionic phosphonium enolate species.[12] However, the
[*] C. Solꢀ, Dr. E. Fernꢁndez
Department Quꢂmica Fꢂsica i Inorgꢃnica, University Rovira i Virgili
C/Marcel lꢂ Domingo s/n, 43007 Tarragona (Spain)
E-mail: mariaelena.fernandez@urv.cat
Homepage: http://www.quimica.urv.cat/tecat/catalytic_
organoborane_chemistry.php
[**] We thank the MEC for funding (CTQ2010-16226) and C.S for a FPU
grant.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305098.
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Table 1: Optimization of reaction conditions for the b-amination of
[a]
ꢀ
4-hexen-3-one with Bpin NMe2 (2a).
ꢀ
Figure 2. In situ 11B NMR spectra of the suggested [MeO !Bpin
ꢀ
ꢀ
ꢀ
NMe2] and [tBuO !Bpin NMe2] adducts.
Entry
Base
(mol%)
Additive
(mol%)
Solvent/
T [8C]
Conv.
[%][b]
was established when heating at 608C and the small signal
disappeared to show the original sp2 Bpin signal. However,
when 2 equivalents of MeOH were added, a total shift was
observed towards the single signal at d = 8.74 ppm, at both
room temperature and 608C.
The scope of substrates for the preparation of b-dimethyl-
amino carbonyl compounds was next examined. As shown in
Table 2, the substrate trans-1-phenyl-2-buten-1-one (4) was
1
2
3
4
5
6
7
8
–
–
–
MeOH/70
MeOH/70
MeOH/70
MeOH/70
MeOH/70
MeOH/70
tBuOH/70
MeOH/70
MeOH/25
THF[c]/70
–
28
67
90
92 (88)
95 (90)
93
7
19
NaOtBu(5)
NaOtBu (5)
NaOtBu(5)
NaOtBu(10)
NaOtBu(15)
NaOtBu(15)
–
NaOtBu(5)
NaOtBu(5)
CsCO3(15)
PPh3(10)
PCy3(10)
PCy3(10)
PCy3(10)
PCy3(10)
PCy3(10)
PCy3(10)
PCy3(10)
PCy3(5)
9
10
11
20
95
MeOH/70
ꢀ
Table 2: b-Amination of a,b-unsaturated substrates with Bpin NMe2 (2a)
ꢀ
[a] Reaction conditions: 1 (0.25 mmol), Bpin NMe2 (0.275 mmol), base
(5–15 mol%), PR3 (5–10 mol%), MeOH (2 mL), 708C, 17 h. [b] Con-
version calculated by GC/MS from an average of two reactions. The yield
of the isolated product is given in parentheses. [c] THF (2 mL) with
2 equiv of MeOH added to the reaction.
[a]
ꢀ
and Bpin NEt2 (2b).
Entry Substrate
Reducing Product
agent
Conv.
[%][b]
ꢀ
Bpin
NMe2
93
(84)
1
2
sole addition of phosphine, with no base, does not guarantee
the b-amination reaction (Table 1, entry 8). A temperature of
708C seems to be required for quantitative transformation of
1 into 3a (Table 1, entries 4 and 9). It was also demonstrated
that MeOH as the solvent had a greater positive influence
than THF with 2 equivalents of MeOH as an additive
(Table 1, entries 4 and 10). The nature of the base was also
studied and under optimized reaction conditions Cs2CO3 was
also efficient at promoting the amination of 1 (Table 1,
entry 11). Overall, the reaction conditions shown in entry 6 of
Table 1 were found to be optimal for the methodology to be
extended to other a,b-unsaturated carbonyl compounds. The
benefits of this methodology involve the direct and clean
conjugate amination, because when we conducted the same
reaction with LiNMe2 instead of the aminoborane/base/
MeOH system, the substrate 1 was transformed into a com-
plex mixture of products, and product 3a was not formed (see
the Supporting Information for details).
ꢀ
Bpin
75
(68)
NMe2
ꢀ
Bpin
NMe2
70
(65)
3
4
5
6
ꢀ
Bpin
NMe2
20
25
ꢀ
Bpin
NMe2
ꢀ
Bpin
NMe2
43
(37)
ꢀ
Bpin
NMe2
63
(58)
7
Spectroscopic evidence has demonstrated the formation
ꢀ
[13]
ꢀ
of the Lewis acid-base adduct [RO !Bpin NMe2]. The
original 11B NMR spectra of the aminoborane 2a, in THF,
shows a clear signal at d = 22.55 ppm which corresponds to
a sp2 Bpin moiety bonded to an amino group (Figure 2). After
the addition of 1 equivalent of NaOtBu, the signal completely
shifted to higher fields (d = 5.88 ppm), even at room temper-
ature. No further changes were observed even in the presence
of 2 equivalents of MeOH at 608C. The new signal might
correspond to the sp3 Bpin moiety of the adduct [tBuOꢀ!
ꢀ
Bpin
NEt2
48
(27)
8
ꢀ
Bpin
NEt2
45
(32)
9
ꢀ
Bpin
NMe2/
HNEt2
10
73[c]
ꢀ
[a] Reaction conditions: substrate (0.25 mmol), Bpin NR2 R=Me,Et
(0.27 mmol), NaOtBu (15 mol%), PCy3 (10 mol%), MeOH (2 mL), 708C,
17 h. [b] Conversion calculated by GC/MS from an average of two
reactions. The yield of the isolated product is given in parentheses. [c] 23%
of product 3b was also formed.
ꢀ
Bpin NMe2]. However, when 1 equivalent of NaOMe was
added to the aminoborane 2a, the signal at d = 22.55 ppm did
not change significantly, and only one small signal appeared at
d = 8.74 ppm. The equilibrium towards the free aminoborane
2
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ꢀ
Table 3: Synthesis of b-amino alcohols through b-dimethylamination with Bpin
quantitatively converted into the corresponding b-
dimethylamino ketone 5a (Table 2, entry 1). How-
ever the aliphatic ketones 3-hepten-2-one (6) and 3-
nonen-2-one (8) were only moderately transformed
into the b-amino ketones 7 and 9, respectively, as
a consequence of the bulkier alkyl Cb substituents
(Table 2, entries 2 and 3). The less efficient b-
amination was observed for the chalcones 10 and
12 (Table 2, entries 4 and 5). It seems that the steric
and electronic properties of the phenyl substituent
on Cb diminished the nucleophilic attack of the
activated aminoborane. Next, we turned our atten-
tion to explore the b-amination of the a,b-unsatu-
rated esters methylcrotonate (14) and ethylcrotonate
(16). In both cases the conversion towards the
desired product was only moderate (Table 2,
entries 6–7).
NMe2 (2a)/reduction.[a]
Entry Substrate
Reducing Product
agent
Conv. syn/
[%][b] anti[c]
90
1
2
NaBH4
93:7
(85)
95
DIBALH
85:15
(88)
75
3
4
NaBH4
NaBH4
72:28
(62)
We also extended the organocatalytic addition of
the diethylamino moiety when the aminoborane
69
ꢀ
62:38
(58)
involved in the reaction was the analogue Bpin NEt2
(2b). When 2b was activated with MeOH and base,
the diethylamino moiety became nucleophilic
enough to b-aminate substrates 1 and 4, and isolate
the corresponding b-diethylamino ketones in mod-
erate yields (Table 2, entries 8 and 9). This reactivity
[a] Reaction conditions: substrate (0.25 mmol), Bpin-NMe2 (0.27 mmol), NaOtBu
(15 mol%), PCy3(10 mol%), MeOH (2 mL), 708C, 17 h. Reducing reagent was
added at ꢀ788C for 2 h. [b] Conversion calculated by GC/MS from an average of two
reactions. The yield of the isolated syn isomer is given in parentheses. [c] The
1
d.r. value was calculated from H NMR spectroscopy. DIBALH=diisobutylalumi-
seems to be related to the less accentuated nucleo-
num hydride.
ꢀ
ꢀ
philic character of the NEt2 moiety in [RO !Bpin
ꢀ
ꢀ
NEt2] versus [RO !Bpin NMe2]. To make the
reaction system more suitable, we followed the
elegant design by Suginome and co-workers,[8] thus taking
advantage of the amino group exchange on aminoboranes
with external amines. When we performed the optimized
reaction on 1 (Table 2, entry 10), with 1 equivalent of [ROꢀ!
a result of the less hindered NMe2 versus Bpin nucleophilic
counterpart.
The in situ reduction of the b-dimethylamino ketones 3a,
5a, 7, and 9, led to the corresponding b-dimethyl amino
alcohol with a favored syn/anti diastereomeric ratio depend-
ing on the reducing agent involved. Table 3 shows the high
d.r. value of the syn b-dimethyl amino alcohols 18 and 19
when NaBH4/MeOH or DIBALH respectively, was used as
the reducing agent (Table 3, entries 1 and 2).
ꢀ
Bpin NMe2] and 1 equivalent of NEt2, the product formed
was 3a (73%) together with 3b (27%). These preliminary
experiments demonstrate the possible in situ iminium ion
generation,[8] to be extended to bulkiers amines.
To highlight the role of the phosphine in this reaction, as
we recently pointed out,[2b] chiral phosphines can assist the
asymmetric organocatalytic b-boration of a,b-unsaturated
carbonyl compounds. In this study, we explored this possibil-
ity and we conducted a parallel (b-boration with B2pin2) and
Alternatively, a complete diastereoselection was achieved
on the direct amination ring-opening, through an SN2’
ꢀ
ꢀ
reaction between the adduct [RO !Bpin NMe2] and the
cyclic vinyl epoxide 3,4-epoxy-1-cyclohexene (22; Scheme 3).
Therefore, 1,4-cyclohexenyl dimethylamino alcohol (23) was
exclusively formed . When 23 was isolated and compared with
the reported NMR data for this polyfunctionalized com-
pound,[14] it was characterized as the trans isomer.
ꢀ
b-amination (with Bpin NMe2) of the model substrate 1 in
the presence of a Josiphos-type ligand. Scheme 2 shows that,
under optimized reaction conditions, the asymmetric induc-
tion on the organocatalytic b-amination is lower than the
corresponding organocatalytic b-boration, probably as
We then went on to explore the b-amination of the
electron-deficient a,b-acethylenic carbonyl substrates in an
attempt to find a direct methodology which exclusively forms
Scheme 2. Asymmetric b-amination reactions assisted by a Josiphos-
type ligand. Comparison with the corresponding b-boration reaction.
Scheme 3. Diastereoselectꢀive amination ring opening of 3,4-epoxy-1-
ꢀ
cyclohexene (22) with [RO !Bpin NMe2].
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Communications
the E isomer of the corresponding b-enamino ester. Although
b-enamino derivatives are of considerable interest, both as
bioactive leads and versatile building blocks,[15] they have
mainly been synthesized by the direct condensation of 1,3-
dicarbonyl compounds with ammonia and primary amines.[16]
In these protocols, the Z isomer of the b-enamino ester was
the main product formed. However, when we conducted the
b-amination of ethyl-2-butynoate (24) and ethyl-2-pentynoate
3 h, respectively; Table 4, entries 6 and 7). Finally, E-b-
enamino esters were selectively formed from the reaction of
ꢀ
Bpin NEt2 with ethyl-2-butynoate (24) and ethyl-2-penty-
noate (26; Table 4, entries 8 and 9). Interestingly, test experi-
ments based on direct reactivity of dimethylamine or diethyl-
amine with the model substrate 24 did not lead to the
formation of either the E- or Z-b-enamino ester. The
implication of the aminoborane methodology opens a a new
pathway towards direct synthesis of E-b-enamino ester
through conjugate addition.
ꢀ
(26) with Bpin NMe2, the E isomer of the b-enamino ester
was preferentially formed (Table 4, entry 1–3), probably as
ꢀ
ꢀ
a consequence of the bulky properties of the [RO !Bpin
To complete this survey of amination through amino-
ꢀ
NMe2] adduct. For those substrates which had bulkier
substituents on Cb, the E-b-enamino ester was exclusively
formed (Table 4, entries 4 and 5). The total b-amination of the
terminal a,b-acethylenic ester 32, by adding Bpin-NMe2 or
Bpin-NEt2, was carried out in shorter reaction times (1 h and
boranes, we became interested in reacting Bpin NMe2 with b-
lactones to elucidate the nucleophilic character of the amino
ꢀ
ꢀ
moiety in the adduct [RO !Bpin NMe2]. As it has been
described,[17] hard nucleophiles express a strong preference
for addition to the lactone carbonyl group, thus providing ring
opening by an addition-elimination pathway.[18] Alternatively,
soft nucleophiles achieve better electronic matching with the
electrophilic Cb, thereby promoting SN2 displacement of the
Table 4: b-Amination of the electron-deficient a,b-acethylenic carbonyl
[a]
carboxylate residue (Scheme 4).[19] When we reacted Bpin
NMe2 with b-butyrolactone (34) in MeOH/base media, the
only product observed was the b-hydroxy N-dimethyl amide,
thus evidencing the hard nucleophilic character of the amino
ꢀ
ꢀ
substrates with Bpin NMe2 (2a) and Bpin NEt2 (2b).
ꢀ
Entry Substrate
Product
Conv.
[%][b]
E/Z[c]
1[d]
2
75
85:15
81 (68) 84:16
3
4
80 (74) 88:12
Scheme 4. Hypothetical reactivity of activated aminoboranes with
b-butyrolactone.
moiety. This type of compound has also been recently
prepared from the copper-[20a] or nickel-catalyzed[20b] b-bora-
tion of a,b-unsaturated amides with B2pin2 with a subsequent
oxidation pathway. We were delighted to see the comple-
mentary organocatalytic fomation of the desired product in
a one-step reaction.
75 (63) 99:1
5
93 (85) 99:1
99 (93) 99:1
99 (93) 99:1
70 (62) 99:1
67 (60) 99:1
In conclusion, the simple Lewis acid–base interaction of
aminoboranes with alkoxides, which forms the adduct
6[e]
7[f]
8
ꢀ
ꢀ
[RO !B(OR)2 N(R’)2] in situ, seems to be the platform to
enhance the nucleophilic attack of amino moieties towards
electron-deficient olefins such as a,b-unsaturated carbonyl
compounds and cyclic vinyl epoxides. Following a simple one-
pot reaction, the 1,3-dialkylamino alcohols and 1,4-dialkyl-
amino alcohols can be isolated in moderate to high yield.
Extension of this methodology to more sterically demanding
aminoboranes is currently under study.
9
Received: June 13, 2013
Revised: August 2, 2013
Published online: && &&, &&&&
[a] Reaction conditions: substrate (0.25 mmol), Bpin-NR2 (1.5 equiv),
NaOtBu (25 mol%), MeOH (2 mL), reflux, 17 h. [b] Conversion calcu-
lated by GC/MS from an average of two reactions. The yield of the
isolated product is given in parentheses. [c] Z/E ratio calculated from
1H NMR spectroscopy. [d] NaOtBu(15 mol%). [e] 1 h. [f] 3 h.
Keywords: aminoboranes · diastereoselectivity · Lewis acid ·
.
organocatalysis · synthetic methods
4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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.
Angewandte
Communications
Communications
Organocatalysis
C. Solꢁ, E. Fernꢂndez*
&&&& — &&&&
Alkoxide Activation of Aminoboranes
towards Selective Amination
Piece of the (inter)action: The interaction
of alkoxides with the sp2 Bpin (pin=
pinacol) moiety in aminoboranes forms
ophile with several electrophiles, thus
providing amino alcohols, b-enamino
esters, and b-hydroxy amides in a direct
and remarkably selective way (see
scheme).
the in situ Lewis acid–base adduct
ꢀ
ꢀ
[RO !B(OR)2 N(R’)2] which enables the
amino moiety to react as a strong nucle-
6
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!