Alkenes. sp 2 C 120 o. Electrophilic addition to alkenes. Nucleophilic addition to alkenes (chapt. 23) Radical react. (Polymerization etc, Chapt 7.

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1 Alkenes Double bond: π and σ sp 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.

2 + 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.

3 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)

4 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

5 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)

6 ation earrangement - Sec carbocation ydride shift Alkyl shift 1,2 ydride shift tert carbocation - Sec carbocation 1,2-Methyl group shift tert carbocation

7 arbenes McM chapt 7.6, lab ex. 7 arbanion - sp 3 B sp 3 arbocation - sp 3 sp 2 sp sp 2 arbene Neutral Divalent ± 6 valence electrons - ighly reactive t 1/2 < 1 s Electron deficient /electrophilic properties

8 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

9 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)

10 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

11 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

12 Enentioselective Simmons Smith Allylic alcohols (hiral auxilary or catalyst) Other carbenoids

13 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

14 stereochemistry Polypropylene isotactic more crystalline / harder syndiotactic atactic (random)

15 hiral zirconocene derivatives Stereoselective polymerisation of alkenes Zr-complex MAO * n atactic Zr PhMe, 60 o * n 91% isotactic Zr

16 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

17 * 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

18 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

19 E2 and stereochemistry Base Base # Alkene eactant Anti Periplanar conformation Anti transition state B cannot react E2

20 B B Most subst alkene B Only possible product

21 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

22 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

23 Alkynes ' Addition react. on alkynes: Add of etc ydratization ydrogenation / reductions Oxidative cleavage

24 + Vinylic carbocation, sp + 3 May be stoppet at this stage With excess Internal alkynes may not be so selective ' ' + Different stability?? ' ' '

25 + 2 O O 2 O 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

26 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

27 eduction of alkynes 2 / cat. ' 2 / cat. ' 2 / cat ' 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)

28 Li ' radical anion ' + Li Li ' N 2 Less hindered anion formed ' 2 N '

29 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 O 3 O O