Direct synthesis of an iridabenzene from a nucleophilic 3-vinyl-1- cyclopropene [1]

Full text of DOI:10.1021/ja984420o

J. Am. Chem. Soc. 1999, 121, 2597-2598
2597
Communications to the Editor
Scheme 1
Direct Synthesis of an Iridabenzene from a
Nucleophilic 3-Vinyl-1-cyclopropene
Robert D. Gilbertson, Timothy J. R. Weakley, and
Michael M. Haley*
Department of Chemistry, UniVersity of Oregon
Eugene, Oregon 97403-1253
ReceiVed December 28, 1998
and a metal-carbon double bond. The use of a vinyl lithiate would
provide the former, whereas rearrangement of a cyclopropene unit
to a vinyl carbene should afford the latter. Our approach would
be a one-step route to metallabenzenes since both of the essential
metal-carbon bonds can be formed from separate ends of a C5
ligand. Combining these features into a single molecule suggested
the use of a functionalized 3-vinyl-1-cyclopropene, such as 2, as
the appropriate precursor. The Z-geometry about the vinyl group
should ensure that both the nucleophilic addition and the cy-
clopropene rearrangement occur with the same organometallic
fragment.
Synthesis of 2 was achieved as shown in Scheme 1. Reduction
of Weinreb amide 39 at low temperature with LiAlH4 gave the
corresponding aldehyde in 93% yield. Wittig reaction of the
aldehyde with iodomethyltriphenylphosphorane10 furnished cy-
clopropene 2 in 81% yield with high stereoselectivity (>95% Z
as determined by 1H NMR integration). Lithium-iodine exchange
of 2 with 1 equiv of butyllithium at -78 °C, followed by trapping
of the intermediate vinyl lithiate with (PPh3)2Ir(CO)Cl directly
resulted in formation of iridabenzene 1. When the initial suspen-
sion was slowly warmed to 0 °C, a color change from yellow to
red was observed. Excess lithiate was quenched with ethanol, and
the insoluble salts were removed by filtration through a glass frit.
After removal of solvent, the crude reaction mixture was dissolved
in a mixture of toluene and hexanes (1:1) at room temperature
and then cooled to -35 °C, giving 1 as X-ray quality red prisms
in 72% yield.
The reactions of cyclopropenes with transition-metal complexes
often generate interesting organometallic products,1 mainly due
to the large amount of strain energy (>50 kcal/mol) contained
within the three-membered ring.2 For example, 3,3-disubstituted
cyclopropenes form metal alkylidenes3 and metallacyclobutenes,4
while ring-opening reactions of 3-vinyl-1-cyclopropenes produce
metallacyclohexadienes and η5-cyclopentadienyl complexes as
well as metallacyclobutenes.5 One particularly attractive aspect
of this chemistry is that a metallacyclohexadiene has been
transformed into a metallabenzene,6 a transition-metal analogue
which is isolobal with benzene and retains aromatic physical and
chemical properties.6,7 We sought a method that would allow
direct entry into the metallabenzene manifold using a cyclopro-
pene precursor. Reported herein is the preparation of iridabenzene
18 directly from the reaction of a nucleophilic 3-vinyl-1-cyclo-
propene with Vaska’s complex and characterization of the metalla-
cycle by X-ray crystallography.
Considering the aforementioned chemistry, this approach
should favor the formation of both a metal-carbon single bond
(1) (a) Goldschmidt, Z. In The Chemistry of the Cyclopropyl Group;
Rappoport, Z., Ed.; Wiley: New York, 1995; Vol. 2, pp 495-695. (b) Binger,
P.; Bu¨ch, H. M. In Topics in Current Chemistry; de Meijere, A., Ed.; Springer-
Verlag: Berlin, 1987; Vol. 135, pp 77-151.
(2) For a comprehensive review of cyclopropenes, see: (a) Methods of
Organic Chemistry (Houben-Weyl); de Meijere, A., Ed.; Thieme: Stuttgart,
1996; Vol. E17d. (b) Halton, B.; Banwell, M. G. In The Chemistry of the
Cyclopropyl Group; Rappoport, Z., Ed.; Wiley: New York, 1987; pp 1223-
1339.
(3) (a) Binger, P.; Muller, P.; Benn, R.; Mynott, R. Angew. Chem., Int.
Ed. Engl. 1989, 28, 610-611. (b) Nguyen, S. T.; Johnson, L. K.; Grubbs, R.
H. J. Am. Chem. Soc. 1992, 114, 3974-3975. (c) Gagne, M. R.; Grubbs, R.
H.; Feldman, J.; Ziller, J. W. Organometallics 1992, 11, 3933-3935. (d)
Johnson, L. K.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993, 115,
8130-8145. (e) Flatt, B. T.; Grubbs, R. H.; Blanski, R. L.; Calabrese, J. C.;
Feldman, J. Organometallics 1994, 13, 2728-2732. (f) Li, R. T.; Nguyen, S.
T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1994, 116, 10032-10040.
(4) (a) Hermond, R. C.; Hughes, R. P.; Robinson, D. J.; Rheingold, A. L.
Organometallics 1988, 7, 2239-2241. (b) Hughes, R. P.; King, M. E.;
Robinson, D. J.; Spotts, J. M. J. Am. Chem. Soc. 1989, 111, 8919-8920. (c)
Binger, P.; Muller, P.; Hermann, A. T.; Phillips, P.; Gabor, B.; Langhau, F.
Chem. Ber. 1991, 124, 2165-2170.
Unambiguous confirmation of the structure of 1 was provided
by single-crystal X-ray diffraction (Figure 1). Selected bond
lengths and bond angles are given in Table 1. The structure is
similar in many respects to that reported for Bleeke’s iridabenzene
(4),6 which has been thoroughly studied. Iridabenzene 1 has a
4
(5) Hughes, R. P.; Trujillo, H. A.; Gauri, A. J. Organometallics 1995, 14,
4319-4324 and references therein.
slightly distorted square pyramidal coordination geometry with
P1 occupying the axial site and C1, C5, C6, and P2 filling the
basal sites. The metallabenzene ring is essentially planar (mean
deviation 0.024 Å) and delocalization of the π system is evident
in bond lengths throughout the ring. The Ir-C1 (2.021(8) Å) and
Ir-C5 (2.025(8) Å) bonds are nearly identical in length, and the
carbon-carbon bonds of the metallacycle are typical of a benzene
derivative. Although the structure of 1 is quite similar to 4,
iridabenzene 1 is air stable in both the solid state and in solution,
whereas 4 reacts with atmospheric oxygen to produce a unique
(6) Bleeke, J. R.; Behm, R.; Xie, Y.-F.; Chiang, M. Y.; Robinson, K. D.;
Beatty, A. M. Organometallics 1997, 16, 606-623 and references therein.
(7) Thorn, D. L.; Hoffmann, R. NouV. J. Chim. 1979, 3, 39-45.
(8) 1 (72% from toluene/hexanes 1:1): mp 196-198 °C. 1H NMR
(benzene-d6) δ 10.79 (ddt, H(5), JH-P ) 11.3 Hz, J5-4 ) 10.1 Hz, J5-3 ) 1.3
Hz, 1H), 8.44 (ddt, H(3), JH-P ) 7.0 Hz, J3-4 ) 7.5 Hz, J3-5 ) 1.3 Hz, 1H),
7.79 (dd, H(4), J4-5 ) 10.1 Hz, J4-3 ) 7.5 Hz, 1H), 7.45 (dd, J ) 8.1, 1.2
Hz, 2H), 7.25-7.13 (m, 16H), 7.03-6.92 (m, 21H), 6.75 (t, J ) 7.5 Hz, 1H).
13C NMR (benzene-d6) δ 203.26 (t, C(6), JC-P ) 51 Hz), 187.74 (t, C(1),
JC-P ) 6.1 Hz), 187.49 (s, C(5)), 169.46 (t, C(4), JC-P ) 4.0 Hz), 147.93 (t,
JC-P ) 4.0 Hz), 142.83 (t, JC-P ) 6.0 Hz), 141.16 (t, C(3), JC-P ) 8.1 Hz),
136.86 (m, PPh3), 134.83 (t, PPh3, JC-P ) 4.5 Hz), 132.02 (s), 130.02 (s,
PPh3), 127.64 (s), 126.50 (s), 125.28 (s), 125.21 (t, JC-P ) 5.0 Hz), 123.49
(s) (two carbon signals are obscured by the benzene solvent). 31P NMR
(benzene-d6, chemical shift reported relative to external H3PO4) δ 17.88 (s).
IR (KBr) 1989 cm-1 (CO). Anal. Calcd for C54H43IrOP2: C, 67.41; H, 4.50.
Found: C, 66.89; H, 4.43.
(9) Hughes, R. P.; Robinson, D. J. Organometallics 1989, 8, 1015-1019.
(10) (a) Stork, G.; Zhao, K. Tetrahedron Lett. 1989, 30, 2173-2174. (b)
Bestmann, H. J.; Rippel, H. C.; Dostalek, R. Tetrahedron Lett. 1989, 30, 5261-
5262.
10.1021/ja984420o CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/09/1999

2598 J. Am. Chem. Soc., Vol. 121, No. 11, 1999
Communications to the Editor
Scheme 2
5
7
6
ment of the η2-cyclopropene in 7 could give 1 directly,14 or
indirectly by first rearranging to 6.15 However, the equilibrium
between 5 and 7 likely favors uncoordinated cyclopropene 5 to a
large degree because of the steric interaction that would result
between the PPh3 ligands and the cyclopropene phenyl groups
when forming 7.16 Direct formation of 6 by insertion of Ir into a
cyclopropene σ-bond would conceivably be a sterically less
demanding process and, therefore, appears more likely than
formation and rearrangement of 7 in our view. Cyclopropenes
frequently undergo rearrangement to vinyl carbenes,2 a reaction
that would allow formation of 1 without involving either 6 or 7.
However, this process typically occurs only at elevated temper-
atures or photochemically and is, therefore, an unlikely event in
the transformation of 5 to 1. Coordination of the cyclopropene
to Ir prior to nucleophilic attack of the vinyl lithiate on the Ir
atom is also unlikely in the present case due to the large cone
angle of the triphenylphosphine ligands.16
In summary, we have prepared iridabenzene 1 by a novel, direct
method. The reaction involves an intramolecular Ir-mediated
rearrangement of a cyclopropene. The individual reactions
involved in our preparation of iridabenzene 1 (i.e., addition of
carbon nucleophiles to low-valent metals and metal-cyclopropene
rearrangements) are commonly used in conjunction with numerous
transition metals.1,3-5,9,12 For this reason, we are currently at-
tempting to extend the methodology described to prepare met-
allabenzenes incorporating different transition metals.
Figure 1. Molecular structure of iridabenzene 1; ellipsoids drawn at the
30% probability level.
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for
Iridabenzene 1
Ir-P1
2.317(2)
2.373(2)
2.021(8)
2.025(8)
1.925(9)
1.41(1)
1.41(1)
1.38(1)
1.33(1)
1.14(1)
P1-Ir-P2
P1-Ir-C1
P1-Ir-C5
P1-Ir-C6
P2-Ir-C1
C5-Ir-C6
C1-Ir-C5
Ir-C1-C2
C1-C2-C3
C2-C3-C4
C3-C4-C5
C4-C5-Ir
99.2(2)
112.6(2)
95.4(3)
Ir-P2
Ir-C1
Ir-C5
Ir-C6
C1-C2
C2-C3
C3-C4
C4-C5
C6-O
99.3(3)
148.2(2)
165.0(4)
86.9(3)
128.8(6)
123.6(8)
124.2(8)
125.5(8)
130.7(7)
Acknowledgment. We gratefully acknowledge the National Science
Foundation (CHE-9502588) and The Camille and Henry Dreyfus Founda-
tion (Teacher-Scholar Award 1998-2003 to M.M.H.) for support of this
research.
dioxygen-bridged species.6 Iridabenzene 1 does, however, react
with maleic anhydride in a [4 + 2] fashion to give an octahedral
adduct.11
A likely reaction pathway is shown in Scheme 2. Initial
substitution of the chloride ligand by the vinyl lithiate should
give 5.12 Bicyclic, Dewar-type metallabenzene 6 could then be
formed by direct insertion of the Ir atom of 5 into one of the
cyclopropene σ-bonds. An intermediate such as 6 could rapidly
valence isomerize to 1.13 Alternatively, the free cyclopropene in
complex 5 could coordinate to the iridium through the cyclopro-
pene π electrons to give an intermediate similar to 7. Rearrange-
Supporting Information Available: General experimental procedures
and spectral data for 1 and 2, X-ray crystal structure of 1, tables of atomic
coordinates, thermal parameters, bond lengths and bond angles (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
JA984420O
(14) Direct conversion of transition metal η2-cyclopropenes to metal
carbenes has been observed. See ref 3d and: Fischer, H.; Hofmann, J.; Mauz,
E. Angew. Chem., Int. Ed. Engl. 1991, 30, 998-999.
(15) Direct rearrangement of η2-cyclopropene complexes to metallacy-
clobutenes has been observed. See ref 4c.
(11) Cycloaddition reactions involving iridabenzene 4 are described in ref
6.
(16) Coordination complexes of 3,3-diphenylcyclopropene to Vaska-type
complexes are in equilibrium with the uncoordinated starting materials. With
small cone angle phosphines the equilibrium lies far to the side of the metal
η2-cyclopropene complex. However, with larger cone angle phosphines, such
as PPh3, the equilibrium lies exclusively to the side of the free cyclopropene.3f
Additionally, NMR experiments in our lab have shown that (PPh3)2Ir(CO)Cl
does not coordinate with 2 in benzene at ambient temperature.
(12) Schwartz, J.; Hart, D. W.; McGiffert, B. J. Am. Chem. Soc. 1974, 96,
5613-5614.
(13) For a discussion concerning the interconversion of metallacyclobutenes
and vinylcarbene complexes see: Doxsee, K. M.; Juliette, J. J. J.; Mouser, J.
K. M.; Zientara, K. Organometallics 1993, 12, 4742-4744.