Ligand Substitution Reactivity of Coordinated Ligands
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1 Reactivity of Coordinated Ligands 2 C 2 H 4 (0) + H e (Cu 2 Cu) H CH 3 CH H "βh elim" ins βh elim H Peter H.M. Budzelaar
2 Why care about substitution? Basic premise about metalcatalyzed reactions: Reactions happen in the coordination sphere of the metal Reactants (substrates) come in, react, and leave again Binding or dissociation of a ligand is often the slow, ratedetermining step This premise is not always correct, but it applies in the vast majority of cases. Notable exceptions: Electrontransfer reactions Activation of a single substrate for external attack peroxyacids for olefin epoxidation C and olefins for nucleophilic attack 2
3 issociative ligand substitution Example: L n M C L n M + C L' L n M L' 18 e 16 e 18 e Factors influencing ease of dissociation: 1 st row < 2 nd row > 3 rd row d 8 ML 5 > d 10 ML 4 > d 6 ML 6 stable ligands (C, olefins, ) dissociate easily (as opposed to e.g. CH 3, Cp). 3
4 issociative substitution in ML 6 16e ML 5 complexes are usually fluxional; the reaction proceeds with partial inversion, partial retention of stereochemistry. or 18e oct The 5coordinate intermediates are normally too reactive to be observed unless one uses matrix isolation techniques. SP 16e distorted TBP 4
5 Associative ligand substitution Example: L' L n M L n M L' L L n1 M L' 16 e 18 e 16 e Sometimes the solvent is involved. Reactivity of cisplatin: Br (NH 3 ) 2 Pt 2 slow (NH 3 ) 2 Pt()(Br) Br fast (NH 3 ) 2 Pt()() + NucleoBase H2 fast slow (NH 3 ) 2 Pt()(NB) + 5
6 Ligand rearrangement Several ligands can switch between ne and (n2)e situations, thus enabling associative reactions of an apparently saturated complex: M N M N 3e 1e M C R M R 5e M 3e M (1+2)e 1e 6
7 Redoxinduced ligand substitution Unlike 18e complexes, 17e and 19e complexes are labile. xidation and reduction can induce rapid ligand substitution. L n M 18e e + e L' L n M + L n M L' + 17e 19e L n M 19e 17e L n1 M + L Reduction promotes dissociative substitution. xidation promotes associative substitution. In favourable cases, the product oxidizes/reduces the starting material redox catalysis. 7
8 Redoxinduced ligand substitution Fe(C) 4 L Fe(C) 5 C Fe(C) 4 Fe(C) 5 Fe(C) 4 L L Initiation by added reductant. Sometimes, radical abstraction produces a 17e species (see C103). 8
9 Photochemical ligand substitution Visible light can excite an electron from an ML bonding orbital to an ML antibonding orbital (Ligand Field transition, LF). This often results in fast ligand dissociation. M(C) 6 d σ hν d π Requirement: the complex must absorb, so it must have a colour! or use UV if the complex absorbs there 9
10 Photochemical ligand substitution Some ligands have a lowlying π* orbital and undergo MetaltoLigand Charge Transfer (MLCT) excitation. This leads to easy associative substitution. The excited state is formally (n1)e! Similar to oxidationinduced substitution M(C) 4 (bipy) d σ π* hν HM d π LUM MM bonds dissociate easily (homolysis) on irradiation (n1)e associative substitution 10
11 Electrophilic and nucleophilic attack on activated ligands Electronrich metal fragment: ligands activated for electrophilic attack N N N Rh N H + S N N N Rh N S is acidic enough to protonate this coordinated ethene. Without the metal, protonating ethene requires H 2 S 4 or similar. 11
12 Electrophilic and nucleophilic attack on activated ligands Electronpoor metal fragment: ligands activated for nucleophilic attack. BuLi Bu H Li + C C Cr C C C Cr C BuLi does not add to free benzene, it would at best metallate it (and even that is hard to do). 12
13 Electrophilic attack on ligand Hapticity may increase or decrease. Formal oxidation state of metal may increase. + M I H + M I + M (0) H + M II 13
14 Electrophilic addition Et + Et 3 + Fe(C) 3 Fe(C) 3 Is formally oxidation of Fe (0) to Fe II (the ligand becomes anionic). Ligand hapticity increases to compensate for loss of electron. 14
15 Electrophilic abstraction Electrophilic abstraction also by Ph 3 C +, H + Cp 2 Zr Me Me B(C 6 F 5 ) 3 Cp 2 Zr + Me + MeB(C 6 F 5 ) 3 16 e 14 e Alkyl exchange also starts with electrophilic attack: Me Zn Me Me Zn Me Me Me Zn Zn Me Me 15
16 Electrophilic attack at the metal If the metal has lone pairs, it may compete with the ligand for electrophilic attack Transfer of the electrophile to the ligand may then still occur in a separate subsequent step + Fe H + Fe H + Ni H + Ni?via? 16
17 Electrophilic attack at the metal Can be the start of oxidative addition I 2 (C) 2 Rh Me I I 2 (C) 2 RhMe + I I 3 (C) 2 RhMe (although this could also happen via concerted addition) CH 3 C HI CH 3 CH 3 CI CH 3 I Key reaction in the Monsanto acetic acid process: Me + C HI "Rh" MeC MeCRh(C) 2 I 3 Rh(C) 2 I 2 MeRh(C) 2 I 3 C MeCRh(C)I 3 17
18 Nucleophilic attack on ligand Nucleophile "substitutes" metal hapticity usually decreases xidation state mostly unchanged Competition: nucleophilic attack on metal usually leads to ligand substitution 18
19 Nucleophilic abstraction Mostly ligand deprotonation Na + NaH C C Cr C C C Cr C Cp 2 WH 2 BuLi Cp 2 WH Li 19
20 Nucleophilic addition () 2 + () 2 Key reaction of the Wacker process: C 2 H 4 + ½ 2 2, CH3 CH Cu 2 20
21 The Wacker process 2 C 2 H 4 (0) + H e (Cu 2 Cu) H CH 3 CH H "βh elim" ins βh elim H 21
22 The Wacker process Characteristics of the Wacker process: The oxygen in the product derives from water, not directly from the oxygen used as oxidant higher olefins yield ketones, not aldehydes large amounts of halides required: corrosive side products resulting from nucleophilic attack of halide on olefin No longer important for acetic acid synthesis Several variations (with more complicated nucleophiles) used in organic synthesis 22
23 Nucleophilic attack on the ligand How can you distinguish between internal and external attack of??? Use transch=ch and trap the intermediate CC with C: ins C H C ins nucl attack H 23
24 Using isotopic labelling to study mechanisms H H H C C 24
25 Using isotopic labelling to study mechanisms Could acetaldehyde be formed directly as vinyl alcohol? Perform reaction in 2 : H H diss H CH 3 ins H "βh elim" diss CH 3 H H diss 2 CH 2 ins H "βh elim" diss CH 3 25
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