A direct route to platinum NCN-pincer complexes derived from 1,3-bis(imino)benzenes and an investigation into their activity as catalysts for carbon-carbon bond formation

Article abstract of DOI:10.1021/om020566r

1,3-Bis(imino)lbenzenes [1,3-C₆H₄(CH=NR)₂], obtained from condensation of 1,3-isophthalaldehyde with primary amines (R = ^tBu, Cy, Bu, Bn, Ph), were heated with K₂PtCl₄ at reflux for 48 h in glacial acetic acid to give ((2,6-bis(N-R-substituted)imino)phenyl)platinum-(II) chloride complexes (15-52% yield). X-ray crystal structures of the R = ^tBu and R = Ph complexes are reported. The byproducts were found to be 1,3-isophthalaldehyde and N-acetylamines. Quantitative chloride abstraction with AgBF₄ (R = ^tBu) or AgOTf (R = ^tBu, Cy) provided the corresponding cationic complexes containing water coordinated to platinum, as established by an X-ray crystal structure of ((2,6-bis(N-tert-butyl)imino)phenyl)aquoplatinum(II) trifluoromethanesulfonate. Use of 4-5 mol % of ((2,6-bisu(N-cyclohexyl)imino)phenyl)aquoplatinum(II) trifluoromethanesulfonate accelerated the rate of the Michael reaction between ethyl α-cyanoacetate and methyl vinyl ketone and the Diels-Alder reaction between acrylonitrile and cyclopentadiene.

Full text of DOI:10.1021/om020566r

Organometallics 2002, 21, 5259-5264
5259
A Dir ect Rou te to P la tin u m NCN-P in cer Com p lexes
Der ived fr om 1,3-Bis(im in o)ben zen es a n d a n
In vestiga tion in to Th eir Activity a s Ca ta lysts for
Ca r bon -Ca r bon Bon d F or m a tion
J ohn S. Fossey and Christopher J . Richards*
Department of Chemistry, Queen Mary, University of London, Mile End Road,
London, E1 4NS, U.K.
Received J uly 16, 2002
1,3-Bis(imino)lbenzenes [1,3-C6H4(CHdNR)2], obtained from condensation of 1,3-isoph-
thalaldehyde with primary amines (R ) tBu, Cy, Bu, Bn, Ph), were heated with K2PtCl4 at
reflux for 48 h in glacial acetic acid to give ((2,6-bis(N-R-substituted)imino)phenyl)platinum-
(II) chloride complexes (15-52% yield). X-ray crystal structures of the R ) tBu and R ) Ph
complexes are reported. The byproducts were found to be 1,3-isophthalaldehyde and
t
N-acetylamines. Quantitative chloride abstraction with AgBF4 (R ) Bu) or AgOTf (R )
tBu, Cy) provided the corresponding cationic complexes containing water coordinated to
platinum, as established by an X-ray crystal structure of ((2,6-bis(N-tert-butyl)imino)phenyl)-
aquoplatinum(II) trifluoromethanesulfonate. Use of 4-5 mol % of ((2,6-bis(N-cyclohexyl)-
imino)phenyl)aquoplatinum(II) trifluoromethanesulfonate accelerated the rate of the Michael
reaction between ethyl R-cyanoacetate and methyl vinyl ketone and the Diels-Alder reaction
between acrylonitrile and cyclopentadiene.
In tr od u ction
Platinum group metal based pincer complexes of
general structure 1 are currently the subject of much
attention due to their application as catalysts and their
incorporation into novel organometallic materials.1
A
major subcategory of 1 is terdentate NCN complexes of
group 10 metals incorporating amino 2,2 oxazolinyl 3,3
and pyridinyl 44 trans-chelating functionalities. In the
(1) For recent reviews see: (a) Steenwinkel, P.; Gossage, R. A.; van
Koten, G. Chem. Eur. J . 1998, 4, 759 (b) Albrecht, M.; van Koten, G.
Angew. Chem., Int. Ed. 2001, 40, 3750.
(2) van Koten G.; Timmer, K.; Noltes, J . G.; Spek, A. L. J . Chem.
Soc., Chem. Commun. 1978, 250. (b) Grove, D. M.; van Koten, G.;
Ubbels, H. J . C.; Zoet, R.; Spek, A. L. Organometallics 1984, 3, 1003.
(c) Grove, D. M.; van Koten, G.; Verschuuren, A. H. M. J . Mol. Catal.
1988, 45, 169. (d) van de Kuil, L. A.; Veldhuizen, Y. S. J .; Grove, D.
M.; Zwikker, J . W.; J enneskens, L. W.; Drenth, W.; Smeets, W. J . J .;
Spek, A. L.; van Koten, G. Recl. Trav. Chim. Pays-Bas 1994, 113, 267.
(e) Gossage, R. A.; van de Kuil, L. A.; van Koten, G. Acc. Chem. Res.
1998, 31, 423. (f) Steenwinkel, P.; Gossage, R. A.; Maunula, T.; Grove,
D. M.; van Koten, G. Chem. Eur. J . 1998, 4, 763. (g) Schlenk, C.; Kleij,
A. W.; Frey, H, van Koten, G. Angew, Chem., Int. Ed. 2000, 39, 3445.
(h) Albrecht, M.; Kocks, B. M.; Spek, A. L.; van Koten, G. J . Organomet.
Chem. 2001, 624, 271. (i) Tsubomura, T.; Tanihata, T.; Yamakawa,
T.; Ohmi, R.; Tamane, T.; Higuchi, A.; Katoh, A.; Sakai, K. Organo-
metallics 2001, 20, 3833. (j) Rodgr´ıguez, G.; Lutz, M.; Spek, A. L.; van
Koten, G. Chem. Eur. J . 2002, 8, 46. (k) Guillena, G.; Rodr´ıguez, G.,
van Koten. G. Tetrahedron Lett. 2002, 43, 3895. (l) Rodgr´ıguez, G.;
Albrecht, M.; Schoenmaker, J .; Ford, A.; Lutz, M.; Spek, A. L.; van
Koten, G. J . Am. Chem. Soc. 2002, 124, 5127.
synthesis of these complexes, the metals are introduced
by either (i) transmetalation of M(II) (M ) Ni, Pd, Pt)
with a 2-metallo-1,3-disubstituted benzene2a,3b-e (ii)
oxidative addition of M(0) (M ) Ni, Pd, Pt) with a 2-halo-
1,3-disubstituted benzene,2b,i,j,3a or (iii) directing group
mediated cyclometalation of M(II) (M ) Pd) with a
2-trimethylsilyl-1,3-disubstituted benzene.2f In general,
the direct cyclometalation of 1,3-disubstituted benzenes
does not occur at C-2 to give two fused five-membered
rings; instead 4,6-bismetalated complexes are obtained
preferentially.5 A notable exception is 4, obtained by
platination of 1,3-bis(2-pyridyl)benzene; palladation of
this same substrate proceeded by the alternative 4,6-
disubstitution pathway.4
(3) Denmark, S. E.; Stavenger, R. A.; Faucher, A.-M.; Edwards, J .
P. J . Org. Chem. 1997, 62, 3375. (b) Motoyama, Y.; Makihara, N.;
Mikami, Y.; Aoki, K.; Nishiyama, H. Chem. Lett. 1997, 951. (c) Stark,
M. A.; Richards, C. J . Tetrahedron Lett. 1997, 38, 5881. (d) Motoyama,
Y.; Mikami, Y.; Kawakami, H.; Aoki, K.; Nishiyama, H. Organome-
tallics 1999, 18, 3584. (e) Stark, M. A.; J ones, G.; Richards, C. J .
Organometallics 2000, 19, 1282.
Prompted by this latter result, we sought to investi-
gate the platination of simple 1,3-bis(imino)benzenes.
(4) Ca´rdenas, D. J .; Echavarren, A. M.; de Arellano, M. C. R.
Organometallics 1999, 18, 3337. In contrast, palladation to give two
fused six-membered rings occurs readily: (b) Canty, A. J .; Minchin,
N. J .; Skelton, B. W.; White, A. H. J . Chem. Soc., Dalton Trans. 1987,
1477. (c) Hartshorn, C. M.; Steel, P. J . Organometallics 1998, 17, 3487.
(d) Canty, A. J .; Patel, J .; Skelton, B. W.; White, A. H. J . Organomet.
Chem. 2000, 607, 194.
(5) Trofimenko, S. J . Am. Chem. Soc. 1971, 93, 1808. (b) Trofimenko,
S. Inorg. Chem. 1973, 12, 1215. (c) Chakladar, S.; Paul, P.; Nag, K.
Polyhedron 1991, 10, 1513. (d) Chakladar, S.; Paul, P.; Venkatsubra-
manian, K.; Nag, K. J . Chem. Soc., Dalton Trans. 1991, 2669. (e)
Meijer, M. D.; Kleij, A. W.; Lutz, M.; Spek, A. L.; van Koten, G. J .
Organomet. Chem. 2001, 640, 166.
10.1021/om020566r CCC: $22.00 © 2002 American Chemical Society
Publication on Web 11/01/2002

5260 Organometallics, Vol. 21, No. 24, 2002
Fossey and Richards
Sch em e 1
F igu r e 1. Representation of the crystal structure of 7a .
Selected bond distances (Å) and angles (deg): Pt(1)-C(11)
) 1.915(6), Pt(1)-Cl(2) ) 2.4192(19), Pt(1)-N(3) ) 2.093-
(5), Pt(1)-N(4) ) 2.097(5), N(3)-C(12) ) 1.277(8), N(4)-
C(5) ) 1.285(8), C(11)-Pt(1)-N(3) ) 78.4(2), C(11)-Pt(1)-
N(4) ) 78.9(2), N(3)-Pt(1)-N(4) ) 157.2(2), C(11)-Pt(1)-
Cl(2) ) 175.89(17).
Reaction of these substrates with palladium acetate had
previously been reported to give 4,6-bispalladation,5c-d
with a later paper describing predominantly monopal-
ladation of position 4 plus formation of small amounts
of an inseparable pincer complex.6 At the onset of this
program no general routes to platinum group pincer
complexes incorporating Schiff base derived anionic
terdentate NCN complexes were known. Recently the
generation of such systems through the oxidative ad-
dition of Pt(0) into 2-bromo-1,3-bis(imino)benzenes was
reported.7 In this paper we describe the synthesis of a
range of such complexes obtained by direct platination.
Activation of the resulting complexes by halide abstrac-
tion and a study into the use of the resulting cationic
species as Lewis acid catalysts are also described.
Resu lts a n d Discu ssion
Starting from commercially available 1,3-isophthal-
dehyde 5 and a series of primary amines, the corre-
sponding 1,3-bis(imino)benzenes 6a -e were synthesized
without difficulty (Scheme 1). After addition to a
suspension of potassium tetrachloroplatinate in glacial
acetic acid, the resulting reaction mixtures were heated
at reflux for 48 h to give yellow-orange solutions
containing a black platinum deposit. These conditions
were required to ensure the consumption of potassium
tetrachloroplatinate; shorter reaction times or lower
temperatures resulted in reduced yields. Removal of the
solvent, column chromatography, and recrystallization
from CH2Cl2 provided new NCN Schiff base derived
complexes as either yellow-orange (7a -d ) or red (7e)
air-stable crystalline solids. Their identity was initially
established by the absence the aryl singlet in the 1H
NMR spectrum, and occupancy of this 1-position by
platinum was revealed by coupling of the 195Pt isotope
to the protons at positions 3 and 5 (4J ) 6.6-8.2 Hz).
Coordination of nitrogen to platinum was indicated by
coupling to the iminyl protons (3J ) 141-152 Hz). Only
a single platinacycle was isolated from each reaction
mixture; the formation of alternative systems through
metalation of position 4 or positions 4 and 6 was not
observed. Repetition of the reaction with 6a and exami-
nation of the resultant product mixture by 1H NMR
F igu r e 2. Representation of the crystal structure of 7e.
Selected bond distances (Å) and angles (deg): Pt(1)-C(11)
) 1.908(6), Pt(1)-Cl(2) ) 2.3914(18), Pt(1)-N(3) ) 2.064-
(5), Pt(1)-N(4) ) 2.061(5), N(3)-C(12) ) 1.307, N(4)-C(5)
) 1.298(8), C(11)-Pt(1)-N(3) ) 79.1(2), C(11)-Pt(1)-N(4)
) 79.2(2), N(3)-Pt(1)-N(4) ) 157.9(2), C(11)-Pt(1)-Cl-
(2) ) 174.93(19).
spectroscopy revealed 7a (39%) and a 1:2 ratio of 1,3-
isophthaldehyde and N-acetyl-tert-butylamine (both
56%). Thus the yield of the platinacycle is limited by
competitive reaction of the imine functionalities with
the solvent. Attempts to avoid this by use of toluene,
acetonitrile, or methanol as reaction solvents did not
lead to any product formation.
Slow evaporation of separate solutions of 7a and 7e
in CH2Cl2 gave crystals suitable for X-ray analysis,
which confirmed the structures as NCN-pincer com-
plexes (Figures 1 and 2). The structure of 7a reveals
significantly longer Pt-N bonds (2.093(5) and 2.097(5)
Å) than those found in related platinum NCN-pincer
i
complexes 3 (R ) Pr, X ) Cl, 2.032(10) and 2.035(10)
Å3d and 4 (2.033(6) and 2.041(6) Å).4a The corresponding
bond lengths are shorter in 7e (2.061(5) and 2.064(5)
Å). The distortion from the ideal square-planar geom-
etry caused by the two fused five-membered rings is
most apparent from the N(3)-Pt-N(4) bond angles of
157.2° (7a ) and 157.9° (7e).
(6) Vila, J . M.; Gayoso, M.; Pereira, T.; Torres, M. L.; Ferna´ndez, J .
J .; Ferna´ndez, A.; Ortigueira, J . M. J . Organomet. Chem. 1996, 506,
165.
(7) Hoogervorst, W. J .; Elsevier, C. J .; Lutz, M.; Spek, A. L.
Organometallics 2001, 20, 4437.
Although the yields arising from the platination step
are moderate, the simplicity of the overall two-step

Platinum NCN-Pincer Complexes
Organometallics, Vol. 21, No. 24, 2002 5261
Sch em e 2
procedure makes this an attractive and direct route to
new platinum NCN-pincer complexes. Existing routes
based on oxidative addition or transmetalation require
the relatively nontrivial synthesis of functionalized
1,2,3-trisubstituted benzenes. Furthermore, diversity of
structure is readily introduced through variation of the
starting amine. We have demonstrated here that pri-
mary amines containing adjacent tertiary, secondary,
primary, benzylic, and aromatic groups may all be
accommodated in this new procedure.
F igu r e 3. Representation of the crystal structure of 9.
Selected bond distances (Å) and angles (deg): Pt(1)-C(11)
) 1.889(5), Pt(1)-O(2) ) 2.188(4), Pt(1)-N(3) ) 2.089(5),
Pt(1)-N(4) ) 2.083(4), N(3)-C(12) ) 1.298(7), N(4)-C(5)
) 1.290(7), C(11)-Pt(1)-N(3) ) 79.9(2), C(11)-Pt(1)-N(4)
) 79.9(2), N(3)-Pt(1)-N(4) ) 159.11(18), C(11)-Pt(1)-
O(2) ) 178.7(2).
Sch em e 3
Nitrogen-based pincer complexes of group 10 metals
have been applied as catalysts for the Kharasch
addition,2c-e the aldol reaction,2g,h cyclopropanation,3a
and the Michael reaction.3c,e,8 With the exception of the
Kharasch addition, which proceeds via a radical mech-
anism catalyzed by nickel complexes of general struc-
ture 2, the other carbon-carbon bond forming reactions
have all utilized cationic palladium complexes obtained
by halide abstraction. Although platinum cationic com-
plexes derived from 3 have been employed as stoichio-
metric stereocontrollers for asymmetric alkylation of
aldimines,3d no examples of catalysis with platinum
NCN complexes have been reported.
confirms the identity of 8 and is further supported by
the number of peaks in the B-F stretching region of
the IR spectrum due to hydrogen bonding with coordi-
nated water.10
For the first use of these complexes in catalysis we
initially sought comparison with related palladium-
based NCN cationic complexes derived from 3 which we
have previously shown to be excellent catalysts for the
Michael reaction with activated nitriles.3c,e Combination
of ethyl cyanoacetate with 3 equiv of methyl vinyl
ketone, together with 4 mol % 10 and 10 mol % Hu¨nig’s
base (Scheme 3), resulted in an almost colorless reaction
mixture in which clean conversion to the double Michael
adduct 12 occurred within 24 h (Figure 4). Without 10
the same reaction conditions gave some uncatalyzed
background reaction in this time period; yet in the
earlier stages the formation of 12 is accelerated ap-
proximately 4-fold by the platinum catalyst. This reac-
tion is significantly slower than those catalyzed by
palladium NCN systems under similar conditions (e.g.,
88% conversion after 3 h with 1 mol % catalyst3c).11 The
lower reactivity of platinum species compared to their
palladium congeners is well known.12 One of the ad-
vantages of this is the potential to observe intermediates
in a catalytic cycle, and in this instance the mono
Michael adduct 11 was detected, a species we had not
Halide abstraction was readily achieved by addition
of either AgBF4 to a solution of 7a in acetone or AgOTf
to solutions of 7a or 7b in dichloromethane (Scheme 2).
No color changes occurred, but after removal of silver
chloride by filtration, the resulting cationic complexes
were isolated as orange or yellow air-stable crystalline
solids. The identities of 8-10 were initially indicated
-
by the presence of the groups BF4 and TfO- in their
respective IR specta, and that for 9 was confirmed by
X-ray analysis of a crystal obtained by slow evaporation
of an acetone solution (Figure 3). Compared to 7a this
displays a shorter platinum-carbon bond (1.889(5) vs
1.915(6) Å) consistent with an increase in the N(3)-
Pt-N(4) bond angle from 157.2° in 7a to 159.11(18)° in
9. Significantly, water was revealed to be occupying the
exchangeable coordination site with triflate as a hydro-
gen-bonded counterion. This differs from the assignment
made to the complex obtained from addition of AgOTf
to a platinum species of general structure 3 (X ) Cl),
the structure of which is drawn with the triflate
coordinated to platinum.3d That the anion BF4 is
-
known to be a weaker metal coordinator than triflate9
(10) Olgemo¨ller, B.; Olgemo¨ller, L.; Beck, W. Chem. Ber. 1981, 114,
2971.
(8) Dijkstra, H. P.; Meijer, M. D.; Patel, J .; Kreiter, R.; van Klink,
G. P. M.; Lutz, M.; Spek, A. L.; Canty, A. J .; van Koten, G. Organo-
metallics 2001, 20, 3159.
(11) Half-lifes of 33-86 min have recently been determined for
amine- and pyrazole-containing cationic palladium NCN complexes,
ref 8.
(9) Strauss, S. H. Chem. Rev. 1993, 93, 927.
(12) Clarke, M. L. Polyhedron 2001, 20, 151.

5262 Organometallics, Vol. 21, No. 24, 2002
Fossey and Richards
F igu r e 6. Diels-Alder reaction to give 15 and 16.
F igu r e 4. Michael reaction to give 11 and 12.
Sch em e 5
related Diels-Alder reaction between 2-chloroacrylo-
nitrile and cyclopentadiene.13 In the early stages of the
reaction use of 5 mol % of 10 again resulted in an
approximately 4-fold increase in the rate of formation
of adducts 15 and 16, the ratio of these two isomers
changing from 1.5:1 in the absence of 10 to 2.1:1 in the
faster reaction (Scheme 5, Figure 6). Under the condi-
tions employed no dimerization of cyclopentadiene was
observed.
F igu r e 5. Aldol reaction to give 13 and 14.
Sch em e 4
Con clu sion
Reaction of 1,3-bis(imino)benzenes with K2PtCl4 in
glacial acetic acid provides direct access to 2-platinum-
substituted NCN-pincer complexes. The success of this
procedure is a further example of a trend for direct
platinations, leading to fused 5,5-metallocycles, proceed-
ing where the corresponding palladations are unsuc-
cessful. Activation of the resulting complexes by chloride
abstraction proceeded smoothly, producing water-
coordinated adducts with either tetrafluoroborate or
triflate counterions. The triflate salt displays moderate
activity as a catalyst for the Michael and Diels-Alder
transformation of nitrile substrates. We are currently
investigating more active platinacycles for application
in these useful C-C bond forming reactions.
observed in the previously reported palladium-catalyzed
reaction. Attempted catalysis with the BF4- salt under
the same conditions resulted in the deposition of a black
precipitate from a green solution, and this reaction was
not investigated further.
The aldol reaction between methyl isocyanoacetate
and benzaldehyde has also been reported to be catalyzed
by NCN palladium cationic pincer complexes.2g,h,k,3c Use
of 5 mol % of 10 and 10 mol % Hu¨nig’s base was found
to give some rate enhancement, although with little
change in the diastereoselectivity between 13 and 14
(ca. 4:1 in both cases), and the rate of uncatalyzed
reaction under the conditions employed is substantial
even at 4 °C (Scheme 4, Figure 5). This result is
consistent with a previous observation that cationic
palladium complexes derived from 3 are also relatively
poor catalysts for this transformation.3c
Exp er im en ta l Section
All organometallic reactions were performed under an
atmosphere of nitrogen employing standard Schlenk tech-
niques. Glacial acetic acid of 99.8% purity was used as
obtained. Dichloromethane was distilled from calcium hydride
under nitrogen. Other solvents employed were not specifically
dried. Petroleum ether refers to that fraction boiling in the
range 40-60 °C. Column chromatography was performed on
SiO2 (40-63 µm). Coupling to 195Pt (xJ Pt) in NMR data is
reported as the 34% component of the signal.
The potential for Lewis-acid activation of other nitrile-
containing substrates was investigated with the Diels-
Alder reaction between acrylonitrile and cyclopentadi-
ene. A cationic platinum phosphinoaryloxazoline complex
has previously been shown to act as a catalyst for the
(13) Blacker, A. J .; Clarke, M. L.; Loft, M. S.; Mahon, M. F.;
Williams, J . M. J . Organometallics 1999, 18, 2867.

Platinum NCN-Pincer Complexes
Organometallics, Vol. 21, No. 24, 2002 5263
5.24. IR (νmax; CH2Cl2): 1595 (CdN) cm-1 1H NMR (δ; 250
Gen er a l Meth od for th e Syn th esis of 1,3-Bis((N-su b-
stitu ted )im in o)p h en ylen es 6a -e Is Exem p lified by th e
Syn th esis of 1,3-Bis((N-ter t-bu tyl)im in o)p h en ylen e 6a .
Isophthaldehyde (2.5 g, 18.6 mmol) and tert-butylamine 6a (4.5
mL, 3.1 g, 42.8 mmol) were refluxed in absolute ethanol (200
mL) for 30 min. The solvent was removed in vacuo, the
residual material taken up in ethyl acetate and rapidly filtered
through a small plug of silica, and the solvent removed in
vacuo to give 6a as a white solid containing 6% isophthalde-
hyde (4.34 g, 92% yield). An analytically pure sample was
obtained by recrystallization from ethyl acetate/petroleum
.
MHz, CDCl3): 1.26 (3 H, m, CH2), 1.48 (3 H, m, CH2), 1.70 (3
H, m, CH2), 1.79 (3 H, m, CH2), 1.87 (4 H, m, CH2), 2.20 (4 H,
m, CH2), 4.16 (2 H, m, NCH(CH2)-), 7.10 (1 H, t, J 7.7, 4-H),
4
7.39 (2 H, (66%) d, J 7.7, (34%) t, J PtH 8.03, 3- and 5-H), 8.32
3
(2 H, (66%) s, (34%) d J PtH 147.4, NdCH). 1H NMR (δ; 250
MHz, acetone-d6): 1.20-1.53 (8H, m, CH2), 1.70 (2 H, app d,
J 11.0, CH2), 1.85 (7 H, m, CH2), 1.98 (3 H, m, CH2), 4.0-4.3
(2 H, m, NCH(CH2)-), 7.13 (1 H, t, J 7.6, 4-H), 7.53 (2 H, (66%)
4
d, J 7.6, (34%) t, J PtH 7.8, 3- and 5- H), 8.65 (2 H, (66%) s,
3
(34%) d J PtH 146.4, NdCH). 13C{1H} NMR (δ; 63 MHz,
ether. Mp: 40 °C. IR (νmax; CH2Cl2): 1641 (NdC) cm-1
.
1H
CDCl3): 25.4 (CH2), 25.4 (CH2), 33.3 (CH2), 67.6 ((34%) d, 2J PtC
3
NMR (δ; 250 MHz, CDCl3): 1.30 (18 H, s, CH3), 7.42 (1 H, t,
J 7.62, 5-H), 7.79 (2 H, dd, J 7.64 and 1.66, 4- & 6-H), 8.08 (1
H, s, 2-H), 8.32 (2 H, s, NdCH). 13C{1H} NMR (δ; 63 MHz,
CDCl3): 29.7 (CH3), 57.4 (C(CH3)3), 127.7 (Ar, 5-C), 128.7 (Ar,
2-C), 129.4 (Ar, 4- and 6-C), 137.5 (Ar, 1- and 3-C). MS (m/z;
FAB): 245 (M+, 100%). High-resolution MS (m/z, FAB): found
for MH+ 245.2030. C16H25N2 requires 245.2018. Similarly
prepared were 6b (>99%), 6c (82%), 6d (>99%), and 6e
(>99%). These materials were used without further purifica-
tion.
17.2, NCH(CH2)-), 122.3 (4-C), 126.3 ((34%) d, J PtC 26.9 3-
2
and 5-C), 142.0 ((34%) d, J PtC 120, 2- and 6-C), 172.4 ((34%)
2
d, J PtC 122.7, NdC), 1-C not observed. MS (m/z; FAB): 490
([M - Cl]+, 100%).
Syn th esis of ((2,6-Bis(N-bu tyl)im in o)p h en yl)p la tin u m -
ch lor id e 7c. 6c (0.25 g, 1.0 mmol) and potassium tetrachlo-
roplatinate (0.437 g, 1.05 mmol) were refluxed under nitrogen
in distilled glacial acetic acid (20 mL) for 48 h. The solvent
was removed in vacuo, and the residue was purified by column
chromatography (25% acetone/petroleum ether) to give 7c as
a sticky orange solid, which was allowed to crystallize by slow
evaporation of a CH2Cl2 solution (0.098 g, 20% yield, based
on 1,3-bis((N-butyl)imino)benzene). Mp: 106 °C Anal. Found:
C, 40.43; H, 4.77; N, 5.49. Calcd for C16H23ClN2Pt: C, 40.55;
H, 4.89; N, 5.91. IR (νmax; CH2Cl2): 1593 (CdN) cm-1. 1H NMR
(δ; 250 MHz, CDCl3): 0.96 (6 H, t, J 7.3, CH3), 1.38 (4 H, sextet,
J 7.4, CH2), 1.94 (4 H, pent, J 7.4, CH2), 4.02 (4 H, (66%) t, J
Syn th esis of ((2,6-Bis(N-ter t-bu tyl)im in o)p h en yl)p la ti-
n u m ch lor id e 7a . Potassium tetrachloroplatinate (0.466 g,
1.12 mmol) and 6a (0.30 g, 1.2 mmol) were refluxed under
nitrogen in glacial acetic acid for 48 h. After cooling the solvent
was removed in vacuo and the residue was purified by column
chromatography (CH2Cl2) and recrystallized (CH2Cl2) to give
7a as a yellow-orange crystalline solid (0.159 g, 30% yield
based on K2PtCl4). Mp: 300 °C (decomp). Anal. Found: C,
40.30; H, 4.81; N, 5.62. Calcd for C16H23ClN2Pt: C, 40.59; H,
3
7.1, (34%) dt, J PtH 36.3, CH2), 7.10 (1 H, dd, J 7.6 and 8.0,
4
4-H), 7.41 (2 H, (66%) d, J 7.6, (34%) app t, J PtH 8.2, Ar, 3-
4.86; N, 5.92. IR (νmax; CH2Cl2): 1599 (CdN) cm-1
(δ; 250 MHz, CDCl3): 1.72 (s, 18 H, CH3), 7.13 (1 H, t, J 7.6,
.
1H NMR
and 5-H), 8.26 (2 H, (66%) s, (34%) d J PtH 143.5, NdCH). 13C-
3
{1H} NMR (δ; 63 MHz, CDCl3): 13.7 (CH3), 19.7 (CH2), 32.6
4
2
4-H), 7.38 (2 H, (66%) d, J 7.7, (34%) app t, J PtH 6.6, 3- and
(CH2), 61.1 (NCH2, (34%) d, J PtC 21.4), 122.3 (4-C), 126.2
5-H), 8.26 (2 H, (66%) s, (34%) d, 3J PtH 152.2, NdCH). 1H NMR
(δ; 250 MHz, acetone-d6): 1.70 (18 H, s, CH3), 7.16 (1 H, t, J
((34%) d, 3J PtC 28.0, 3- and 5-C), 141.5 ((34%) d, 2J PtC 113.2, 2-
and 6-C), 175.4 ((34%) d, 2J PtC 126.0, NdC), 1-C not observed.
MS (m/z; FAB): 436 ([M - Cl]+, 100%).
4
7.6, 4-H), 7.52 (2 H, (66%) d, J 7.6, (34%) dd, J 7.6 and J PtH
3
6.1, 3- and 5-H), 8.55 (2 H, (66%) s, (34%) d, J PtH 152.1, Nd
Syn t h esis of ((2,6-Bis(N-b en zyl)im in o)p h en yl)p la t i-
n u m ch lor id e 7d . 6d (0.47 g, 1.5 mmol) and potassium
tetrachloroplatinate (0.523 g, 1.26 mmol) were refluxed under
nitrogen in distilled glacial acetic acid (20 mL) for 48 h. The
solvent was removed in vacuo, and the residue was purified
by column chromatography (gradient elution, neat CH2Cl2 to
5% EtOAc/CH2Cl2) to give 7d as a bright yellow solid, which
was further purified by recrystallization from CH2Cl2 (0.100
g, 15% yield, based on K2PtCl4). Mp: 212 °C dec. Anal.
Found: C, 48.02; H, 3.53; N, 4.94. Calcd for C22H19ClN2Pt-
(2H2O): C, 47.96; H, 3.66; N, 5.08. IR (νmax; CH2Cl2): 1603
CH). 13C{1H} NMR (δ; 63 MHz, CDCl3): 29.8 ((34%) d, J PtC
3
12.3, CH3), 65.6 (34%) d, 2J PtC 88.2, (C(CH3)3), 122.5 (4-C), 127.1
((34%) d, 3J PtC 22.6, 3- and 5-C), 142.4 ((34%) d, 2J PtC 113.2, 2-
and 6-C), 171.5 ((34%) d, 2J PtC 118.3, NdCH), 1-C not observed.
13C{1H} NMR (δ; 63 MHz, acetone-d6): 45.1 (C(CH3)3), 56.1
(34%) d, 2J PtC 25.0, (C(CH3)3), 122.8 (4-C), 128.2 ((34%) d, 3J PtC
2
22.4, 3- & 5-C), 143.5 ((34% d, J PtC 120.3, 2- and 6-C) 173.9
2
((34%) d, J PtC 113.6, NdCH), 1-C not observed. MS (m/z;
FAB): 473 (M+, 9%), 438 (M - Cl, 100).
Repetition of this complexation with potassium tetrachlo-
roplatinate (0.557 g, 1.34 mmol) and 6a (0.35 g, 1.4 mmol)
followed by filtration of the crude reaction mixture through
Celite and evaporation of the solvent gave 0.56 g of a dark
brown amorphous solid. Examination of this by 1H NMR
spectroscopy revealed a 0.7:1:2 ratio of 7a , isophthaldehyde,
and N-acetyl-tert-butylamine corresponding to a 39% yield of
7a and 56% of the two organic products (i.e., an overall yield
of 95% from 6a ). The structure of the amide was confirmed
following its isolation by column chromatography (CH2Cl2/
EtOAc). IR (νmax; CH2Cl2): 1674 (CdO) cm-1. 1H NMR (δ; 250
MHz, CDCl3): 1.30 (9 H, s, (CH3)3), 1.96 (3H, s, OdC-CH3),
5.45 (1 H, br s, NH). 13C{1H} NMR (δ; 63 MHz, CDCl3): 24.47
(C(CH3)3), 28.77 (C(CH3)3), 51.11 (OdC-CH3), 169.52 (OdC);
MS (m/z; FAB) 116 (M+, 28%).
Syn t h esis of ((2,6-Bis(N-cycloh exyl)im in o)p h en yl)-
p la tin u m ch lor id e 7b. 6b (0.170 g, 0.57 mmol) and potassium
tetrachloroplatinate (0.204 g, 0.49 mmol) were refluxed under
nitrogen in glacial acetic acid (25 mL) containing 0.25% acetic
anhydride for 48 h. The solvent was removed in vacuo and
the residue purified by column chromatography (CH2Cl2) and
recrystallized (CH2Cl2) to give 7b as an orange crystalline solid
(0.137 g, 53%). Mp: 226 °C Anal. Found: C, 44.95; H, 5.01;
N, 4.90. Calcd for C20H27ClN2Pt(2H2O): C, 44.90; H, 5.28; N,
1
(CdN) cm-1. H NMR (δ; 250 MHz, CDCl3): 5.31 (4 H, (66%)
3
s, (34%) d J PtH 27.7, CH2), 7.05 (1 H, t, J 4.0, 4-H), 7.40 (12
3
H, m, Ar), 8.16 (2 H, (66%) s, (34%) d, J PtH 145.0, NdCH).
2
13C{1H} NMR (δ; 63 MHz, CDCl3): 63.4 ((34%) d, J PtC 22.0,
3
CH2), 122.4 (Ar), 126.7 ((34%) d, J PtC 26.8, Ar, 3- and 5-C),
128.3 (Ar), 129.1 (Ar), 129.7 (Ar), 136.0 (Ar), 141.5 (Ar), 143.7
2
(Ar), 176.4 ((34%) d, J PtC 125.8, NdC). MS (m/z; FAB): 505
([M - Cl]+, 85%), 415 (9.5%), 338 (100%).
Syn th esis of ((2,6-Bis(N-p h en yl)im in o)p h en yl)p la ti-
n u m ch lor id e 7e. 6e (0.35 g, 1.2 mmol) and potassium
tetrachloroplatinate (0.451 g, 1.09 mmol) were refluxed under
nitrogen in glacial acetic acid (20 mL) for 48 h. The solvent
was removed in vacuo, and the residue was initially purified
by column chromatography (70% EtOAc/petroleum ether)
followed by recrystallization from CH2Cl2 to give 7e as a red
crystalline solid (0.174 g, 31% yield based on K2PtCl4). Mp:
270 °C Anal. Found: C, 45.60; H, 3.01; N, 5.19. Calcd for
C
20H15ClN2Pt(2H2O): C, 45.94; H, 3.08; N, 5.36. IR (νmax; CH2-
1
Cl2): 1587 (CdN) cm-1. H NMR (δ; 250 MHz, CDCl3): 7.24
(2 H, t, J 7.7, Ar), 7.35 (1 H, d, J 7.0, Ar), 7.41 (4 H, t, J 7.4,
Ar), 7.52 (4 H, m, Ar), 7.65 (2 H, (66%) d, J 7.7, (34%) app t,
4J PtH7.3, 3- and 5-H), 8.54 (2 H, (66%) s, (34%) d, J PtH 140.7,
3
NdCH). 13C{1H} NMR (δ; 63 MHz, CDCl3): 122.9 (4-C), 124.2

5264 Organometallics, Vol. 21, No. 24, 2002
Fossey and Richards
((34%) d, 3J PtC 12.9, 3- and 5-C), 128.03 (Ar), 128.2 (Ar), 128.5
KBr disk): 1030 (SO), 1161 (CF3), 1597 (CdN) cm-1. 1H NMR
(δ; 250 MHz, acetone-d6): 1.25 (2 H, m, CH2), 1.39 (2 H, m,
CH2), 1.51 (6 H, m, CH2), 1.70 (2 H, m, CH2), 1.88 (4 H, d, J
12.8, CH2), 2.15 (4 H, d, J 11.9, CH2), 3.21 (2 H, brs, Pt-OH2),
3.74 (2 H, m, NCH(CH2)-), 7.23 (1 H, t, J 7.9, 4-H), 7.57 (2 H,
(66%) d, J 7.7, (34%) app t, 4J PtH 9.0, 3- and 5-H). 13C{1H} NMR
(δ; 63 MHz, acetone-d6): 25.7 (CH2), 25.9 (CH2), 33.2 (CH2),
2
(Ar), 128.6 (Ar), 142.1 (34%) d, J PtC 100.8, 2- and 6-C), 177.2
((34%) d, 2J PtC 110.9, NdC), 1-C not observed. MS (m/z; FAB):
513 (M+, 5%), 478 ([M - Cl], 100%).
Syn th esis of ((2,6-Bis(N-ter t-bu tyl)im in o)p h en yl)a qu o-
p la tin u m (II) Tetr a flu or obor a te 8. 7a (0.158 g, 0.33 mmol)
and silver tetrafluoroborate (0.066 g, 0.34 mmol) were stirred
in acetone (25 mL) protected from light for 14 h, by which time
the starting complex had been completely consumed (TLC,
silica, CH2Cl2). The mixture was filtered through Celite to
remove silver chloride and evaporated to dryness to give 8 as
a yellow crystalline solid (0.180 g, 97%). Mp: 234 °C Anal.
Found: C, 34.25; H, 4.48; N, 4.59. Calcd for C16H25BF4N2OPt-
(H2O): C, 34.24; H, 4.85; N, 4.99. IR (νmax; KBr disk): 1067
3
68.3 (NCH(CH2)-), 124.7 (4-C), 128.6 ((34%) d, J PtC 33.6, 3-
and 5-C), 142.8 ((34%) d, 2J PtC 125.79, 2- and 6-C), 176.1 ((34%)
2
d, J PtC 131.3, NdC), 1-C not observed. MS (m/z; FAB): 490
([M - OTf - OH2]+, 100%).
Mich a el Rea ction to Give 11 a n d 123e. A stock solution
was prepared containing ethyl cyanoacetate (0.10 mL, 1.0
mmol), methyl vinyl ketone (0.24 mL, 2.9 mmol), Hu¨nig’s base
(0.017 mL, 0.1 mmol), and CD2Cl2 (3 mL). An aliquot (1 mL)
of this solution was transferred to a NMR tube (the blank
reaction), and a further aliquot (1.5 mL) was transferred to a
second NMR tube containing 10 (0.014 g, 0.02 mmol). Both
samples were stored at 4 °C and their NMR spectra recorded
periodically. The presence of 11 was revealed by a character-
istic peak at 3.64 ppm (1 H, dd, J 8.7 and 6.1, NCCH-).
Ald ol Rea ction to Give 13 a n d 14.14 A stock solution was
prepared containing methyl isocyanoacetate (0.5 mL, 6 mmol),
benzaldehyde (0.8 mL, 8 mmol), Hu¨nig’s base (0.1 mL, 0.6
mmol), and CD2Cl2 (3 mL). An aliquot (1 mL) of this solution
was transferred to a NMR tube (the blank reaction). A further
aliquot (1 mL) was transferred to a second NMR tube contain-
ing 10 (0.044 g, 0.07 mmol). Both samples were stored at 4 °C
and their NMR spectra recorded periodically.
1
(BF4-), 1599 (CdN) cm-1. H NMR (δ; 250 MHz, acetone-d6):
1.49 (18 H, s, CH3), 7.24 (1 H, t, J 7.6, Ar, 4-H), 7.61 (2 H,
4
(66%) d, J 7.5, (34%) app t, J PtH 7.8, Ar, 3- and 5-H), 8.70 (2
3
H, (66%) s, (34%) d J PtH 163.9, NdCH). 13C{1H} NMR (δ; 63
2
MHz, acetone-d6): 29.1 (CH3), 64.6 ((34%) d, J PtC 18.2,
3
(C(CH3)3), 125.1 (4-C), 129.6 ((34%) d, J PtC 30.1, 3- and 5-C),
2
143.0 (2- and 6- C), 162.6 (1-C), 175.0 ((34%) d, J PtC 126.7,
NdC). MS (m/z; FAB): 438 ([M - OH2 - BF4]+, 100%).
Syn th esis of ((2,6-Bis(N-ter t-bu tyl)im in o)p h en yl)a qu o-
p la tin u m (II) Tr iflu or om eth a n esu lfon a te 9. 7a (0.155 g,
0.33 mmol) and silver triflate (0.1 g, 0.4 mmol) were stirred
in CH2Cl2 (25 mL) protected from the light and left for 14 h,
by which time all the starting complex had been consumed
(TLC, silica, CH2Cl2). The mixture was filtered through Celite
to remove silver chloride, washing with acetone, and evapo-
rated to dryness to give 9, as a yellow crystalline solid (0.198
g, >99%). Mp: 244 °C. Anal. Found: C, 33.76; H, 4.12; N, 4.63.
Calcd for C17H25F3N2O4PtS(2H2O): C, 33.72; H, 4.16; N, 4.49.
IR (νmax; KBr disk): 1034 (SO), 1163.0 (CF3), 1282.6 (SO),
Diels-Ald er Rea ction to Give 15 a n d 16.15 A stock
solution was prepared containing acrylonitrile (0.1 mL, 1.5
mmol), freshly cracked cyclopentadiene (0.3 mL, 3.7 mmol),
and CD2Cl2 (3.5 mL). An aliquot (1 mL) of this solution was
transferred to a NMR tube (the blank reaction). A further
aliquot (1 mL) was transferred to a second NMR tube contain-
ing 10 (0.015 g, 0.02 mmol). Both samples were stored at 4 °C
and their NMR spectra recorded periodically.
1
1600.8 (CdN) cm-1. H NMR (δ; 250 MHz, acetone-d6): 1.70
(18 H, s, CH3), 7.16 (1 H, t, J 7.6, 4-H), 7.53 (2 H, (66%) d, J
4
7.6, (34%) dd, J 7.6 and J PtH 6.1, 3- and 5-H), 8.55 (2 H, (66%)
3
s, (34%) d J PtH 152.1, NdCH). 13C{1H} NMR (δ; 63 MHz,
2
acetone-d6): 20.1 (CH3), 64.6 ((34%) d, J PtC 17.7, C(CH3)3),
1
3
124.9 (4-C), 127.4 (q, J CF 328.4, CF3), 129.5 ((34%) d, J PtC
29.2, 3- and 5-C), 142.9 ((34%) d, 2J PtC 129.4, 2- and 6-C), 162.6
((34%) d, 1J PtC 1078.2, 1-C), 174.7 ((34%) d, 2J PtC 126.8, NdC).
MS (m/z; FAB): 438 ([M - OH2 - OTf]+, 100%).
Ack n ow led gm en t. We wish to thank the EPSRC
for a studentship (J .S.F.), Majid Motevalli for the X-ray
crystal structure determinations, and Greg Coum-
barides for assistance in determining the NMR spectra.
Syn t h esis of ((2,6-Bis(N-cycloh exyl)im in o)p h en yl)-
a qu op la tin u m (II) Tr iflu or om eth a n esu lfon a te 10. 7b (2.70
g, 5.1 mmol) and silver triflate (1.5 g, 6 mmol) were stirred in
CH2Cl2 (45 mL) protected from the light and left for 24 h, by
which time all the starting complex had been consumed (TLC,
silica, CH2Cl2). The mixture was filtered through Celite eluting
with acetone to remove silver chloride and evaporated to
dryness to give 10, as a red-orange crystalline solid (3.35 g,
99%). Mp: 262 °C. Anal. Found: C, 38.22; H, 4.37; N, 4.10.
Su p p or tin g In for m a tion Ava ila ble: Details of the X-ray
structure determinations of 7a , 7e, and 9. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM020566R
(14) Hayashi, T.; Sawamura, M.; Ito, Y. Tetrahedron 1992, 48, 1999.
(15) Dilling, W. L.; Kroening, R. D.; Little, J . C.; J . Am. Chem. Soc.
1970, 92, 928.
Calcd for C21H29F3N2O4PtS: C, 38.36; H, 4.44; N, 4.26. IR (νmax
;