Unprecedented rhodium-mediated tetramerization of bulky terminal alkynes leading to hydropentalenylrhodium complexes

Article abstract of DOI:10.1246/cl.2001.998

The reactions of bulky terminal alkynes, RC≡CH [R= ^tBu, 1a; Me₃Si, 1b; (cod)Rh(η⁵-Me₂C₅H₂), 1c] with [RhCl(cod)]₂ in the presence of Et₃N have been found to provide novel hydropentalenylrhodium complexes, (cod)Rh(η⁵-R₄C₈H₃) (2, 4 and 5). The structures have been determined by a single-crystal X-ray diffraction analysis. A mechanism involving the migration of an α-tert-butyl group or an α-hydrogen atom of the metal-vinyl intermediates is proposed.

Full text of DOI:10.1246/cl.2001.998

998
Chemistry Letters 2001
Unprecedented Rhodium-Mediated Tetramerization of Bulky Terminal
Alkynes Leading to Hydropentalenylrhodium Complexes
Hiroaki Komatsu, Yasunobu Suzuki, and Hiroshi Yamazaki*
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551
(Received August 1, 2001; CL-010734)
t
The reactions of bulky terminal alkynes, RC≡CH [R= Bu,
1a; Me3Si, 1b; (cod)Rh(η5-Me2C5H2), 1c] with [RhCl(cod)]2 in
the presence of Et3N have been found to provide novel
hydropentalenylrhodium complexes, (cod)Rh(η5-R4C8H3) (2, 4
and 5). The structures have been determined by a single-crystal
X-ray diffraction analysis. A mechanism involving the migra-
tion of an α-tert-butyl group or an α-hydrogen atom of the
metal–vinyl intermediates is proposed.
Oligomerization of the terminal alkynes by various transition
metal complexes has been extensively studied.1 The formation
of linear dimer and cyclic trimer are very common but the selec-
tive formation of a higher oligomer is rather scarce.2 Recently,
we and others reported the highly selective synthesis of buta-
trienes from bulky terminal alkynes by ruthenium, osmium and
iridium complex catalysts.3 As a continuation of our study, we
now report the rhodium-mediated tetramerization of bulky termi-
exhibits a different NMR spectrum from 2.4 The X-ray structure
determination of 2 revealed it to be the novel hydropentalenyl-
rhodium complex (Figure 1).5 It is noteworthy that two of the
t
four Bu groups in 2 are accommodated on the same carbon
t
t
nal alkynes, RC≡CH [R= Bu, (1a); Me3Si, (1b); (cod)Rh(η5-
atom suggesting the involvement of a Bu group migration dur-
Me2C5H2), (1c)] by [(cod)RhCl]2, leading to hydropentalenyl-
ing the reaction. There are some reports on the formation of
hydropentalenyl and pentalenyl metal complexes from the corre-
sponding hydropentalene anion, pentalene dianion and cyclooc-
tatetraene dianion.6 As far as we know, this is a first example of
the direct formation of the hydropentalenyl-metal complex from
terminal alkynes, although there is a precedent for the palladium
mediated formation of dihydropentalene itself.7
rhodium complexes, (cod)Rh(η5-R4C8H3) (Scheme 1).
Treatment of [RhCl(cod)]2/Et3N with excess amount of 1b
and 1c provided 4 (orange crystals, 50% yield) and 5 (yellow
crystals, 74% yield), having the same empirical formula,
(cod)Rh(R4C8H3). The 1H NMR spectrum of 4 shows four sin-
glets ascribed to four different Me3Si groups (δ = –0.05, 0.16,
0.27, 0.36) and that of 5 reveals eight singlets ascribed to eight
different methyl groups on the cyclopentadienyl groups (δ =
1.70, 1.72, 1.76, 1.81, 1.84, 1.87, 1.93, 1.99), suggesting a low
symmetry for the molecules. The structures of 4 and 5 were
determined by a single-crystal X-ray diffraction analysis.8 The
ORTEP view of 4 is shown in Figure 2. They have the same
tert-Butylacetylene 1a (0.096 g, 1.17 mmol) was kept at
room temperature for one day in the presence of [RhCl(cod)]2
(0.014 g, 0.028 mmol) and Et3N (0.182 g, 1.80 mmol) in cyclo-
hexane. No oligomers of 1a were detected in the reaction mix-
ture. After removal of the solvent, the residue was chro-
matographed on alumina. The yellow-orange eluate by hexane
was evacuated and the residue was crystallized from ethanol to
produce orange-yellow crystals (2) (0.004 g, 13% yield). The
1H NMR spectrum of 2 (in CDCl3) showed four singlet peaks
ascribed to tBu groups (δ = 0.83, 1.08, 1.31, 1.40). The reaction
t
of [Rh(NCMe)2(cod)][BF4] with BuC≡CH in THF has been
known to give a cyclic tetramer-rhodium complex (3) which
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
999
migration of an α-hydrogen of a vinyl to the metal.11 A similar
α-hydrogen migration step (from C to F) may be involved dur-
ing the formation of 4 and 5. The subsequent steps are similar
to those for 2 although the regioselectivity of the insertion step
of the fourth alkyne is dependent on the employed alkyne. The
mechanism including an sp3 C–H bond activation as an impor-
tant step was proposed by Green et al. for the formation of 3 as
depicted together in Scheme 2. We were not successful in
detecting 3 under our reaction conditions.
hydropentalenyl skeletons as 2. No migration of the R groups
was observed in these cases. They differ from one another with
respect to their regio- and stereochemistries. A more bulky
alkyne, (cod)Rh(η5-Me4C5C≡CH), resulted in the recovery of
the starting alkyne. Under similar reaction conditions men-
tioned above, mesitylacetylene afforded a dinuclear acetylide-
rhodium complex [(µ-η1:η2-Me3C6H2C≡C)Rh(cod)]2 (6) (30%
yield); the structure was determined by X-ray analysis.9 Simple
phenylacetylene provided an insoluble polymer.10
For the present rhodium mediated tetramerization of termi-
nal alkynes, we tentatively suggest the mechanism summarized
in Scheme 2. The first step may be the formation of alkynyl-
rhodiacyclopentadienes (A). Reductive coupling of A occurs in
two ways, giving B and C. It is necessary to consider the
migration of a tBu group from the parent carbon during the for-
mation of 2. We assume the formation of a vinylidene interme-
diate (D) from B by the migration of an α-tBu group of the
vinyl to the rhodium metal. Intramolecular addition of the
acetylene moiety to the rhodium–carbon double bond followed
by reductive coupling may provide E. Insertion of the fourth
tert-butylacetylene into the rhodium–carbon bond in E and the
subsequent intramolecular cyclization may result in 2. There is
a precedent for the formation of a metal vinylidene complex by
This work was supported by the Promotion and Mutual Aid
Corporation for Private Schools of Japan (2000–2002).
References and Notes
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–
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7
8
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18.456(3) Å , V = 3310(1) Å3; Z = 4; Dcalc = 1.210 g·cm–3 ; R =
0.033, Rw = 0.046. Crystallographic data. 5: C72H96Rh5, fw =
–
1476.08; triclinic, space group P1 (No.2); a = 16.762(3) Å, b =
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9
6: FT-IR (KBr); ν(C≡C) 2005 cm–1. 1H NMR (CDCl3) δ 2.24 (s,
Me, 6H), 2.45 (s, Me2, 12H), 6.83 (s, Ph, 4H), 4.48 (br, cod, 8H),
2.34 (m, cod, 8H), 2.02 (pseudo-q, cod, 8H). Crystallographic data.
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10.108(2) Å, b = 13.722(4) Å, c = 23.804(2) Å, β = 98.007(19), V =
3269(1) Å3; Z = 4; Dcalc = 1.439 g·cm–3; R = 0.030, Rw = 0.025.
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