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DOI: 10.1002/cctc.201300113
Base-Free b-Boration of a,b-Unsaturated Imines Catalysed
by Cu O with Concurrent Enhancement of Asymmetric
2
Induction
[b]
[a]
[b]
[a]
Adam D. J. Calow, Cristina Solꢁ, Andrew Whiting,* and Elena Fernꢂndez*
The stereoselective synthesis of g-aminoalcohols from the cata-
lytic asymmetric b-boration of a,b-unsaturated imine precur-
formation of the imines, which allows access to cyclic and acy-
clic b-boryl imines. The reaction was followed by using in situ
IR spectroscopy, which demonstrated the imine formation–
b-boration sequence and that the new catalytic system is supe-
rior to those employed for this reaction previously.
sors has been streamlined with the use of Cu O as the catalyst,
2
readily accessible (R)-(+)-2,2’-bis(diphenylphosphino)-1,1’-bi-
naphthyl as the chiral ligand and no additional base. The new
simplicity of the catalytic system has the added value of in situ
Introduction
I
Cu-catalysed asymmetric b-boration reactions have received
erocyclic carbene ligands) and Cu(OH) /L are used to activate
2
[7,8]
2
3
considerable attention since Yun et al. discovered that CuCl
B2pin2
or sp –sp -hybridised mixed diboron reagents trans-
(
(
3 mol%) modified with bidentate Josiphos-type chiral ligands
3 mol%) could activate bis(pinacolato)diboron (B pin ) in the
metallated with CuCl are used to provide the CuBpin reactive
[9]
species. We became interested in exploring the use of Cu O
2
2
2
presence of base (9 mol%) to deliver the Bpin moiety enantio-
as a precursor of the active catalytic system for the b-boration
of a,b-unsaturated imines. Most importantly, this could poten-
tially behave as a base-free system and it has the potential to
be asymmetric if used in the presence of suitable compatible
selectively to the b-position of a,b-unsaturated nitrile com-
[
1]
pounds (Scheme 1a). Further efforts have been devoted to
increase the scope of the application of this convenient meth-
[2,3]
odology to poly-functional organoboron compounds.
We
chiral ligands. This hypothesis is based on the possibility that
I
became interested in and focussed on the enantio- and
Cu O could interact with MeOH to generate a CuÀalkoxide or
2
I
diastereoselective preparation of g-aminoalcohols by the Cu -
hydroxide species. To the best of our knowledge, there is only
mediated b-boration of a,b-unsaturated imines followed by
boron-assisted in situ imine reduction and BÀC oxidation steps
one example of asymmetric induction upon CÀB bond forma-
tion mediated by Cu O in the b-boration of a,b-unsaturated
2
[
4]
(
Scheme 1b). We extended this strategy of the in situ forma-
N-acyloxazolidinones by using a chiral bi-cyclic 1,2,4-triazolium
[10]
tion of a,b-unsaturated imines from a,b-unsaturated aldehydes
salt (Scheme 2) and Cs CO as a base. Our objective was to
2 3
[
5]
and ketones and trapped them by using b-boration. In addi-
tion, especially for certain water-soluble g-amino alcohol prod-
ucts, a further protection step could be performed in situ to
give the readily isolated 1,3-oxazine derivatives in a five-step
investigate and highlight the benefits of Cu O as a cheap cata-
2
lyst precursor, which avoids the addition of an external base,
and modify Cu O with commercially available chiral ligands,
2
such as (R)-(+)-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl
((R)-BINAP, L0), to promote an efficient enantioselective catalyt-
ic system.
[
5]
one-pot sequence (Scheme 1c).
For all the Cu-mediated b-borations of electron-deficient ole-
fins reported to date, the addition of base has always been re-
[
6]
quired unless pre-formed (NHC)CuOR species (NHC =N-het-
Results and Discussion
Our study began with the b-boration of 4-phenyl-3-buten-2-
one (1) as a model substrate and B pin as the diboron re-
[
a] C. Solꢀ, Dr. E. Fernꢁndez
Dept Quꢂmica Fꢂsica i Inorgꢃnica
University Rovira i Virgili
C/Marcel·lꢂ Domingo s/n 43005 Tarragona (Spain)
Fax: (+34)977-559563
E-mail: mariaelena.fernandez@urv.cat
2
2
I
agent. Two Cu sources were selected, CuCl (3 mol%) and Cu O
2
(
1.5 mol%), to compare their relative activities as catalyst pre-
cursors in the presence of L0. Initially, 1 was not converted
into the b-borated ketone 2 in the absence of BnNH (Bn=
[
b] A. D. J. Calow, Prof. A. Whiting
Centre for Sustainable Chemical Processes
Chemistry Department
2
benzyl group; Table 1, entries 1 and 6). However, with the in-
creasing addition of BnNH2 (10–100 mol%), the progressive
formation of the b-borated imine 3a occurred with different
efficiencies, which depended on the Cu source. If the CuCl/L0
catalytic system was used, 2 was still the main product at low
amine loadings (Table 1, entries 2–3). If the percentage of
Durham University
South Road, Durham DH1 3LE (United Kingdom)
E-mail: andy.whiting@durham.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300113.
ꢀ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 8
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