M. C. Holthausen, S. Ghosh et al.
all cases, the 1,2,4-isomer is predominant, except for p-
ꢀ
ꢀ
NH2C6H4C CH. The reaction of compound 2 with MeC
CPh yields 1,2,4-trimethyl-3,5,6-triphenylbenzene as the
1
only isomer that is detectable by H NMR spectroscopy.
We found that electron-withdrawing substituents generally
enhanced the reactivity of an alkyne. For example, with
ꢀ
HC CCO2Et, we observed a high yield of the cyclotrimer-
ized product, which was comparable to the results for rhoda-
ꢀ
borane 1b. The reactions of HC CR (R=p-CF3C6H4 or 2,4-
F2C6H3) gave insoluble materials and no cyclotrimerized
products.
To evaluate the scope of this catalytic process, the reac-
tion was also carried out by using the symmetrical alkyne
ꢀ
PhC CPh, thereby affording the fully substituted arene,
hexaphenylbenzene. Notably, rhodaborane 1b, which cata-
lyzes the cyclotrimerization of a variety of internal alkynes,
does not yield the cyclotrimerization product of this
alkyne.[19] The decreasing activities of compound 2 for RC
CSiMe3 (R=H, Ph, SiMe3, and C4H9) imply the presence of
a significant steric factor in the reaction mechanism.
ꢀ
There is one striking aspect of these observations with re-
spect to the earlier work of Fehlner and co-workers:[19] in
any presumable mechanistic picture, a key requirement for
the metal-catalyzed cyclotrimerization is the coordination of
the alkyne to the metal center, which, in turn, requires the
presence of a vacant metal site or labile ligands. We note
that neither compound 2 nor compound 1b contain vacant
metal sites, yet compound 2 contains two potentially labile
CO ligands. To establish their role, we tested the cyclotrime-
rization of alkynes by using [(Cp*Ru)(Cp*RuPMe2Ph)B2H6]
(3),[25] in which the CO ligands were substituted by a more-
labile PMe2Ph ligand. The reaction led to decomposition at
room temperature and no sign of the cyclotrimerized prod-
uct was observed. Up to this point, the catalytic activity re-
mained puzzling and it is still not clear why the presence of
the Cp*RuH site in compound 1a leads to alkyne insertion,
the presence of the Cp*RuCO site in compound 2 leads to
catalytic cyclotrimerization, and the presence of a Cp*RuP-
Scheme 2. Four pathways that were investigated for the initial interaction
between the alkyne and the catalyst.
the Cp* ligands in compound 2 with Cp ligands). Calcula-
ꢀ ꢁ
tions were performed with HC C COOEt as a representa-
tive example for the cyclotrimerization reaction. Scheme 2
shows four conceivable pathways that we considered for the
initial alkyne interactions with the catalyst. First, a dissocia-
tive path (path A) involves the initial release of one of the
carbonyl ligands to provide a vacant Ru-coordination site
for subsequent alkyne addition. The associative paths
(paths B–D) commence with the coordination of the alkyne
to three different sites in compound 2’. Path B starts with
the attack of the alkyne on one of the boron atoms of the
B2H5 group. Alternatively, addition to one of the metal cen-
ters involves a concomitant change of the coordination
mode of either the B2H5 group (from bridging to terminal,
path C) or of the adjacent Cp ring (change in hapticity from
h5 to h1, path D).
Me2Ph fragment in compound
(Scheme 1).
3 offers no reaction
Thus, we performed a detailed computational study to es-
tablish the mechanism of the cyclotrimerization reaction
that is catalyzed by binuclear ruthenaborane compound 2.
Although numerous theoretical papers have focused on the
cyclotrimerization of alkynes catalyzed by mononuclear
complexes,[26] to the best of our knowledge, only one study
has focused on catalysis by binuclear complexes.[27] This
latter computational study,[27] which was on the tetrameriza-
tion of acetylene by a nickel catalyst, revealed that terminal-
ly coordinated acetylene ligands insert more-readily into
Path A involves the release of CO from compound 2’,
thereby leading to either structures A1 or A2 (Scheme 3).
The calculations revealed that this dissociation is highly en-
dothermic (DRH298 =39.7 and 44.6 kcalmolꢁ1, respectively),
and, therefore, we did not consider this pathway any further.
Along path B, we examined the potential reactivity of the
B2H5 group in compound 2’ with respect to alkyne attack.
The only transition state that we could locate in extensive
potential-energy-surface scans was TS2’–B1, in which the
alkyne attacks the terminal boron atom (Scheme 3). In this
ꢁ
Ni C bonds than bridging acetylene ligands and, thus, the
most-favored dinuclear path strongly resembles the mono-
nuclear pathway.
ꢁ
ꢁ
transition state, one of the Ru B bonds is broken and a B
C bond is formed synchronously to yield intermediate B1
and a boracyclopropene, that is, the alkyne abstracts a BH
Density functional study on cyclotrimerization pathways:
The reaction pathways have been explored by employing
[(CpRuCO)2B2H6] (2’) as a model catalyst (that is, replacing
moiety from compound 2’. However, the calculated energy
¼
barrier of this transformation is prohibitively high (DH
=
298
8484
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8482 – 8489