Activation of metal-bound η5-C5Me5 groups to Diels-Alder addition of 3O2 and other dienophiles

Article abstract of DOI:10.1039/b818537a

Pt(η⁵-C₅Me₅)(PR₃)I (PR ₃ = PPh₃, PPhCy₂, PPhMe^tBu) reacts with ³O₂ to form the metal-bound endoperoxide Pt(η¹,η²-C₅

Full text of DOI:10.1039/b818537a

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Activation of metal-bound g5-C5Me5 groups to Diels–Alder addition
3
of O2 and other dienophilesw
Neil M. Boag* and K. Mohan Raoz
Received (in Cambridge, UK) 20th October 2008, Accepted 24th December 2008
First published as an Advance Article on the web 27th January 2009
DOI: 10.1039/b818537a
Pt(g5-C5Me5)(PR3)I (PR3 = PPh3, PPhCy2, PPhMetBu)
were the unique methyl resonance in the 1H NMR spectra
2
reacts with 3O2 to form the metal-bound endoperoxide
(B0.3 ppm, JPtH = B60 Hz) and the unique ring carbon in
Pt(g1,g2-C5Me5O2)(PR3)I;
analogous
reactions
of
the 13C{1H} NMR spectra (B40 ppm), which exhibited a
platinum carbon constant characteristic of a s-bound carbon
(B500 Hz). Although these parameters are indicative of
Z5 - Z1 ring slippage, the magnitudes of the platinum and
phosphorus coupling constants to the remaining pentamethyl-
cyclopentadienyl resonances were inconsistent with a ring
slipped product.4 Furthermore, the long-term stability of
solutions of Pt(Z5-C5Me5)(PPh3)I under anaerobic conditions
suggested to us that the new products arose from aerial
oxidation.
Pt(g5-C5Me5)(PPh3)I with a variety of other dienophiles to form
Pt(g1,g2-C5Me5CH2CHR)(PPh3)I (R = C(O)H, CO2Me,
C(O)Et) in which the metal is anti to the incoming
dienophile are indicative of direct Diels–Alder addition to the
g5-C5Me5 ring.
Although Diels–Alder [4 + 2] cycloadditions to metal-bound
Z1-cyclopentadienyl rings are not uncommon and have been
utilised in the synthesis of substituted norbornenes and
norbornadienes,1 formal Diels–Alder [4 + 2] cycloadditions
to metal-bound Z5-cyclopentadienyl rings are rare and are
only found when the dienophile is an electron deficient alkyne,
the classic case being the addition of dimethylacetylene
dicarboxylate to nickelocene.2 In all examples, however, the
double bond originating from the dienophile ends up bound to
the metal in the final metal-bound Z1,Z2-norbornadiene and a
mechanism involving a metal mediated condensation of an
Z1-cyclopentadienyl intermediate that also contains the metal-
bound alkyne is thought to be operative. This is unfortunate
because direct Diels–Alder addition to an Z5-ring would offer
an inherently face-selective methodology.3 Herein we describe
this specific reactivity of a platinum-bound Z5-pentamethyl-
cyclopentadienyl ring to a variety of dienophiles including
dioxygen.
On
a preparative scale, stirred green solutions of
Pt(Z5-C5Me5)(PR3)I (PR3 = PPh3, 1a, PPhCy2, 1b, PMetBuPh, 1c)y
in open vessels completely react at room temperature over
2–3 h affording, on work-up, pale yellow complexes, 2, with
identical NMR spectroscopic properties to the species formed
in situ.z It was found that the same reaction takes place in the
dark with no diminution in rate and also in the presence of
Whereas solutions of Pt(Z5-C5Me5)(CO)I in aerated CDCl3
are stable for several hours as might be expected for a Pt(II)
complex, the corresponding solutions of Pt(Z5-C5Me5)(PR3)I
(R = aryl, alkyl), 1, rapidly change from an initial green
colour to pale yellow over a few minutes. The NMR spectra
of these pale yellow solutions revealed that the fivefold
degeneracy of the cyclopentadienyl ring had been broken with
the new, quantitatively generated species exhibiting a 1 : 2 : 2
ratio of both the ring carbon and ring methyl resonances,
further reduced to 1 : (1 : 1) : (1 : 1) when the phosphine
contained three differing R groups. Particularly distinctive
Functional Materials, Institute for Materials Research,
Cockcroft Building, University of Salford, Salford, UK M5 4WT.
E-mail: N.M.Boag@salford.ac.uk
w Electronic supplementary information (ESI) available: Full syn-
thetic procedures, analytical and spectroscopic data for complexes 2
and 3 as well as CIF files for complexes 2b and 3c. CCDC 706170 and
706171. For ESI and crystallographic data in CIF or other electronic
format, see DOI: 10.1039/b818537a
z Current address: Department of Chemistry, North Eastern Hill
University, Shillong 793 022, India; E-mail: mohanrao59@hotmail.com;
Tel: 91 364 272 2620
Fig. 1 An ORTEP diagram of the molecular structure of 2b showing
50% probability thermal ellipsoids. Selected bond distances (A) and
angles (1). Pt–I 2.6913(17), Pt–C2 2.08(2), Pt–P 2.286(5),
Pt–(midpt)C4/C5 2.129, C4–C5 1.41(3), O1–O2 1.52(3), C2–Pt–P
105.6(5), C2–Pt–I 158.0(5), P–Pt–I 96.33(14), I–Pt–(midpt)C4/C5
97.86, C2–Pt–(midpt)C4/C5 60.19.
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Scheme 1
Scheme 2
radical scavengers as well as in the solid state, albeit over
several weeks.
addition to a variety of other standard dienophiles (Scheme 2).
For example, hexane solutions of Pt(Z5-C5Me5)(PPh3)I
react with acrolein, methyl acrylate or ethyl vinyl ketone
at elevated temperatures over 1–2 days to afford
Pt(Z1,Z2-C5Me5CH2CHR)(PPh3)I (R = C(O)H, 3a, R =
CO2Me, 3b, R = C(O)Et, 3c) in essentially quantitative yields.
These show similar NMR spectroscopic features to the endo-
peroxides, with additional resonances attributable to the
fragment arising from the dienophile.** In all cases, both exo
and endo isomers were formed; however, only with the less
sterically demanding acrolein were significant quantities
(B15%) of the exo isomer generated. The endo-ethyl vinyl
ketone derivative, 3c, (Fig. 2) was also characterised by X-ray
crystallography.ww The structure and structural parameters are
very similar to those of 2b with the peroxide linkage replaced by
the endo-CH2CHC(O)Et group.
The structure of 2b was subsequently identified by X-ray
crystallography (Fig. 1) as a metal-bound endoperoxide aris-
ing from dioxygen addition to the pentamethylcyclo-
pentadienyl ring, Pt(Z1,Z2-C5Me5O2)(PPh3)I 2b (Scheme 1).8
The dioxygen has essentially undergone Diels–Alder addition
to the ring on the opposite face to the metal forming an
endoperoxide group coordinated to the metal via a s and a p
bond with a ‘‘bite’’ of 60.21 resulting in a platinum atom in a
distorted square planar environment. The iodide is trans to
the s-bound carbon of the endoperoxide group. The O–O
distance 1.52(3) A is within the range expected for a cyclic
peroxide linkage.6
The endoperoxides 2 are light sensitive and decompose over
several days. The only platinum species we have been able to
identify after decomposition are the known halo phosphine
complexes trans-[Pt2(PR3)4I2] and trans-[Pt(PR3)2I2].7 As yet,
the fate of the organic fragment is unknown.
It is difficult to envisage how the anti stereochemistry (with
respect to the metal) found for the bicyclic ring in the crystal
structures of 2b and 3c can be generated other than by direct
[4 + 2] addition as a metal templated addition would generate
There are only sporadic reports of reactions of metal-bound
carbocyclic ligands with dioxygen. The radicals Co(Z5-C5H5)2
and Fe(Z5-C6H6)(Z5-C5H5) react with O2 to form dimeric
complexes containing bridging peroxo bridges between the
C5H5 and C6H6 rings, respectively.8 [Rh(Z4-diphenylfulvene)2]+
reacts with O2 generating a bis-cyclopentadienyl cation with a
peroxide bridge between the exocyclic carbons of the diphenyl
substituent,9 while an iridium metallabenzene undergoes
1,4-addition of O2 across the ring generating a bicyclic
peroxide.10 Insertion into metal alkyl and metal hydrogen
bonds generating linear peroxides is also well established.11
In other examples, reaction with oxygen leads to oxygen
atom transfer12 or deprotonation of the carbocyclic ring.13
Pentamethylcyclopentadiene itself reacts with O2 to form a
pyrylium salt via the intermediacy of a peroxide arising from
O2 insertion into the ring C–H bond,14 whereas with singlet
oxygen an endoperoxide is formed.15
Diels–Alder addition of dioxygen to an Z5-bound cyclo-
pentadienyl group is unprecedented, but the reaction is
analogous to Diels–Alder addition of singlet dioxygen to
cyclopentadienes. Diels–Alder addition of triplet dioxygen,
although formally spin forbidden, is known to occur for
several diene systems, particularly those in which there is a
degree of ring strain, and is thought to follow a radical
pathway.16 The degenerate ‘‘diene’’ and ‘‘allyl-ene’’ distortions
of the ring in d8-M(Z5-C5R5)L2 systems are well established17
and it is possible that on platinum the distortion has become
sufficiently diene-like, with a corresponding inherently high
degree of ring strain, that the ring is now susceptible to
spontaneous oxidation through the established radical
pathway.
Fig. 2 An ORTEP diagram of the molecular structure of 3c showing
50% probability thermal ellipsoids. Selected bond distances (in A) and
angles (1). Pt–I 2.6969(9), Pt–C2 2.105(8), Pt–P 2.262(2),
Pt–(midpt)C4/C5 2.164, C2–Pt–P 102.0(2), C2–Pt–I 159.6(2), P–Pt–I
98.01(6), I–Pt–(midpt)C4/C5 98.80, C2–Pt–(midpt)C4/C5 60.83.
In support of this, we have found that the platinum-bound
Z5-pentamethylcyclopentadienyl ring will undergo Diels–Alder
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a norbene fragment with the corresponding syn stereo-
chemistry. Such a fragment would be unable to coordinate
to the metal in the manner found. By contrast, reaction
of nickelocene with the activated alkenes methylketene or
tetrafluoroethylene results in [2 + 2] cycloaddition not
[4 + 2] cycloaddition.18 The reactivity found for these
platinum systems suggests that group 10 metal mediated
addition may provide a solution to the long sought goal of
facial selectivity in Diels–Alder addition reactions to cyclic
systems.
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Notes and references
y The species 1 were obtained from the hexane mediated reaction of
Pt(Z5-C5Me5)(CO)I with the appropriate phosphine. Pt(Z5-C5Me5)(CO)I
was prepared by reaction of Pt2(Z5-C5Me5)2(CO)2 with I2 at ꢁ116 1C
(ether slush).5
10 J. R. Bleeke, Y. F. Xie, L. Bass and M. Y. Chiang, J. Am. Chem.
Soc., 1991, 113, 4703–4704.
z Spectroscopic data for 2b: 1H NMR (CD2Cl2) d 7.5–7.4 (m 5H, Ph),
2.8–1.0 (m 22H, Cy), 1.97 (d, 6H, Me, JPH = 3.0 Hz, JPtH = 27.7 Hz),
1.34 (s, 6H, Me), 0.39 (d, 3H, Me, JPH = 0.9 Hz, JPtH = 61.5 Hz);
selected 13C{1H} (CD2Cl2) d 113.2 (d, 2C, QCMe, JPC = 13 Hz,
JPtC = 78 Hz), 95.9 (d, 2C, C(O)Me, JPC = 4 Hz, JPtC = 69 Hz), 42.0
(s, 1C, CMe, JPtC = 503 Hz), 13.3 (br s, 2C, Me, JPtC = 11 Hz), 10.7
(s, 2C, Me, JPtC = 17 Hz), 9.1 (s, 1C, Me, JPC = 4 Hz); 195Pt{1H}
(C6D6, X = 21.4), d 444 (d, JPtP = 4461 Hz); 31P{1H}(C6D6) d 28.3
(s, JPtP = 4459 Hz).
11 W. M. Cleaver and A. R. Barron, J. Am. Chem. Soc.,
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8 Crystal data for 2b: C28H42IO2PPt, M = 763.58, monoclinic, a =
9.954(3), b = 11.961(4), c = 12.053(5) A, b = 93.41(3)1, U =
1432.5(8) A3, T = 203(2) K, space group P21, Z = 2, 3645 reflections
measured, 3455 unique (Rint = 0.0480); reflections with I 4 2s(I) =
2440, R1 = 0.063 (I 4 2s(I)), wR2 = 0.142 (all data), GOF = 1.039.
** Spectroscopic data for 3b (major isomer): selected 1H NMR (C6D6)
d 1.98 (d, 3H, Me, JPH = 3.3 Hz, JPtH = 30.8 Hz) 1.81 (d, 3H, Me,
JPH = 3.2 Hz, JPtH = 36.6 Hz), 1.47 (s, 3H, Me), 1.33 (s, 3H, Me),
ꢁ0.19 (d, 3H, Me, JPH = 1.3 Hz, JPtH = 64.0 Hz); selected 13C{1H}
(CD2Cl2) d 173.4 (s, 1C, C(O)), 113.8 (d, 1C, QCMe, JPC = 15 Hz,
JPtC = 68 Hz), 110.0 (d, 1C, CMe, JPC = 13 Hz, JPtC = 73 Hz), 67.4
(s, 1C, CMe, JPtC = 84 Hz), 63.2 (s, 1C, CMe, JPtC = 88 Hz), 52.6
(s, 1C, CH2, JPtC = 111 Hz), 51.2 (s, 1C, CO2Me), 39.9 (s, 1C, CH2,
JPtC = 110 Hz), 36.6 (s, 1C, CMe, JPtC = 468 Hz), 15.5 (d, 1C, Me,
JPC = 2 Hz, JPtC = 17 Hz), 15.2 (s, 1C, Me, JPtC = 34 Hz), 14.4 (s, 1C,
Me, JPtC = 32 Hz), 13.5 (d, 1C, Me, JPC = 3 Hz, JPtC = 17 Hz), 8.9
(d, 1C, Me, JPC = 6 Hz); 195Pt{1H} (C6D6, X = 21.4), d 719
(d, JPtP = 4587 Hz); 31P{1H}(C6D6) d 19.9 (s, JPtP = 4588 Hz).
ww Crystal data for 3c: C28H42IO2PPt, M = 803.59, a = 9.490(3), b =
15.263(4), c = 21.282(7) A, b = 92.55(2)1, U = 3079.5(15) A3, T =
293(2) K, space group monoclinic, P21/c, Z = 4, 7499 reflections
measured, 7094 unique (Rint = 0.0314); reflections with I 4 2s(I) =
4258, R1 = 0.049 (I 4 2s(I)), wR2 = 0.115 (all data), GOF = 1.053.
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