Aminooxazolinate; a chiral amidinate analogue

Article abstract of DOI:10.1039/b409113b

High levels of diastereoselection with respect to chirality-at-metal are achieved at equilibrium for complexes containing a new and available range of diazaallyl ligands.

Full text of DOI:10.1039/b409113b

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Aminooxazolinate; a chiral amidinate analogue{
Ian J. Munslow, Andrew R. Wade, Robert J. Deeth and Peter Scott*
Department of Chemistry, University of Warwick, Coventry, UK. E-mail: peter.scott@warwick.ac.uk;
Fax: 144 (0)24 7657 2710; Tel: 144 (0)24 7652 3238
Received (in Cambridge, UK) 17th June 2004, Accepted 18th August 2004
First published as an Advance Article on the web 30th September 2004
High levels of diastereoselection with respect to chirality-at-
metal are achieved at equilibrium for complexes containing a
new and available range of diazaallyl ligands.
Complexes of the amidinate ligand I have an extensive stoichio-
metric1 and now a considerable catalytic chemistry.2,3 In particular
for the latter aspect, Sita and co-workers have developed a class of
cyclopentadienyl complexes e.g. II that are capable of highly
stereocontrolled a-olefin polymerisation4 and have potential for the
development of reagents for asymmetric hydrozirconation.5 Also
of interest is the development by Nagashima et al. of Kharasch
cyclisation catalysed by III.6 These results and others indicate that
chiral diazaallyl ligands have many potential applications in
enantioselective synthesis using their metal complexes. However,
while ligands with chiral alkyl substituents such as IV are readily
prepared, free rotation about the N–C single bond(s) limits the
degree to which the ligand-centred chirality is expressed in the
structure of the complex. For chiral-at-metal systems7 this leads to
modest diastereoselectivity.8 We thus sought cyclic analogues of IV.
Fig. 1 Molecular structure of (SRu,SC)-[Ru(g-C6H6)L1Cl]. Selected bond
lengths (A) and angles (u): Ru(1)–N(1) 2.150(3), Ru(1)–N(2) 2.154(3),
Ru(1)–C(arene) 2.16–2.20, Ru(1)–Cl(1) 2.3997(11), N(1)–Ru(1)–N(2)
62.71(12), N(1)–Ru(1)–Cl(1) 85.74(8), N(2)–Ru(1)–Cl(1) 84.83(8).
˚
4-membered chelate unit at the N–N vector such that O(1) is tilted
slightly toward the RuCl unit.
In order to probe the possibility of metal-centred epimerisation
processes and the issue of thermodynamic diastereoselection7 in
these compounds we have synthesised chiral racemic [Ru(g-
p-iPrMeC6H4)L3Cl] [Scheme 1(a)]. The 1H NMR spectrum of this
complex at 298 K in d8-toluene contains two resonances separated
by ca. 0.03 ppm for the oxazolinyl 4-methyl groups, these being
rendered inequivalent on complexation. As a result of this small
chemical shift difference we were able to observe coalescence at
373 K. This corresponds (at 400 MHz) to a DG{ of ca. 95 kJ mol21
for interconversion of enantiomers.
Using Kim and co-workers’ synthesis of 2-phenylamino-2-
oxazolines9 we prepared a range of proligands HLn in two steps
from commercially available isothiocyanates and optically-pure
(S)-aminoalcohols in ca. 60% overall yield.{ Treatment of HL1
with NaH in THF led to quantitative conversion to Na(THF)mL1
which may be isolated by filtration and evaporation or used directly
as a ligand transfer agent. Subsequent treatment of Na(THF)mL1
with [{Ru(g-arene)Cl}2-m-Cl}2] (arene ~ C6H6, p-iPrMeC6H4)
in THF gave crystalline [Ru(g-arene)L1Cl] in good yield.
[Ru(g-C6H6)L2Cl] was prepared similarly. 1H NMR spectra of
crude reaction mixtures indicate that single diastereomers are
formed (d.e. w 95%, vide infra).
The pure diastereomers (SRu,SC)-[Ru(g-arene)L1Cl] were also
studied by variable temperature NMR spectroscopy. The 1H
spectra in d8-toluene did not alter significantly up to 373 K, and
were identical to the starting spectra on returning to 298 K. Storage
at 323 K for 14 days had no noticeable effect; no new diastereomers
were observed.
Hence an epimerisation process for optically pure (SRu,SC)-
The molecular structure of (SRu,SC)-[Ru(g-C6H6)L1Cl] is shown
˚
in Fig. 1.{ The two N–Ru bond lengths are very similar (ca. 2.15 A)
as are the N–Ru–Cl angles (ca. 85u), but there is a 17.5u fold in the
{ Electronic supplementary information (ESI) available: synthesis and
characterisation data for all complexes; crystal data and structure files for
(SRu,SC)-[Ru(g-C6H6)L1Cl], (D,SC)-[ZrL4 (CH2Ph)2] and [Ru(g-p-iPr-
Scheme 1 (a) Inversion of chirality-at-metal observed for racemic
[Ru(g-p-iPrMeC6H4)L3Cl]; (b) only (SRu,SC)-[Ru(g-arene)L1Cl] (A)
detected under equilibrium conditions.
2
MeC6H4)L3Cl]; DFT calculated structure files for eight diastereomers of
[ZrL4 (CH2Ph)2]. See http://www.rsc.org/suppdata/cc/b4/b409113b/
2
2 5 9 6
C h e m . C o m m u n . , 2 0 0 4 , 2 5 9 6 – 2 5 9 7
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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diastereoselection is excellent. We are currently investigating
stereoselective catalytic applications of this type of complex
guided by the chemistry of the functionally related amidinates.
We thank EPSRC for support and D. L. Davies (Leicester) for
helpful discussions.
Notes and references
{ Crystaldata:(SRu,SC)-[Ru(g-C6H6)L1Cl]:C19H23ClN2ORu,M~431.91,
˚
orthorhombic, a ~ 10.1427(4), b ~ 11.1134(4), c ~ 16.1543(6) A, U ~
3
˚
1820.91(12) A , T ~ 180 K, space group P212121, Z ~ 4, m(Mo–Ka) ~
1.015 mm21
4368 independent reflections (Rint 0.0574). Final
R1 0.0368 [I
2s(I)] CCDC 246168. [ZrL4 (CH2Ph)2]:
C44H56N4O2Zr, M ~ 764.15, orthorhombic, a ~ 10.1903(2), b ~
,
~
~
w
2
Fig. 2 Molecular structure of (D,SC)-[ZrL4 (CH2Ph)2]; most H atoms have
been removed and benzyl ligands hatched for clarity. Selected bond lengths
3
2
˚
˚
17.3251(4), c ~ 25.7931(4) A, U ~ 4553.72(16) A , T ~ 180 K, space
group P212121, Z ~ 4, m(Mo–Ka) ~ 0.277 mm21,11289 independent
reflections (Rint ~ 0.1133). Final R1 ~ 0.0892 [I w 2s(I)]. CCDC 246169.
[Ru(g-p-iPrMeC6H4)L3Cl]: CCDC 246170. See http://www.rsc.org/supp-
data/cc/b4/b409113b/ for crystallographic data in .cif or other electronic
format.
˚
(A) and angles (u): Zr(1)–N(1) 2.319(6), Zr(1)–N(2) 2.233(5), Zr(1)–N(3)
2.244(6), Zr(1)–N(4) 2.259(5), Zr(1)–C(38) 2.276(7), Zr(1)–C(31) 2.278(7),
N(1)–C(3) 1.305(8), N(2)–C(3) 1.321(9), N(3)–C(18) 1.319(8), N(4)–C(18)
1.316(9), N(3)–Zr(1)–N(4) 59.6(2), C(38)–Zr(1)–C(31) 94.2(3), N(2)–Zr(1)–
N(1) 59.3(2).
§ The molecular structure of racemic [Ru(g-p-iPrMeC6H4)L3Cl] (see ESI{)
provides evidence of this type of steric compression. We suggest that the
low synthetic yield of this compound (17%) is a direct result of this
unfavourable interaction.
[Ru(g-arene)L1Cl] [Scheme 1(b)] and related chiral ligand com-
plexes is kinetically feasible on the chemical timescale, but the
thermodynamic diastereoselection for the observed (SRu,SC)
isomers is very high. Examination of molecular models of the
unobserved (RRu,SC) diastereomers reveals a strong steric inter-
action between the oxazolinyl 4-alkyl substituent and the Ru–Cl
unit [Scheme 1(b)].§}
} In Brunner et al.15 and Davies et al.16 related ruthenium salicyloxazoline
complexes the opposite situation prevailed, i.e. the major (or exclusive)
product was in most cases the equivalent of isomer B [Scheme 1(b)].
Examination of molecular structures for both series indicates that this
disparity arises from the variance in chelate ring size. For the 6-membered
chelate salicyloxazolines, avoidance of steric compression between the alkyl
group and the g-arene dominates. For the present system with 4-membered
chelates, this potential steric interaction with the g-arene is minimised, but
that with the Ru–Cl unit depicted in Scheme 1(b) is exacerbated. See also
ref. 17.
We were interested to investigate the synthesis and stereochemi-
cal behaviour of aminooxazolinate complexes of earlier transition
metals. Direct reactions of two equivalents of HLn (n | 3) with
t
[Zr(CH2R)4] (R ~ Ph, Bu) gave a range of chiral zirconium
complexes [ZrLn (CH2R)2] in ca. 90% yield. Good control over
2
metal/ligand stoichiometry is evident in these reactions, in contrast
to the related (achiral) aminopyridinates.10
1 F. T. Edelmann, Coord. Chem. Rev., 1994, 137, 403; J. Barker and
M. Kilner, Coord. Chem. Rev., 1994, 133, 219.
2 E. A. C. Brussee, A. Meetsma, B. Hessen and J. H. Teuben, Chem.
Commun., 2000, 497.
3 V. Volkis, M. Schmulinson, C. Averbuj, A. Livovskii, F. T. Edelmann
and M. S. Eisen, Organometallics, 1998, 17, 3155; C. Averbuj,
E. Tish and M. S. Eisen, J. Am. Chem. Soc., 1998, 120, 8640;
D. Herskovics-Korine and M. S. Eisen, J. Organomet. Chem., 1998,
503, 307.
4 L. R. Sita, R. J. Keaton, K. C. Jayaratne, D. A. Henningsen and
L. A. Koterwas, J. Am. Chem. Soc., 2001, 123, 6197; L. R. Sita
and K. C. Jayaratne, J. Am. Chem. Soc., 2000, 122, 958; L. R. Sita and
Y. Zhang, Chem. Commun., 2003, 2358.
5 L. R. Sita, R. J. Keaton and Y. Zhang, J. Am. Chem. Soc., 2003, 125,
8746.
6 H. Nagashima, M. Gondo, S. Masuda, H. Kondo, Y. Yamaguchi and
K. Matsubara, Chem. Commun., 2003, 442.
7 (a) H. Brunner, Eur. J. Inorg. Chem., 2001, 905; (b) C. Ganter, Chem.
Soc. Rev., 2003, 32, 130; (c) J. W. Faller and A. R. Lavoie, New J. Chem.,
2003, 27, 899.
The molecular structure of [ZrL4 (CH2Ph)2] determined by
2
X-ray diffraction is shown in Fig. 2. The overall structure is closely
related to achiral bis(benzamidinate) complexes of zirconium11,12
and our achiral ligand bis(aminopyridinate) complexes.13 The
zirconium-bound benzyl methylene groups in [ZrL4 (CH2Ph)2]
2
occupy mutually cis positions and the C2-symmetry is apparent
from Fig. 2; the metal is stereogenic with D configuration (vide
infra). The conformation of the benzyl ligands serves to minimise
steric interactions between their phenyl groups and the oxazolinyl
units, which project over the open face of the metal centre. The
chelate units are folded at the N–N vectors in the same sense as in
(SRu,SC)-[Ru(g-C6H6)L1Cl] above, by 26.2 and 22.4u.
Investigation of metal-centred epimerisation of these zirconium
compounds was hampered by the inaccessibility of complexes
1
containing achiral L3, presumably for steric reasons.§ H NMR
spectra of (D,SC)-[ZrL4 (CH2Ph)2] in d8-toluene varied little in
2
appearance from 363 K to the onset of viscosity broadening. Only
one set of signals was evident throughout. While we expect
inversion of chirality-at-metal to be rapid relative to that in the
ruthenium compounds above, we have previously reported chiral-
at-zirconium systems which exhibit slow exchange at ambient
temperature on the NMR chemical shift timescale.14 We thus
turned to computational methods as a probe of the relative stability
of possible diastereomers.
8 For example, the two diastereomers of [CpTi(IV)Me2] are observed in
approximately equal quantities (L. A. Koterwas, J. C. Fettinger and
L. R. Sita, Organometallics, 1999, 18, 4183 ), and diastereoselection in
similar molybdenum cyclopentadienyls is relatively low (M. Draux and
I. Bernal, Inorg. Chim. Acta, 1986, 114, 75 and references therein).
9 T. H. Kim, N. Lee, G.-J. Lee and J. N. Kim, Tetrahedron, 2001, 57, 7137.
10 R. Kempe, Eur. J. Inorg. Chem., 2003, 5, 791.
11 J. R. Hagdahorn and J. Arnold, J. Chem. Soc., Dalton Trans., 1997,
3087.
12 D. Herskovics-Korine and M. S. Eisen, J. Organomet. Chem., 1998, 503,
307.
13 C. Morton, P. O’Shaughnessy and P. Scott, Chem. Commun., 2000,
2099.
14 I. J. Munslow, A. J. Clarke, R. J. Deeth, I. Westmoreland and P. Scott,
Chem. Commun., 2002, 1868.
15 H. Brunner, B. Nuber and M. Prommesberger, Tetrahedron: Asym-
metry, 1998, 9, 3223.
16 A. J. Davenport, D. L. Davies, J. Fawcett and D. R. Russell, Dalton
Trans., 2004, 1481.
[ZrL4 (CH2Ph)2] has eight possible six-coordinate SC-diastere-
2
omers. DFT calculations{ on the D-cis,trans,cis isomer led to an
optimised structure which was indistinguishable from that observed
in the solid state (Fig. 2). The remaining seven isomers were found
to be 45–111 kJ mol21 less stable; only in the observed isomer are
the oxazolinyl alkyl substituents able to point into regions of steric
space. In other words, the same effects dominate the stereochemical
preference here as in the ruthenium compounds above.
Hence in complexes of aminooxazolinate with zirconium
and ruthenium the chirality of the ligand is expressed very
efficiently in the structure of the complex. The thermodynamic
17 H. Brunner, R. Oeschey and B. Nuber, Organometallics, 1996, 15, 3616;
H. Brunner, R. Oeschey and B. Nuber, Dalton Trans., 1996, 1499.
C h e m . C o m m u n . , 2 0 0 4 , 2 5 9 6 – 2 5 9 7
2 5 9 7