Chiral-at-metal organolanthanides: Enantioselective aminoalkene hydroamination/cyclisation with non-cyclopentadienyls

Article abstract of DOI:10.1039/b305105f

Chiral non-racemic complexes [ML{N(SiMe₂H)₂}(thf)] (M = Y, La, H₂L = salicylaldimine ligands derived from 2,2′-diamino-6,6′-dimethylbiphenyl) are found not to be effective catalysts for the intramolecular hydroamination of

Full text of DOI:10.1039/b305105f

Chiral-at-metal organolanthanides: enantioselective aminoalkene
hydroamination/cyclisation with non-cyclopentadienyls†
Paul N. O’Shaughnessy, Paul D. Knight, Colin Morton, Kevin M. Gillespie and Peter Scott*
Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL.
E-mail: peter.scott@warwick.ac.uk; Fax: 024 7657 2710; Tel: 024 7652 3238
Received (in Cambridge, UK) 7th May 2003, Accepted 9th June 2003
First published as an Advance Article on the web 18th June 2003
Chiral non-racemic complexes [ML{N(SiMe2H)2}(thf)] (M =
Y, La, H2L = salicylaldimine ligands derived from 2,2A-
diamino-6,6A-dimethylbiphenyl) are found not to be effective
catalysts for the intramolecular hydroamination of ami-
noalkenes, but new amino/phenoxide ligand designs without
reducible functional groups led to long-lived and enantiose-
lective catalysts.
by use of the 6-methylsalicylaldimine unit in L2.6d Accordingly,
the complexes [ML2{N(SiMe2H)2}(THF)] (M = Y, La) were
prepared, and while significantly better conversions were
obtained in hydroamination/cyclisation, the reaction was very
slow (Table 1, entries 3, 4).
The molecular structure of the complex [YL2{N(Si-
Me2H)2}(THF)] (Figure 1) is similar to that observed in our
group 4 metal complexes in that the Schiff base is disposed with
C2 symmetry about the metal in the rare cis-a orientation. The
chirality arising from the biaryl unit is thus very well expressed
in the (potentially) active sites at the metal, and this encouraged
us to investigate alternative methods of generating this type of
architecture with ligands that do not contain reactive electro-
philic groups. This problem has been noted by Evans.5d
The new chiral aminophenoxido proligand (S)-H2L3 was
prepared conveniently and in high yield as shown in Scheme 2.
The diamine (S)-110 reacted with 3,5-di-tert-butylcatechol
under acid catalysed conditions to give 2 directly.§ The N–H
groups in this compound were efficiently methylated using
LiBun/MeI to give H2L3. Protonolysis reactions with the metal
amides gave [ML3{N(SiMe2H)2}(THF)2] (M = Y, Sm, La).
The lability of group 3 and lanthanide element–ligand bonds
and the flexibility of their coordination geometries make these
elements highly suitable for use in catalysis, but these factors
also make it difficult to generate well-defined chiral archi-
tectures that will lead to efficient enantioselective reactions. It
has been pointed out that the number of truly effective chiral
ligands in lanthanide coordination chemistry is severely
limited.1 For organolanthanide systems‡ the chiral cyclopenta-
dienyls, particularly Marks’ planar chiral menthyl and neomen-
thyl Cp complexes2 are essentially peerless, and while several
groups worldwide are working on the synthesis of new chiral
organolanthanide complexes, actual enantioselective catalyses
are exceptionally rare.3 It is becoming clear that for organolan-
thanides to be successful in this type of application there is a
requirement that the chirality of the system is exceptionally well
expressed in the active sites. This presents a major challenge to
lanthanide chemists.1
Polydentate Schiff base ligands have been used in a wide
range of metal-catalysed reactions,4 and well defined com-
plexes of these ligands with lanthanide and group 3 elements
have been reported.5 Chiral salicylaldimines6 such as H2L1 and
similar binaphthyls7 coordinate to transition metals giving
chiral-at-metal8 structures. We envisioned this type of structure
could lead to enantioselective organolanthanide catalysts for
aminoalkene hydroamination,2 particularly since they give
stereoselective ring-opening polymerisation of meso-lactide.9
Treatment of H2L1 with [M{N(SiMe2H)2}3(THF)2] (M = Y,
La)5a led to clean formation of [ML1{N(SiMe2H)2}(THF)] (M
= Y, La). (Figure 1). Both complexes mediated the cyclisation
of 2,2-dimethylaminopent-4-ene at 70° (Table 1 entries 1, 2),
but conversion ceased after a few turnovers. Since this catalytic
reaction involves the intermediacy of a metal alkyl species,2c we
conjectured that the catalyst could be decomposing by 1,2-mi-
gratory insertion at an imine unit.5b We have recently shown
how a similar reaction in group 4 complexes can be prevented
Fig. 1 Molecular structure of [YL2{N(SiMe2H)2}(THF)].
Table 1 Hydroamination/cyclisation dataa
Conv./
Entry Catalyst
M
t/h
%
ee/%
1
2
3
4
5
6
7
8
9
[ML1{N(SiMe2H)2}(THF)]
Y
La
Y
La
Y
Sm
La
Y
24b
24c
12
9
45
—
—
—
—
5
0
0
11
27
61
[ML2{N(SiMe2H)2}(THF)]
[ML3{N(SiMe2H)2}(THF)2]
[ML4{N(SiMe2H)2}(THF)2]
49
72
44
40
24
30
40
100
100
100
100
100
100
Scheme 1 Synthesis of Schiff base complexes.
Sm
La
10
† Electronic supplementary information (ESI) available: complete experi-
mental procedures and characterising data for all ligands and complexes,
crystal data for [YL2{N(SiMe2H)2}(THF)] and [SmL4{N(SiMe2H)}]. See
http://www.rsc.org/suppdata/cc/b3/b305105f/
a All reactions were performed in C6D6 at 70 °C in presence of ca 1%
catalyst. b No further turnover was observed after this time. c Very slow, but
catalysis still occurring after 4 weeks.
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CHEM. COMMUN., 2003, 1770–1771
This journal is © The Royal Society of Chemistry 2003

system is chemically robust, the chirality of the biaryl unit is
poorly communicated to the prochiral reagent, probably as a
result of insufficient chelate “reach-around”. In contrast L4
provides a non-cyclopentadienyl environment for enantiose-
lective organolanthanide catalysis.‡ It is worth noting that the
cis-b structure adopted by complexes of L4 is analogous to that
of the ruthenium complexes of e.g. L1 which give exceptionally
high stereoselectivity in catalytic alkene cyclopropanation.6c
We believe that the chiral-at-metal architecture is the origin of
the selectivity in both cases; as in the case of planar-chiral
cyclopentadienyl systems,2 the chirality is strongly expressed in
the active site.
PS thanks EPSRC and BP Chemicals for support of this
work.
Scheme 2 Synthetic route to aminophenols H2L3 and metal complexes.
A further novel chiral ligand (S)-H2L4 (Scheme 3) was
prepared via reduction of the corresponding Schiff-base and N-
methylation as before. The subsequent complexes [ML4{N(Si-
Me2H)2}(THF)2] (M = Y, Sm, La) were accessible. NMR
spectra indicate that the complexes are C2 symmetric on this
timescale in the presence of THF. In the case of the Y
compound, prolonged drying in vacuo removed the majority of
THF and a single diastereomeric C1 symmetric system was
observed by NMR spectroscopy. The structure of the THF-free
samarium complex (Figure 2) indicates that the biaryl unit has
predetermined in this case a (S)-L-cis-b1 structure6c,8 consistent
with the NMR data. Notably, the stereogenic NMe units have
opposite absolute configurations. One amido SiH group is
involved in an agostic Si–H–Sm interaction.11
Notes and references
‡ The distinction between lanthanide coordination complexes and organo-
lanthanides (broadly alkyls, hydrides and amido compounds) is important
since in the former all catalyses are expected to be Lewis acid processes,
whereas in the latter there exists the possibility of other reactions such as
migratory insertion.
§
Our development of this reaction type will be reported elsewhere.
Electronic Supplementary Information (ESI) available.†
Crystal data: C46H64N3O3Si2Y, M = 852.09, monoclinic, a = 15.92(2),
b = 17.51(3), c = 16.80(2) Å, b = 95.16(13), U = 4666(12) Å3, T =
180(2) K, space group P2(1)/a, Z = 4, m(Mo–Ka) = 1.341 mm21, 20959
reflections measured, 6070 unique (Rint = 0.3134). The final R1 = 0.1607
[I > 2s (I)]. CCDC 210134. C57H92N3O2Si2Sm, M = 1057.87, triclinic, a
= 11.1290(13), b = 14.1565(17), c = 19.349(2) Å, a = 101.030(2), b =
101.798(3), g = 94.828(3), U = 2904.7(6) Å3, T = 180(2) K, space group
All of the above complexes of L3–4 are catalysts for
hydroamination/cyclisation of dimethylaminopentene at 70 °C,
giving complete conversion of the substrate (Table 1, entries
5–10). In the case of the L3 complexes we were surprised to find
that the products were essentially racemic. This situation was
not improved significantly by operating the catalysts at lower
temperatures. In contrast, the complexes of L4 gave significant
enantiomeric excess in this reaction, and in the case of the
lanthanum complex [ML4{N(SiMe2H)] the selectivity is com-
parable with the best result thus far obtained by Marks using the
C1 symmetric (S)-menthylcyclopentadienyl ansa-metallocene
system,2 although the activity is somewhat lower.
P1, Z = 2, m(Mo–Ka) = 1.092 mm21, 28296 reflections measured, 9141
¯
unique (Rint = 0.0661). The final R1 = 0.0524 [I > 2s (I)]. CCDC 211979.
See http://www.rsc.org/suppdata/cc/b3/b305105f/ for crystallographic data
in .cif or other electronic format.
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Hence it appears that (i) the Schiff base ligands L1,2 are too
reactive to support this type of reaction, and (ii) while the L3
5 (a) O. Runte, T. Priermeier and R. Anwander, Chem Commun., 1996,
1385; (b) D. J. H. Emslie, W. E. Piers, M. Parvez and R. McDonald,
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Scheme 3 Synthetic route to aminophenols H2L3 and metal complexes.
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Fig. 2 Molecular structure of (S)-L-cis-b1-[SmL4{N(SiMe2H)}]
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