Electrophilic Addition Reactions

transcript

Part 2Susan Morante

All diagrams in this module are drawn using ISIS Draw software or are taken by permission from the textbook for this course by Hornback.

Hydroboration

“anti-Markovnikov” addition provides a source of hard to get alcohols

why do we need another way of hydrating alkenes?

C C C C

1)BH3, THF

2) H2O2, NaOH

Hydroboration

Hydroboration BH3 has a reversal of the usual polarity for

hydrogen Usually exists as a dimer (diborane)

Hydroboration Also exists as a complex with THF

Hydroboration mechanism

Hydroboration of alkynes internal alkyne -C-CC-C-

Works in the same way as alkenes Intermediate is an enol (see oxymercuration) Final product is a ketone

Hydroboration of alkynes

CH3CHCH2C CCH3

1)BH3, THF

2)H2O2, NaOH(CH3)2CHCH2CH2CCH3

5-methyl-2-hexyne 5-methyl-2-hexanone

BH3, THF

(CH3)2CHCH2

C=CH BH2

H2O2, NaOH

(CH3)2CHCH2

C=CH OH

(vinylborane) (enol)

tautomerism

Hydroboration of alkynes Problem: In example above another

product is also possible. Draw it. Is this second product more or less likely, or equally as likely as 5-methyl-2-hexanone?

Hydroboration of alkynes terminal alkyne -C-CC-H

Vinylborane produced is very reactive Reacts with a second molecule of borane When treated this gives a gem diol (gem diol = geminal diol = 1,1-diol)

C C HCH3C C

NaOHH2O2

CH3CH2CH(OH)2

(propyne)

CH3CH2-C-H

BH2(1,1-propanediol)

Hydroboration of alkynes to convert alkyne to

aldehyde (instead of diol) need to use a sterically hindered borane

i.e. BHR2 with R as a

large bulky group as in diisoamylborane

CH-CH-CH3

CH3CH3

CH-CH-CH3

Hydroboration of alkynes made by reaction of BH3 with 2 moles of 2-

methyl-2-butene adding a group like this to alkyne stops a

second borane from adding to the vinyl borane and leads to the aldehyde

C C HCH3

HNaOHH2O2

CH3CH2CH

(propyne)

(propanal)

HB(CH(CH3)CH(CH3)2)2 CHCHCH3

CH3CH3

CHCHCH3

CH3CH3

Addition of Carbene What is Carbene?

C with 2 bonds, 1 pair nonbonding e-

sp2 hybridized unstable transient intermediate simplest carbene :CH2 methylene

Addition of Carbene

unhybridized p orbitalsp2 hybridized orbital

Addition of Carbene Reacts in a similar fashion to Br and Hg

Addition of Carbene How is carbene formed: method 1: most commonly from

diazomethane species is so reactive that the alkene must be

present before the carbene is formed.

Addition of Carbene Method 2: formed from chloroform and

bromoform by 1,1-elimination in the presence of a strong

base again the alkene must be present in solution

to allow the next reaction to occur reaction is stereoselective

Addition of Carbene Problem: E-2-butene in reaction with

CHBr3 and tert-BuOK gives 68% yield of a

racemic mixture of trans-1,1-dibromo-2,3,-dimethylcyclopropane isomers. Show the reaction including stereochemistry.

Addition of Carbene Method 3: use a carbenoid to achieve the

same result. Gives stereospecific products Simmons - Smith reagent

Addition of Carbene mechanism

Epoxidation Use percarboxylic

acid reagents

R group does not affect reaction

Epoxidation

Hydroxylation of Alkenes Colour change of KMnO4 (purple) to MnO2

(brown) is a lab test to distinguish alkenes OsO4 oxidises three times better than

Hydroxylation of Alkenes Partial Mechanism

Hydroxylation of Alkynes Gives 1,2-dicarbonyl compounds unless

the triple bond is terminal in which case a carboxylic acid is obtained

Usually uses osmium tetroxide (because it is a more powerful oxidising agent than potassium permanganate)

Additional reagents are KClO3 with water and diethyl ether as the solvent system

Ozonolysis of alkenes A cleavage reaction O3 formed in upper atmosphere, during

lightning flashes, or by machine Bubble gas stream into a low temperature

solution of alkene which is dissolved in an inert solvent

Reactive intermediate is explosive

Ozonolysis of Alkenes Often water is added to destroy the reactive

intermediate and convert to carbonyl and H2O2

Then Zn dust and acetic acid is added to destroy H2O2

This is known as a reductive workup and produces aldehydes and ketones

An oxidative workup results in the production of ketones and carboxylic acids

Question 15The ozonolysis of 2,4-dimethyl-2-pentene

with oxidative workup will produce:

Question 16The ozonolysis of 2,4-dimethyl-2-pentene

with reductive workup will produce:

Ozonolysis of Alkynes Always produces carboxylic acids

Question 17What product is formed when 2-butyne is

subjected to ozonolysis?

Addition of Hydrogen (Hydrogenation) Alkenes

Addition of Hydrogen (Hydrogenation) Usual catalysts are Ni, Pd or Pt (as PtO2)

because of solid catalyst both H's add to the same side of the =

Addition of Hydrogen (Hydrogenation) reaction of alkynes with H2 in the presence

of Ni, Pd or Pt gives alkanes

Addition of Hydrogen (Hydrogenation) to obtain alkene must use Lindlar catalyst

(a special Pd catalyst)

Question 18Which one of the following terms best applies

to the hydrogenation of an alkene in thepresence of finely divided platinum? (in ethanol as solvent)A) anti additionB) concerted reactionC) heterogeneous catalysisD) endothermic reaction

Question 19How would you accomplish the following

conversion?

A) NaNH2

B) H2, Lindlar Pd

Question 20What is the structure of Compound Y in the

following synthetic sequence?

A) pentane

B) cis-2-pentene

C) trans-2-pentene

D) 2-pentyne

Question 21Which would be the best sequence of reactions to

use in order to prepare cis-3-nonenefrom 1-butyne?

A) 1. NaNH2 in NH3; 2. 1-bromopentane; 3. H2, Lindlar Pd

B) 1. H2, Lindlar Pd; 2. NaNH2 in NH3; 3. 1-bromopentane

Other Catalytic Hydrogenation Reactions

RCN + RaNi/H2 + heat gives RCH2NH2

R=NOH + Rh/C + H2 + EtOH gives RNH2

Carbonyl compounds are also subject to catalytic hydrogenation but there are better ways to reduce a C=O

Retrosynthesis

Synthesis

Retrosynthesis

Synthesis

Electrophilic Addition Reactions

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