electrophilic addition reactions
DESCRIPTION
Electrophilic Addition Reactions. Part 2 Susan 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 - PowerPoint PPT PresentationTRANSCRIPT
Electrophilic Addition Reactions
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
H OH
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
CH3
1)BH3, THF
2)H2O2, NaOH(CH3)2CHCH2CH2CCH3
O
5-methyl-2-hexyne 5-methyl-2-hexanone
BH3, THF
(CH3)2CHCH2
C=CH BH2
CH3
H2O2, NaOH
(CH3)2CHCH2
C=CH OH
CH3
(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)
Hydroboration of alkynes
C C HCH3C C
H
CH3
BH2
H
NaOHH2O2
CH3CH2CH(OH)2
(propyne)
BH3
THF
CH3CH2-C-H
BH2
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
B
CH3
CH-CH-CH3
CH3
H
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
Hydroboration of alkynes
C C HCH3
C CH
CH3
B
HNaOHH2O2
CH3CH2CH
(propyne)
(propanal)
O
ether
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
C :H
H
E
Nu
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
R-C
O-OH
O
Epoxidation
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
KMnO4
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:
A) B)
C) D)
Question 16The ozonolysis of 2,4-dimethyl-2-pentene
with reductive workup will produce:
A) B)
C) D)
Ozonolysis of Alkynes Always produces carboxylic acids
Question 17What product is formed when 2-butyne is
subjected to ozonolysis?
A) B)
C) D)
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
Retrosynthesis
Synthesis
Retrosynthesis
Retrosynthesis
Synthesis