10.1002/chem.201603979
Chemistry - A European Journal
COMMUNICATION
In contrast to the mechanism Fernández and co-workers
reported, we only observed a single transition state (TS) for the
addition. This could be due to the shallowness of the potential
energy landscape or to the fact that in the study of Fernández
and co-workers the catalyst was a methoxy group, which
conferred a negative charge to the system. Our study uses
amines as a catalyst and thus the overall system remains
neutral.
Acknowledgements
AB would like to thank the University of Hull for the starting grant
which supported this research. AF would like to thank the EU for
the Erasmus+ grant and University of Hull fellowship. KS would
like to thank Brazil council for SwB (239091/2013-2) funding. We
thank AllyChem for the gift of diboranes. Solid-State 11B NMR
was obtained at EPSRC National Solid-state NMR Service at
Durham. We acknowledge the VIPER high-performance
computing facility of the University of Hull.
Keywords: organoboron • organocatalysis • Lewis base •
amines • density functional calculations
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Scheme 5. Interaction energy of the fragments compared to the
dissociated products at the RI-TPSS-D3/def2-TZVPP level of theory. This is
equivalent to considering each step in the cycle as a separate reaction step.
The energies in the scheme are those of Adduct 1 compared to B2Cat2 + NR3
at infinite separation (Adduct 1); TS compared to Adduct 1 + olefin at infinite
separation (TS); Adduct 2 compared to Adduct 1 + olefin at infinite separation
(Adduct 2).
As can be seen from Scheme 5, the formation energy of
Adduct 1 is favourable and similar for all amines with a slight
preference for the secondary amine. However, the TS has a
higher energy for the tertiary amine compared to the secondary
and primary amines. This is likely caused by the increased steric
bulk of the amine in this case and is in line with the decreased
reactivity that we observe experimentally. The reaction then
proceeds to form Adduct 2, in a predicted exothermic process
(–144.5 kJ/mol for NH2Me) before regenerating the amine
catalyst.
In summary, we have developed a simple method for the
activation of diboronic esters and their use in diboration
reactions. Our methodology relies in the use of amines as the
catalyst and overcomes the many limitations of current methods
in terms of both substrate scope and functional group tolerance.
This method opens the door to the use of amines for chiral
borylation reactions that are currently under study in our
laboratory.
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