HYDROGENOLYSIS OF CYCLOALKANES OVER TbCL3·3H2O·3(RO)2AlOH
1255
TbCl3·6H2O (67) > Tb(NO3)3·6H2O (14)
fore (i-Bu)2AlH may be rationalized on the assumption
that cyclohexane is coordinated to Tb3+ ion thus ham-
pering formation of Tb–Cl–Tb(Al) after dehydration of
terbium ion [reaction of crystallization water with
(i-Bu)2AlH]. In this case catalytically active centers are
formed before Tb–Cl–Tb(Al).
> Tb2(SO4)3·8H2O (0) = Tb2(CO3)3·6H2O (0).
Considerably lower catalytic activity of terbium(III)
nitrate compared to chloride may be understood taking
into account that NO3– is a bulkier anion which ham-
pers formation of catalytically active hydrogenolysis
centers as a result of change of coordination environ-
ment of Tb3+ ion. The nature of the R radical (R = Et,
i-Bu) in the akoxy fragment (RO) of Tb/Al did not
affect the conversion of cyclohexane. The conversion
of cyclohexane decreased in going from dioxane
(67%) to aromatic solvents, toluene (11%) and ben-
zene (9%). Presumably, higher efficiency of hydro-
genolysis in dioxane is related to enhanced mobility of
the hydrogen atom in (i-Bu)2AlH due to hydrogen
bonding with oxygen atom in dioxane molecule.
Higher efficiency of dehalogenation of halogenated
hydrocarbons by the action of (i-Bu)2AlH in ether
solvents in the presence of a number of metal complex
catalysts was rationalized in a similar way [4].
The conversion of cycloalkanes increased with rise
in the strain energy in the ring.
Catalytic hydrogenolysis of alkyl-substituted cyclo-
hexane over Tb/Al occurred more readily than analo-
gous reaction with unsubstituted cyclohexane; the
conversion was 75 and 67%, respectively. The only
hydrogenolysis product obtained from methylcyclo-
hexane was n-heptane. This means that the reaction
with methylcyclohexane involves cleavage of the C–C
bond at the carbon atom linked to the methyl group
[(H3C)C–CH2] rather than cleavage of H2C–CH2 bond.
Higher efficiency of hydrogenolysis of methylcyclo-
hexane is determined by lower strength of the
(H3C)C–CH2 bond. This is confirmed by the results of
RI-MP2/L2 quantum-chemical calculations (PRIRODA
program) of the energies of the (H3C)C–CH2 and
H2C–CH2 bonds (396 and 410 kJ/mol, respectively).
These results are consistent with the data of [6],
according to which the apparent energies of activation
for hydrogenolysis of methylcyclohexane and cyclo-
hexane over Ru/C are equal to 59 and 67 kJ/mol,
respectively.
Hydrogenolysis of cycloalkanes (general proce-
dure). A glass reactor was charged with 0.08 mmol of
LnCl3 ·6 H2O, 0.24 mmol of Al(OR)3 in 12.5 ml of
dioxane was added, and the mixture was stirred until
LnCl3·6H2O disappeared and a homogeneous solution
was formed (25°C, 4 h). The corresponding cycloal-
kane, 4.8 mmol, was added, the mixture was purged
with argon over a period of 10 min, 7.2 mmol of
(i-Bu)2AlH was added, and the mixture was heated for
6 h at 80°C. The mixture was then cooled to 10°C,
treated with 15 ml of 10% hydrochloric acid, and
extracted with diethyl ether, the extract was dried over
sodium sulfate, and the solvent was distilled off.
The effect of temperature was studied in the range
from 25 to 80°C. No hydrogenolysis occurred below
40°C, and the conversion of cyclohexane at 60 and
80°C was 30 and 67%, respectively. In the absence of
(i-Bu)2AlH thermal cracking of cyclohexane did not
observed even under pressure at 180°C (6 h).
The conversion of cyclohexane strongly depended
on the reactant ratio and order of their addition. At
Ln3+–cyclohexane–(i-Bu)2AlH molar ratios of 1:60:90
and 1:60 :60 the conversions of cyclohexane were
67 and 43%, respectively. If cyclohexane was added
prior to (i-Bu)2AlH, its conversion was 67%, and it de-
creased to 40% when cyclohexane was added after
(i-Bu)2AlH (in both cases, the reactants were added
after generation of the Ln/Al catalyst). The lower con-
version of cyclohexane in the latter case is likely to
result from dehydration of Tb3+ ion due to reaction of
crystallization water in the catalyst with (i-Bu)2AlH;
ion Tb3+ thus becomes coordinately unsaturated and
gives rise to catalytically inactive Tb–Cl–Tb(Al) units.
Addition of (i-Bu)2AlH to Tb/Al leads to appreciable
shortening of the photoluminescence lifetime of
terbium ion τ(Tb3+*) from 1000 to 15 μs. The forma-
tion of Tb–Cl–Tb(Al) units was observed by us previ-
ously [5] in the reaction of TbCl3 · 6 H2O with
(i-Bu)3Al, which was also accompanied by reduction
of τ(Tb3+* ) from 420 to <15 μs.
Pentane. Conversion of cyclopentane 77%, nD20
=
1.3582 (1.3580 [7]). 1H NMR spectrum, δ, ppm: 0.80 t
(6H, CH3), 1.33 br.s (6H, CH2). 13C NMR spectrum, δC,
ppm: 13.73 q (C1, C5), 22.93 t (C2, C4), 34.58 t (C3).
Hexane. Conversion of cyclohexane 67%, nD20
=
1.3752 (1.3751 [7]). 1H NMR spectrum, δ, ppm: 0.96 t
(6H, CH3), 1.35 br.s (8H, CH2). 13C NMR spectrum, δC,
ppm: 14.03 q (C1, C6), 23.04 t (C2, C5), 32.13 t (C3, C4).
In keeping with the above stated, higher efficiency
of catalytic hydrogenolysis of cyclohexane added be-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 8 2010