Organometallics in water: Three-coordinate [Pt0(N,N-chelate)(η2-olefin)] complexes containing new chiral ligands based on α-D-mannose

Article abstract of DOI:10.1021/om980350i

Summary: New bidentate nitrogen ligands based on α-D-mannose were prepared and investigated by preparing a family of platinum(0) complexes of formula [Pt(N,N-chelate)(η²-olefin)]. The ability of one N,N-chelate to induce a stereoselective reaction in water was assessed.

Full text of DOI:10.1021/om980350i

3832
Organometallics 1998, 17, 3832-3834
Or ga n om eta llics in Wa ter : Th r ee-Coor d in a te
[P t0(N,N-ch ela te)(η2-olefin )] Com p lexes Con ta in in g New
Ch ir a l Liga n d s Ba sed on r-D-Ma n n ose
Maria L. Ferrara, Ida Orabona, and Francesco Ruffo*
Dipartimento di Chimica, Universita` di Napoli “Federico II”,
via Mezzocannone 4, I-80134 Napoli, Italy
Maria Funicello
Dipartimento di Chimica, Universita` della Basilicata,
via Nazario Sauro 85, I-85100 Potenza, Italy
Achille Panunzi
Facolta` di Agraria and Dipartimento di Chimica, Universita` di Napoli “Federico II”,
via Mezzocannone 4, I-80134 Napoli, Italy
Received May 5, 1998
Summary: New bidentate nitrogen ligands based on R-D-
mannose were prepared and investigated by preparing
a family of platinum(0) complexes of formula [Pt(N,N-
chelate)(η2-olefin)]. The ability of one N,N-chelate to
induce a stereoselective reaction in water was assessed.
mannoside as starting material, a known procedure
allowed the synthesis of azide 3.3 Addition of PMe2Ph
to 3 gave the iminophosphorane 44,5 (path i), which in
turn was converted into N,N-diimines 16 and 2 by
condensation with 6-methyl-2-pyridinecarboxaldehyde
and glyoxal, respectively (ii and iii). Both 1 and 2 are
readily soluble in organic solvents. They were coordi-
nated to {Pt0(η2-olefin)} fragments by reaction with [Pt-
(η2-norbornene)3]7 in the presence of diphenyl or di-
methyl fumarate (iv).8 The corresponding complexes
[Pt(1)(η2-(E)-R′′O2CCHdCHCO2R′′)] and [Pt(2)(η2-(E)-
R′′O2CCHdCHCO2R′′)] could be isolated in high yield
as orange-red microcrystalline solids. The analogous
The design of environmentally “friendly” processes
plays a prominent role in contemporary chemistry. A
plausible approach to the problem involves the exten-
sion to aqueous media of the processes now performed
in organic solvents.1 With reactions promoted by met-
als, the ancillary ligands should (i) confer solubility in
water, (ii) be economically affordable, and (iii) possibly
display chirality. On this basis, we undertook the
synthesis of new N,N-bidentate ligands possessing the
aforementioned requirements. With the assumption
that natural products are often readily accessible as well
as generally chiral, we decided to examine derivatives
of the easily available carbohydrate R-D-mannose.2 An
attractive feature of this type of ligand is the possibility
of either lipophilic or hydrophilic behavior, depending
on whether the alcoholic functions in the sugar residues
are protected.
In this report we describe preliminary results dealing
with the synthesis of new ligands and their use in
preparing a series of organometallic platinum(0) com-
plexes of formula [Pt(N,N-chelate)(η2-olefin)].
The chemistry developed in this work is depicted in
Scheme 1, and a list of the new complexes is provided
in Table 1. Our effort was initially directed toward the
synthesis of protected ligands 1 and 2, which are
expected to display a fair solubility in common organic
solvents. With the commercial precursor methyl-R-D-
(3) Horton, D.; Luetzow, A. E. Carbohydr. Res. 1968, 7, 101.
(4) Compounds similar to 4 are known. For a recent example, see:
Garc`ıa Ferna`ndez, J . M.; Mellet Ortiz, C.; D`ıaz Pe`rez, V. M.; Fuentes,
J .; Kova`cs, J .; Pinte`r, I. Tetrahedron Lett. 1997, 38, 4161.
(5) Synthesis of 4: to a stirred solution of PPhMe2 (0.14 g, 1.0 mmol)
in 5 mL of dry dichloromethane kept in an ice bath was added dropwise
a solution of 3 (0.42 g, 1.0 mmol) in 5 mL of dry dichloromethane. After
the addition was complete, the ice bath was removed and formation of
nitrogen was observed. After 10 h of stirring removal of the solvent
under vacuum afforded the product as
a white glassy solid in
quantitative yield. Selected 1H NMR resonances [in CDCl3, CHCl3 (δ
7.26) as internal standard, at 250 MHz, δ]: 5.48 (m, 2H), 5.27 (m, 1H),
4.70 (d, 1H), 3.95 (m, 1H), 3.41 (s, 3H, OMe), 3.3-3.1 (m, 2H), 2.13 (s,
3H, MeCO2), 1.85 (s, 3H, MeCO2), 1.60 [d, 3H, PPh(Me)Me′], 1.56 [d,
3H, PPh(Me)Me′]. Selected 13C NMR resonances [in CDCl3, CDCl3 (δ
77) as internal standard, at 62.9 MHz, δ]: 98.3 (1C, C1 of mannoside),
73.5, 70.1, 69.7, and 68.8 (4C, C2-C5 of mannoside), 55.0 (1C, OMe),
46.7 (1C, NCH2), 20.9 (1C, MeCO2), 20.6 (1C, MeCO2), 15.9 [1J P-C
21 Hz, PPh(Me)Me′], 14.8 [1J P-C ) 15 Hz, PPh(Me)Me′].
)
(6) Synthesis of 1: to a solution of 4 (1.0 g, 2.0 mmol) in 5 mL of
dry toluene containing A4 molecular sieves was added a solution of
6-methyl-2-pyridinecarboxaldehyde (0.24 g, 2.0 mmol) in 3 mL of dry
toluene. After 1 h of stirring at 363 K the solvent was removed under
vacuum. The residue was filtered through a column of Florisil (15 ×
1.5 cm) with 1:1 petroleum ether/ethyl acetate to give the product as
a light yellow glassy solid (0.80 g, yield 80%). [R]298 ) +50°. Selected
1H NMR resonances [in CDCl3, CHCl3 (δ 7.26) as internal standard,
at 200 MHz, δ]: 8.36 (s, 1H, CHdN), 5.60 (d, 2H), 5.31 (narrow m,
1H), 4.81 (narrow m, 1H), 4.29 (m, 1H), 3.96 (d, 1H), 3.75 (dd, 1H),
3.35 (s, 3H, OMe), 2.55 (s, 3H, Me-py), 2.15 (s, 3H, MeCO2), 1.89 (s,
3H, MeCO2). Selected 13C NMR resonances [in CDCl3, CDCl3 (δ 77) as
internal standard, at 50.3 MHz, δ]: 164.8 (1C, CHdN), 98.3 (1C, C1
of mannoside), 69.8, 69.7, 69.2, and 68.7 (4C, C2-C5 of mannoside),
61.6 (1C, NCH2), 55.0 (1C, OMe), 24.3 (1C, Me-py), 20.9 (1C, MeCO2),
20.7 (1C, MeCO2). Anal. Calcd for C25H28N2O8: C, 61.98; H, 5.82; N,
5.78. Found: C, 62.25; H, 5.74; N, 5.89.
(1) For recent examples, see: (a) Nait Ajjou, A.; Alper, H. J . Am.
Chem. Soc. 1998, 120, 1466. (b) Lynn, D. M.; Mohr, B.; Grubbs, R. H.
J . Am. Chem. Soc. 1998, 120, 1627.
(2) Ligands based on carbohydrates are known. For recent examples,
see: (a) Tanase, T.; Yasuda, Y.; Onaka, T.; Yano, S. J . Chem. Soc.,
Dalton Trans. 1998, 345 and references therein. (b) Stolmar, M.;
Floriani, C.; Gervasio, G.; Viterbo, D. J . Chem. Soc., Dalton Trans.
1997, 1119. (c) Tschoerner, M.; Trabesinger, G.; Albinati, A.; Pregosin,
P. S. Organometallics 1997, 16, 3447.
(7) Crascall, L. E.; Spencer, J . L. Inorg. Synth. 1990, 28, 126.
S0276-7333(98)00350-1 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 07/31/1998

Communications
Organometallics, Vol. 17, No. 18, 1998 3833
Sch em e 1
Ta ble 1. [P t(N,N-ch ela te)(η2-olefin )] Com p lexes
a n d Th eir Dia ster eom er ic Equ ilibr iu m
Com p osition a
sponding water-soluble complex [Pt(1*)(η2-(E)-MeO2-
CCHdCHCO2Me)] in high yield (vii).10 By using this
compound as a precursor, water-soluble complexes of
other olefins were obtained according to path viii.11
All the compounds were characterized by elemental
analysis and NMR spectroscopy.12 They are stable in
solution for several hours, until the appearance of
signals pertaining to the free olefin suggests the occur-
rence of some decomposition process. In no case were
aldehyde signals observed, which indicates that the
imine bond in the coordinated ligands is stable.
[Pt(1)(η2-(E)-MeO2CCHdCHCO2Me)]b
[Pt(1)(η2-(E)-NCCHdCHCN)]b
[Pt(1)(η2-C4H2O3)]b
1.3:1
1.2:1
1.3:1
1:2.7
6:1
[Pt(2)(η2-(E)-PhO2CCHdCHCO2Ph)]b
[Pt(2)(η2-(E)-MeO2CCHdCHCO2Me)]b
[Pt(2)(η2-(E)-NCCHdCHCN)]b
[Pt(2)(η2-C4H2O3)]b
1:2.5
c
[Pt(1*)(η2-(E)-MeO2CCHdCHCO2Me)]d
[Pt(1*)(η2-(E)-NCCHdCHCN)]d,e
[Pt(1*)(η2-C4H2O3)]d
1.3:1
1.2:1
1:2.5
The number of possible isomers for each platinum
complex is dictated by the symmetry of both ligands.
More precisely, prochiral olefins (fumarodinitrile and
a
At 298 K. The first figure refers to the first-formed isomer,
quantitatively obtained through a second-order transformation.
b In CDCl3. c Formation of only one isomer is theoretically possible.
In D2O. e A freshly dissolved sample already disclosed significant
d
amounts of both diastereomers.
(8) Synthesis of [Pt(1)(η2-(E)-MeO2CCHdCHCO2Me)]: to a stirred
solution of the olefin (0.14 g, 1.0 mmol) in dry diethyl ether (10 mL)
was added solid [Pt(η2-norbornene)3] (0.48 g, 1.0 mmol). The resulting
solution was added to a suspension of the N,N-ligand (0.50 g, 1.0 mmol)
in 3-4 mL of dry diethyl ether. After 24 h of stirring the orange-red
crystals of the product were separated, washed with diethyl ether (3
× 4 mL), and dried under vacuum (yield >75%). Selected 1H NMR
resonances [in CDCl3, CHCl3 (δ 7.26) as internal standard, at 250 MHz,
derivatives of fumarodinitrile ((E)-NCCHdCHCN) or
maleic anhydride (C4H2O3) were obtained by displacing
the fumaric ester according to path v. The complexes
were found to be readily soluble in chlorinated solvents
but insoluble in water.
δ]: major isomer 9.03 (3J Pt-H ) 53 Hz, s, 1H, CHdN), 3.72 (2J Pt-H
)
As expected, the hydrophilicity of the chelate was
strongly enhanced by removing the alcoholic functions
on the sugar residue. When 1 was treated in methanol
containing a catalytic amount of sodium methoxide, the
corresponding water-soluble ligand 1* formed (vi). This
ligand was always accompanied by another species,
which made difficult the isolation of 1* in pure form.9
For this reason, hydrolytic treatment was more
conveniently performed on [Pt(1)(η2-(E)-MeO2C-
CHdCHCO2Me)], which selectively afforded the corre-
90 Hz, s, 2H, HCdCH), 3.79, 3.57, and 3.35 (s, 9H, OMe), 2.92 (s, 3H,
Me-py), 2.18 and 1.89 (s, 6H, MeCO2); minor isomer 9.09 (3J Pt-H ) 55
Hz, s, 1H, CHdN), 3.68 (AB q, 2H, HCdCH), 3.55, 3.46, and 3.12 (s,
9H, OMe), 2.95 (s, 3H, Me-py), 2.16 and 1.88 (s, 6H, MeCO2). Selected
13C NMR resonances [in CDCl3, CDCl3 (δ 77) as internal standard, at
50.3 MHz, δ]: major isomer 167.3 (1C, CHdN), 25.1 (1J Pt-C ) 415 Hz,
1C, HCdCH), 24.3 (1J Pt-C ) 430 Hz, 1C, HCdCH); minor isomer 163.0
(1C, CHdN), 25.2 (1J Pt-C ) 411 Hz, 1C, HCdCH), 24.5 (1J Pt-C ) 414
Hz, 1C, HCdCH). Anal. Calcd for C31H36N2O12Pt: C, 45.20; H, 4.40;
N, 3.40. Found: C, 45.18; H, 4.55; N, 3.26.
(9) The NMR spectrum of the crude product displayed a small
amount of aldehyde, while preliminary attempts to purify 1* resulted
in extensive hydrolysis of the imine bond.

3834 Organometallics, Vol. 17, No. 18, 1998
Communications
fumaric esters) can afford two diastereomers, depending
on whether the alkene coordinates re or si. In all cases,
the NMR spectra of freshly dissolved samples suggested
that the complexes crystallized in pure diastereomeric
form13 due to second-order asymmetric transforma-
tion.14 On standing in solution a slow epimerization
process led to an equilibrium mixture within a few hours
(see Table 1). According to the diastereomeric ratios
at equilibrium, ligand 2 was more effective than 1 or
1* in inducing stereoselective coordination of prochiral
olefins. The equilibrium ratio was always higher than
2.5 in the presence of 2, while in the case of 1 or 1* the
value dropped to ca. 1.3:1. The most pronounced case
was [Pt(2)(η2-(E)-MeO2CCHdCHCO2Me)], which con-
sisted of diastereomers in a 6:1 ratio. This value is
slightly higher than that recently measured for a series
of trans-stilbene complexes of chiral diphosphines
[Pt(P,P-chelate)(η2-(E)-PhCHdCHPh)].15
that olefin Pt(0) complexes undergo electrophilic attack
by protic acids,16 we added HBF4 to the diastereomeric
equilibrium mixture of [Pt(1*)(η2-(E)-MeO2CCHdCHCO2-
Me)] in D2O (path ix). A transient product was observed
in the early stages of the reaction. The presence of one
methoxy resonance at δ 4.20 suggests formation of the
cyclometalated product 5,17 similar to what is found in
related insertion processes.16 Subsequent disappear-
ance of the signal at δ 4.20 and stoichiometric concomi-
tant formation of methanol are compatible with the
hydrolysis of one carboxymethyl group,18 possibly acti-
vated by its coordination to Pt. After 24 h formation of
6 was quantitative (x).19 According to the NMR spectra
formation of the new stereogenic carbon (indicated with
an asterisk) occurred with significant stereoselectivity
(ee > 70%).20
Maleic anhydride can give rise to only one type 2
derivative, while diastereomers are theoretically pos-
sible for the complexes of 1 and 1*. The corresponding
equilibrium ratios are reported in Table 1.
The ability of 1* to induce a stereoselective reaction
in water was preliminarily investigated. As it is known
Ack n ow led gm en t. We thank the Centro Interdi-
partimentale Ricerche: Ambiente (CIRAM) of the Uni-
versita` di Napoli “Federico II”, the Consiglio Nazionale
delle Ricerche, and the MURST for financial support
and the Centro Interdipartimentale di Metodologie
Chimico-fisiche, Universita` di Napoli “Federico II”, for
NMR facilities.
(10) Synthesis of [Pt(1*)(η2-(E)-MeO2CCHdCHCO2Me)]: to a stirred
suspension of [Pt(1)(η2-(E)-MeO2CCHdCHCO2Me)] (0.50 g, 0.60 mmol)
in 5 mL of dry methanol was added a catalytic amount of sodium
methoxide in methanol. After 30 min formation of a solution was
observed, followed by precipitation of the yellow product. After a further
16 h of stirring the complex was separated, washed with cold methanol
(3 × 3 mL), and dried under vacuum (0.30 g, yield: 79%). Selected 1H
NMR resonances [in D2O, DHO (δ 4.8) as internal standard, at 250
MHz, δ]: major isomer 9.40 (3J Pt-H ) 60 Hz, s, 1H, CHdN), 3.75, 3.69,
and 3.47 (s, 9H, OMe), 2.91 (s, 3H, Me-py); minor isomer 9.35 (3J Pt-H
) 60 Hz, s, 1H, CHdN), 3.78, 3.71, and 3.28 (9H, s, OMe), 2.95 (s, 3H,
Me-py). Selected 13C NMR resonances [in CDCl3, CDCl3 (δ 77) as
internal standard, at 50.3 MHz, δ]: major isomer 166.5 (1C, CHdN),
26.6 (1C, HCdCH), 23.4 (1J Pt-C ) 416 Hz, 1C, HCdCH); minor isomer
166.6 (1C, CHdN), 25.5 (1J Pt-C ) 405 Hz, 1C, HCdCH), 24.3 (1J Pt-C
) 414 Hz, 1C, HCdCH). Anal. Calcd for C20H28N2O9Pt: C, 37.80; H,
4.44; N, 4.41. Found: C, 37.93; H, 4.57; N, 4.27.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving experimental details, NMR data, and elemental analy-
ses of the complexes (7 pages). Ordering information is given
on any current masthead page.
OM980350I
(16) De Felice, V.; De Renzi, A.; Ruffo, F.; Tesauro, D. Inorg. Chim.
Acta 1994, 219, 169.
3
(17) The magnitude (90 Hz) of the J Pt-H coupling constant of the
imine proton in 5 points to the imine nitrogen atom being trans to
oxygen. See: Albano, V. G.; Braga, D.; De Felice, V.; Panunzi, A.;
Vitagliano, A. Organometallics 1987, 6, 517.
(18) This result was confirmed by performing the addition of HBF4
to [Pt(1*)((E)-CD3O2CHdCHCO2CD3)].
(19) Product 5 could be isolated by carrying out the reaction in dry
toluene. Dissolution of this complex in D2O resulted in hydrolysis of
one carboxymethyl group and formation of 6 within 12 h.
(20) The reaction was accompanied by the occurrence of the equi-
librium C1(R) a C1(â), involving the anomeric carbon atom of the
carbohydrate residue. The isomerization, which is in keeping with the
presence of an acidic reagent, may represent a drawback for the use
of 1* or other carbohydrate-based ligands in processes promoted by
metals, since the stereochemical stability of the ancillary ligands is
an important requirement. Nevertheless, the enantiomeric excess
observed in this preliminary test is encouraging. Furthermore, it
should be noted that the equilibrium C1(R) a C1(â) takes place only in
acidic solutions, while in neutral or alkaline media the anomeric carbon
atoms are known to be configurationally stable.
(11) Water-soluble platinum(0) complexes are rare. For some recent
examples, see: (a) Darensbourg, D. J .; Decuir, T. J .; Stafford, N. W.;
Robertson, J . B.; Draper, J . D.; Reibenspies, J . H.; Katho`, A.; J oo`, F.
Inorg. Chem. 1997, 36, 4218. (b) Ellis, J . W.; Harrison, K. N.; Hoye, P.
A. T.; Orpen, A. G.; Pringle, P. G.; Smith, M. B. Inorg. Chem. 1992,
31, 3026.
(12) Elemental analyses and 1H and 13C NMR spectra for the new
complexes are reported in Tables S1-3 of the Supporting Information.
(13) The only exception was represented by [Pt(1*)(η2-(E)-NCCHd
CHCN)]. Actually, the NMR spectrum of a fresh solution of the complex
already disclosed significant amounts of both diastereomers.
(14) Eliel, E. L. Stereochemistry of Carbon Compounds; McGraw-
Hill: London, 1962; p 49.
(15) Wicht, D. K.; Zhuravel, M. A.; Gregush, R. V.; Glueck, D. S.;
Guzei, I. A.; Liable-Sands, L. M.; Rheingold, A. L. Organometallics
1998, 17, 1412.