CHM3930 Lecture Notes - Lecture 12: Reagent, Stereoselectivity, Triethylamine

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Week 12
Reagent selection
oAfter choosing synthetic route, must choose reagents
There are hundreds of diff reagents that do the same thing
oAim to produce high yield with minimal work up effort
oSuitability gaged from:
Reactivity (fast/slow)
Selectivity
Cost
Availability (multiple suppliers)
Ease of use on large scale
Waste disposal -diff by-products
Safety
oHow to pick the ideal reagent:
(1) Balanced equations -atom efficiency
- ie. Swern oxidation of secondary alcohols (see pic.)
oUses DMSO to oxidise (MeS=O-R)
oBut if R group is Me, then MeSMe is a by-product
gas at room temp & smells
oBut if R group is a 12-chain carbon
by-product is liquid & has no odour
oBut if R group is a ring
odourless, crystalline and easily recovered and
recycled
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- See slide 43 for characteristics of ideal reagent
(2) Safety and toxicity
- Toxicity screens should be conducted on all intermediates and by-
products
- Corrosivity (break down container, harm to humans)
- Metabolic poisons (Harm humans)
- Electrophilicity (React with proteins and DNA (nucleophile))
- Bioactivation (via metabolism -functional groups becoming
hazardous)
(3) Atom efficiency
- Minimising environmental impact and disposal costs
- Sometimes green reagents have properties that disadvantage their
use
- ie. Methylating agents (add a methyl group to something) **on
exam
otoo low bp -gas at room temp
either need to cool everything down, or use it as a
gas
oToo high boiling point -will breakdown organic component
before it gets boiled off
oIdeal: 41 degrees (MeI) which has atom efficiency of 16%
(4) Cost
- Differences amongst a family of reagents
oDifferent cost per mol
oDifferent pros and cons
- ie. Alkali metal hydroxides and (cost per mol)
oLiOH.H2O (3.8)
oNaOH pellets (1.0)
oNaOH 50% aq. (1.1)
oKOH 85% aq. (2.0)
Families of reagents
o(1) Reagents for deprotonation
NaH and BuLi used in small scale but not large scale (H2 -flammable)
o(2) Alkoxide bases
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Document Summary

Reagent selection: after choosing synthetic route, must choose reagents. There are hundreds of diff reagents that do the same thing: aim to produce high yield with minimal work up effort, suitability gaged from: See slide 43 for characteristics of ideal reagent (2) safety and toxicity. Toxicity screens should be conducted on all intermediates and by- products. Bioactivation (via metabolism -functional groups becoming hazardous) (3) atom efficiency. Differences amongst a family of reagents: different cost per mol, different pros and cons ie. alkali metal hydroxides and (cost per mol, lioh. h2o (3. 8, naoh pellets (1. 0, naoh 50% aq. (1. 1, koh 85% aq. (2. 0) Families of reagents: (1) reagents for deprotonation. Nah and buli used in small scale but not large scale (h2 -flammable: (2) alkoxide bases. Kotertbu used small scale but not large scale (poor solubility in organic solvents) tertamoh (changing a butane into a long carbon chain) used instead: (3) amine bases.

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