2 research outputs found
Hydrosilylation of Aldehydes and Formates Using a Dimeric Manganese Precatalyst
The
formally zero-valent Mn dimer [(<sup>Ph2PEt</sup>PDI)ÂMn]<sub>2</sub> has been synthesized upon reducing (<sup>Ph2PEt</sup>PDI)ÂMnCl<sub>2</sub> with excess Na/Hg. Single crystal X-ray diffraction analysis
has revealed that [(<sup>Ph2PEt</sup>PDI)ÂMn]<sub>2</sub> possesses
a κ<sup>4</sup>-PDI chelate about each Mn center, as well as
η<sup>2</sup>-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 <i>S</i> = <sup>1</sup>/<sub>2</sub> Mn centers.
At ambient temperature in
the absence of solvent, [(<sup>Ph2PEt</sup>PDI)ÂMn]<sub>2</sub> 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<sup>–1</sup> (4,950 min<sup>–1</sup> per Mn). Moreover, the [(<sup>Ph2PEt</sup>PDI)ÂMn]<sub>2</sub>-catalyzed
dihydrosilylation of formates has been found to proceed with turnover
frequencies of up to 330 min<sup>–1</sup> (165 min<sup>–1</sup> relative to Mn). These metrics are comparable to those described
for the leading Mn catalyst for this transformation, the propylene-bridged
variant (<sup>Ph2PPr</sup>PDI)ÂMn; however, [(<sup>Ph2PEt</sup>PDI)ÂMn]<sub>2</sub> is more easily inhibited by donor functionalities. Carbonyl
and carboxylate hydrosilylation is believed to proceed through a modified
Ojima mechanism following dimer dissociation
A Pentacoordinate Mn(II) Precatalyst That Exhibits Notable Aldehyde and Ketone Hydrosilylation Turnover Frequencies
Heating (THF)<sub>2</sub>MnCl<sub>2</sub> in the presence of the pyridine-substituted bisÂ(imino)Âpyridine
ligand, <sup>PyEt</sup>PDI, allowed preparation of the respective
dihalide complex, (<sup>PyEt</sup>PDI)ÂMnCl<sub>2</sub>. Reduction
of this precursor using excess Na/Hg resulted in deprotonation of
the chelate methyl groups to yield the bisÂ(enamide)ÂtrisÂ(pyridine)-supported
product, (κ<sup>5</sup>-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-<sup>PyEt</sup>PDEA)ÂMn. This complex was characterized by single-crystal X-ray diffraction
and found to possess an intermediate-spin (<i>S</i> = <sup>3</sup>/<sub>2</sub>) MnÂ(II) center by the Evans method and electron
paramagnetic resonance spectroscopy. Furthermore, (κ<sup>5</sup>-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-<sup>PyEt</sup>PDEA)Mn was determined
to be an effective precatalyst for the hydrosilylation of aldehydes
and ketones, exhibiting turnover frequencies of up to 2475 min<sup>–1</sup> when employed under solvent-free conditions. This
optimization allowed for isolation of the respective alcohols and,
in two cases, the partially reacted silyl ethers, PhSiHÂ(OR)<sub>2</sub> [R = Cy and CHÂ(Me)Â(<sup>n</sup>Bu)]. The aldehyde hydrosilylation
activity observed for (κ<sup>5</sup>-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-<sup>PyEt</sup>PDEA)Mn renders it one of the most efficient first-row
transition metal catalysts for this transformation reported to date