S-based Alkene Formation Flashcards by Barney Burden

Give the name of this reaction

The Julia Olefination
- highly selective for E alkenes

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Give the mechanism for step 4

The reaction is stereoconvergent
- beginning stereochemistry is irrelevant

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Give the name of this reaction

The Julia Olefination: Kocienski modification

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Give the mechanism for the Kocienski modification

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Through what reaction is the tetrazole unit made of the Kocienski modification?

The Mitsunobu reaction
- thiol then oxidised to sulphone using m-CPBA

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What is important about the final elimination step of the Kocienski modification?

Elimination is via antiperiplanar arrangement

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What’re the main differences between sulfur and oxygen chemistry?

O-H bond s much stronger than S-H bond (homolysis much easier)

Oxidants convert alcohols to carbonyls whereas RSH is converted to RSSR.

RSH and RSR are much more nucleophilic than O analogues in SN2

S stabilises carbanions more effectively than O

S has low-lying unoccupied MOs, allowing formation of hypervalent compounds

Carbonyls more stable than thiocarbonyls (2p-2p pi bonding vs 3p-2p pi bonding)

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How many lone pairs do S(II) compounds have?

2 lone pairs on S

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How many lone pairs do S(IV) compounds have?

1 lone pair on S

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Give the names of these following sulfur asked compounds

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What are thioketones susceptible to?

Hydrolysis

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What’s a common use of thiolate anions?

Due to their high nucleophilicity, they can be used to demethylate aryl methyl ethers and methyl esters

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Give the mechanism for this reaction

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What is the product of this reaction?

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What is the product of the following reaction?

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What effect does increasing the oxidation of a sulfur centre have on the acidity of alpha C-H bonds?

Increasing oxidation of S centre increases the acidity of alpha C-H bonds

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What factors affect acidity of alpha C-H bonds to sulfur?

Inductive effects (major)

Hyper conjugation with S-R sigma* orbital

Resonance/hyperconjugation with S=O sigma* orbital
- resonance with pi* (S=O) is not a factor that affects C-H acidity

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Give the Newman projection (down C-S bond) and explain significance to acidity

Due to antiperiplanar arrangement, -ve charge can hyperconjugate with Me sigma* - stabilising the anion

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Give the product of the following reaction

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How can this not very acidic proton be made more acidic?

By oxidising the S alpha to it
- this can then be alkylated using appropriate reagent (alkyl halide

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Give the product of the following reaction

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Give the mechanism of the following reaction

Hg coordinates to 1 of the S

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What is used commercially instead of Hg to convert carbonyls to thiocarbonyls?

Lawesson’s reagent

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Give the mechanism of Lawesson’s reagent with a ketone bound to R1 and R2 respectively. What drives this process?

P=O bond formation is driving force

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What catalyst is used to reduce C-S bonds?

Raney Nickel (RaNi) - sometimes in presence of H2

How can this amide be converted to a 2˚ amine?

Give the product of the following reaction

Give the products of the following reactions

Give the mechanism of PhSCl with a cis alkene

Give the mechanism of PhSCl with a trans alkene

Give the mechanism of the following reaction, (Hint: the reaction is heated to +20˚C after initial step)

What’s the benefit in synthetic chemistry of episulfonium ions?

They are highly electrophilic at adjacent carbon

How do analogous selenyl chlorides (ArSeCl) compare to sulfenyl chlorides (ArSCl)?

Selenyl chlorides proceed at lower temperatures (-78˚C) This is because weaker Se-Cl bond (vs S-Cl) allows reaction to proceed at lower T

Give the name of the following reaction

The Swern oxidation Used for the oxidation of alcohols to aldehydes and ketones

Give the mechanism of the Swern oxidation

What is the driving force of the Swern oxidation?

The loss of CO and CO2

What’s a drawback of the Swern oxidation?

The active electrophile, the chlorosulfonium species, is only stable at low temperatures Above -60˚C side reactions occur (including the Pummerer reaction)

What is the name of the following reaction? What the use of it?

The Parikh-Doering oxidation It is a more user friendly option to the Swern oxidation, occurring at 0˚C to rt instead of -60˚C

Give the mechanism of the Parikh-Doering oxidation

The less good LG makes the intermediate more stable, allowing the reaction to occur at 0˚C to rt

Give the name of the reaction shown

The Pummerer reaction Converting sulfoxide to alpha-acyloxy sulfides

Give the mechanism of the Pummerer reaction followed by acid workup

How can the alpha proton be made more acidic to promote the Pummerer reaction?

By using EWGs at R1

This reaction is reversible, what is often added to ensure the reaction goes to completion?

A scavenger - Na2CO3 - is often added to sequester the sulfenic acid - deprotonating sulfenic acid so reverse reaction is not possible

What is the name of the following reaction?

The Chugaev elimination it dehydrates alcohols to give alkenes

Give the mechanism of the Chugaev elimination

Give the name of this reaction

The Mislow-Evans rearrangement This is the name given to the method of trapping O-allylsulfenate by product to force completion in sulfoxide [2,3]-rearrangements

Give the mechanism of the Mislow-Evans rearrangement and identify intermediate X

Give the products of the following reactions

Ag+ blocks sulfur lone pair and so reacts via oxygen

What does reaction of sulfides with strong alkylating agents like Me3O+ . BF4- give?

Sulfonium and sulfoxonium salts can be deprotonated to form ylides, how do they differ?

Sulfoxonium ylides are softer Sulfonium ylides are harder This is due to sulfoxonium ylides having more diffuse, hence more polarisable, HOMOs

What do both sulfoxonium and sulfonium ylides both form when reacting with aldehyde or ketones?

Epoxides (Known as the Corey-Chaykovsky epoxidation)

Give the product of the following epoxidation

Give the regio-selective products of the following reactions

Differing selectivity has been proposed to be due to hard/soft effects

Give mechanisms of the following reaction at both hard and soft sites

The cyclopropane product is dominant as this is irreversible, whereas the 1,2-addition is reversible in stabilised sulfonium ylides

Give the mechanism of the following [2,3]-sigmatropic rearrangement. What is the driving force?

Driving force is: Gain of C-C bond Loss of C-S bond (Specific deprotonation as anion is stabilised by resonance of alkene and S+)

Give the name of the following reaction

The Kornblum oxidation Converting alpha-haloketones to (mostly) aldehydes and ketones

What are Cheletropic reactions?

Sigma-bonds are broken/formed at only 1 atom of 1 component

Give the 2 possible products (SO2 is released)

Give the Fukui HOMO-LUMO analysis of linear and non-linear cycloaddition of sulfones to form episulfones

Through what reactions can sulfonic acids be made?

Give the mechanism of the following reaction

What’s the significance of sulfoxide acids like MeSO3H, TolSO3H & CF3SO3H?

They are very strong organic acids (pKa = -2.0, -2.7 & -13.0 respectively)

Give the mechanism of the following reaction, including identification of the intermediate.

Why is diimide useful in organic synthesis?

Because it reduces alkenes (and some alkynes) under metal free conditions

Give the mechanism of how diimide reduces alkenes

What factors make alkene reduction by diimide faster?

Unhindered and electron rich alkenes are reduced fastest

Give the geometric isomer products of the following reaction.

Give the name of the following reaction

The Shapiro reaction It involves base-mediated decomposition of a sulfenylhydrazone to an alkynylation lithium species

Give the mechanism of the Shapiro reaction

Give the name of the following reaction

The Eschenmoser fragmentation It is facilitated by decomposition of sulfonylhydrazones

Give the mechanism of the Eschenmoser fragmentation