3.2.1 MDA reaction on 6-6 double bond of C60:
As evident from Figure 5 , the stability of the first DA adduct
for the alternative approach, i.e.,
A16-6S (-4.1 kcal/mol) is comparable
with A16-6O. The activation barrier
involved for R16-6S formation via
TS16-6S is calculated to be 17.3
kcal/mol, which is only 1.6 kcal/mol higher than the corresponding
direct pathway. Thus, we can say that a fruitful second DA reaction can
be done by placing the butadiene molecule not only at the opposite of
the first functionalization but also at the adjacent position. The
addition of third butadiene molecule can be done either opposite to the
first functionalization or second functionalization in
R16-6S, but eventually both generate
the same tri-functionalized product
(R26-6) obtained in the ‘Direct’
approach. The associated adduct complex,
A26-6S is placed at -36.5 kcal/mol on
the energy profile diagram, which is energetically comparable with
A26-6O. The transition state
TS26-6S (Figure 4 ) associated
with the formation of R26-6 requires a
barrier height of 18.0 kcal/mol, which is exactly the same as the direct
one. Similar to the second functionalization, the third
functionalization is also highly exothermic as the associated product,
R26-6 is -24.2kcal/mol more stable
than A26-6S. For the attachment of the
fourth butadiene, it will precisely follow similar pathway proposed for
the ‘Direct’ approach.
Like ‘Direct’ approach, for the ’Alternative’ approach also, as shown inFigure 4 , all three DA reactions are synchronous processes as
the vibrations associated with the new C-C bonds occur to an equal
extent in the TS geometry.