Alkenes Double bond: π and σ sp 2 120 o Electrophilic addition to alkenes E- E Alkene Funktionalized "alkane" Nucleophilic addition to alkenes (chapt. 23) EWG Nu- EWG Nu adical react. (Polymerization etc, hapt 7.10) Alkene EWG: -O, -N, -NO 2 etc Funktionalized "alkane" onjugate add., 1,4-add., Michael add.
+ Energy TS # 1 TS # 2 1. step. rate limiting egiosel.: Formation of the most stable cation intermediate (Markovnikov) ΔG # 2 ΔG # 1 Intermediate(s) Stabilization: Inductive yperkonjugation esonace + eactant(s) ΔG o Product(s) eact. prog.
Energy of TS # 1 or intermediate? Intermediate higher in energy + TS # 1 TS # 1 TS # 1 ΔG # 1 ΔG # 1 ΔG # 1 + eactant(s) + eactant(s) + eactant(s)
The ammond Postulate The structure of the TS # resembles the structure of the nearest (in energy) stable species (Stable specie = reactant or product / intermediate) eactant like TS # Product like TS # eactant TS # TS # Prod Prod ΔG>0 ΔG>0 Exergonic step eactant Endergonic step
The ammond Postulate Endergonic react. TS # resembles cationic intermed. TS # 1 Most stable cation Lowest energy TS # TS # 1 Least stable cation ighest energy TS # Least stable cation Lowest energy TS TS # 1 ΔG # 1 ΔG # 1 ΔG # 1 + eactant(s) + eactant(s) + eactant(s)
ation earrangement - Sec carbocation ydride shift Alkyl shift 1,2 ydride shift tert carbocation - Sec carbocation 1,2-Methyl group shift tert carbocation
arbenes McM chapt 7.6, lab ex. 7 arbanion - sp 3 B sp 3 arbocation - sp 3 sp 2 sp 3 - - sp 2 arbene Neutral Divalent ± 6 valence electrons - ighly reactive t 1/2 < 1 s Electron deficient /electrophilic properties
singlet carbene triplet carbene p-orbital bonding sp 2 -orbitals non-bonding sp 2 -orbital electron pair, opposite spin cation / anion properties 2 half-filled p-orbitals electrons with parallell spin diradical properties Gives triplet signal in esr spektrum Triplet normally somewhat more stable than singlet Singlets more reactive dihalocarbene Stabilizing overlap in singlet state
arbenes and carbeniods in synthesis Generation of carbenes Base cf lab ex 7 Not in McM N N N N hν or heat + N 2 Diazomethane (toxic, explosive) (From g-species)
eaction with alkenes - Synth. of cyclopropanes One-step - oncerted (see also hapt 30) Stereospesific arbenoids arbenoid carbene. Free carbene (probably) not formed, species reacts carbene ' Met
arbenoids - Simmons Smith reaction 2 I 2 + Zn(u) Zn-u alloy Insertion I- 2 -ZnI (arbenioid) cf. Grignard 3 + Mg 3 -Mg Other methods I- 2 -ZnI One-step - oncerted (see also hapt 30) Stereospesific TS #? ZnI 2 I
Enentioselective Simmons Smith Allylic alcohols (hiral auxilary or catalyst) Other carbenoids
Polymerization of alkenes (McM 7.10) Polymers - Synthetic macromolecules adical polymerization ationic polymerization n 2 2 ethene (Ethylene) polyethylene onformation of polymer - Properties linear polymer crystalline solid Amorphous solid Both linear and amorph areas hardness - drawn to fibers elasticity
stereochemistry Polypropylene isotactic more crystalline / harder syndiotactic atactic (random)
hiral zirconocene derivatives Stereoselective polymerisation of alkenes Zr-complex MAO * n atactic Zr PhMe, 60 o * n 91% isotactic Zr
Polymerization of dienes (McM 14.7) Natural and synthetic rubber Acid cat polymerization of 1,3-butadiene adical polymerization ationic polymerization Allylic cation esonans stab. and so on 1,4-addition Nu Nu 1,2 addition 1,4 addition Nu Nu
* evea brasiliensis, tropical Americas. autchuc * n Nattural rubber Isoprene Gutta-percha * n resin from the Isonandra Gutta tree (South east Asia) Less elastic than nat. rubber Isolation under-water cables Little use today
Elimination reactions - epetition E2: mechanism B ' ' ' ' + B + One step 2. order Stereospecific E1: mechanism ' ' ' + ' B ' ' and / or + B + ' ' Two steps 1. order 1. step rate limiting 1.step = 1. step in S N 1 Not stereospecific Elimination in competition with substitution
E2 and stereochemistry Base Base # Alkene eactant Anti Periplanar conformation Anti transition state B cannot react E2
B B Most subst alkene B Only possible product
Deuterium Isotope Effect (McM 11.14) (Kinetic Isotope Effect) Important in elucidation of reaction mechanisms eavage of - and -D requires different amount of energy ookes Law, Stretching frequencies, I ν = 1 2πc f(m 1 +m 2 ) ν -: ca 3000 cm -1 m 1 m 2 m 1 m 2 ν -D: ca 2200 cm -1 elationship between Streching frequency (ν) and zero-point energy (E 0 ) E 0 = 1/2 hν E 0-18 kj/mol TS # E 0 -D 13 kj/mol igher activation energy for cleavage of -D eavage of - will be faster
Mechanistic Information from Deuterium Isotope Effect? E2 - broken in rate limiting step Same prod. no info from regio / stereosel. D D D Fastest if E2, - more easily broken
Alkynes ' Addition react. on alkynes: Add of etc ydratization ydrogenation / reductions Oxidative cleavage
+ Vinylic carbocation, sp + 3 May be stoppet at this stage With excess Internal alkynes may not be so selective ' ' + Different stability?? ' ' '
+ 2 O O 2 O - + + 2 O 2 O - + O Enol g(ii)-cat, required with internal alkynes (see also Fig 8.3) g + g 2+ 2 O 2 O g - + Taut. O keto No second add. Ketone O O g
B 2 B 3 B [ox] O 3 + B(O) 3 "Anti Markovnikov" Terminal alkyne - aldehyde B3 B 2 B 2 B 2 [ox] O O - 2 O O gem-diol Internal alkyne -ketone B3 B 2 [ox] O O Sel. probl. if not symmetrical
eduction of alkynes 2 / cat. ' 2 / cat. ' 2 / cat. - 2-2 -' Difficult to stop here, cf. hydrogenation of alkenes Lindlar (deactivated Pd-cat.) ' 2 /Lindlar ' cis cis-alkenes difficult to get, i.e. Wittig etc Li / N 3 (l) ' Li / N 3 (l) ' trans (mech. see Fig 8.4)
Li ' radical anion ' + Li Li ' N 2 Less hindered anion formed ' 2 N '
Oxidative cleavage of alkynes 1 O 3 reductive work-up 1 2 ketone O + O aldehyde 2 oxidative work-up 1 2 O + O O ketone carboxylic acid ' O 3 or KMnO 4 O 2 + O 2 ' O 3 or KMnO 4 O 2 + 2 O 3 O 2 + 2 O