alkene: versatile function group X Alcohol Alkane alohydrin 1,2-Diol X X C C Carbonyl compound 1,2-Dihalide X C Cyclopropane alide Epoxide
7.1 Preparation of Alkenes: A Review of Elimination Reactions Elimination addition C C + X Y X Y elimination
Dehydrohalogenation: -X under strong basic condition (eg. K) Br K C 3 C 2 + KBr + 2
Dehydration: - 2 under strong acidic condition (eg. 2 S 4 ) C 3 2 S 4 C 3 2 TF, 50 o C + 2 TF Tetrahydrofuran
7.2 Addition of alogens to Alkenes Cl 2, Br 2 add readily to alkenes F 2 is too reactive and difficult to control, and I 2 does not react with most alkenes + Cl 2 Cl Cl 1,2-Dichloroethane
Bromination mechanism: carbocation intermediate cannot explain the stereochemistry Br Br Br - Br Br Br
Anti stereochemistry: only trans product Br trans Br Br Br Br - Br X Br Br cis NT formed
Bromonium ion: explain anti stereochemistry similarly, chloronium ion Br Br Br Br bromonium ion The neighboring bromo substituent stabilizes the positive charge by using two of its electrons to overlap the vacant carbon p orbital, giving a three-membered-ring bromonium ion.
top side is shielded from attack Br Br Br Br Br Br - trans bottom side is open to attack
stable bromonium ion: recently, George lah has prepared stable bromonium ion solution ; strong evidence for the existence of bromonium ion 3 C 3 C F SbF 5 Br SbF 5 C 3 liquid S 2 Br 3 C C 3 3 C SbF 6 bromonium ion (stable in S 2 solution) SbF 5 : strong Lewis acid, activate C-X bond to generate carbocation
7.3 alohydrin Formation alohydrin: 1,2-halo alcohol X 2 2 X halohydrin
In the presence of an additional nucleophile, the intermediate bromonium ion can be intercepted by the added nucleophile. Br Br Br 2 Br Br - Br 2 2 δ + δ - Br Br 2 Br a bromohydrin + 3 + + Br -
In practice, few alkenes are soluble in water, use polar solvent; aqueous dimethyl sulfoxide (DMS) and NBS (Nbromosuccinimide) as a source of Br + Styrene N Br (NBS) Br 2 /C 3 SC 3 (DMS) 76% 2-Bromo-1-phenylethanol -Br 2 is toxic, difficult to handle - NBS is stable, easy to handle Note that the aromatic ring is inert to reaction condition (aromatic ring is more stable than the isolated alkenes)
7.4 Addition of Water to Alkenes: xymercuration ydration: addition of water, need a strong acid catalyst, similar mechanism to that of X addition limitation; strong acid and high temperature 3 P 4 catalyst 2 C 3 C 2 250 o C suitable for large-scale industrial procedures
mechanism of the acid catalyzed hydration of an alkene 3 C 3 C -A 3 C 3 C A - C 3 3 C 2 C 3 3 C A - 3 C 3 C C 3 + A
xymercuration: hydration of alkenes in the laboratory; g(ac) 2 C 3 1. g(ac) 2, 2 -TF 2. NaB 4 C 3 92% electrophilic addition of g 2+ (mercuric) ion to alkene give mercurinium ion (similar to bromonium ion) g(ac) 2 is ionic salt like Na + - Ac g(ac) 2 g 2+ + 2 - Ac g 2+ ion is much more electrophilic than + ; reaction can occur at low temperature (rt)
mechanism Ch.7 Alkenes: Reactions and Synthesis Ac g Ac g Ac g Ac 2 C 3 C 3 C 3 - Ac - Ac C 3 - g(0) g C 3 NaB 4 g Ac C 3 + -Ac - the final reductive demercuration involves radicals
- the regiochemistry: Markovnikov addition of water (- group attaches to the more highly substituted carbon) Ac - Ac g δ + C 3 δ + Ac g C 3 - Ac NBS δ + Br Br 2
7.5 Addition of Water to Alkenes: ydroboration ydroboration: addition of B- bond of borane (B 3 ) to an alkene; 1959,.C. Brown B 3 2 B an organoborane
borane is highly reactive because the boron atom has only 6 electrons in its valence shell. ; borane accepts an electron pair from a solvent molecule in a Lewis-acid reaction to complete its octet ; commercial borane reagents are available as complexes B + 3 B B 3 -TF complex 3 B N B 3 -pyr C 3 3 B S C 3 B 3 -DMS
ydroboration: addition occurs three times to give trialkylborane, R 3 B hydroboration / oxydation 3 B 3 TF B 2 2 Na, 2 3 + B() 3 87%
mechanism of oxidation R 2 B R Na + - R 2 B R BR 2 R B(-R) 3 Na B() 3 + 3 R the net result of hydroboration / oxidation is hydration of alkene double bond
Regiochemistry of hydroboration: syn stereochemistry with boron attaching to the less substituted carbon C 3 B 3 B 2 2 TF C 3 Na Allkylborane intermediate 85% C 3 trans-2-methylcyclopentanol the C-B bond is replaced by the C- bond with the retention of stereochemistry
This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration. 1. g(ac) 2 C 3 2 2. NaB 4 B 3 ; C 3 Na C 3
Mechanism of hydroboration borane is electron defficient, electrophilic; alkene is nucloephilic Borane becomes negative in the transition state, as electrons shift from the alkene to boron, but is positive in the product. syn stereoselectivity: concerted mechanism B 2 B 3 2 B Addition of borane to the alkene π-bond occurs in a single step through a cyclic four-membered-ring transition state.
Regiochemical stereoselectivity electronic factors δ + C 3 B 3 C 3 B δ - partial 3 o cation (more stable TS) 2 B favored C 3 δ + C 3 B δ - partial 2 o cation (less stable TS) C 3 B 2 disfavored
Steric factor: explain regioselectivity in hydroboration steric factors C 3 B sterically less hindered (favored) C 3 B sterically more hindered (disfavored) Sterically bulky organoboranes (R 2 B): high regioselectivity
7.6 Addition of Carbenes to Alkenes: Cyclopropanation Carbon Species tetrahedral carbons carbanion radical carbene cabocation R 4 C R 3 C R 3 C R 2 C R 3 C 8 e - 8 e - 7 e - 6 e - 6 e - sp 3 sp 3 sp 2 ~ sp 3 sp 2 sp 2 R R singlet planar R X triplet planar
Carbene: neutral, highly reactive, electron-deficient, electrophilic ; generated only in situ cyclopropanation: concerted one step reaction + R C R' R R' carbene cyclopropane
generation of carbene: dichlorocarbene Cl Cl C K Cl Cl C Cl C Cl + Cl - Cl Cl Chloroform (acidic C-) dichlorocarbene vacant p orbital Cl Cl sp 2 R R R Dichlorocarbene carbocation
carbenes generated in the presence of alkenes add to alkenes Et (Z) Me + CCl 3 K Et Cl Cl Me syn + KCl K Cl + CCl + KCl 3 Cl
carbene addition is a stereospecific reaction: a single stereoisomer is formed Me + Et (E) CCl 3 K Et Cl Cl Me trans + KCl
Stereospecific reaction: generally concerted reactions - stereoisomeric starting materials afford stereoisomerically different products under the same reaction conditions CCl 2 Cl Cl CCl 2 Cl Cl
Stereoselective reaction: a single reactant has the capacity of forming two or more stereoisomeric products in a particular reaction but one is formed preferentially B 3 ; Na C 3 Ch 3 C 3 + major minor stereospecific: G > 25 kcal/mol stereoselective: ~ 1-5 kcal/mol
Simmos-Smith reaction: involve carbenoid- a metal-complexed reagent with carbene-like reactivity C 2 I 2 + Zn(Cu) Et 2 " " I-C 2 -Zn-I C 2 (Iodomethyl)zinc iodide (a carbenoid) + C 2 I 2 Zn(Cu) Et 2 + ZnI 2 92%
7.7 Reduction of Alkenes: ydrogenation Reduction in organic chemistry: addition of hydrogen or removal of oxygen xidation in organic chemistry: addition of oxygen or removal of hydrogen ydrogenation: addition of 2 to a unsaturated bond; reduction + catalyst catalyst: Pt 2, Pd/C
hydrogenation is heterogeneous reaction and takes place on the surface of solid catalyst syn stereoselectivity C 3 C 3 2, Pt 2 C 3 C 2 C 3 C 3 82%
Mechanism 2 +
hydrogenation is extremely sensitive to the steric environment around the double bond: reduction from sterically less hindered face 3 C C 3 X 2 3 C C 3 C 3 Pd/C C 3
ketone, aldehyde, ester, nitrile: stable under normal hydrogenation condition. ; vigorous condition (high T and high pressure of 2 ) can reduce them 2 Pd/C Et C 3 2 Pd/C Et C 3 C N 2 Pd/C Et C N - typical method to reduce double bond only in unsaturated carbonyls
commercial saturation of double bonds: unsaturated vegetable oils to saturated fats Ester of linoleic acid (a constituent of vegetable oil) 2 Pd/C Ester of stearic acid
7.8 xidation of Alkenes: ydroxylation and Cleavage xidation in organic chemistry: addition of oxygen or removal of hydrogen Epoxidation : RC 3 (peroxyacid) RC 3 epoxide
concerted formation of epoxide: mcpba (m-chloroperbenzoic acid) mcpba C 2 Cl 2 C 3 Cl mcpba
Alkene Dihydroxylation: 1,2-diol (glycol); use s 4 (osmium tetroxide) s 4 NaS 3 1,2-diol concerted formation of osmate C 3 C 3 s 4 Pyridine C 3 s C 3 a cyclic osmate intermediate NaS 3 2 C 3 C 3 87% cis-1,2-dimethyl-1,2- cyclopentanediol
Alkene Cleavage: ozonolysis electric 3 2 2 3 discharge ozone 3 C 2 Cl 2-78 o C a molozonide an ozonide reduction + reducing reagent: Zn, Ac, 2
1. 3 2. Zn, 3 + + C 3 (C 2 ) 7 C=C(C 2 )C 2 C 3 1. 3 2. Zn, 3 + C 3 (C 2 ) 7 C + C(C 2 ) 7 C 2 C 3
KMn 4 (potassium permanganate, strong oxidant) in neutral or acidic solution can cleave double bonds ; if hydrogens are present on the double bond, carboxylic acids are produced KMn 4 3 + + KMn 4 3 + + KMn 4 3 + + C 2
Cleavage of 1,2-Diol 1,2-diol I 4 2, TF + I 4 (periodic acid) or NaI 4 (sodium periodate) cleaves diols a mild alternative to direct cleavages of alkenes with KMn 4 or 3 s 4 NaS 3 1,2-diol NaI 4 2, TF +
C- is not further oxidized; compare with KMn 4 C 3 I 4 2, TF C 3 I C 3 cyclic periodate intermediate I 4 2, TF 2
in cyclic diols, only cis-diol is cleaved; trans-diol is not cleaved ; trans-diols cannot form the cyclic periodate C 3 trans-diol I 4 2, TF No Reaction
7.9 Biological Alkene Addition Reactions biological reaction: aqueous medium, catalyzed by enzymes the kinds of biological reactions are similar to laboratory reactions enzyme catalyzed reactions are highly substrate selective for example, fumarase catalyzes the addition of water to fumaric acid in citric acid cycle, which our bodies use to metabolize food Fumaric acid Maleic acid 2 p 7.4 Fumarase 2 p 7.4 Fumarase Malic acid No Reaction
7.10 Addition of Radicals to Alkenes: Polymers polymer: built up by repetitive bonding of monomers Many 2 C C 2 A section of polyethylene ethylene polymerization: high pressure (1000-3000atm), high temperature (100-250 o C) in the presence of a catalyst (eg. benzoyl peroxide)
radical polymerization Ch.7 Alkenes: Reactions and Synthesis step 1. Initiation: homolytic cleavage of - bond thermally or photolytically heat 2 = Bz Benzoyl peroxide Benzoyloxy radical - benzoyloxy radical adds to ethylene to generate an alkyl radical; Bz 2 C C 2 Bz 2 C C 2 Bz 2 C C 2
step 2. propagation: repetition of radical addition 2 C C 2 Bz C 2 C 2 Bz-C 2 C 2 C 2 C 2 Bz-(C 2 C 2 ) n C 2 C 2 step 3. termination: two radicals combine to form a stable product R + R' R R'
other monomers Ch.7 Alkenes: Reactions and Synthesis 2 C CC 3 Propylene Polypropylene 2 C CPh Styrene Ph Ph Ph Ph Polystyrene
- unsymmetrically substituted alkene monomer: more stable radical formation is favored - radical species are electron defficient; stabilized by alkyl groups like carbocations Bz 2 C C Bz 2 CC R R Bz C C 2 R primary radical (less stable)
Some polymers (Table 7.1) Ethylene Propylene Vinyl chloride Styrene 2 C C 2 2 C CC 3 2 C CCl 2 C CPh PVC Tetrafluoroethylene F 2 C CF 2 Teflon Acrylonitrile Methyl methacrylate Vinyl acetate 2 C C-CN 2 C C C 2 C 3 C 3 2 C C CC 3
Chain Branching During Polymerization in practice, polymerization is complicated by several problems branching: radicals can abstract hydrogen from C- bond, which can lead branched polymers Intramolecular C- abstraction: 1,5- abstraction short-chain branching a short chain C 2 C2 2 C C 2 2 C C 2 C 3 branched
Intermolecular C- abstraction long-chain branching C 2 C 3 2 C C 2 a long chain C 2 branched short-chain branching occurs about 50 times as often as long-chain branching. Why?
Cationic Polymerization - initiated by cationic species: strong protic or Lewis acid catalyt - electrophilic addition sequence - stable, tertiary carbocation intermediates are commonly used acid catalyst C 3 Polyisobutylene C 2 C C 3 n
Anionic Polymerization: (Chapter 31) Nu - 2 C C Nu 2 CC EWG EWG 2 C C EWG EWG Nu C 2 C C 2 C EWG 2 C C EWG EWG C 2 C n
Chemistry @ Work Natural Rubber Rubber: natural alkene polymer produced by plants Isoprene A segment of natural rubber n crude rubber: called latex = ~5000 monomers, MW = 200,000-500,000 ; too soft, tacky vulcanization: hardening and stiffening by heating with elemental sulfur; cross linking between the rubber chain by forming C-S bonds The remarkable ability of rubber to stretch and then contract to its original shape is due to the irregular shapes of the polymer chains caused by the double bonds.