4.5 Cross Metathesis Between Two Different Alkanes
Cross metathesis between two different alkanes represent one of the most difficult
challenges in organic chemistry [53]. In 2001, Basset et al. first demonstrated the
possibilities of sigma bond metathesis between two different alkanes [55]. In 2004,
this same group has reported the cross metathesis between ethane and toluene [81]
and methane and propane [82]. Silica-supported tantalum hydride catalyst
SiO
ð
Þ 2 TaH
Â
à РSiO
ð
Þ 2 TaH 3
Â
Ã
was employed for cross-metathesis reaction
between toluene and ethane at 250
C. Under static condition, it produced mainly
ethyl benzene and xylenes as major product along with propane and methane
(Scheme 24).
In order to have better understanding, the same reaction was performed with
100%
13 C-enriched methyl in toluene and found mainly
13 C-mono-labeled ethylbenzene at the α-position (92%), while xylenes were 98%
13 C-mono-labeled.
Besides this isolation of stable intermediates, such as [(SiO) 2 Ta–C 2 H 5 ],
[(SiO) 2 Ta–CH 2 C 6 H 5 ], or [(SiO) 2 Ta–C 6 H 4 CH 3 ] by
13 C CP MAS NMR spectroscopy, the mechanism of the reaction leading to various xylenes is given in
Scheme 25.
In
this
scheme,
we
assume
that
in
the
equilibrium
SiO
ð
Þ 2 Ta
III
ð Þ H
Â
à РSiO
ð
Þ 2 Ta
V
ð Þ H 3
Â
Ã
, it is [(SiO) 2 TaH 3 ], d
0 , which is
Scheme 23 Proposed mechanism for metathesis of 2-methylpropane
Scheme 24 Cross metathesis of toluene and ethane with silica-supported tantalum hydride
180
M.K. Samantaray et al.
Cross metathesis between two different alkanes represent one of the most difficult
challenges in organic chemistry [53]. In 2001, Basset et al. first demonstrated the
possibilities of sigma bond metathesis between two different alkanes [55]. In 2004,
this same group has reported the cross metathesis between ethane and toluene [81]
and methane and propane [82]. Silica-supported tantalum hydride catalyst
SiO
ð
Þ 2 TaH
Â
à РSiO
ð
Þ 2 TaH 3
Â
Ã
was employed for cross-metathesis reaction
between toluene and ethane at 250
C. Under static condition, it produced mainly
ethyl benzene and xylenes as major product along with propane and methane
(Scheme 24).
In order to have better understanding, the same reaction was performed with
100%
13 C-enriched methyl in toluene and found mainly
13 C-mono-labeled ethylbenzene at the α-position (92%), while xylenes were 98%
13 C-mono-labeled.
Besides this isolation of stable intermediates, such as [(SiO) 2 Ta–C 2 H 5 ],
[(SiO) 2 Ta–CH 2 C 6 H 5 ], or [(SiO) 2 Ta–C 6 H 4 CH 3 ] by
13 C CP MAS NMR spectroscopy, the mechanism of the reaction leading to various xylenes is given in
Scheme 25.
In
this
scheme,
we
assume
that
in
the
equilibrium
SiO
ð
Þ 2 Ta
III
ð Þ H
Â
à РSiO
ð
Þ 2 Ta
V
ð Þ H 3
Â
Ã
, it is [(SiO) 2 TaH 3 ], d
0 , which is
Scheme 23 Proposed mechanism for metathesis of 2-methylpropane
Scheme 24 Cross metathesis of toluene and ethane with silica-supported tantalum hydride
180
M.K. Samantaray et al.