Lectures in Heterocyclic Chemistry

Lectures in Heterocyclic ChemistryChem. 4239

Collected and organized by Prof. Dr. Adel Awadallah

Islamic University of Gaza

(2011)

Lectures in Heterocyclic Chemistry

(Collected and organized by Prof. Dr. Adel Awadallah)

Text BookHeterocyclic Chemistry, T. L. GilchristOther Books and References

* Heterocyclic Chemistry, R. Gupta, M. Kumar, V. Gupta* Heterocyclic Chemistry, J. A. Joule, G. F. Smith* An Introduction to the chemistry of Heterocyclic compounds, R. M. Acheson

* Comprehensive Heterocyclic Chemistry, edited by: A. R. Katritzky and C. W. Rees

* Journals in organic and heterocyclic chemistry such as

J. Heterocyclic Chem.

Heterocycles

Molecules

Synthetic communications

J. Organic ChemistryNomenclature of Heterocyclic CompoundsSystematic Nomenclature system: (Hantzsch-Widman System)

Heterocycles with recognized trivial names

Naming HeteromonocyclesPrefix (heterotoms, number, positions) + Stem (ring size + saturation)

Examples: Name the following compounds

Indication of saturated positions1 position (H)

2 positions (dihydro)

3 positions (dihydro + H)

4 posit ions (tetrahydro)

5 positions (tetrahydro + H)

Saturated positions receive the lower number

Examples:

Compounds containing exocyclic C=O and C=S

Nomenclature of fused ring systemsPrefix(O) + Base component

Base Component1) One ring only contains N, Choose it

2) No, Nitrogen, oxa , thia, aza

3) One consists of two or more rings, choose it

4) Two rings of different size, choose the larger

5) Choose the one with more heteroatoms

6) Same number of heteroatoms, choose oxa > thia > aza

7) Same number of heteroatoms, same oxa, thia, aza, then choose lower numbering

Indicate the fusion by giving letters to the base components and numbers to the prefix (go in the same direction)

Examples:

Numbering substituents on fused rings:

1) Use rectangular coordinates2) As many rings as possible lie in a horizontal row

3) A maximum number of rings are in the upper right quadrant4) The system is numbered in a clockwise direction commencing with that atom which is not engaged in the ring fusion and is furthest to the left:

in the uppermost ring or

in the ring furthest to the right in the upper row

5) C atoms which belong to more than one ring are omitted

6) Heteroatoms in such positions are, however, included

7) If there are several possible orientations in the coordinate system, a))) the one in which the heteroatoms bear the lowest locants is valid,,,,,, b))) or the one in which the C atom that belongs to more than one ring has the lowest locant

Examples:

Chapter 4

Ring Synthesis

Cyclization Reactions Cycloaddition Reactions Ring transformation

1) Displacement at saturated carbons

Examples

Feist-Benary Furane Synthesis

More Examples

Intramolecular Nucleophilic Addition to Carbonyl Groups

Hinzberg Synthesis of Thiophene

(Carbon nucleophile)

Pall-Knorr Synthesis of Furane

Pall-Knorr Synthesis of Pyrrole

Cyclization onto the ortho position of a phenyl ring

A free ortho position act as a nucleophilic center

Intramolecular Nucleophilic Addition to Other double bonds

(C=S, C=N, C=C)

Cyclization onto triple bonds

Cyclization onto nitriles (CN)

Cyclization onto Isonitriles (R-NC)

Cyclization onto triple bonds

Cyclization onto nitriles (CN)

Cyclization onto Isonitriles (R-NC)

Radical Cyclization Five- and six-membered rings are most commonly formed by preferential exo-cyclization.

Kinds of Radicals:

Neutral (generated by tributyltin hydride for carbon radicals, or by photolysis of N-Cl bond).

This radical is very reactive and unselective.

Protonated radicals(add efficiently to many types of double bonds, mainly C=C)

Radicals complexed to metal ions (moderate reactivity)

Neutral aminyl radical

Neutral carbon radical

Protonated aminyl radical

Radicals complexed to metal ions

More Examples:

Carbene and nitrene cyclization

Carbenes are uncharged, electron deficient molecular species that contain a divalent carbon atom surrounded by a sextet of electrons.

Nitrenes are uncharged, electron deficient molecular species that contain a monovalent nitrogen atom surrounded by a sextet of electrons.

Generally there are two types of carbenes; singlet or triplet carbenes. Singlet carbenes have a pair of electrons and an sp2 hybrid structure. Triplet carbenes have two unpaired electrons. They may be either sp2 hybrid or linear sp hybrid. Most carbenes have a nonlinear triplet ground stateCarbenes are called singlet or triplet depending on the electronic spins they possess. Triplet carbenes are paramagnetic and may be observed by electron spin resonance spectroscopy if they persist long enough. The total spin of singlet carbenes is zero while that of triplet carbenes is one (in units of ). Bond angles are 125-140 for triplet methylene and 102 for singlet methylene (as determined by EPR). Triplet carbenes are generally stable in the gaseous state, while singlet carbenes occur more often in aqueous media.

For simple hydrocarbons, triplet carbenes usually have energies 8 kcal/mol (33 kJ/mol) lower than singlet carbenes (see also Hund's rule of Maximum Multiplicity), thus, in general, triplet is the more stable state (the ground state) and singlet is the excited state species.Formation Reactions of Carbenes

Reactions of Carbenes

1) Addition to multiple bonds

Singlet carbenes generally participate in cheletropic reactions as either electrophiles or nucleophiles. Singlet carbene with its unfilled p-orbital should be electrophilic. Triplet carbenes should be considered to be diradicals, and participate in stepwise radical additions. Triplet carbenes have to go through an intermediate with two unpaired electrons whereas singlet carbene can react in a single concerted step. Addition of singlet carbenes to olefinic double bonds is more stereoselective than that of triplet carbenes. Addition reactions with alkenes can be used to determine whether the singlet or triplet carbene is involved.

Reactions of singlet methylene are stereospecific while those of triplet methylene are not. For instance the reaction of methylene generated from photolysis of diazomethane with cis-2-butene and trans-2-butene is stereospecific which proves that in this reaction methylene is a singlet.[4]

Insertions are another common type of carbene reactions.

The carbene basically interposes itself into an existing bond. The order of preference is commonly: 1. X-H bonds where X is not carbon 2. C-H bond 3. C-C bond. Insertions may or may not occur in single step.

Carbene insertion

Intramolecular insertion reactions present new synthetic solutions. Generally, rigid structures favor such insertions to happen. When an intramolecular insertion is possible, no intermolecular insertions are seen. In flexible structures, five-membered ring formation is preferred to six-membered ring formation.

Carbene intramolecular reaction

Carbene intermolecular reaction

Nitrenes

Formation

Nitrenes are very reactive and not isolated as such. They are formed as reactive intermediates in the reactions:

1) from thermolysis or photolysis of azides with expulsion of nitrogen gas, analogues to the formation of carbenes from diazo compounds.

2) from isocyanates, with expulsion of carbon monoxide, analogues to carbene formation from ketenes

R-N=C=O gives R-N

3) From N-amino heterocycles

4) From photolysis of Sulfilimines:

Examples:

Carbene and nitrene cyclization

Carbenes are uncharged, electron deficient molecular species that contain a divalent carbon atom surrounded by a sextet of electrons.

Nitrenes are uncharged, electron deficient molecular species that contain a monovalent nitrogen atom surrounded by a sextet of electrons.

Generally there are two types of carbenes; singlet or triplet carbenes. Singlet carbenes have a pair of electrons and an sp2 hybrid structure. Triplet carbenes have two unpaired electrons. They may be either sp2 hybrid or linear sp hybrid. Most carbenes have a nonlinear triplet ground stateCarbenes are called singlet or triplet depending on the electronic spins they possess. Triplet carbenes are paramagnetic and may be observed by electron spin resonance spectroscopy if they persist long enough. The total spin of singlet carbenes is zero while that of triplet carbenes is one (in units of ). Bond angles are 125-140 for triplet methylene and 102 for singlet methylene (as determined by EPR). Triplet carbenes are generally stable in the gaseous state, while singlet carbenes occur more often in aqueous media.

For simple hydrocarbons, triplet carbenes usually have energies 8 kcal/mol (33 kJ/mol) lower than singlet carbenes (see also Hund's rule of Maximum Multiplicity), thus, in general, triplet is the more stable state (the ground state) and singlet is the excited state species.Formation Reactions of Carbenes

Reactions of Carbenes

1) Addition to multiple bonds

Singlet carbenes generally participate in cheletropic reactions as either electrophiles or nucleophiles. Singlet carbene with its unfilled p-orbital should be electrophilic. Triplet carbenes should be considered to be diradicals, and participate in stepwise radical additions. Triplet carbenes have to go through an intermediate with two unpaired electrons whereas singlet carbene can react in a single concerted step. Addition of singlet carbenes to olefinic double bonds is more stereoselective than that of triplet carbenes. Addition reactions with alkenes can be used to determine whether the singlet or triplet carbene is involved.

Reactions of singlet methylene are stereospecific while those of triplet methylene are not. For instance the reaction of methylene generated from photolysis of diazomethane with cis-2-butene and trans-2-butene is stereospecific which proves that in this reaction methylene is a singlet.[4]

Insertions are another common type of carbene reactions.

The carbene basically interposes itself into an existing bond. The order of preference is commonly: 1. X-H bonds where X is not carbon 2. C-H bond 3. C-C bond. Insertions may or may not occur in single step.

Carbene insertion

Intramolecular insertion reactions present new synthetic solutions. Generally, rigid structures favor such insertions to happen. When an intramolecular insertion is possible, no intermolecular insertions are seen. In flexible structures, five-membered ring formation is preferred to six-membered ring formation.

Carbene intramolecular reaction

Carbene intermolecular reaction

Nitrenes

Formation

Nitrenes are very reactive and not isolated as such. They are formed as reactive intermediates in the reactions:

1) from thermolysis or photolysis of azides with expulsion of nitrogen gas, analogues to the formation of carbenes from diazo compounds.

2) from isocyanates, with expulsion of carbon monoxide, analogues to carbene formation from ketenes

R-N=C=O gives R-N

3) From N-amino heterocycles

4) From photolysis of Sulfilimines:

Evidence of Singlet Nitrene C-H Insertion Selectivity

Examples:

Electrocyclic Reactions

Formation of a -bond at the termini of a fully conjugated -system by heat or light.

Examples

More Examples

Cycloaddition Reactions

1,3-Dipolar Cycloaddition Reactions

Resonance Structures of 1,3-Dipoles

Each molecule has at least one resonance structure which indicates separation of opposite charges in 1,3-relationship.

Mechanism of Cycloaddition:

1,3-Dipolar cycloaddition reactions were found to be stereoselective. Most of them are regioselective.

2 -electrons of the dipolarophile and 4 electrons of the dipolar compound participate in a concerted, pericyclic shift. The addition is stereoconservative

1) Concerted Mechanism (suggested by R. Huisgen)

2) Biradical mechanism (Stepwise mechanism by Firestone)

Regiochemistry

Generation of 1,3-Dipoles

Nitrile oxides

Nitrile Sulfides

Nitrile Imides (Nitrilimines)

Examples

Hetero-Diels-Alder ReactionsReaction between cyclopentadiene and diethyl azodicarboxylate

Diens and Dienophiles

2 + 2 Cycloaddition

Paterno-Buechi Reaction

Cheletropic Reaction

Heterocyclic Synthesis

Pyridine

********************************************************************

Ring Synthesis1) The Hantzsch Synthesis

1,3-dicarbonyl compound + ammonia + aldehyde

2) Reaction of Ammonia + 1,5-diketone

3) Diels-Alder Reaction

4) Kroehnke Synthesis

Chemistry of Pyridine

a) Reaction at nitrogen

Electrophilic SubstitutionPyridine is million times less reactive than benzene

Nitration (less than 5%, Chlorination in moderate yield, Bromination in a good yield)

3-position is usually attacked preferably

ChiChibabin Reaction

Amination of pyridine and related heterocycles at the 2-position by sodamide

Quinoline and Isoquinoline

Quinoline Skraup Synthesis

Doebner-von Millar

Combes Synthesis

Friedlaender Synthesis

Isoquinoline Synthesis

Bischler-Napierlaski

Pictet-Spengler Synthesis

Pomeranz-Fritsch Synthysis

Chemistry of Quinoline and Isoquinoline

Nucleophilic Substitution (ChiChibabin Reaction)

Electrophilic Substitution

Occurs at the 5- or 8-positions, or both

Quinoline N-Oxides can be nitrated at the 4-position or photoisomerize as follows

Heterocyclic Synthesis

Preparation of Pyrylium Salts

Reactions of Pyrylium Salts

Synthesis of (-Pyrones

Diels-Alder Reactions of (-Pyrones

(-Pyrone

Cliasen Condensation of Ethylpropiolate with Acetone

Pyrrole

b. p. 129

Synthesis of pyrrole

Knorr Synthesis

L. Knorr, Ber. 17, 1635 (1884); Ann. 236, 290 (1886); L. Knorr, H. Lange, Ber. 35, 2998 (1902).

The Knorr pyrrole synthesis is a widely used chemical reaction that synthesizes substituted pyrroles (3).[1]

HYPERLINK "http://en.wikipedia.org/wiki/Knorr_pyrrole_synthesis" \l "cite_note-1" \o "" [2]

HYPERLINK "http://en.wikipedia.org/wiki/Knorr_pyrrole_synthesis" \l "cite_note-2" \o "" [3] The method involves the reaction of an -amino-ketone (1) and a compound containing a methylene group - to (bonded to the next carbon to) a carbonyl group (2).[4]The original Knorr synthesis employed two equivalents of ethyl acetoacetate, one of which was converted to ethyl 2-oximinoacetoacetate by dissolving it in glacial acetic acid, and slowly adding one equivalent of saturated aqueous sodium nitrite, under external cooling. Zinc dust was then stirred in, reducing the oxime group to the amine. This reduction consumes two equivalents of zinc and four equivalents of acetic acid.

Modern practice is to add the oxime solution resulting from the nitrosation and the zinc dust gradually to a well-stirred solution of ethyl acetoacetate in glacial acetic acid. The reaction is exothermic, and the mixture can reach the boiling point, if external cooling is not applied. The resulting product, diethyl 3,5-dimethylpyrrole-2,4-dicarboxylate, has been called Knorr's Pyrrole ever since. In the Scheme above, R2 = COOEt, and R1 = R3 = Me represent this original reaction.

Paal-Knorr Pyrrole Synthesis

The Paal-Knorr Pyrrole Synthesis is the condensation of a 1,4-dicarbonyl compound with an excess of a primary amine or ammonia to give a pyrrole.

The Hantzsch pyrrole synthesisThe Hantzsch pyrrole synthesis, named for Arthur Rudolf Hantzsch, is the chemical reaction of -ketoesters (1) with ammonia (or primary amines) and -haloketones (2) to give substituted pyrroles (3).[1]

HYPERLINK "http://www.answers.com/topic/hantzsch-pyrrole-synthesis" \l "cite_note-1" \o "" [2]Note: direct reaction of -ketoesters (1) with -haloketones (2) gives furan [Fiest-Benary furan synthesis], and this can be a troublesome side reaction.

References1. ^ Hantzsch, A. Ber. 1890, 23, 1474.

2. ^ Feist, F. Ber. 1902, 35, 1538.

Reactions of Pyrrole

Substitution at nitrogen

A) Metallation of Pyrrole

B) Formation of N-substituted pyrrole

N-substituted products are normally isolated only from reaction of pyrrole anions with electrophiles

Electrophilic Substitution

Intermediates in the electrophilic substitution of pyrrole

The Vilsmeier Haack reaction

Cycloaddition Reactions with dichlorocarbene

Reimer-Tieman Reaction

Ring Expansion

Diels-Alder Reactions of pyrrole

Pyrroles normally do not undergo DA reactions

Exception

[2 +2] Cycloaddition

Furan

b.p. = 31 oC

Natural products containing furane

Synthesis of Furan

Paal-Knorr Synthesis

Feist-Benary Furane Synthesis

Reactions of Furana) Protonation

b) Electrophilic aromatic substitution

Bromination of furane:

Nitration of Furane

Vilsmeier-Haack reaction produces 2-formylfuran

Cycloaddition Reactions

Diels-Alder reaction with maleic anhydride

Reaction with Acrylonitrile

Reaction with dimethylacetylendicarboxylate

Thiophene

b. p. 84 oC from coal tar

electron rich aromatic compound which is more aromatic than benzene.

Pyrantal 49, is a broad spectrum anthelmintic agent ( ) effective against pinworm and hookworm

Bioten (Vitamin H), 50, occurs in yeast and egg

Thiophene also occurs in organic conducting polymers

Ring Synthesis

a) The Pall Synthesis

b) The Hinzberg Synthesis

c) The Gewald Synthesis

Lawesson's reagent can be used also in the first synthesis.

Simple carbonyl compounds can be used in the third synthesis in the presence of elemental sulfur

Lawesson's reagent

From Wikipedia, the free encyclopedia

Jump to: navigation, searchLawesson's reagent

IUPAC name[show]2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane2,4-disulfide

Other namesLawesson's reagent, LR

Lawesson's reagent, or LR, is a chemical compound used in organic synthesis as a thiation agent. Lawesson's reagent was first made popular by Sven-Olov Lawesson, who did not, however, invent it. Lawesson's reagent was first made in 1956 during a systematic study of the reactions of arenes with P4S10.[1]

Preparation

Lawesson's reagent is commercially available. It can also be conveniently prepared in the laboratory by heating a mixture of anisole with phosphorus pentasulfide until the mixture is clear and no more hydrogen sulfide is formed,[2] then recrystallized from toluene or xylene.

As Lawesson's reagent has a strong and unpleasant smell, it is best to prepare the compound within a fume-hood and to treat all glassware used with a decontamination solution before taking the glassware outside the fume-hood. One common and effective method of destroying the foul smelling residues is to use an excess of sodium hypochlorite (chlorine bleach).

[edit] Mechanism of action

Lawesson's reagent has a four membered ring of alternating sulfur and phosphorus atoms. With heating, the central phosphorus/sulfur four-membered ring can open to form two reactive dithiophosphine ylides (R-PS2). Much of the chemistry of Lawessons's reagent is in fact the chemistry of these reactive intermediates.

In general, the more electron rich a carbonyl is, the faster the carbonyl group will be converted into the corresponding thiocarbonyl by Lawesson's reagent.

[edit] Applications

The chemistry of Lawesson's reagent and related substances has been reviewed by several groups.[3]

HYPERLINK "http://en.wikipedia.org/wiki/Lawesson's_reagent" \l "cite_note-3" \o "" [4]

HYPERLINK "http://en.wikipedia.org/wiki/Lawesson's_reagent" \l "cite_note-4" \o "" [5]

HYPERLINK "http://en.wikipedia.org/wiki/Lawesson's_reagent" \l "cite_note-5" \o "" [6] The main use of Lawesson's reagent is the thionation of carbonyl compounds. For instance, Lawesson's reagent will convert a carbonyl into a thiocarbonyl.[7] Additionally, Lawesson's reagent has been used to thionate enones, esters[8], lactones[9], amides, lactams[10], and quinones.

In one study, reaction of maltol with LR results in a selective oxygen replacement in two positions.[11]

A combination of silver perchlorate and Lawesson's reagent is able to act as an oxophilic Lewis acid with the ability to catalyze the Diels-Alder reaction of dienes with ,-unsaturated aldehydes.

Reactions of Thiophene

Electrophilic Substitution Substitution takes place at the 2- position

Reactivity pyrrole >> furan > thiophene > benzene

Thiophene tends to undergo substitution rather than addition reactions and it is not so readily cleaved by acids as is furan.

Substitution of 2-substituted thiophene

Nucleophilic Substitution

Cycloaddition ReactionThiophene is a poor diene

Nicotine

(s)-proline

Insect pheromone

Pyoluteorin

Pyrrolnitrin

Porphobilinnogen

Haem (iron (II) complex)

haemin

Porphyrin

uroporphyrinogen

Chlorophyll

Vitamin B12

Bilirubin

Ascorbic Acid

Rosefuran

PAGE 16

Mechanistic Aspects of Nitrene Chemistry

Cycloaddition - Observations

Evidence of Singlet Nitrene C-H Insertion Selectivity