26 research outputs found
Stabilization of NaBH<sub>4</sub> in Methanol Using a Catalytic Amount of NaOMe. Reduction of Esters and Lactones at Room Temperature without Solvent-Induced Loss of Hydride
Rapid reaction of
NaBH<sub>4</sub> with MeOH precludes its use
as a solvent for large-scale ester reductions. We have now learned
that a catalytic amount of NaOMe (5 mol %) stabilizes NaBH<sub>4</sub> solutions in methanol at 25 °C and permits the use of these
solutions for the reduction of esters to alcohols. The generality
of this reduction method was demonstrated using 22 esters including
esters of naturally occurring chiral γ-butyrolactone containing
dicarboxylic acids. This method permits the chemoselective reductions
of esters in the presence of cyano and nitro groups and the reductive
cyclization of a pyrrolidinedione ester to a fused five-membered furoÂ[2,3-<i>b</i>]Âpyrrole and a (−)-crispine A analogue in high optical
and chemical yields. Lactones, aliphatic esters, aromatic esters containing
electron-withdrawing groups, and heteroaryl esters are reduced more
rapidly than aryl esters containing electron-donating groups. The <sup>11</sup>B NMR spectrum of the NaOMe-stabilized NaBH<sub>4</sub> solutions
showed a minor quartet due to monomethoxyborohydride (NaBH<sub>3</sub>OMe) that persisted up to 18 h at 25 °C. We postulate
that NaBH<sub>3</sub>OMe is probably the active reducing agent. In
support of this hypothesis, the activation barrier for hydride transfer
from BH<sub>3</sub>(OMe)<sup>−</sup> onto benzoic acid methyl
ester was calculated as 18.3 kcal/mol
Biotic Carbon Sequestration and the Kyoto Protocol: The Construction of Global Knowledge by the Intergovernmental Panel on Climate Change
biotic carbon sequestration, boundary work, carbon sequestration, climate change, climate change policy, forests, global governance, global knowledge, Intergovernmental Panel on Climate Change (IPCC), international boundary organization, international knowledge organization, Kyoto Protocol, uncertainty management,