[3+3] cyclodimerization of methylenecyclopropanes: Stoichiometric and catalytic reactions of Nickel(0) with electron-deficient alkylidenecyclopropanes

Article abstract of DOI:10.1021/om100317y

Stoichiometric treatment of Ni(cod)₂ with ethyl cyclopropylideneacetate (ECPA) in the presence of PCy₃ resulted in an unpredicted formation of a Ni(0) complex bearing an (E,E)-1,2-bis(exo- alkylidene)cyclohexane ligand, which stemmed from the [3 + 3] cyclodimerization of ECPA. The reaction could be expanded to a Ni(0)-catalyzed [3 + 3] cyclodimerization reaction of ester-substituted methylenecyclopropanes, giving the corresponding cyclohexane derivatives in excellent yields.

Full text of DOI:10.1021/om100317y

2386 Organometallics 2010, 29, 2386–2389
DOI: 10.1021/om100317y
[3 þ 3] Cyclodimerization of Methylenecyclopropanes: Stoichiometric and
Catalytic Reactions of Nickel(0) with Electron-Deficient
Alkylidenecyclopropanes
Masato Ohashi,*,†,‡ Tomoaki Taniguchi,† and Sensuke Ogoshi*,†
†Department of Applied Chemistry, Faculty of Engineering and ‡Center for Atomic and Molecular
Technologies, Osaka University, Suita, Osaka 565-0871, Japan
Received April 20, 2010
Chart 1. Ni(0)-Catalyzed Cyclodimerization of Methylenecyclo-
propane and Proposed Intermediates
Summary: Stoichiometric treatment of Ni(cod)2 with ethyl
cyclopropylideneacetate (ECPA) in the presence of PCy3
resulted in an unpredicted formation of a Ni(0) complex bear-
ing an (E,E)-1,2-bis(exo-alkylidene)cyclohexane ligand, which
stemmed from the [3 þ 3] cyclodimerization of ECPA. The
reaction could be expanded to a Ni(0)-catalyzed [3 þ 3]
cyclodimerization reaction of ester-substituted methylenecy-
clopropanes, giving the corresponding cyclohexane derivatives
in excellent yields.
ethyl cyclopropylideneacetates in the presence of Ni catalysts
was developed by Saito.4b Although a number of the cyclo-
dimerizations of MCPs have been reported as described
above, there is a very limiting example for [3 þ 3] cyclodi-
merization in which two proximal C-C bonds are cleaved
(Chart 1).4b
Methylenecyclopropanes (MCPs) are strained reactive
molecules, and transition-metal-catalyzed cyclodimerization
or cycloaddition reactions of MCPs have provided an atom-
efficient, powerful strategy for the synthesis of a variety of
cyclic compounds.1 Nickel(0) is also known to catalyze
cyclodimerization reactions of MCPs to afford cyclobutane
and cyclopentane derivatives via [2 þ 2] and [3 þ 2] cyclo-
dimerizations, respectively (Chart 1).2-4 Pioneering work on
the cyclodimerization of methylenecyclopropane in the pre-
sence of Ni(cod)2, giving a mixture of dispiro[2.1.2.1]octane
([2 þ 2] product) and 5-methylenespiro[2.4]heptane ([3 þ 2]
product), was reported by Binger.2a The usage of maleic
anhydride as an additive with the Ni(0) catalyst was found to
be effective for the selective formation of the [3 þ 2] cyclo-
dimerization product,2c whereas the introduction of tertiary
phosphines into the Ni(0) catalytic system resulted in the
preferential formation of MCP trimers.2b Interestingly, em-
ploying dialkyl fumarates as the solvent in the Ni-catalyzed
reaction was devised to give another dimer, 1,3-dimethyle-
necyclohexane, as a minor product.2c The selective [3 þ 2]
cyclodimerization of methylenecyclopropane, on the other
hand, was also caused by treating with Pd(0) catalyst.3 In
contrast, the limiting studies of cyclodimerization reactions
of substituted MCPs have been described thus far: the
cyclodimerization of 1-methylene-2-vinylcyclopropane with
Pd(0) catalysts was reported by Binger,4a and the reaction of
In these related reactions, some attempts to confirm the
reaction intermediates have been made. Based on the isola-
tion and characterization of a nickelacyclopentane inter-
mediate (Chart 1, A) by using (bipy)Ni(cod) as a Ni(0)
precursor, oxidative cyclization of methylenecyclopropane
with Ni(0) has been regarded as a key reaction step.5 The
C-C bond cleavage of the cyclopropane ring could be
achieved via β-carbon elimination of A, which is known as
a “cyclopropyl-butenyl rearrangement”, during the cataly-
tic reaction. Another possibility was proposed that the
oxidative addition of a proximal bond to Ni(0) took place
at the initial stage of the reaction to give 2-methylene-1-
nickelacyclobutane (Chart 1, B), while the corresponding
intermediate has not been isolated.6,7 In addition, the sole
example of a nickel(0) complex containing an η2-methylene-
cyclopropane ligand has been reported.8
(5) (a) Doyle, M. J.; McMeeking, J.; Binger, P. J. Chem. Soc., Chem.
Commun. 1976, 376–379. (b) Binger, P.; Doyle, M. J.; Benn, R. Chem. Ber.
1983, 116, 1–10.
(6) (a) Noyori, R.; Odagi, T.; Talaya, H. J. Am. Chem. Soc. 1970, 92,
5780–5781. (b) Noyori, R.; Kumagai, Y.; Umeda, I.; Talaya, H. J. Am.
Chem. Soc. 1972, 94, 4018–4020.
(7) In contrast, the oxidative addition of a distal bond, affording
a 3-methylene-1-palladacyclobutane intermediate, occurred in the
Pd(0)-catalyzed cyclodimerization reaction. See also ref 3.
(8) Isaeva, L. S.; Peganova, T. A.; Petrovskii, P. V.; Furman, D. B.;
Zotova, S. V.; Kudryashev, A. V.; Bragin, O. V. J. Organomet. Chem.
1983, 258, 367–72.
(9) Isolated examples of heteronickelacycles proposed as reaction
intermediates: (a) Ogoshi, S.; Oka, M.-a.; Kurosawa, H. J. Am. Chem.
Soc. 2004, 126, 11082–11803. (b) Ogoshi, S.; Ueta, M.; Arai, T.; Kurosawa,
H. J. Am. Chem. Soc. 2005, 127, 12810–12811. (c) Ogoshi, S.; Tonomori,
K.-i.; Oka, M.-a.; Kurosawa, H. J. Am. Chem. Soc. 2006, 128, 7077–7086.
(d) Ogoshi, S.; Ikeda, H.; Kurosawa, H. Angew. Chem., Int. Ed. 2007, 46,
4930–4932. (e) Ogoshi, S.; Arai, T.; Ohashi, M.; Kurosawa, H. Chem.
Commun. 2008, 1347–1349. (f) Ohashi, M.; Kishizaki, O.; Ikeda, H.;
Ogoshi, S. J. Am. Chem. Soc. 2009, 131, 9160–9161.
*To whom correspondence should be addressed. ohashi@chem.eng.
osaka-u.ac.jp (M.O.); ogoshi@chem.eng.osaka-u.ac.jp (S.O.).
€
(1) Reviews: (a) Binger, P.; Buch, H. M. Top. Curr. Chem. 1987, 135,
77–151. (b) Dzhemilev, U. M.; Khusnutdinov, R. I.; Tolstikov, G. A. J.
Organomet. Chem. 1991, 409, 15–65. (c) Binger, P.; Schmidt, T. Methoden
Org. Chem. Houben-Weyl 1997, E17c, 2217–2294.
(2) (a) Binger, P. Angew. Chem., Int. Ed. 1972, 11, 309–310. (b) Binger,
P.; McMeeking, J. Angew. Chem. 1973, 85, 1053–1054. (c) Binger, P.
Synthesis 1973, 427–428. For a review, see: (d) Modern Organonickel
Chemistry; Tamaru, Y., Ed.; Wiley-VCH: Weinheim, Germany, 2005.
(3) Binger, P.; Schuchardt, U. Angew. Chem., Int. Ed. 1977, 16, 249–
250.
(4) (a) Binger, P.; Gerner, A. Chem. Ber. 1981, 114, 3325–3335. (b)
Kawasaki, T.; Saito, S.; Yamamoto, Y. J. Org, Chem. 2002, 67, 4911–4915.
r
pubs.acs.org/Organometallics
Published on Web 04/30/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 11, 2010 2387
In the course of our continuous studies on the develop-
ment of Ni-catalyzed transformation reactions and isolation
of heteronickelacycle key intermediates,9 we recently demon-
strated that nickel(0) complexes bearing a strong σ-donor
ligand, such as PCy3 and IPr, promote oxidative addition of
the cyclopropane ring adjacent to a carbonyl group to yield
six-membered oxanickelacycles.10 We therefore investigated
the stoichiometric reaction of Ni(cod)2 with ethyl cyclopro-
pylideneacetate (1a; ECPA)11 in the presence of PCy3 in
anticipation of smooth cleavage of a proximal carbon-
carbon bond of the cyclopropane ring. It was, as a result,
found that selective formation of a 1,2-bis(exo-alkylidene)-
cyclohexane framework was achieved as a result of a [3 þ 3]
cyclodimerization reaction via cleavage of two proximal
C-C bonds (Chart 1).12-14 We also herein report a novel
Ni-catalyzed [3 þ 3] cyclodimerization of electron-deficient
alkylidenecyclopropanes.
Figure 1. Molecular structures of 3a with thermal ellipsoids at
the 30% probability level. H atoms are omitted for clarity.
Scheme 1. Reaction of 1a with Ni(cod)2 in the Presence of PR3
In the presence of 2 equiv of PCy3, the reaction of 1a with
Ni(cod)2 in toluene-d8 at -15 °C led to the quantitative
formation of an η2-ECPA complex (2a) (Scheme 1). On
further monitoring of the reaction at room temperature by
means of NMR spectroscopy, the gradual decomposition of
2a was observed, while no other reaction intermediates were
detectable, to yield a 1:1 mixture of Ni(cod)2 and an un-
expected Ni(0) complex (3a). X-ray crystallography of 3a
revealed that an (E,E)-1,2-bis(exo-alkylidene)cyclohexane
unit, which arose from the [3 þ 3] cyclodimerization of 1a
via selective cleavage of the proximal C-C bond trans to the
ethoxycarbonyl group, coordinated to the nickel atom in
η2:η2 fashion (Figure 1). The nickel center in 3a adopted a
tetrahedral coordination geometry, which is often observed
in four-coordinated Ni(0) complexes.
By employing 2 equiv of PPh3 instead of PCy3 as a ligand,
on the other hand, the corresponding η2-ECPA complex
(2a0) was quantitatively obtained (Scheme 1). Unlike the
PCy3-ligated complex 2a, the isolated 2a0 is stable, regardless
of the presence or absence of a 1,5-COD molecule, in C6D6
solution at room temperature, and therefore, it was not
converted into further compounds at all. The X-ray crystal-
lography of 2a0 clearly demonstrated a three-coordinated
Ni(0) structure coordinated by the η2-ECPA and two PPh3
molecules (Figure 2). Complex 2a0 is the first example of a
structurally well-defined nickel complex coordinated by an
η2-methylenecyclopropane derivative,8 while some prece-
dents for other transition-metal complexes having an η2-
methylenecyclopropane ligand have been reported.15 It
should be mentioned that the reaction of 1a with 10 mol %
of 2a0 as a catalytic precursor gave the [3 þ 2] cyclodimeriza-
tion product 6a.16
(10) (a) Ogoshi, S.; Nagata, M.; Kurosawa, H. J. Am. Chem. Soc.
2006, 128, 5350–5351. (b) Tamaki, T.; Nagata, M.; Ohashi, M.; Ogoshi, S.
Chem. Eur. J. 2009, 15, 10083–10091. The six-membered oxanickelacycle
was found to be a key intermediate in the Ni(0)-catalyzed cyclodimerization
of cyclopropyl phenyl ketone; see also ref 4b and: (c) Liu, L.; Montgomery, J.
J. Am. Chem. Soc. 2006, 128, 5348–5349. (d) Liu, L.; Montgomery, J. Org.
Lett. 2007, 9, 3885–3887. (e) Lloyd-Jones, G. C. Angew. Chem., Int. Ed.
2006, 45, 6788–6790.
(11) The unique role of 1a as a three-carbon source has also been
developed in Ni-catalyzed [3 þ 2 þ 2] and [4 þ 3] cycloaddition reactions.
(a) Saito, S.; Masuda, M.; Komagawa, S. J. Am. Chem. Soc. 2004, 126,
10540–10541. (b) Komagawa, S.; Saito, S. Angew. Chem., Int. Ed. 2006, 45,
2446–2449. (c) Saito, S.; Takeuchi, K. Tetrahedron Lett. 2007, 48, 595–598.
(d) Maeda, K.; Saito, S. Tetrahedron Lett. 2007, 48, 3173–3176. (e) Saito,
S.; Komagawa, S.; Azumaya, I.; Masuda, M. J. Org. Chem. 2007, 72,
9114–9120. (f) Komagawa, S.; Yamasaki, R.; Saito, S. J. Synth. Org. Chem.
Jpn. 2008, 66, 974–982. (g) Yamasaki, R.; Sotome, I.; Komagawa, S.;
Azumaya, I.; Masu, H.; Saito, S. Tetrahedron Lett. 2009, 50, 1143–1145.
(h) Komogawa, S.; Takeuchi, K.; Sotome, I.; Azumaya, I.; Masu, H.;
Yamasaki, R.; Saito, S. J. Org. Chem. 2009, 74, 3323–3329. (i) Fukusaki,
Y.; Miyazaki, J.; Azumaya, I.; Katagiri, K.; Komagawa, S.; Yamasaki, R.;
Saito, S. Tetrahedron 2009, 65, 10631–10636. (j) Yamasaki, R.; Terashima,
N.; Sotome, I.; Komagawa, S.; Saito, S. J. Org. Chem. 2010, 75, 480–483.
(k) Saito, S.; Maeda, K.; Yamasaki, R.; Kitamura, T.; Nakagawa, M.; Kato,
K.; Azumaya, I.; Masu, H. Angew. Chem., Int. Ed. 2010, 49, 1830–1833.
(12) Formation of bis(exo-alkylidene)cyclohexanes via hydrolysis of
titana- or zirconabicycles: (a) Nugent, W. A.; Calabrese, J. C. J. Am.
Chem. Soc. 1984, 106, 6422–6424. (b) Negishi, E.; Cederbaum, F. E.;
Takahashi, T. Tetrahedron Lett. 1986, 27, 2829–2832. (c) Nugent, W. A.;
Thorn, D. L.; Harlow, R. L. J. Am. Chem. Soc. 1987, 109, 2788–2796. (d)
Negishi, E.; Holmes, S. J.; Tour, J. M.; Miller, J. A.; Cederbaum, F. E.;
Swanson, D. R.; Takahashi, T. J. Am. Chem. Soc. 1989, 111, 3336–3346. (e)
Urabe, H.; Sato, F. J. Org. Chem. 1996, 61, 6756–6757.
In contrast, the reaction with 1-cyclopropylidene-2-pro-
panone (1b) was found to proceed in a different manner.
(15) The following precedents were found in the CSD database
(version 5.31, Nov 2009). For Fe complex: (a) Whitesides, T. H.; Slaven,
R. W.; Calabrese, J. C. Inorg. Chem. 1974, 13, 1895–1899. (b) Pinhas,
A. R.; Samuelson, A. G.; Riesmberg, R.; Arnold, E. V.; Clardy, J.; Carpenter,
B. K. J. Am. Chem. Soc. 1981, 103, 1668–1675. For W complex: (c) Fischer,
H.; Bidell, W.; Hofmann, J. J. Chem. Soc., Chem. Commun. 1990, 858–859.
For Rh complex: (d) Osakada, K.; Takimoto, H.; Yamamoto, T. J. Chem.
Soc., Dalton Trans. 1999, 853–860. (e) Green, M.; Howard, J. A. K.;
Hughes, R. P.; Kellett, S. C.; Woodward, P. J. Chem. Soc., Dalton Trans.
1975, 2007–2014. For Co complex: (f) Foerstner, J.; Kozhushkov, S.; Binger,
(13) Formation of bis(exo-alkylidene)cyclohexanes via transition-
metal-catalyzed cocyclization of 1,7-diynes: (a) Tamao, K.; Kobayashi,
K.; Ito, Y. J. Am. Chem. Soc. 1989, 111, 6478–6480. (b) Onozawa, S.;
Hatanaka, Y.; Tanaka, M. Chem. Commun. 1997, 1229–1230. (c) Onozawa,
S.; Hatanaka, Y.; Choi, N.; Tanaka, M. Organometallics 1997, 16, 5389–
5391. (d) Suginome, M.; Matsuda, T.; Ito, Y. Organometallics 1998, 17,
5233–5235. (e) Uno, T.; Wakayanagi, S.; Sonoda, Y.; Yamamoto, K. Synlett
2003, 1997–2000. (f) Miura, T.; Yamauchi, M.; Murakami, M. Synlett 2007,
2029–2032.
€
(14) Other examples for synthesis of bis(exo-alkylidene)cyclo-
hexanes: (a) Bhatarah, P.; Smith, E. H. J. Chem. Soc., Perkin Trans.1
1992, 114, 2163–2168. (b) Piers, E.; McEachern, E. J.; Romero, M. A. J.
Org. Chem. 1997, 62, 6034–6040. (c) Lomberget, T.; Bouyssi, D.; Balme, G.
Synthesis 2005, 311–329. (d) Gandon, V.; Aubert, C.; Malacria, M.;
Vollhardt, K. P. C. Chem. Commun. 2008, 1599–1601.
P.; Wedemann, P.; Noltemeyer, M.; de Meijere, A.; Butenschon, H. Chem.
Commun. 1998, 239–240. (g) Kozhushkov, S. I.; Foerstner, J.; Kakoschke,
A.; Stellfeldt, D.; Yong, L.; Wartchow, R.; de Meijere, A.; Butenschon, H.
Chem. Eur. J. 2006, 12, 5642–5647.
(16) The [3 þ 2] cyclodimerization of 1a in the presence of Ni(cod)2
and PPh3 has been reported to yield the same product. See also ref 4b.

2388 Organometallics, Vol. 29, No. 11, 2010
Ohashi et al.
Table 1. Ni(0)/PCy3-Mediated [3 þ 3] Cyclodimerization of 1a
yield (%)b
run
R
conditions
5 [EE:EZ]
6 [E:Z]
1
2
3
4
OEt (1a)
room temp, 7 h
60 °C, 1 h
38 [95:5]
81 [87:13]
62 [100:0]
19 [75:25]
10 [20:80]
4 [100:0]
4 [100:0]
7 [100:0]
0
60 °C, 6 hc
60 °C, 6 hc
60 °C, 6 hc
60 °C, 6 hc
100 °C, 3 h
90 (70) [92:8]
96 (64) [93:7]
96 (55) [93:7]
93 (61) [>99:1]
50 (40) [0:100]
OnBu (1c)
OiPr (1d)
OtBu (1e)
Me (1b)
5
6
Figure 2. Molecular structures of 2a0 with thermal ellipsoids at
the 30% probability level. H atoms, except for those attached to
C1, C3, and C4 carbon atoms, and the solvated molecules (0.5
C7H8) are omitted for clarity. C7A:C7B = 0.600(14):0.400(14).
7d
a General conditions: [1] = 0.2 M. b Determined by 1H NMR analysis
of the crude reaction mixture. Cited yields in parentheses are of the
isolated major regioisomer. c After slow addition of 1 (for 5 h) was
terminated, the reaction mixture was further stirred for 1 h. d Ni(cod)2
and PCy3 (10 mol % each) were used.
catalytic amount of Ni(cod)2 and PCy3 (10 and 20 mol %,
respectively) was conducted at room temperature, resulting
in full consumption of 1a within 7 h. However, the expected
[3 þ 3] cycloaddition product (5a) was obtained in 38% yield
and a [3 þ 2] cyclopentane derivative (6a) was formed as a
major product (Table 1, run 1). Elevating the reaction
temperature (60 °C) improved the yield of 5a to 81% (run
2). Finally, slow addition of 1a to the toluene solution of a
Ni(cod)2 and PCy3 mixture over 5 h led to the formation of
5a in 90% yield, and the major product, (E,E)-5a, was
isolated in 70% yield (run 3). This catalytic reaction can be
applied for primary, secondary, and tertiary ester-substi-
tuted MCPs to give the corresponding cyclohexane deriva-
tives in excellent yields (runs 4-6). For esters 1a,c-e, the
geometry of the [3 þ 3] cyclodimerization product was highly
controlled to be E,E, which was consistent with the geometry
observed in the stoichiometric reaction. In contrast, the
ketone derivative 1b required higher reaction temperature
for its consumption, and the major product, (E,Z)-5b, was
acquired in 50% yield (Table 1, run 7). The difference in
diastereoselectivities of 5 between the reactions of 1a,c-e
and that of 1b reflects the difference in structures of inter-
mediates 3 and 4.
Figure 3. Molecular structures of 4 with thermal ellipsoids at
the 30% probability level. H atoms are omitted for clarity.
Scheme 2. Reaction of 1b with Ni(cod)2 in the Presence of PCy3
To elucidate the mechanism for the formation of the reaction
side product 6, the catalytic reaction with 1a was monitored by
NMR spectroscopy, revealing a nickelacyclohexane intermedi-
ate (7a) whose 31P resonance appeared at 34.5 ppm (Scheme
3).17 The generation of 7a was also supported by exposing 7a
(prepared in situ) to carbon monoxide (5 atm), giving the
expected six-membered ketone (8) with the concomitant for-
mation of Ni(PCy3)(CO)3 (Scheme 3).18 These observations
convinced us that 6a stemmed from the reductive elimination of
7a, which would be formed by the CdC bond insertion of 1a into
a Ni-C bond of a transient 2-alkylidene-1-nickelacyclobutane
Stoichiometric treatment of Ni(cod)2 and PCy3 with 2 equiv
of 1b led to the quantitative formation of a Ni(II) complex
(4), although neither the corresponding η2-olefin complex
similar to 2a nor the analogue of 3a was observed through
the reaction (Scheme 2). Unlike the case for complex 3a, the
1,2-bis(exo-alkylidene)cyclohexane fragment in 4 coordi-
nated to the nickel center in the η1-oxo-η3-oxaallyl mode
(Figure 3). Interestingly, treatment of (E,E)-5b with Ni(cod)2
in the presence of PCy3 smoothly afforded 4 while the reac-
tion with its isomer, (E,Z)-5b, did not proceed (Scheme 2),
which suggested that (E,E)-5b was a prior product in the
stoichiometric [3 þ 3] cyclodimerization reaction of 1b (vide
infra).
(17) Except for the unidentified minor species (δP 32 ppm), reso-
nances attributable to 7a and free PCy3 appeared as detectable signals in
the 31P NMR spectrum (see also Figures S1 and S2 in Supporting
Information).
(18) The treatment of nickel compounds with carbon monoxide can
yield [Ni(CO)4] (extremely toxic) due to the addition of insufficient
amounts of PCy3, careless handling, or an accident. The reaction
mixture must be handled in a well-ventilated fume hood.
A catalytic application of the selective [3 þ 3] cyclodimer-
ization of 1a was examined. The reaction of 1a with a

Communication
Organometallics, Vol. 29, No. 11, 2010 2389
Scheme 3. Observation of the Nickelacyclohexane Intermediate
7a
Scheme 4. Plausible Mechanism for Catalytic Formation of 5a
species (C) (Scheme 4).6,19,20 Although the β-carbon elim-
ination of 7a might occur in parallel to give the same [3 þ 3]
product 5a as a result of the reductive elimination from a
seven-membered nickelacycle, the results of the stoichio-
metric reaction to give 3a, depicted in Scheme 1, and the
suppression of the formation of 6a by the slow addition of 1a
support the notion that oxidative addition of the proximal
C-C bond to the electron-rich Ni(0) center would be a key
reaction step, leading to the selective [3 þ 3] cyclodimeriza-
tion without the generation of 7a (Scheme 4, left circle).
In summary, we have demonstrated the unique nickel-
catalyzed [3 þ 3] cyclodimerization of alkylidenecyclopro-
panes to yield 1,2-bis(exo-alkylidene)cyclohexanes in excel-
lent yields.
Acknowledgment. We thank Prof. Shinichi Saito and
Mr. Koichiro Kawakami (Tokyo University of Science)
for the gift of some compounds and fruitful discussions.
This work was supported by a Grant-in-Aid for Scientific
Research (No. 21245028) and Encouragement for Young
Scientists (B) (No. 21750102) from the MEXT.
Supporting Information Available: Text, figures, and CIF files
giving detailed experimental procedures, analytical and spectral
data for all new compounds, and crystallographic data for 2a0, 3,
and 4. This material is available free of charge via the Internet at
http://pubs.acs.org.
(19) We could not rule out the possibility that 7a would be formed via
another intermediate (D) by the oxidative cyclization of 1a with 2a.
€
€
(20) Binger, P.; Muller, P.; Podubrin, S.; Albus, S.; Kruger, C. J.
Organomet. Chem. 2002, 656, 288–298.