Cycloaddition Reactions• In this reaction two π bond lost and two σ bonds formed• One step mechanism, no intermediates (aromatic transition state*) arrows start from π bond and go around the ring.• Can occur thermally or photochemically.• All cycloaditions are suprafacial ( occur on one face of π system).
Suprafacial and antarafacial geometries• Suprafacial if overlap occur at two lobes located on the same side the nodal plane• Antarafacial if on opposite sides
Ene Reaction• Alder Ene reaction• Alkene with –I group (enophile) and alkene with allylic hydrogen.• Formation of one new C-C and C-H bonds• No ring formed here ????????• Mainly , similar features of D.A reaction. O O O O H H O O
Woodward – Hoffmann rulesFor Cycloadition reactions Total number of Reaction Allowed mode(4q + 2)s and (4r)a conditions of ring closure Odd number Thermal Suprafacial Odd number Photochemical Antarafacial Even number Thermal Antarafacial Even number Photochemical Suprafacial
Enophiles• Can also be aldehyde, ketone and imines.• The product in this case is hydroxylated. R3 S N R4 R2 O R2 R1 R2 R1 R2 R1 R3 R1 O O X X H H
Ene Reaction• Can be catalyzed by Lewis acids to increase electrophilicity of enophile.• Intramolecular ene reaction MeAlCl2 O O H R R O OH H
Carbonyl Ene Reaction• Carbonyl compounds are good enophiles.• L.A catalysis is possible (lowering LUMO energy)• Using L. A enhances stereoselectivity. O O L.A MeO MeO H O OH
Diels-Alder ReactionOtto Diels Kurt Alder • Method for synthesis of 6-membered ring • One-step, concerted reaction • Termed [4+2] cycloaddition reaction where 4π and 2π electrons react. +
Diels-Alder Reaction• Discovered by O. Diels and K. Alder in 1928.• Occur between a conjugated diene and substituted alkene (dienophile) to form cyclohexene ring system.• Concerted reaction (single step), can be accelerated by heating or using some catalysts.• [ 4+2 ] cycloaddition reaction.• In retro Diels-Alder reaction, the six membered ring is break down to regenerate the diene and dienophile using high temperature usually.• Stereoselective reaction ( mainly one product formed).
Diels-Alder Reaction• Stereospecific reaction ( reactants can keep their stereochemistry).• No transition states or charged intermediates.• All electrons moving in same time to form two new σ bonds.• 100 % economic ( No. of reactants atoms = No. of products atoms).• If one or more of diene or dienophile atoms are not carbon ,the reaction is hetero-Diels-Alder reaction.
How it works?• Concerted reaction via aromatic transition state.• There are two approches, the first depends on the interaction between HOMO of The diene and LUMO of the dienophile.• The second, depends on +I, -I groups affect, since they form negative and positive cherges on the diene and dienophile.
Frontier orbital interactions in the[π4s+π2s] cycloaddition ψ4Antibonding Ψ2׳ ψ3 ψ3 LUMO Ψ 2׳ B A HOMO ψ1׳Bonding ψ2 ψ1׳ ψ2 ψ1 A B Suprafacial
Photochemical [2+2] Cycloaddition• 4n cycloaddition type (not allowed thermally).• Light excitation• Stereospecific reaction• Forward reaction ψ2 ψ2׳ Antibonding HOMO LUMO LUMO ψ2׳ HOMO ψ2 Bonding *
Photochemical [2+2] Cycloaddition• Mechanism of most of theses reactions are not clear due to isomerisation. O O O hv . . O O MeO2 C O CO2 Me CO2 Me hv CO2 Me CO2 Me MeO2 C
Regioselectivity• LUMO - LUMO Interaction O O hv • HOMO - LUMO Interaction O O- + hv X
Thermal [2+2] Cycloaddition• Mainly, via ketene chemistry.• Formation four memebered ring.• Alkene with elecrtophilic alkene O O C C C N Cl Cl R O C heat
Frontier orbital interactions in the[π2s+π2s] cycloadditionA- Thermal reaction ψ2 Antibonding LUMO ψ2 ψ1 Bonding HOMO ψ1 Suprafacial Antarafacial
Thermal [2+2] Cycloaddition• Cyclopentadiene can react in [2+2] fashion??? faster than Diels-Alder. O O C NO2 NO2 NC OAc OAc CN
1,3 dipolar cycloaddition• [(2╥ +2e) + 2 ╥ ] to form five membered ring• 1,3 dipolar component and polarophile (substituted alkene).• Nitrone ( 3 atoms ) + Alkene (2 atoms).• 4s+2s suprafacail cycladdition R +N _ R O N O O R
1,3 dipolar cycladdition• [2+3] means no. of atoms NOT [2+3] cycloaddition• Other examples of 1,3 dipoles• Could we say nitrone similar to the diene ???
Comparison• 1.3 dipolar C.A • Diels-Alder reaction• Nitrone and alkene • Diene + alkene• [(2╥ +2e) + 2 ╥ ] • [4╥ + 2 ╥ ]• 3+2 =Five membered ring • 4+2= six membered ring• Works with simple alkene • HOMO of diene and LUMO• Use HOMOs or LUMOs, of the alkene.* depending on the polarophile
1,3 dipolar cycloaddition R RLUMO +N _ +N _ HOMO O O R O LUMOHOMO R
Ozonolysis• Cleveage of alkenes with ozone to form carbonyl compounds mainly.• So, 1,3 dipolar cycloaddirion O O O O O O
Ozonolysis• The product can be controlled by work up step.
Ozonolysis O O OO O 1,3 dipolar cycloaddition O O O O R1 R2 R1 R2R1 R2 Ozonide carbonyl oxide O R1 R1 O O 1,3 dipolar cycloaddition O O O R2 R2 O O R2 Me Me R1 O SMe2 S O R1 O O R2 unstable
Oxidation with OsO4• OsO4, is yellow solid,used as oxidizing agent and as a catalyst.• High compatibility for compexation due to multi oxidation states.• Os (76e) has 11 different oxidation states,. O O Os O O R OsO4 R OH R HO R
Mechanism O O R Os O O O O Os R O O R1 R2 viii O O Os HO OH OH H 2O O O O O Os R R HO OH R1 R2R1 R2 OH vi Sy nOsmate ester
Oxidation with OsO4• OsO4 can react with both –I ,+I alkenes• So, it uses HOMO or LUMO*.• Because of its toxicity and expense, its used with catalytic amount with co oxidant. OH O OsO 4, NMO t-BuOH, H2 O OH N Me O
References• Organic chemistry; Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Oxford University Press, Oxford, 2006.• Strategic Applications of Named Reactions in Organic Synthesis; Korti, L.; Czako, B. Elsevier Press, USA, 2005.• Wikipedia.co.uk.• Organic Chemistry Portal.Org