VenPure® solution - The Dow Chemical Company

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O 1 NaBH4, 1 LiCl R diglyme, 162 o C OH H R Time Yield Me 5 85 i Pr 5 83 t-Amyl 5 0 Mita, N.; Nagase, H.; Iizuka, H.; Oguchi, T.; Sakai, K.; Horikomi, K.; Miwa, T.; Takahashi, S. EP 0839805; 1998 3. Assistance from Protic Solvents. When sodium borohydride is dissolved in a protic solvent, it will react with the solvent according to the equation NaBH4 + x HOR � NaBHx(OR)4-x + x H2 The resulting borohydride derivative, NaBHx(OR)4-x, is a stronger reductant than neat NaBH4 (this is due to electron-donation from the oxygen ligands). The art of using this technology resides in finding the optimum reaction conditions that allow for the formation of the strongly reducing borohydride derivative, without loosing all of the hydrides to solvolysis. Indeed, if all 4 hydrides react with the protic solvent, then NaB(OR)4 is formed, which has ZERO reducing strength : NaBH4 + 4 HOR � NaB(OR)4 + 4 H2 Methanol is by far the most efficient solvent in enhancing borohydride’s reductive strength. Water and ethanol are second best, however, water is typically not a good solvent for (longer chain) esters. Ethanol would be a good solvent, however NaBH4 only dissolves up to 4% in it. If one wants to use e.g. a 10% NaBH4 dispersion in ethanol, then we recommend the use of Rohm and Haas’ VenPure SF powder, which has a small particle size (increased active surface). The solubility of NaBH4 in water and methanol is higher than 15%. Hence, for making large quantities of a NaBH4 solution in these 2 solvents, we recommend to use an easier-to-handle, larger particle size, such as provided by VenPure SF granules or VenPure AF caplets. VenPure SF powder VenPure SF granules Venpure AF caplets H Hereunder are some examples of the use of protic solvents for enhancing Borohydride’s Reducing Strength. A. Mixture : Water / Dioxane 15 O Me R O 7-10 hydride eq NaBH 4 Water or Water/Dioxane, RT, 2-24 h R O H Substrate Time(h) Yield PhCO2Me 12 80 2-ClPhCO2Me 6 100 2-NO2PhCO2Me 2 100 2-MePhCO2Me 12 80 2-MeOPhCO2Me 24 60 Bianco, A.; Passacantilli, P.; Righi, G. Synth. Commun. 1988, 18, 1765 B. Mixture : an organic solvent / methanol 16 MeO O OH OH NaBH4 OMe O US 6,479,714 B1, Nov 12, 2002 Solvent, MeOH RT OH Solvent Temp Time Ratio Yield (°C) Sub /NaBH4 % THF/MeOH 65 1h 1 / 3.5 99 THF/MeOH 68 2.5 1 / 1.8 95 Spirits/MeOH 0 22h 1 / 1.79 100 THF/ i PrOH 50 6h 1 / 1.8 85 iPrOH 50 7h 1 / 1.80 85 THF 25 17h 1 / 1.80 90 This technique is often used on larger-scale, as it is a compromise between using the solubility characteristics of THF, and the Reductive Strength Enhancing capability of methanol. C. Ethanol In the above two examples the reactive protic solvent, methanol or water, is diluted with a non-reactive solvent This is done in order to minimize the amount of sodium borohydride that is lost due to solvolysis decomposition. In some instances, a 2-solvent system may not be optimal. It is then possible to replace it by one protic solvent that decomposes NaBH4 less rapidly. Ethanol 17 and polyethylene glycol 18 are such solvents. They have the advantage that borohydride’s solvolysis rate is substantially slower then in methanol. On the other hand, their enhancing effect will be less pronounced. . OH
D. Methanol Another way to minimize the rate of solvolysis of NaBH4 in protic solvent is to increase the solvent’s alkalinity. An example of this is to add sodium methoxide to a sodium borohydride solution in methanol. The rate of solvolysis is then reduced by more then 90 %. % NaBH4 consumed 100 80 60 40 20 5. Overview 0 0 10 20 30 40 Time (minutes) No NaOMe added 0.010 N NaOMe As a summary, please find hereunder a table with proven Reductive Technologies for reduction of Ethyl Benzoate. Metal moles Time Temp Additive Solvent Yield Hydride h o C (
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Page 3: Cl 2. Assistance from Lewis Acids T

VenPure® solution - The Dow Chemical Company

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