Rhodacarboranes as catalysts
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 4, April, 2014
985
E/kcal mol–1
moderate catalytic activity in the oxidative coupling of
benzoic acid with diphenylacetylene to give 1,2,3,4ꢀtetraꢀ
phenylnaphthalene as the major product. Apparently, the
selectivity of this reaction is determined mainly by the
electronic rather than steric properties of the ligand at the
rhodium atom.
TS2
1
1
1
4
2
0
8
6
4
2
0
2
2
TS1
1
Experimental
The reactions were carried out under argon. oꢀXylene was
purified by distillation over Na metal. The products were isolatꢀ
A
B + CO2
3
4
5
6
7
8
–
ed in air. Complexes 1, 2, 3(BF ) , 4BF , 5BF , [CpRhI ] ,
[
4
5
2
4
4
2 2
8
9
CpRh(C H )](BF ) , and [( ꢀC H Me )Rh(C H )](BF )
6 6 4 2 9 2 5 6 6 4 2
were synthesized according to known procedures. Column chroꢀ
matography was performed on Merck silica gel (70—230 mesh).
Fig. 1. Intrinsic reaction coordinate of the decarboxylation of
the intermediate A for the complexes CpRh (1) and Cp*Rh (2);
TS1 and TS2 are the first and second transition states (see Fig. 2),
respectively.
1
The H NMR spectra were recorded on a Bruker Avanceꢀ400
1
instrument (400.13 MHz for H).
Oxidative coupling of benzoic acid with diphenylacetylene (genꢀ
eral procedure). oꢀXylene (2 mL) was added to a mixture of the
catalyst (0.005 mmol), Cu(OAc) (182 mg, 1.00 mmol), benzoic
2
acid (31 mg, 0.25 mmol), and diphenylacetylene (89 mg, 0.5 mmol).
The reaction mixture was refluxed with vigorous stirring for 6 h.
Then the solvent was removed in vacuo, and the residue was
extracted with diethyl ether. The extract was applied to a 15×1 cm
column packed with silica gel. Unreacted diphenylacetylene was
washed off with petroleum ether. Then the yellow fraction was
collected using diethyl ether as the eluent. After the removal of
the solvent in vacuo, 1,2,3,4ꢀtetraphenylnaphthalene was obꢀ
a
b
1
tained as an oily yellow substance. H NMR (CDCl ), : 6.95—6.97
3
Fig. 2. Calculated structures of TS1 (a) and TS2 (b) for unsubstiꢀ
tuted acetylene. Hydrogen atoms and the Cp ligand are omitted
for clarity.
(
(
m, 10 H); 7.32—7.34 (m, 10 H); 7.47—7.49 (m, 2 H); 7.76—7.78
m, 2 H) (cf. lit data ).
1
0
DFT calculations. The geometry optimization was carried
1
1
out without symmetry restrictions using the Priroda 6 program,
1
2
13
the case of the methylated derivative, the free energy of
the activation is 5.1 kcal mol higher and the total energy
the PBE functional, the scalarꢀrelativistic Hamiltonian,
1
4
–
1
atomic basis sets composed of Gaussian functions, and the
densityꢀfitting scheme. The all electron threeꢀexponent basis
set L2 with two polarization functions was used. The frequenꢀ
1
5
–
1
gain (G) is 4.4 kcal mol smaller compared to the unꢀ
substituted analog (Table 2). However, such a small enerꢀ
gy difference should have no substantial effect on the seꢀ
lectivity of the reaction proceeding at rather high temperꢀ
ature (120 C). Presumably, the donor effect of five methꢀ
yl groups of the Cp* ligand reduces the oxidation potential
of the intermediate A, due to which the reductive eliminaꢀ
tion of the Rh atom giving isocoumarin 6 becomes more
favorable than the decarboxylation.
1
6
1
7
cies and the intrinsic reaction coordinate (IRC) were calculatꢀ
ed at the same level of theory. The molecular modeling and the
18
visualization were carried out using the ChemCraft program.
This study was financially supported by the Russian
Academy of Sciences (program of the Presidium of the
Russian Academy of Sciences Pꢀ8).
Therefore, in the present study we showed that diꢀ
carbollide and tricarbollide complexes of Rh exhibit
References
III
1
. K. Ueura, T. Satoh, M. Miura, J. Org. Chem., 2007, 72,
362—5367.
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. M. M. Vinogradov, Z. A. Starikova, D. A. Loginov, A. R.
Kudinov, J. Organomet. Chem., 2013, 738, 59—65.
5
Table 2. Free energies (G298/kcal mol–1) of the
2
3
transition states TS1 and TS2 (GTS1 and GTS2
)
and the reaction (G ) for the decarboxylation of the
r
intermediate A in the presence of CpRh and Cp*Rh
complexes (in the case of unsubstituted acetylene)
4
5
. D. A. Loginov, Z. A. Starikova, P. V. Petrovskii, J. Holub,
A. R. Kudinov, Inorg. Chem. Commun., 2011, 14, 313—315.
. M. Corsini, S. Losi, E. Grigiotti, F. Rossi, P. Zanello, A. R.
Kudinov, D. A. Loginov, M. M. Vinogradov, Z. A. Starikoꢀ
va, J. Solid State Electrochem., 2007, 11, 1643—1653.
Ligand
GTS1
GTS2
Gr
Cp
Cp*
8.29
7.54
7.76
13.43
11.4
7.03
6
. D. A. Loginov, A. O. Belova, Z. A. Starikova, P. V. Petrovꢀ
skii, A. R. Kudinov, Mendeleev Commun., 2011, 21, 4—6.