Sequential electrophile/nucleophile additions for η2-cyclopentadiene complexes of osmium(II), ruthenium(II), and rhenium(I)

Article abstract of DOI:10.1021/om960905e

A series of d⁶ transition-metal complexes of the type ML₅(η²-CpH), where ML₅ = [Os^II(NH₃)₅]²⁺, [Ru^II-(NH₃)₅]²⁺, and[Re^I(PPh₃)(PF₃)(dien)]⁺, were synthesized as their triflate salts and combined with electrophiles (HOTf, CH₂(OMe)₂) to form η³-allyl complexes. Treatment of these π-allyl complexes with the mild carbon nucleophile 1-methoxy-2-methyl-1-(trimethylsiloxy)propene (MMTP) followed by decomplexation affords substituted η²-cyclopentene derivatives with excellent regio-and stereocontrol. Deuteration and NOE studies for the π-allyl complexes along with stereochemical analysis of the organic products confirm that both electrophilic and nucleophilic addition occurs exclusively from the exo face of the ring (opposite to metal coordination) for all three systems.

Full text of DOI:10.1021/om960905e

Organometallics 1996, 15, 5447-5449
5447
Sequ en tia l Electr op h ile/Nu cleop h ile Ad d ition s for
η2-Cyclop en ta d ien e Com p lexes of Osm iu m (II),
Ru th en iu m (II), a n d Rh en iu m (I)
Michael L. Spera, R. Martin Chin, Mark D. Winemiller, Katharine W. Lopez,
Michal Sabat, and W. Dean Harman*
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
Received October 24, 1996X
Summary: A series of d6 transition-metal complexes of
the type ML5(η2-CpH), where ML5 ) [OsII(NH3)5]2+, [RuII-
(NH3)5]2+, and [ReI(PPh3)(PF3)(dien)]+, were synthesized
as their triflate salts and combined with electrophiles
(HOTf, CH2(OMe)2) to form η3-allyl complexes. Treat-
ment of these π-allyl complexes with the mild carbon
nucleophile 1-methoxy-2-methyl-1-(trimethylsiloxy)pro-
pene (MMTP) followed by decomplexation affords sub-
stituted η2-cyclopentene derivatives with excellent regio-
and stereocontrol. Deuteration and NOE studies for the
π-allyl complexes along with stereochemical analysis of
the organic products confirm that both electrophilic and
nucleophilic addition occurs exclusively from the exo face
of the ring (opposite to metal coordination) for all three
systems.
Although η4-coordinate triene complexes have been
observed to react with electrophiles at an uncoordinated
olefinic carbon,1 the analogous reaction for a simple η2-
diene is far less common.2 Electrophilic additions to η4-
dienes have been reported, but the commonly invoked
reaction mechanism for this process involves electro-
philic addition (typically a proton) to the metal followed
by transfer to the endo face of the diene.3 As a π-base,
pentaammineosmium(II) has demonstrated an ability
to activate η2-coordinated aromatic π systems toward
electrophilic addition,4 and we questioned whether this
or other electron-rich 16e- systems might promote the
reaction of unactivated dienes toward the direct addition
of an electrophile.5 Herein we describe the reactions of
an η2-cyclopentadiene ligand with a Brønsted acid and
a carbon electrophile (an acetal) for three d6-transition-
metal systems: {Os(NH3)5}2+, {Ru(NH3)5}2+, and {Re-
(dien)(PPh3)(PF3)}+ (Figure 1; all complexes are isolated
as their triflate salts).6
F igu r e 1. Reaction scheme for η2-diene complexes where
(a) M ) {Os(NH3)5}2+, (b) M ) {Ru(NH3)5}2+, and (c) M )
{fac-Re(dien)(PPh3)(PF3)}+ (all complexes isolated as tri-
flate salts).
Desiring the simplest cyclic diene possible, we chose
cyclopentadiene for our initial study. The corresponding
pentaammineosmium(II) complex 1a is readily prepared
(95%) using established methods.7 Treatment of a
methanol or an acetonitrile solution of 1a with 1.2 equiv
of HOTf generates the π-allyl species 2a in 72% isolated
yield. 1H and 13C spectral features are typical for a
symmetrical π-allylic species8 and are in good agreement
X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Brookhart, M.; Volpe, A. F., J r.; Yoon, J . In Comprehensive
Organic Synthesis; Trost, B. M., Ed.; Pergamon Press: Oxford, U.K.,
1991; Vol. 4, Chapter 3.5.
(2) (a) Pearson, A. J .; Srinivasan, K. J . Org. Chem. 1992, 57, 3965.
(b) Brookhart, M.; Noh, S. K.; Timmers, F. J .; Hong, Y. H. Organo-
metallics 1988, 7, 2458. (c) J ohnson, B. F. G.; Lewis, J .; Randall, G. L.
P. Chem. Commun. 1969, 1273.
(3) (a) J ohnson, B. F. G.; Lewis, J .; Yarrow, D. J . J . Chem. Soc.,
Dalton Trans. 1972, 2084. (b) Pearson, A. J . Metallo-organic Chemistry;
Wiley: New York, 1985; p 267.
(4) (a) Kopach, M. E.; Gonzalez, J .; Harman, W. D. J . Am. Chem.
Soc. 1991, 56, 4321. (b) Hodges, L. M.; Gonzalez, J .; Myers, W. H.;
Koontz, J . I.; Harman, W. D. J . Org. Chem 1995, 60, 2125. (c) Kolis,
S. P.; Gonzales, J .; Bright, L. M.; Harman, W. D. Organometallics 1996,
15, 245.
(7) Synthesis and characterization of 1a : Os(NH3)5(OTf)3 (3.02 g,
4.18 mmol) was reduced (Mg0, 3.02 g) in a solution of cyclopentadiene
(3.15 g, 49.2 mmol) and DMAc (2.95 g). Yield upon precipitation in
CH2Cl2: 2.54 g (95%). 1H NMR (CD3CN): δ 6.32 (dd, J ) 6.8 Hz, 1.9
Hz, 1H), 5.97 (dd, J ) 6.8 Hz, 2.1 Hz, 1H), 4.65 (d, J ) 5.9 Hz, 1H),
4.13 (d, J ) 5.9 Hz, 1H), 4.10 (br s, 3H), 2.89 (br s, 12H), 2.45 (m, 1H),
2.38 (m, 1H). Anal. Calcd for C7H27N5O6F6S2Os: C, 13.15; H, 3.31; N,
10.95. Found: C, 12.96; H, 3.60; N, 10.78.
(5) Shepherd has demonstrated that addition of Br2 occurs on the
uncoordinated portion of butadiene, but this reaction readily occurs
for organic olefins. See: Elliot, M. G.; Shepherd, R. E. Inorg. Chem.
1988, 27, 3332.
(8) Charaterization data for 2a : 1H NMR (CD3CN) δ 5.72 (d, J )
2.9 Hz, 1H), 4.99 (t, J ) 2.9 Hz, 1H), 4.88 (br s, 3H), 3.71 (br s, 12H),
2.94 (d, J ) 16.2 Hz, 1H), 1.63 (d, J ) 16.2 Hz, 1H); 13C NMR (CD3-
(6) The complex [Os(NH3)5(η2-1,3-cyclohexadiene)]2+ was previously
reported to undergo protonation to form an allyl species; however,
nothing was reported about the mechanism. See: Harman, W. D.;
Hasegawa, T.; Taube, H. Inorg. Chem. 1991, 30, 453.
CN) δ 94.41 (CH), 83.63 (CH), 31.86 (CH2). Anal. Calcd for C8H22
-
N5O9S3F9Os: C, 12.17; H, 2.81; N, 8.87. Found: C, 11.95; H, 2.47; N,
9.14.
S0276-7333(96)00905-3 CCC: $12.00 © 1996 American Chemical Society

5448 Organometallics, Vol. 15, No. 26, 1996
Communications
In a similar manner, a π-allyl species was prepared
from the cyclopentadiene complex 1a with a carbon
electrophile.11 Treatment of an acetonitrile solution of
1a with the acetal dimethoxymethane (1.2 equiv) and
Lewis acid (TBSOTf, 1.1 equiv) results in the formation
of π-allyl 3a . A full spectroscopic analysis12 of 3a
reveals the methoxymethylcyclopentadienium species
shown in Figure 1, where electrophilic addition again
occurs to the exo face of the diene. Subsequent treat-
ment of either allyl species 2a or 3a with a mild carbon
nucleophile13 generates a substituted cyclopentene com-
plex. For example, when a solution of 2a or 3a is
combined with 1-methoxy-2-methyl-1-(trimethylsiloxy)-
propene (MMTP), alkene complexes are formed (4a
(79%) and 5a (81%), respectively) that upon oxidation
with DDQ (0.6 equiv) release the alkylated cyclo-
pentenes 6 (70%) and 7 (75%), respectively. In the latter
case, nucleophilic addition occurs to the allyl species to
form a single regio- and diastereomer (>10:1) that 13C
1
and H NMR, COSY, and NOE data indicate is the cis-
3,5-disubstituted cyclopentene product.
The weaker π-donor properties of ruthenium make
protonation of the diene complex 1b considerably more
difficult than its heavy-metal congener.14 Whereas 1a
can be protonated in methanol, the formation of the
π-allyl species 2b requires protonation of the diene
complex 1b in acetonitrile at -40 °C. Spectral features
for 2b (-40 °C, CD3CN) correlate well with those of 2a .15
Although attempts to isolate 2b failed due to its thermal
instability, it can be trapped by a suitable nucleophile
to generate substituted cyclopentenes. For example,
when a solution of 1b (-50 °C) is treated with triflic
acid followed by MMTP, the cyclopentene complex 4b
is recovered in good yield (83%). Heating 4b (30 min,
60 °C) delivers the organic cyclopentene 6 in 63% yield.
Repeating this sequence for 1b with dimethoxymethane
(TBSOTf; -45 °C) delivers the disubstituted cyclopen-
tene complex (5b) that when heated yields the cis-3,5-
disubstituted cyclopentene 7 as a single regioisomer
with good stereoselectively (>10:1) in overall 83% yield
from 1b.
Hoping to show this novel diene reactivity for a metal
outside of group VIII, our focus shifted to rhenium,
where our intention was to examine a system with
similar electronic structure to the pentaammineosmium-
(II) moiety. The system {fac-Re(dien)(PPh3)(PF3)}+ is
isoelectronic with the osmium system, shows a similar
electrochemical profile,16 and does not have chemically
accessible electrons on its ancillary ligands that might
be a target for an electrophile. When a sample of [fac
-Re(dien)(PPh3)(PF3)(OTf)](OTf)17 is reduced with Mg0
in the presence of cyclopentadiene, the compound [fac-
F igu r e 2. ORTEP drawing for the π-allyl complex [Os-
(NH3)5(η3-C6H9)]3+ (8). Selected bond lengths (Å): Os-C(1),
2.25 (1); Os-C(2), 2.19(1); Os-C(3), 2.26(1); C(1)-C(2),
1.40(2); C(2)-C(3), 1.42(2). Selected bond angles (deg):
C(1)-C(2)-C(3), 111(1).
with what has been reported for the cyclohexadiene
analog, 8 (Figure 2).6 Although efforts to grow a crystal
of 2a suitable for structural analysis were unsuccessful,
well-formed crystals of 8 were eventually obtained from
acetonitrile solution and an X-ray structure determi-
nation was carried out.9 Carbon-metal bond lengths
are virtually identical (2.25, 2.26 Å) for the two terminal
carbons (Figure 2), while the central carbon is drawn
in considerably closer to the osmium (2.19 Å).
To establish the facial selectivity of diene protonation,
a sample of 1a was treated with DOTf in CD3OD to
1
generate 2a -d1. A H NMR spectrum (CD3CN) estab-
lishes that only one of the four diastereotopic methylene
positions incorporates a significant amount of deute-
rium. For 2a -d1, the overall intensity of the resonance
Ha is lowered by half, while the signal for Hb becomes
a superposition of the original doublet (1.67 ppm; 3.7
Hz) and a new a singlet (1.62 ppm). When the cis-
ammine signal is irradiated, an 11% NOE is measured
for Hb. Taken together, these observations confirm that
protonation occurs from the exo face of the diene.10
(11) Preliminary evidence suggests that Michael acceptors, acetals,
and alkyl triflates, all react with η2-diene complexes of osmium(II).
(12) These experiments include 1H, 13C, DEPT, and NOE.
(13) Nucleophiles that react with 2, 3, or 8 include silated enols,
hydride, and cyanide reagents. Spera, M. L.; Lopez, K. W.; Harman,
W. D. Preliminary results.
(14) Synthesis of 1b is accomplished by reducing Ru(NH3)5(OTf)3
in acetone and CpH with Zn/Hg amalgam. Yield ) 79%. See Supporting
Information.
(15) Characterization of 2b: Partial 1H NMR (CD3CN, -40 °C) δ
5.18 (m, 1H), 5.02 (m, 1H), 4.75 (br s, 3H), 3.38 (dd, J ) 20.2 Hz, 4.9
Hz, 1H), 2.92 (br s, 12H). One proton resonance was not assigned.
(16) For a number of examples, the reduction potentials of [Os-
(NH3)5L]2+ and [Re(dien)(PPh3)(PF3)(L)]+ are within 200 mV of each
other. See ref 17.
(17) Chin, R. M.; Dubois, R. H.; Helberg, L. E.; Sabat, M.; Bartucz,
T. Y.; Lough, A. J .; Morris, R. H.; Harman, W. D. Manuscript submitted
for publication.
(9) Crystal data for 8, [Os(NH3)5(C6H9)](OTf)3‚2CH3CN: C11H24F9N6-
O9S3Os; M ) 841.7; triclinic, P1h (No. 2); a ) 12.105(4), b ) 15.648(6),
c ) 8.457(3) Å; R ) 105.46(2)°, â ) 102.71°, γ ) 103.97°; V ) 1427 Å3;
Z ) 2; Dcalc ) 1.96 g cm-3. Unique reflections: 5015. R ) 0.064, Rw
0.090, and GOF ) 2.46.
)
(10) Exo addition of electrophiles is also observed with aromatic
systems bound to pentaammineosmium(II). See ref 4.

Communications
Organometallics, Vol. 15, No. 26, 1996 5449
of isomers.21 When 5c is heated in an acetonitrile
solution, compound 7 is recovered as the same stereoi-
somer (de >90%) recovered from ruthenium or os-
mium.22 That 7 is recovered exclusively as the cis
isomer confirms that electrophilic addition occurs from
the exo face in the rhenium system as is observed for
the group VIII metals.
For three complimentary d6 octahedral transition-
metal systems we have shown that η2-dienes undergo
direct electrophilic addition of both a proton and carbon
electrophiles to form cationic π-allyl complexes. Given
the general reactivity of allyl cations with mild nucleo-
philes, this sequential stereocontrolled addition of elec-
trophile and nucleophile to an η2-diene complex is
potentially a useful new synthetic tool for the stereo-
selective preparation of substituted cycloalkenes. Work
in our laboratories is currently focusing on the scope of
this reaction sequence and the possibility of enantiose-
lective transformations with the described rhenium
system.
F igu r e 3. Diastereomers for the diene complex 1c.
Re(dien)(PPh3)(PF3)(η2-CpH)](OTf) (1c) is isolated (59%)
as a 5:2 equilibrium mixture of diastereomers (Figure
3).18 Treatment of this mixture with triflic acid in
methanol or acetonitrile generates the π-allyl complex
2c (84%).19 In a similar manner, when diene complex
1c is combined with dimethoxymethane (22 °C) and
TBSOTf with 2,6-di-tert-butylpyridine, a 5:2 diastereo-
meric mixture of allyl species 3c (86%) is formed,
consistent with the original ratio of diene diastereo-
mers.20 Treatment of the allyl species 2c and 3c with
MMTP (CH3CN) generates the cyclopentene complexes
4c (62%) and 5c (59%), respectively. In the latter case,
the cyclopentene complex is isolated as a 5.5:1 mixture
Ack n ow led gm en t is made to the Camille and Henry
Dreyfus Foundation, the National Science Foundation
(NSF Young Investigator program), the Alfred P. Sloan
Foundation, and Colonial Metals Inc. (Elkton, MD) for
their generous support of this work.
(18) Synthesis and characterization of 1c: fac-[Re(OTf)(PPh3)(PF3)-
(dien)](OTf) (4.56 g, 4.86 mmol), cyclopentadiene (5.2 g, 78.8 mmol),
and Mg0 were stirred in DME (35 mL; 2 h) and then filtered. The
filtrate was added to Et2O to precipitate the product as a white solid.
Yield: 2.46 g (59%, 5:2 mixture of two isomers). 1H NMR (CD3CN,
both isomers): δ 7.64-7.31 (m, 30H), 6.23 (br s, 1H), 6.13 (br s, 1H),
5.93 (m, 1H), 5.27 (m, 1H), 5.09 (m, 1H), 4.00 (m, 1H), 3.84 (m, 1H),
3.69-2.29 (m, 12H), 2.18 (m, 1H), 2.02-1.64 (m, 3H). Anal. Calcd for
Su p p or tin g In for m a tion Ava ila ble: Text giving full
experimental procedures and characterizations for all com-
pounds described in this account and complete tables of
crystallographic data and an ORTEP diagram for 8 (13 pages).
Ordering information is given on any current masthead page.
C
28H34F6O3N3P2ReS: C, 39.34; H, 4.01; N, 4.92. Found: C, 39.82; H,
4.28; N, 4.90.
(19) Characterization of 2c: 84% yield from the protonation (HOTf)
of 1c in acetonitrile. 1H NMR (CD3CN): δ 7.66-7.49 (m, 15 H), 5.78
(br s, 1 H), 4.60 (br s, 1 H), 4.49 (br s, 1 H), 4.43 (br s, 1 H), 4.25 (br
s, 1 H), 4.02 (br s, 1 H), 3.96-3.79 (m, 2H), 3.26-3.04 (m, 5H), 3.03-
2.75 (m, 2H), 2.57 (m, 1H), 2.41 (m, 1H), 0.93 (dd, J ) 6.1 Hz, 15.9 Hz,
1H). Anal. Calcd for C29H35F9O6N3P2ReS2: C, 34.66; H, 3.51; N, 4.18.
Found: C, 34.63; H, 3.15; N, 4.10.
OM960905E
(21) The ratio of isomers in isolated 5b is significantly different than
the 5:2 ratio of its precursor. However, only 59% of the product is
isolated from solution, and it is likely that one diasteromer is less
soluble than the other.
(22) Characterization of 7 (75% from 5a): 1H NMR (CDCl3) δ 5.69
(m, 1H), 5.65 (m, 1H), 3.66 (s, 3H), 3.35 (s, 3H), 3.29 (m, 2H), 3.06 (m,
1H), 2.94 (m, 1H), 2.08 (m, 1H), 1.2 (m,1H), 1.16 (s, 3H), 1.10 (s, 3H);
13C NMR (CDCl3) δ 177.95 (CO), 133.02 (CH), 132.37 (CH), 77.19
(CH2), 72.42 (CH), 58.92 (CH), 53.79 (CH3), 51.66 (CH3), 44.43 (C),
29.12 (CH2), 22.79 (CH3), 22.21 (CH3). Anal. Calcd for C12H20O3: C,
67.89; H, 9.50. Found: C, 67.62; H, 9.88. The regio- and stereochem-
istry was confirmed by 1D-NOE, COSY, and 1H-1H decoupling data.
(20) Characterization of 3c (both isomers): 86% yield from 1c. 1H
NMR (CD3CN): δ 7.64-7.39 (m, 15H), 6.04 (br s, 1H), 5.92 (br s, 1H),
4.74 (br s, 1H), 4.68 (br s, 1H), 4.42 (br s, 1H), 4.35 (br s, 1H), 4.16 (br
s, 1H), 3.98 (br s, 1H), 3.79 (br s, 1H), 3.66 (m, 1H), 3.24 (s, 3H), 3.02
(s, 3H), 3.31-2.71 (m, 20H), 2.40-2.03 (m, 9H), 1.25 (m, 3H). Anal.
Calcd for C29H39N3F9O7P2S2Re: C, 33.99; H, 3.84; N, 4.10. Found: C,
33.71; H, 3.96; N, 4.11.