chemistry 125: lecture 67 april 12, 2010 oxidizing/reducing alcohols grignard reactions green...
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Chemistry 125: Lecture 67April 12, 2010
Oxidizing/Reducing AlcoholsGrignard ReactionsGreen Chemistry
This
For copyright notice see final page of this file
(Loudon)
Alcohol Oxidation (sec. 16.14)
Stopping at Aldehyde (p. 805-6)
Vicinal Diol Cleavage by IO4- (sec. 16.14b)
Pyridinium Chlorochromate (Cl-CrO2O-)
“In practice the chromium byproduct deposits with pyridine as a sticky black tar, which can complicate matters.” (Wikipedia)
no water; no diol; no
overoxidation
O
“Versatility” of Grignard Reagent
R-OH R-Br R-MgBrPBr3 Mg
O
CH2
O
CH
O
C
Is there a preferred order?
O
C
nucleophiliccarbon?
from alkene
(3 steps from CH3OH)
or
(1 step from CH3OH)
“Versatility” of Grignard Reagent
1) CH3MgBrO
OH
CH3
95%
2) H+ / H2O MgBr
OH
t-Bu
0%
1) t-BuMgBr
2) H+ / H2OO
1) t-BuCH2MgBr
2) H+ / H2OO
OH
CH2-t-Bu
4%
OMgBrH
H
OH
65%
H- reduction
H-CH2-t-Bu
H
H-t-Bu+ ketone
35%
H+
+ enolate ketone90%
from Roberts & Caserio (1965)
Cf. 2 t-Bu t-Bu-H
+
no H
avoid steric hindrance
H
:-(
+
“Versatility” of Grignard Reagent
from Roberts & Caserio (1965)
Risk of Reduction
H
and steric
hindrance
“Lithium aluminum hydride, having a molecular weight of 38 and four hydrides per molecule, has the highest hydride density and is frequently
used, even though it cogenerates an inorganic by-product which is difficult to separate from the product…slow filtration and product loss
through occclusion or adsorption are typical problems…”
Pharmaceuticals generate < 0.2% of the chemical industry’s product mass, but some 25% of its $ value,
and >50% of its chemical waste.
Table 1 Reactions companies use now but would strongly prefer better reagents
Amide formation avoiding poor atom economy reagents 6 votes
OH activation for nucleophilic substitution 5 votes
Reduction of amides without hydride reagents 4 votes
Oxidation/Epoxidation methods without the use of chlorinated solvents 4 votes
Safer and more environmentally friendly Mitsunobu reactions 3 votes
Friedel-Crafts reaction on unactivated systems 2 votes
Nitrations 2 votes
“Key green chemistry research areas - a perspective from pharmaceutical manufacturers” Green Chemistry, 2007, 9, 411-420
Pharmaceuticals generate < 0.2% of the chemical industry’s product mass, but some 25% of its $ value,
and >50% of its chemical waste.
Research AreaNumber of roundtable companies voting for this research area as a priority area
“…the use of stoichiometric high-valent metals (Mn, Os, Cr) have virtually been eliminated
from pharmaceutical processes…”
End of Lecture 67April 12, 2010
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