[2 + 2 + 1] alkyne cyclotrimerization: A metallacyclopentadiene route to fulvenes

Full text of DOI:10.1021/ja970007p

J. Am. Chem. Soc. 1997, 119, 3631-3632
3631
[2 + 2 + 1] Alkyne Cyclotrimerizations: A
Metallacyclopentadiene Route to Fulvenes
Joseph M. O’Connor,* Kristin Hiibner, Rich Merwin,
Peter K. Gantzel, and Bella S. Fong
Department of Chemistry (0358), 9500 Gilman DriVe
UniVersity of California, San Diego
La Jolla, California 92093-0358
Melanie Adams and Arnold L. Rheingold*
Department of Chemistry, UniVersity of Delaware
Newark, Delaware 19716
ReceiVed January 2, 1997
Metal-mediated [2 + 2 + 2] cyclotrimerization of alkynes
to produce aromatic six-membered rings is now established as
an important route toward natural products and novel organic
materials.1 In sharp contrast, reports of [2 + 2 + 1] cyclotri-
merization of alkynes to produce the thermodynamically less
stable fulvene products are exceedingly rare (Scheme 1).2,3 In
principle, metallacyclopentadiene complexes may serve as
intermediates for both types of cycloaddition reactions. Direct
carbocycle formation from an η2-alkyne-metallacyclopentadiene
intermediate gives benzene product whereas an η2-alkyne to
vinylidene isomerization followed by carbocycle formation
would lead to fulvene product.
Figure 1. ORTEP drawing of 5 showing selected atom labeling.
Scheme 1
Previously, we reported a number of reactions involving
iridiacyclopentadienes and terminal alkynes (e.g., 1 to 2, Scheme
2) that appear to proceed through vinylidene intermediates (3).4
In no case was coupling observed between the vinylidene ligand
and the butadiendiyl ligand. Intrigued by the possibility that a
facial arrangement of the vinylidene and butadiendiyl ligands
would prove to be a more favorable geometry for fulvene
formation, we set out to prepare a metallacycle complex of the
1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) ligand and
undertake reactivity studies with respect to terminal alkyne
substrates. Ηerein we report the first examples of the desired
metallacyclopentadiene to fulvene transformation.
Scheme 2
are heated at reflux for 24 h in toluene (20 mL), the triphos
When (PPh3)2Ir[CRdCRCRdCR]Cl (R ) CO2CH3)5 (0.96
complex (CH3C(CH2PPh2)3)Ir(CRdCRCRdCR)Cl (4, R )
CO2CH3) precipitates as a white solid in 95% yield (Scheme
3). Addition of phenylacetylene (0.88 mmol) to a methylene
chloride solution of 4 (200 mg, 0.18 mmol, 4.5 mM) and AgBF4
(0.19 mmol) at room temperature (16 h) led to isolation of an
orange-red solid (6)6 which was dissolved in wet chloroform
and stirred for 1 h to give fulvene complex 5 in 80% isolated
yield.7
mmol) and 1,1,1-tris(diphenylphosphinomethyl)ethane (1.2 mmol)
(1) For leading references to late metal alkyne cyclotrimerization see:
(a) Cruciani, P.; Aubert, C.; Malacria, M. Synlett 1996, 105. (b) Bose, R.;
Matzger, A. J.; Mohler, D. L.; Vollhardt, K. P. C. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1478. (c) Saa, C.; Crotts, D. D.; Hsu, G.; Vollhardt, K. P.
C. Synlett 1994, 487. (d) Bianchini, C.; Caulton, K. G.; Chardon, C.;
Doublet, M.-L.; Eisenstein, O.; Jackson, S. A.; Johnson, T. J.; Meli, A.;
Peruzzini, M.; Streib, W. E.; Vacca, A.; Vizza, F. Organometallics 1994,
13, 2010. (e) Sato, Y.; Nishimata, T.; Mori, M. J. Org. Chem. 1994, 59,
6133. (f) Vollhardt, K. P. C. Pure Appl. Chem. 1993, 65, 153. (g) Nambu,
M.; Mohler, D. L.; Hardcastle, K.; Baldridge, K. K.; Siegel, J. S. J. Am.
Chem. Soc. 1993, 115, 6138. (h) Vollhardt, K. P. C. Angew. Chem., Int.
Ed. Engl. 1984, 23, 539.
(2) (a) Moran, G.; Green, M.; Orpen, A. G. J. Organomet. Chem. 1983,
250, C15. (b) Moreto, J.; Maruya, K.; Bailey, P. M.; Maitlis, P. M. J. Chem.
Soc., Dalton Trans. 1982, 1341. (c) It is interesting to note that the
mechanisms suggested for alkyne cyclization in refs 2a and 2b do not involve
metallacyclopentadiene intermediates. (d) See also: Liebeskind, L. S.;
Chidambaram, R. J. Am. Chem. Soc. 1987, 109, 5025.
1
In the H NMR spectrum (CDCl3) of 5 a vinyl hydrogen
resonance was observed as an apparent triplet at δ 6.28 (3JPH
) 7.0 Hz). A 13C-1H heteronuclear correlation 2D NMR
experiment (HMQC)8 established that the hydrogen giving rise
to the δ 6.28 resonance in the 1H NMR spectrum is attached to
a carbon which resonates at 45.8 ppm (2JPC ) 51.2 Hz, 1JCH
)
148 Hz) in the 13C NMR spectrum.9 The downfield chemical
shift of the benzylic hydrogen (δ 6.28) and the 148 Hz carbon-
hydrogen coupling constant argue against an agostic interac-
(3) Cyclotrimerization of 3,3-dimethylbut-1-yne to a fulvene product has
been observed with an early metal catalyst: Prof. I. P. Rothwell, personal
communication.
(6) In a 1H NMR tube scale reaction (CDCl3), 4 and phenylacetylene
gave rise to a new complex (6) that exibits four carboxymethyl singlets at
δ 4.03, 3.67, 3.38, and 2.85. Addition of water to the solution cleanly
converted 6 to 5 and methanol. We tentatively assign a fulvene structure to
6 as indicated in Scheme 3.
(4) (a) O’Connor, J. M.; Pu, L.; Rheingold, A. L. J. Am. Chem. Soc.
1989, 111, 4129. (b) O’Connor, J. M.; Pu, L.; Chadha, R. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 543. (c) O’Connor, J. M.; Pu, L.; Rheingold, A. L.
J. Am. Chem. Soc. 1990, 112, 6232. (d) O’Connor, J. M.; Pu, L. J. Am.
Chem. Soc. 1990, 112, 9013. (e) O’Connor, J. M.; Pu, L.; Chadha, R. K. J.
Am. Chem. Soc. 1990, 112, 9627. (f) O’Connor, J. M.; Pu, L.; Rheingold,
A. L. J. Am. Chem. Soc. 1990, 112, 9663. (g) O’Connor, J. M.; Hiibner,
K.; Rheingold, A. L. J. Chem. Soc., Chem. Commun. 1995, 1209. (h)
O’Connor, J. M; Hiibner, K.; Merwin, R.; Pu, L.; Rheingold, A. L. J. Am.
Chem. Soc. 1995, 117, 8861.
(7) Characterization details are provided as Supporting Information.
(8) Summers, M. F.; Marzilli, L. G.; Bax, A. J. Am. Chem. Soc. 1986,
108, 4285.
(9) The η2-ethylene ligand in (CH3C(CH2PPh2)3)Ir(η2-CH2dCH2)Cl
exhibits resonances at δ 1.23 and 2.37 ppm in the 1H NMR spectrum (CD2-
Cl2) and at δ 26.2 ppm in the 13C NMR spectrum (CD2Cl2): Barbaro, P.;
Bianchini, C.; Meli, A.; Peruzzini, M.; Vacca, A.; Vizza, F. Organometallics
1991, 10, 2227.
(5) Collman, J. P.; Kang, J. W.; Little, W. F.; Sullivan, M. F. Inorg.
Chem. 1968, 7, 1298.
S0002-7863(97)00007-3 CCC: $14.00 © 1997 American Chemical Society

3632 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Communications to the Editor
Scheme 3a
a Complexes 6, 10, and 12 are intermediates.
tion.10 When phenyl acetylene-d1 (PhCtCD) is employed in
a similar reaction with metallacycle 4 and AgBF4, H NMR
-
Ir[CRdCRCRdCR][dC(CH2)3O]+BF4 (8, R ) CO2CH3).4
1
The carbene ligand in 8 is formed by intramolecular trapping
of a vinylidene intermediate by the pendent hydroxyl group.
Remarkably, when 3-butyn-1-ol is added to a methylene chloride
solution of AgBF4 and 4, the π-allyl complex 9 is formed and
isolated as a yellow powder in 48% yield (Scheme 3).7 The
formation of 9 is more complicated than in the case of 5 and 7
due to dehydration of the primary alcohol. In order to
circumvent the dehydration we employed 4-pentyn-1-ol in a
reaction with 4/AgBF4 and observed formation of alkene
complex 11,7 which was isolated in 36% yield. Complex 11 is
presumably derived from a fulvene intermediate such as 12,
which undergoes loss of an allylic proton to generate the cis
η2-alkene ligand. The structures of both 9 and 11 were
established by X-ray crystallographic analysis (Scheme 3).12
The butadiendiyl ligand of 4 was originally derived from 2
equiv of dimethyl acetylenedicarboxylate, thus the net trans-
formations reported here represent [2 + 2 + 1] cyclotrimer-
ization of alkynes to give fulvenoid product, in preference to
the aromatic compounds normally formed in alkyne cyclotri-
merizations. Preliminary results indicate that the terminal alkyne
substituents exert a remarkable influence on the partitioning
between [2 + 2 + 1] and [2 + 2 + 2] cycloaddition pathways
in the reactions of 4/AgBF4. Thus, acetylene, propargyl alcohol,
and ethyl ethynyl ether all give high yields of [2 + 2 +
2]-derived products from reaction with 4 and AgBF4.13 Efforts
are currently underway to determine the scope and mechanism
of these new cyclotrimerization reactions in order to define the
parameters that control partitioning between [2 + 2 + 1] and
[2 + 2 + 2] cycloaddition pathways.
spectroscopic analysis of the product, 5-d1, indicates complete
(>95%) incorporation of deuterium into the hydrogen site,
which gave rise to the δ 6.28 resonance. In the vinyl region of
the 13C{1H} NMR spectrum (CDCl3) of 5, numerous signals
are observed for the carbons of the seven nonequivalent phenyl
groups and the fulvene ring carbons.
The ambiguous spectroscopic data led us to carry out a single-
crystal X-ray diffraction study in order to elucidate the structure
for 5 (Figure 1).7 The complex adopts a distorted T-shaped
trigonal bipyramid geometry with P(1), P(2), and the alkene
ligand (C(1h), C(6h)) in the equatorial plane. The interplanar
angle between the P(2)-Ir-P(1) and C(1h)-Ir-C(6h) planes
is 5.7°. The five-membered carbon ring of the fulvene ligand
is nearly planar with the largest deviation of the ring atoms
from the mean plane at C(3h) (0.02 Å).11 The Ir-C(1h) distance
of 2.403(16) Å, the C(1h)-C(6h) distance of 1.577(25), and
the Ir-C(1h)-C(6h) bond angle of 79.9(9)° all point to a major
contribution to the ground state structure of 5 from charge-
separated resonance structure 5-B.
When 3,3-dimethylbut-1-yne is employed as the alkyne
substrate in reaction with 4 and AgBF4, the cationic fulvene
complex 7 is isolated as a bright-orange solid in 77% yield.7 In
the 1H NMR spectrum of 7 (CDCl3) a vinyl hydrogen resonance
is observed as an apparent triplet at δ 6.36 (dd, 3JPH ) 8.0, 5.5
Hz), and four methyl singlets are observed at δ 4.0, 3.77, 3.37,
and 3.21. As was observed for 5, an HMQC (13C-1H) NMR
experiment established that the hydrogen giving rise to the δ
6.36 resonance in the 1H NMR spectrum is attached to a carbon
that resonates upfield at 57.7 ppm (2JCP ) 53.8 Hz, 1JCH ) 143
Hz) in the 13C NMR spectrum. We assign a fulvene structure
to 7 based on the similarity of the spectroscopic properties to
those for 5.
Acknowledgment. Support by the National Science Foundation
(CHE-9520213) and a generous loan of precious metals from Johnson
Matthey are gratefully acknowledged.
A reasonable mechanism for the formation of 5 and 7
from 4 involves initial coordination of the alkyne to the metal,
rearrangement to a vinylidene ligand, and reductive cyclization
to the carbocycle product. In an effort to trap the vinylidene
ligand in this system we examined the reactions of 3-butyn-
1-ol and 4-pentyn-1-ol with 4. Previously, we observed
the reaction of 1 with 3-butyn-1-ol to give a quantitative
yield of the oxacyclopentylidene complex, (PPh3)2(CO)-
Supporting Information Available: Characterization data for 4,
5, 7, 9, and 11 and details of crystal structure determinations, diagrams,
listings of fractional coordinates, bond distances, bond angles, hydrogen
atom coordinates, and thermal parameters for 5, 9, and 11 (41 pages).
See any current masthead page for ordering and Internet access
instructions.
JA970007P
(12) The alkene-iridium bonding in 11 is much more symmetric than
that observed for 5. For 11: Ir-C(11) 2.166(11) Å, Ir-C(12) 2.123(10)
Å, Ir-C(11)-C(12) 68.8(6)°, and Ir-C(12)-C(11) 72.1(6)°.
(13) (a) O’Connor, Hiibner, unpublished results. (b) See also: Bianchini,
C.; Caulton, K. G.; Chardon, C.; Eisenstein, O.; Folting, K.; Johnson, T.
J.; Meli, A.; Peruzzini, M.; Rauscher, D. J.; Streib, W. E.; Vizza, F. J. Am.
Chem. Soc. 1991, 113, 5127.
(10) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem.
1988, 36, 1.
(11) Deviations from the C(1h)-C(5h) plane: C(1h) 0.001, C(2h)
-0.014, C(3h) 0.022, C(4h) -0.021, C(5h) 0.011, C(6h) -0.054, C(7h)
-0.215.