Hydrosilylation of Aldehydes and Formates Using a Dimeric Manganese Precatalyst
2017-09-13T13:18:49Z (GMT)
by
The
formally zero-valent Mn dimer [(Ph2PEtPDI)Mn]2 has been synthesized upon reducing (Ph2PEtPDI)MnCl2 with excess Na/Hg. Single crystal X-ray diffraction analysis
has revealed that [(Ph2PEtPDI)Mn]2 possesses
a κ4-PDI chelate about each Mn center, as well as
η2-imine coordination across the dimer. The chelate
metrical parameters suggest single electron PDI reduction and EPR
spectroscopic analysis afforded a signal consistent with two weakly
interacting S = 1/2 Mn centers.
At ambient temperature in
the absence of solvent, [(Ph2PEtPDI)Mn]2 has
been found to catalyze the hydrosilylation of aldehydes at loadings
as low as 0.005 mol % (0.01 mol % relative to Mn) with a maximum turnover
frequency of 9,900 min–1 (4,950 min–1 per Mn). Moreover, the [(Ph2PEtPDI)Mn]2-catalyzed
dihydrosilylation of formates has been found to proceed with turnover
frequencies of up to 330 min–1 (165 min–1 relative to Mn). These metrics are comparable to those described
for the leading Mn catalyst for this transformation, the propylene-bridged
variant (Ph2PPrPDI)Mn; however, [(Ph2PEtPDI)Mn]2 is more easily inhibited by donor functionalities. Carbonyl
and carboxylate hydrosilylation is believed to proceed through a modified
Ojima mechanism following dimer dissociation.