TM Catalysis of Organic Flashcards by David B

What is the oxn state of a metal?

Charge left on metal after all ligands have been removed in their normal closed shell config

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What is the 18e- rule?

Total no of e- in shell never exceeds 18
In mononuclear diamagnetic complexes

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What is the formal charge of a ligand?

number of electrons required from metal required for bonding

i.e. -1 for when requires one e- and 0 if 2 are donated to the M

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What is the formal charge of a ligand?

number of electrons required from metal required for bonding

i.e. -1 for when requires one e- and 0 if 2 are donated to the M

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What is oxidative addition?

X-Y added to coordinatively unsaturated metal centre
M increases oxn state by 2

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Name examples of things added in oxidative addition

alkenyl-X
aryl-X
RCO-X
RSO₂-X
H-H, etc.

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How is X-Y added in oxidative addition?

Added in cis fashion
Can rearrange to trans

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How do Pd complexes become active?

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What promotes oxidative addition (OA)?

Bulkyl and good donor ligand -> stabilises oxn+2 state

Reactivity: I > OTf > Br > Cl

e- defficient substrates react faster

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Does inversion of config occur for OA?

Vinyl halides - retention

Allyl, benzyl, alkyl halides - inversion

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What is the rate of OA for Pd(0) and alkyl halides?

Slow as no pre-coordination event is possible

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What is the mechanism for oxidative addition?

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What is reductive elimination (RE)?

Loss of 2xligands of cis-config to give single product
Unimolecular pathway
Oxn state of M reduced by 2

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What is the oxn state of Pd during OA/RE?

Pd(0) <-> Pd(2)

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What type of ligand encourages RE?

e- poor ligands, so reduced e- density of M (more electrophilic)
π-acceptors and bulkyl ligands also increase rate of RE

e.g. PPh₃ > PPh₂Me

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How does RE occur wrt retention?

Retention of stereochem in both fragments

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What is the bite angle of a ligand?

Angle between two ligand atoms in a bidentate

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How does bite angle effect RE?

Increased bite angle (larger bidentate rings) increase RE by bringing two departing ligands closer together

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What is transmettalation?

M-R of main group metal react with Pd, organic group/H is transferred to Pd

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What is the driving force of transmetallation?

Difference in electronegativity of two metals
Main group is more electropositive than Pd

M = Mg, Zn, B, Al, Sn, Si, Hg

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What is the driving force of transmetallation?

Difference in electronegativity of two metals
Main group is more electropositive than Pd

This is often RDS in a process

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What is the change of oxn state in transmetallation?

No change

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What increases the rate of transmetallation?

Ligands increasing nucleophilicity of R of Pd and increasing electrophilicity of Main group metal

e.g. Zn > Sn > B&raquo_space; Li
Transfer faster with alkyne > alkene > aryl > allyl > Benzyl&raquo_space; alkyl

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Name some examples of complexes for transmettalation

LiR, XMgR, RZrClCp₂, RZnCl, RCuLn
RSnR’₃, RB(OR’)₂m, R-9BBN, RSiR’₃

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What is 1,1-migratory insertion of CO?

R groups undergo migratory insertion to CO ligand Rapid and reversible

What is the retention of config for 1,1-migratory insertion of CO?

Retention of config

What is 1,2-migratory insertion of alkenes?

Addition of alkene/alkyne to sigma-bonded Pd complex Syn fashion and can happen multiple times

What is β-hydride elimination?

Syn-elim of H from C at β-position to metal centre to give a H-Pd-X and an alkene Reversible and requires coord unsaturated Pd complexes Resulting H-Pd complex then gives Pd[0] by reductive elim in presence of base

What is required for β-hydride elim?

cis to alkyl group is site of coord unstaturation - corresponds to site of elec unsaturation (empty metal orbital) M-C-C-H unit can take coplanar conformation which brings β-H in close proxim to metal to form an agostic interaction M is electrophilic resulting in agostic interaction primarily e- donation (σ donation >> π-backbonding)

How does Ni and Pd compare for β-hydride elim?

Higher energy Ni(II) vacant d-orbtial means weaker agostic interaction with β CH sigma bond Shown by greater lengthening of this bond in Pd complex Ni alkyl more stable towards β-elim in line with greater electronegativity of Pd(II) than Ni(II)

What is the Mizoroki-Heck reaction?

R-X + monosub-alkene -> E-geometry alkene Cat: Pd(OAc)₂ or Pd₂(dba)₃ Ligands: phosphines Solvents: aprotic Base: PMP, Et₃N, K₂CO₃

Why is the Mizoroki-Heck reaction E-stereoselective?

TS leading to cis-olefin complex involvewd is energetically unfavourable eclipsing interactions between CO₂Me and Ph

What is the mechansim cycle for the Mizoroki-Heck reaction?

What 3 ways can you do in situ reduction of Pd(II) to Pd(0)?

Transmetalation Tertiary alpiphatic amines e- rich phosphines and base

How can Pd(II) be reduced in situ to Pd(0) via transmetallation?

How can Pd(II) be reduced in situ to Pd(0) by 3° aliphatic amines?

Is via a β-hydride elim

How can Pd(II) be reduced in situ to Pd(0) by e- rich phosphines and bases?

XS of PR₃

What are the conditions and result of the cationic Mizoroki-Heck reaction?

Pd(OAc)₂, PPh₃, base, AND AgNO₃ Reduces double-bond isomeration

How does the cationic Mizoroki-Heck reaction reduce isomerisation?

Reduces reversibility of the β-H elim so less isomerism e.g.

What is the mechanism of the cationic mizoroki-heck reaction mechanism

What is the regioselectivity of the Mizoroki-Heck reaction when e- poor or rich alkene and PhX?

e- poor (EWG): Ph attached to β-C, further from EWG e- rich (EDG): Ph attached to α-C, closer to EDG

What is a consideration for the intramolecular heck reaction?

Endo vs exo ring closure exo - double bodn outside the new cycle Large rings - endo Small rings - exo for 5,6,7 membered ring closures

What is the Heck-Tandem reaction?

What is the stereochemn of carbopalladation and β-hydride elim?

syn processes

What is the enantionselective heck reaction?

Asymmetric, uses chiral phosphine BINAP

What are the hybridisations are C which can be connected by cross-coupling reactions?

Csp-Csp² Csp²-Csp² Csp²-Csp³ Csp³-Csp³

Name all the cross-coupling reactions

Kumada coupling Stille reaction Suzuki reaction Negishi coupling Hiyama coupling Sonogashira

What is the general cross-coupling cycle?

Overall: R₁-X + R₂-M -> R₁-₂ Uses [TM] and grignard reagent

What is the Kumada cross-coupling reaction?

C-C bond forming cross-coupling reaction Ni or Pd Catalysis

Why are Ni catalysts advantageous?

More economical Aryl&vinyl chlorides are more reactive unlike normal

Why is the Kamada coupling reaction advantageous?

Direct coupling of Grignard reagents - avoids additional steps scuhsuch as conversion to Zn

What is the mechanism of the kumada coupling?

What is the advantage of using Pd over Ni catalyst?

Transmetallation is stereospecific

What is the issue in the kumada coupling?

Grignard reagent used - not functional group tolerant

How do bidentate phosphine ligands change the activity of a catalyst?

Higher catalytic activity for a wide range of aryl & vinyl halides

What is the Negishi coupling?

R₁-X + R₂ZnX -> R₁R₂ Using ML_n, usually M = Pd

How are the Zn reagents prepared for the Negishi coupling?

alkyne hydroalumination then ZnCl₂ or iodide + ZnCl₂

What is the Stille reaction?

Cross-coupling using organotin reagent

What are the benefits of organotin reagents?

Easily accessible Group tolerant Air/moisture stable, and to large range of organic reagents

What is the ease of ligand transfer for the stille reaction?

alkynyl > alkenyl > aryl > ally or benzyl > alkyl where alkyl is most difficult to transfer

What is the rate of the Stille reaction?

Rate = `k[Pd][Sn][R-I]`⁰

What does Cu added in the Stille reaction do?

Rate enhancement - promote ligand dissociation and transmetallation by binding to free PPh₃ Increased group transfer selectivity

What does Cu added in the Stille reaction do?

Rate enhancement - promote ligand dissociation and transmetallation by binding to free PPh₃ Increased group transfer selectivity

What is the retention for the Stille reaction?

Retention: E->E, and Z->Z

How can you make organostannanes for the Stille reaction?

tBuLi and Bu₃SnCl OTf on alkyl/allyl, Pd and Me₃SnSnMe₃ or radical: alkyne + AIBN (cat) + Bu₃SnH or do a Stille

How can you perform carboalumination?

What is the mechanism for carboalumination?

What happens when a carboaluminate reacts with BuLi and an epoxide?

What can an alkene bonded to a copper grignard reagent make?

How can you perform a carbocupration?

TM Catalysis of Organic Flashcards

(70 cards)