Cobalt-mediated η5-pentadienyl/alkyne [5 + 2] cycloaddition. Synthesis and characterization of unbridged η2, η3-coordinated cycloheptadienyl complexes

Article abstract of DOI:10.1021/ja710568d

A general new metal-mediated [5 + 2] cycloaddition reaction of η⁵-pentadienyl cobalt complexes and alkynes is reported, the first such cycloaddition reaction to provide controlled incorporation of 1 equiv of alkyne. The reaction of terminally substituted pentadienyl complexes proceeds thermally under exceptionally mild conditions, affording cycloheptadienyl complexes cleanly and in high yield. As a consequence of the unusually low kinetic barrier to the cycloaddition, reactions with acetylene at low temperature produce the cycloheptadienyl ring system exclusively as the η²,η³-isomer, an unprecedented coordination mode for unbridged seven-membered ring complexes. Isomerization of the kinetic product to the fully conjugated η⁵-cycloheptadienyl isomer is observed quantitatively upon heating. Disubstituted alkynes, as represented by 2-butyne, proceed through the η²,η³-intermediate to the thermodynamic η⁵-complex, but the rate of initial alkyne incorporation is attenuated to the extent that isomerization is competitive with the cycloaddition, preventing isolation of the η²,η³-intermediates. The reaction with terminal alkynes is relatively insensitive to modest steric or electronic influences, providing mixtures of regioisomeric products. Consistent with these results, a dissociative mechanism is proposed, initiated by the unusually facile η⁵→η³ isomerization of the pentadienyl ligand. The pentadienyl complexes themselves arise from protonolysis of an organic 1,4-pentadien-3-ol in the presence of the labile Co(I) complex, (η⁵-pentamethylcyclopentadienyl)cobalt(ethylene)₂, suggesting that the reaction can be considered a novel interrupted Nazarov cyclization, amenable to further development in an explicitly organic context. Copyright

Full text of DOI:10.1021/ja710568d

Published on Web 01/29/2008
Cobalt-Mediated η5-Pentadienyl/Alkyne [5 + 2] Cycloaddition. Synthesis and
Characterization of Unbridged η2,η3-Coordinated Cycloheptadienyl Complexes
Ross D. Witherell, Kai E. O. Ylijoki, and Jeffrey M. Stryker*
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received December 2, 2007; E-mail: jeff.stryker@ualberta.ca
Scheme 1 a
Novel transition metal-mediated reactivity patterns provide the
conceptual basis for the development of new synthetic reactions.
Particularly for larger ring systems, metal-mediated cycloaddition
pathways can provide convergent access to a range of substituted
and functionalized carbocyclic systems that would be otherwise
laborious to prepare.1 The [5 + 2] cycloaddition of an alkyne and
η5-pentadienyl complex remains an unsolved problem, despite
considerable effort.2 Without exception, the reaction suffers from
limited scope, low yields, and/or poor product control, often
preferentially proceeding to higher order cycloadducts incorporating
additional alkyne.3
To evaluate the hypothesis that a cobalt-mediated [5 + 2] alkyne/
η5-pentadienyl cycloaddition (IIIfIV, eq 1) might be embedded
in the mechanism of our recently reported η3-cyclopentenyl/alkyne
ring expansion reaction (IfIV),4 we initiated an investigation into
the preparation and reactivity of acyclic (open) η5-pentadienyl cobalt
complexes. Although the transformation of agostic η3-cyclopentenyl
complexes to ring-opened η5-pentadienyl products (e.g., IIfIII)
has been demonstrated in the solid state at elevated temperature,5
the reactivity of cobalt η5-pentadienyl complexes toward alkynes
is unknown.
a Conditions: (a) Cp*Co(CH2CH2)2, HBF4•OEt2, acetone, -78 °C; (b)
KPF6, H2O (not performed for 1f).
temperature (eq 2, Table 1). Reactions with excess acetylene
proceed slowly (entries 1, 6, and 9), providing nonconjugated η2,η3-
cycloheptadienyl cycloadducts 2 with excellent selectivity. The
structures of the products were unambiguously determined by
spectroscopic analysis and confirmed for complex 2a by X-ray
crystallography.15
All three η2,η3-cycloadducts isomerize quantitatively to the fully
conjugated η5-cycloheptadienyl complexes 3b, 3f, and 3i upon
heating; the more reactive phenyl derivative 2f converts slowly even
at room temperature.15 Cycloaddition reactions incorporating 2-bu-
tyne proceed slowly but quantitatively (entries 2 and 7). In these
cases, however, the rate of cycloadduct isomerization is competitive
with the rate of the initial cycloaddition, rendering it impossible to
isolate the η2,η3-cycloheptadienyl intermediates selectively.17 This
situation is maintained for the reactions of the terminal alkynes,
1-pentyne and ethoxyacetylene (entries 3 and 4), which afford η5-
cycloadducts in excellent yields. Neither reaction, however, shows
appreciable steric or electronic control over the regioselectivity of
alkyne insertion. Cycloaddition of the more sterically demanding
trimethylsilylethyne with 1-phenylpentadienyl complex 1f proceeds
to a single η5-cycloadduct 3h (entry 8), albeit in greatly diminished
yield.18 Confirmation of the structure of 3h was obtained by X-ray
crystallography.15
To our considerable consternation, terminally unsubstituted
pentadienyl complexes exhibit significantly attenuated reactivity
toward alkynes. Neither the unsubstituted pentadienyl nor 2-
methylpentadienyl complex (1a, 1c) reacts with ethyne or 2-butyne
at or below 40 °C; both convert slowly at 60 °C but give only
intractable product mixtures. The more highly substituted 1,2,4-
trimethylpentadienyl complex 1e is similarly inert, consistent with
the greater stability noted for ancillary pentadienyl ligands alkylated
in the 2- and 4-positions.11,19 Significantly, however, the 1,2-
dimethylpentadienyl complex 1d is more promising, reacting with
ethyne at 60 °C (16 h) to yield a mixture of η2,η3- and η5-
cycloadducts 2e and 3e, stronglysand surprisinglysbiased toward
the kinetic η2,η3-isomer (entry 5).
Here we report the discovery of a general [5 + 2] cycloaddition
reaction, providing seven-membered ring complexes in high yield
under notably mild conditions. For some substituent arrays, the
reaction proceeds with high selectivity to give the cycloheptadienyl
ring as the η2,η3-coordinated isomer, an unprecedented coordination
mode for unbridged cycloheptadienyl systems.6-9 Subsequent
isomerization to the fully conjugated η5-cycloheptadienyl product
occurs under thermodynamic control.
Acyclic η5-pentadienyl complexes of cobalt are known but are
rare.5,10,11 An efficient and reasonably general entry into the
synthesis of the requisite cationic η5-pentadienyl complexes 1 has
been developed by adapting a ligand exchange/protonolysis strategy
previously reported for converting conjugated dienols to pentadienyl
complexes of rhodium and iridium.12
Thus, readily available 1,4-alkadien-3-ol substrates typically
associated with the Nazarov cyclization13 react with HBF4•OEt2
and (C5Me5)Co(C2H4)2 to afford η5-pentadienyl complexes 1 in
14
reasonable isolated yields (Scheme 1).15 The reaction gradually
diminishes in utility with increasing substitution of the substrate,
presumably due to competing off-metal cyclization/oligomerization
pathways. The air-stable complexes are readily isolated and purified
by chromatography on the bench.16 In addition to new complexes
1a-f, the corresponding η5-1-ethylpentadienyl complex 1g was pre-
pared as the BF4 salt according to the previously published pro-
cedure.5b
Mechanistically, these results are consistent with a dissociatiVe
process initiated by an η5fη3 pentadienyl isomerization (Scheme
2), which is unexpectedly facile for pentadienyl ligands bearing
terminal substituents. Subsequent alkyne coordination, insertion,
and transannular cyclization are fully consistent with conceptually
related [3 + 2 + 2] allyl/alkyne cycloaddition reactions.6e-g,7 Most
unusual, however, is the determination that the kinetic barrier to
cycloheptadienyl valence isomerization from η2,η3- to η5-coordina-
Although coordinatively saturated, 1-substituted pentadienyl
complexes 1b, 1f, and 1g incorporate alkyne thermally even at room
9
2176
J. AM. CHEM. SOC. 2008, 130, 2176-2177
10.1021/ja710568d CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Cycloheptadienyl Synthesis by [5 + 2] Cycloadditiona
(2) Alkyne/η5-pentadienyl cycloadditions: (a) Wilson, A. M.; Waldman, T.
E.; Rheingold, A. L.; Ernst, R. D. J. Am. Chem. Soc. 1992, 114, 6252-
6254. (b) Kreiter, C. G.; Koch, E.-C.; Frank, W.; Reiss, G. Inorg. Chim.
Acta 1994, 220, 77-83. (c) Wang, C.; Sheridan, J. B.; Chung, H. J.; Cote´,
M. L.; Lalancette, R. A.; Rheingold, A. L. J. Am. Chem. Soc. 1994, 116,
8966-8972. (d) Kreiter, C. G.; Fiedler, C.; Frank, W.; Reiss, G. J. Chem.
Ber. 1995, 128, 515-518. (e) Kreiter, C. G.; Fiedler, C.; Frank, W.; Reiss,
G. J. J. Organomet. Chem. 1995, 490, 133-141. (f) Kreiter, C. G.; Koch,
E.-C.; Frank, W.; Reiss, G. J. Z. Naturforsch. B 1996, 51B, 1473-1485.
(g) Chen, W.; Chung, H.-J.; Wang, C.; Sheridan, J. B.; Cote´, M. L.;
Lalancette, R. A. Organometallics 1996, 15, 3337-3344. (h) Chung, H.-
J.; Sheridan, J. B.; Cote´, M. L.; Lalancette, R. A. Organometallics 1996,
15, 4575-4585. (i) Tomaszewski, R.; Hyla-Kryspin, I.; Mayne, C. L.;
Arif, A. M.; Gleiter, R.; Ernst, R. D. J. Am. Chem. Soc. 1998, 120, 2959-
2960. (j) Basta, R.; Harvey, B. G.; Arif, A. M.; Ernst, R. D. Inorg. Chim.
Acta 2004, 357, 3883-3888. (k) Harvey, B. G.; Mayne, C. L.; Arif, A.
M.; Tomaszewski, R.; Ernst, R. D. J. Am. Chem. Soc. 2005, 127, 16426-
16435.
entry
substrate
alkyne
2/yield (%)b
3/yield (%)c
1
1b (R1 ) Me;
R, R′ ) H
2a/98
3a/99
R2, R3 ) H)
2
3
1b
1b
R, R′ ) Me
R ) H
R′ ) OEt
-
-
3b/99
3c:3c′ (2:1)/82
3c: R ) H,
R′ ) OEt
3c′: R ) OEt,
R′ ) H
(3) Related [5 + 2] cycloadditions of polar alkenes and 1-vinyl-η3-allyl (η3-
pentadienyl) complexes: (a) Yueh, T.-C.; Lush, S.-F.; Lee, G.-H.; Peng,
S.-M.; Liu, R.-S. Organometallics 1996, 15, 5669-5673. (b) Yin, J.;
Liebeskind, L. S. J. Am. Chem. Soc. 1999, 121, 5811-5812. (c)
Malinakova, H. C.; Liebeskind, L. S. Org. Lett. 2000, 2, 3909-3911. (d)
Malinakova, H. C.; Liebeskind, L. S. Org. Lett. 2000, 2, 4083-4086. (e)
Zhang, Y.; Liebeskind, L. S. J. Am. Chem. Soc. 2006, 128, 465-472.
(4) Dzwiniel, T. L.; Stryker, J. M. J. Am. Chem. Soc. 2004, 126, 9184-
9185.
(5) (a) Bennett, M. A.; Nicholls, J. C.; Rahman, A. K. F.; Redhouse, A. D.;
Spencer, J. L.; Willis, A. C. J. Chem. Soc., Chem. Commun. 1989, 1328-
1330. (b) Nicholls, J. C.; Spencer, J. L. Organometallics 1994, 13, 1781-
1787. (c) Cracknell, R. B.; Nicholls, J. C.; Spencer, J. L. Organometallics
1996, 15, 446-448.
(6) See, for example: (a) Salzer, A.; Werner, H. J. Organomet. Chem. 1975,
87, 101-108. (b) Salzer, A.; Bigler, P. Inorg. Chim. Acta 1981, 48, 199-
203. (c) Williams, G. M.; Fisher, R. A.; Heyn, R. H. Organometallics
1986, 5, 818-819. (d) Chen, W.; Sheridan, J. B.; Cote´, M. L.; Lalancette,
R. A. Organometallics 1996, 15, 2700-2706 and references therein. (e)
Schwiebert, K. E.; Stryker, J. M. J. Am. Chem. Soc. 1995, 117, 8275-
8276. (f) Etkin, N.; Dzwiniel, T. L.; Schwiebert, K. E.; Stryker, J. M. J.
Am. Chem. Soc. 1998, 120, 9702-9703. (g) Dzwiniel, T. L.; Etkin, N.;
Stryker, J. M. J. Am. Chem. Soc. 1999, 121, 10640-10641. See also refs
3a, c, and d.
4
1b
R ) H
-
3d:3d′ (2:1)/91
3d: R ) H,
R′ ) nPr
R′ ) nPr
3d′: R ) nPr,
R′ ) H
5c
6
1d (R1, R2 ) Me; R, R′ ) H
R3 ) H)
2e + 3e
(3.8:1)/68d
1f (R1 ) Ph;
R, R′ ) H
2f/89
3f/99
R2, R3 ) H)
7
8
1f
1f
R, R′ ) Me
R ) H
R′ ) TMS
-
-
3g/96
3h/26e
R ) H,
R′ ) TMS
3i/99
9
1g (R1 ) Et;
R2, R3 ) H)
R, R′ ) H
2i/91
(7) The [3 + 2 + 2] reactions leading to the first unbridged η1,η4-coordinated
cycloheptadienyl complexes have been reported (Ru): Older, C. M.;
McDonald, R.; Stryker, J. M. J. Am. Chem. Soc. 2005, 127, 14202-14203.
(8) The assertion that protonation of Cp*Co(η4-cycloheptatriene) leads to the
formation of the [Cp*Co(η2,η3-cycloheptadienyl)] cation is not supported
by spectroscopic data: Lewis, J.; Parkins, A. W. J. Chem. Soc. (A) 1969,
953-957. The isolated product from this reaction was later established
to be the expected η5-cycloheptadienyl cation.6b
a Detailed conditions reported in the Supporting Information; yields of
isolated products after SiO2 chromatography (3-4% MeOH in CH2Cl2).
b Ethyne (saturated solution in CH2Cl2), rt, 12-20 h. Minor amounts of
the fully conjugated product 3 are detectable by NMR spectroscopy. c As
above but 40-60 °C, 24-72 h (rt, 72 h for 3g). d An additional minor
byproduct, tentatively identified as a [3 + 2 + 2] cycloadduct, is detected
(9) For references to bridged bicyclic η2,η3-cycloheptadienyl ligands, see the
Supporting Information.
1
in the NMR spectrum of this product.15 e Yield determined by H NMR
integration using 1,3,5-trimethoxybenzene as an internal standard.
(10) (a) Bleeke, J. R.; Peng, W.-J. Organometallics 1984, 3, 1422-1426. (b)
Bleeke, J. R.; Peng, W.-J. Organometallics 1986, 5, 635-644. (c) Lee,
G.-H.; Peng, S.-M.; Liao, M.-Y.; Liu, R.-S. J. Organomet. Chem. 1986,
312, 113-120. (d) Ernst, R. D.; Ma, H.; Sergeson, G.; Zahn, T.; Ziegler,
M. L. Organometallics 1987, 6, 848-853. (e) Butovskii, M. V.; Englert,
U.; Herberich, G. E.; Kirchner, K.; Koelle, U. Organometallics 2003, 22,
1989-1991.
(11) Reviews of metal pentadienyl chemistry: (a) Ernst, R. D. Chem. ReV.
1988, 88, 1255-1291. (b) Ernst, R. D. Comments Inorg. Chem. 1999,
21, 285-325.
(12) (a) Powell, P. J. Organomet. Chem. 1981, 206, 239-255. (b) Krivykh,
V. V.; Gusev, O. V.; Petrovskii, P. V.; Rybinskaya, M. I. J. Organomet.
Chem. 1989, 366, 129-145.
Scheme 2
(13) Recent reviews: (a) Pellissier, H. Tetrahedron 2005, 61, 6479-6517. (b)
Frontier, A. J.; Collison, C. Tetrahedron 2005, 61, 7577-7606.
(14) (a) (C5Me5)Co(C2H4)2: Nicholls, J. C.; Spencer, J. L. Inorg. Synth. 1990,
28, 278. (b) (C5Me5)Co(C2H4)2/HBF4: Brookhart, M.; Lincoln, D. M.;
Bennett, M. A.; Pelling, S. J. Am. Chem. Soc. 1990, 112, 2691-2694. (c)
Brookhart, M.; Lincoln, D. M.; Volpe, A. F.; Schmidt, G. F. Organome-
tallics 1989, 8, 1212-1218.
tion is rate-limiting in this system, allowing isolation and, we
presume, exploitation of the novel η2,η3-coordination mode.20
Cobalt-mediated [5 + 2] cycloaddition represents a general new
reaction for the convergent synthesis of seven-membered rings. In
combination with post-cycloaddition alkylation/demetalation strate-
gies, currently in development, the process can be construed as a
novel “interrupted Nazarov” cyclization,21 in which the cationic
intermediate is intercepted prior to the electrocyclization.
(15) Complete experimental details are provided as Supporting Information.
(16) Complexes 1b-f have also been characterized by X-ray crystallography;
details will be provided in a subsequent report.
(17) Spectroscopic analysis of the reaction of 1b with 2-butyne at intermediate
times (ca. 40% conversion) reveals the presence of both η2,η3- and η5-
cycloadducts 2b and 3b, with 2b in low concentration. Complete
conversion to 3b is obtained upon prolonged reaction time, establishing
that the η2,η3-isomer is an intermediate in the formation of the η5-
cycloadduct.
(18) A single cycloheptadienyl product was detected spectroscopically in the
complex reaction mixture. No tractable product is isolated from reactions
of 1-methylpentadienyl complex 1b with tert-butyl-, trimethylsilyl-, or
phenylacetylene, which return complex, partly paramagnetic mixtures.
(19) Preliminary investigation suggests that the reactivity in the parent
(cyclopentadienyl)cobalt series is enhanced, allowing the [5 + 2] cyclo-
addition reaction to proceed even with the unsubstituted pentadienyl
complex.
Acknowledgment. We thank Prof. R. D. Ernst for generous
discussions, and Drs. R. McDonald and M. Ferguson for X-ray
crystallography. Financial support from NSERC, the Province of
Alberta, and the University of Alberta is gratefully acknowledged.
Supporting Information Available: Experimental procedures and
complete characterization data for all new compounds; details of the
X-ray crystallography for complexes 2a and 3h. This material is
available free of charge via the Internet at http://pubs.acs.org.
(20) Alkylation/oxidative decomplexation protocols for demetallation of the
organic remain under investigation; for related reactions, see ref 6f.
(21) See, for example: (a) Song, D.; Rostami, A.; West, F. G. J. Am. Chem.
Soc. 2007, 129, 12019-12022 and references therein. (b) Tius, M. A.
Acc. Chem. Res. 2003, 36, 284-290.
References
(1) An extensive list of reviews and lead references for metal-mediated seven-
membered ring synthesis is provided as Supporting Information.
JA710568D
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2177