Repeated insertion ultiple insertion leads to dimerization, oligomerization or polymerization. k prop Et Key factor: k CT / k prop = κ κ 1: mainly dimerization κ 0.1-1.0: oligomerization (always mixtures) κ «0.1: polymerization κ 0: "living" polymerization Bu k CT + k prop x k CT + k prop c + k prop etc k CT 1
Applications of oligomers and polymers Ethene and propene come directly from crude oil "crackers" Primary petrochemical products, basic chemical feedstocks Dimerization rarely desired aking butene costs $$$! ligomers: surfactants, comonomers igh added value, but limited market Polymers: plastics, construction materials, foils and films Very large market, bulk products 2
Selective synthesis of trimers etc? 1-exene and 1-octene are valuable co-monomers. Selective synthesis of 1-hexene from ethene is not possible using the standard insertion/elimination mechanism. There are a few catalysts that selectively trimerize ethene via a different mechanism ("metallacycle" mechanism). Redox-active metals (Ti, V, Cr, Ta) required Cr systems are used commercially There are also one or two catalysts that preferentially produce 1-octene. The mechanism has not been firmly established. 3
Trimerization via metallacycles = (and others) Ti II + subst II coord II Key issues: Geometrical constraints prevent β-elimination in metallacyclopentane. Formation of 9-membered rings unfavourable. red elim Ligand helps balance (n) and (n+2) oxidation states.? IV IV β-elim coord IV ins IV 4
C/olefin copolymerization C cheaper than ethene Copolymer more polar than polyethene much higher melting point Chemically less inert P P C C P No double C insertion uphill No double olefin insertion C binds more strongly, inserts more quickly Slow β-elimination from alkyl 5-membered ring hinders elimination C P C P = L 2 Pd, L 2 Ni 5
ydroformylation Used to make long-chain alcohols and acids from 1-alkenes ften in situ reduction of aldehydes to alcohols Unwanted side reaction: hydrogenation of olefin to alkane ain issue: linear vs branched aldehyde formation It is possible to make linear aldehydes from internal olefins! C C 2 2 6
Insertion of longer conjugated systems Attack on an η-polyene is always at a terminal carbon. Usually α,ω-insertion R R 7
Insertion of longer conjugated systems A diene can be η 2 bound. 1,2-insertion R R etallocenes often do not have enough space for η 4 coordination: 8
Diene rubbers Butadiene could form three different "ideal" polymers: cis 1,4 trans 1,4 1,2 In practice one obtains an imperfect polymer containing all possible insertion modes. Product composition can be tuned by catalyst variation. Polymer either used as such or (often) after cross-linking and hydrogenation. 9
Addition to enones RLi, Grignards: usually 1,2 "charge-controlled" R rganocu compounds often 1,4 or even 1,6 etc "orbital-controlled" stereoregular addition possible using chiral phosphine ligands frequently used in organic synthesis R 10
Less common elimination reactions α-elimination: Cp 2 Zr - Cp 2 Zr tbu Probably via σ-bond metathesis: Zr ther ligand metallation reactions: tbu Zr - Zr L 2 Pt - L 2 Pt Via σ-bond metathesis or oxidative addition/reductive elimination 11
Less common elimination reactions β-elimination from alkoxides of late transition metals is easy: C3 + C 2 The hydride often decomposes to + and reduced metal: alcohols easily reduce late transition metals. Also, the aldehyde could be decarbonylated to yield metal carbonyls. For early transition metals, the insertion is highly exothermic and irreversible. 12
xidative Addition and Reductive Elimination red elim coord ox add ins 2
xidative Addition Basic reaction: L n + X Y L n X Y The new -X and -Y bonds are formed using: the electron pair of the X-Y bond one metal-centered lone pair The metal goes up in oxidation state (+2) X-Y formally gets reduced to X -, Y - Common for transition metals, rare for main-group metals 14 xidative addition, reductive
ne reaction, multiple mechanisms Concerted addition, mostly with non-polar X-Y bonds 2, silanes, alkanes, 2,... Arene C- bonds more reactive than alkane C- bonds (!) X L n + L n X Y Y Intermediate A is a σ-complex. Reaction may stop here if metal-centered lone pairs are not readily available. Final product expected to have cis X,Y groups. A L n X Y 15 xidative addition, reductive
Concerted addition, "arrested" Cr(C) 5 : coordinatively unsaturated, but metalcentered lone pairs not very available: σ-complex Cr(Pe 3 ) 5 : phosphines are better donors, weaker acceptors: full oxidative addition 16 xidative addition, reductive
ne reaction, multiple mechanisms Stepwise addition, with polar X-Y bonds X, R 3 SnX, acyl and allyl halides,... low-valent, electron-rich metal fragment (Ir I, Pd (0),...) L n X Y L n X Y L n etal initially acts as nucleophile. Coordinative unsaturation less important. Ionic intermediate (B). Final geometry (cis or trans) not easy to predict. B X Y 17 xidative addition, reductive
ne reaction, multiple mechanisms Radical addition has been observed but is relatively rare RIrCl(C)L 2 X R IrCl(C)L 2 RX RIrCl(C)L 2 Tests: Formation R-R CIDNP Radical clocks: Br 18 xidative addition, reductive