Synthesis Using Organometallics

7/21/2019 Synthesis Using Organometallics

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Organometallic Cross-Coupling in Synthesis

• Organometallics play a key role in C-C bond forming reactions

• Negatively polarized carbon-metal bond (C!- M!

+) is suited for this purpose

• Reactivity of organometallic generlaly increases with ionic character of the C-M bond

o

Related to the electronegativity value EN difference between the carbon and themetal (EN = electrostatic force exerted by a nucleus on the valence electrons)

• Ionic character (% ionicity) is related to difference between the EN values of the atoms of

the C-M bond• EN and ionic character are affected by substituents on carbon

• Thus, C-Li, C-Mg, C-Ti and C-Al bonds are more ionic then C-Zn, C-Cu, C-Sn, and C-B

o Latter bonds have more covalent character

• Special techniques are often required based on reactivity of C-M bonds

Organolithium reagents in synthesis

• React with a wide variety of organic substrates to form C-C bonds

• Serve as precursors for preparation of other organometallic reagents

•

Various methods can be used to prepare organolithium species depending on substrate1. Li metal and alkyl halides

• Does not work for allylic, benzylic or propargylic systems (homo coupling)

• Basicity decreases with increasing stability of carbanion

o tBuLi > sBuLi > nBuLi

2. Lithium-Halogen (Li-X) Exchange

• Proceeds forward when R-Li is a weaker base (more stable carbanion) than

R'-Li



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• Alkenyl lithium reagents

o (E) and (Z)-alkenyl halides are configurationally stable @ low temp

o Can react with various electrophiles

• Aryllithium reagents

o Li-halogen exhange is very fast @ low tempo Use of tetramethylethylene diamine (TMEDA) to accelerate

metalation more than M-X exchange

3. Transmetalation [Lithium-Metal (Li-M) Exchange]

• Used to prepare allylic, benzylic and propargylic lithium reagents



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4. Lithium-Hydrogen Exchange (Metalation)

• Use of TMEDA, HMPA, DMPU or crown ether to promote metalation

5. Metalation of "-Heterosubstituted alkenes

• Relative activating effect S > O > N (inductive effect increases acidity of C-H



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6. Directed Ortho-Metalation (DOM)

• Permits regioselective preparation of substituted benzene derivatives andheterocycles



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Organomagnesium reagents in synthesis

• Grignard reagents (Grignard, Novel Prize 1912)

• Prepared by reaction of alkyl halide with (1) activated Mg in ethereal solvent or (2) withRieke Mg (MgCl2 treated with lithium naphthalide --> Mg

0)

•

Alkenyl and phenyl Grignards prepared from corresponding halideso Grignards from (E)- And (Z)-alkenes are configurationally unstable (gives

mixtures of isomers)

• Allylic Grignard reagents

o homo coupling often a side reaction

o Barbier-type reaction is a solution (all components in one pot)

•

Alkynyl Grignard reagentso Prepared by deprotonation of 1-alkynes with EtMgBr

• Reactions of Grignards with carbonyl compounds

o Carbonyl reactivity towards Grignard reagents: aldehyde >ketone >ester > amide

o Proceed through polar-concerted reaction or a step-wise electron transfero

Often accompanied by side reactions

! enolization, reduction, or aldol condensation

BrH

Hn-C4H9

MgBrH

Hn-C4H9

CO2HH

Hn-C4H9Mg, Et2O a. CO2

b. H+, H2O

HR

EtMgBr

MgBrR

O

H

Mg

R

X

H

O

R

MgX

H+, H2O H

OH

RO

H

Mg

R

X

O

H R

Mg

X O

HR

Mg

X

enolization

OMgX

H MgX

O OMgX

H

Reduction



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Organocopper reagents in Synthesis

• Cu is less electropositive then Li or Mg

• C-Cu bond is less polarized than C-Li or C-Mg bonds

• Organocopper reagents react with alkyl, alkenyl and aryl halides to give alkylated

productso More selective

o Can be reacted with acid chlorides to form ketones without further nucleophilic

attacko Reactivity: acid chlorides > aldehydes > halides and tosylates > epoxides >>

ketones > esters > nitriles

o Prefers 1,4-addition with ",#-unsaturated carbonyls

• Class of Organocuprates1. Homocuprates (R 2CuLi, R 2CuMgX)

o Thermally-labile (need low temp)

o

Prepared from lithiates or Grignards (need 2 equivalents) and Cu(I) halide

2. Heterocuprates (R 1R 2CuM)

o More controlled reactivity (tempered)

o

Thermally more stable (less prone to #-elimination of C-H)

o Only one group transferred

! Can use a non-transferable group

o

alkynyl, 2-thienyl, PhS, tBuO, R 2 N

3. Higher Order Cyanocuprates (Lipshutz Reagents)

o

Have reactivity of homocuprates with thermal stability of heterocuprates



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4.

Grignard-Copper(I) Reagents

o Use of catalytic copper with Grignard reagent (complexation)

o Controlled reactivity (tolerates ketone, ester, amide, nitrile groups)o Reacts with alkyl halides to give displacement products (no elimination)

• Reactions of Cuprates

o

Substitution of alkyl halides!

Two plausible mechanisms

• Depends on nature of solvent, substrate and cuprate

o Substitution of allylic halides! Competition between S N2 and S N2'

• due to overlap of Cu d orbital with $* and %* orbitals of allyl

system

! Use RCu-BF3 for predominantly S N2'



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o Reactions with vinyl and aryl halides

! Halogen displacement: more general and gives higher yields than RLi orRMgX

! Useful for biaryl coupling reactions (e.g., Ullman coupling)

o

Acylations (form ketones)

o

1,2-Additions to aldehydes, ketones and imines! Highly diastereoselective

I Me2CuLi Me

90%

NO2

BrNH3

25 ˚C

NO2

NO2

CuOTf

NBOC

OMea) sBuLiTMEDA,Et2O, -45 ˚C

b) CuI-P(OEt)3NCOB

OMe(EtO)3PCu

NRI

MeO OMe

c.

d. H+, H2O

CHO

OMe

MeON

OMe

BOC



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o Epoxide cleavage! R 2Cu(CN)Li2 reagents- mild and efficient enough for epoxide cleavage

! Stereospecific S N2 opening (addition as least substituted carbon)

o

Reactions with alkynes

nBu2CuLi

HH

Cu

H H

nBu I

H H

nBuI2

65-75%

(pentyl)2CuLi

HH

Cu

H H

Pentyl

H H

Pentyl

78%

OEt

O

H

H

CO2Et

EtMgBrHnBu

CuMgBr2

nBu H

Et

CuBr

H+H

nBu H

Et

82%

iPrCuMgBr2HnBu

CuMgBr2

nBu H

I- Pr CN

nBu H

i- Pr

92%

Cl CN



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! Formation of allenes from propargyl acetates, halides and sulfonates

o Conjugate additions

!

Organometallics may add 1,2- or 1,4-manner to ",#-unsaturated carbonyls! 1,4-Addition most successful with "soft" (relatively non-basic)

nucleophileso CN, RNH2, R 2 NH, RSH, malonates, organocuprates

! 1,2-Addition most successful with "hard" (relatively basic) nucleophiles

o

Hydrides, RLi, RMgX

! All cuprates and CuX-catalyzed additions of RMgX give 1,4-addition

!

Chemoselective (addition to less hindered C=C bond)

! Stereoselective (addition of R group from less hindered face) ! Mechanism

! For sterically-demanding cuprates or enones with steric hindrance atreaction site

OAc MeCu-LiBr-MgBrI

C H

Me



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• Add TMSCl to accelerate conjugate addition (activates carbonyl)

•

Can also use Lewis acids to activate carbonyl

!

Can perform tandem 1,4-trapping



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! Understanding conjugate addition

• Sterics and electronic play big role



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Organopalladium reagents in synthesis

• Pd-catalyzed coupling reactions (one of most versatile tools for C-C bond formation

o (Nobel Prize 2010- Heck, Negishi, and Suzuki)

• Organo-Pd species usually generated in situ

•

High chemo-, regio-, and stereoselectivities• Usually low catalyst loading

• Pd chemistry dominated by Pd0-Pd

II:

o Pd(0), d10

: zero valent state

o Pd(II), d8: +2 state

• Palladium (0) complexes

o readily accessible, easily prepared and easily handled

o Phosphine ligands provide high electron density on the metal! Pd-P complex favors oxidative addition

! Pd-P complex favors dissociation of ligand to coordinating unsaturated

complex

! Electron-rich, nucleophilic species!

Prone to oxidation, ligand dissociation, insertion, and oxidative coupling

reactions

! React with a broad range of halides having proximal $-bonds to form %-

organoPd(II) complexes (oxidative addition)



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• Pd(II) complexes

! electrophilic species

! Undergo ligand association and reductive coupling reactions

• Basis for Pd cross-coupling chemistry

o

Oxidative additiono Transmetalation

o Reductive elimination

• Common Pd(0) complexes: Pd(PPh3)4 (yellow, crystalline material); Pd2dba3

• Pd(0) complexes prepared in situ from Pd(II) in presence of phosphines

o PdCl2, Pd(OAc)2, Pd(PPh3)Cl2

• Pd(II) also reduced to Pd(0) by amines, alkenes or R-M

• Oxidative additions occur readily for aryl and vinyl halides at RT

o Vinyl halide retain stereochemical integrity

• Alkyl halides give alkyl-Pd(II)-X complexes

o If substrate contains #-H on sp3 carbon

! Rapid dehydropalladation (#-hydride elimination)!

Many new methods overcoming this issue with choice of catalyst, ligand

• 16-electon Pd(II) complex can acquire second substrate by insertion into alkene or bytransmetalation with an organometallic

o Central theme for Pd chemistryo Plays key role in organic synthesis

• Pd-C % bond is stable to many functional groups (NO2, Cl, CO2R, CN, NR 2, NHCOR)

O

Ph Ph



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Mizoroki-Heck Reaction

• Coupling of alkenyl or aryl halides with alkenes in presence of Pd(0) and base to give

alkenyl- or aryl-substituted alkenes

• Catalytic Cycle

o Generally gives (E)-alkenes (sterics during syn elimination TS)

• Advantages

o Widely used

o Compatible with many functional groups and alkenes

o Modern modifications use Pd/C (cheap), use of water-soluble Pd catalysts



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• Disadvantages

o Cannot have hydrogens on #-carbon

o Often requires high temp (50-160 ˚C) for couplingo Aryl chlorides are not good substrates (react slowly)

o

More substituted alkenes give slower reaction

• Scope and limitationso Common catalysts: Pd(OAc)2, PdCl2, Pd2dba3, Pd(PPh3)4 o Representative ligands: PAr 3, dppp, BINAP

o X: iodide, bromide, triflate

o Polar solvents often used: DMF, CH3CN, MeOH

o Addition of phase-transfer catalysts help reactivity

o Electron-releasing groups on C=C lead to increased addition to most electron-

deficient carbon (electron-poor olefins react faster)

o Reactivity order of aryl halides in oxidative additions:! Ar-I > Ar-OTf > Ar-Br >> Ar-Cl

o Aryl substituents do not interfere with coupling (Cl, CN, CO2R, CHO, NMe2)

o Vinyl triflates prepated from enolizable ketones with PhNTf 2 (N-phenyl

triflamide)

R

O

R

R

O

R

hindered

base Tf2O

or

PhNTf2

R

OTf

R



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• Vinylation of alkenes

•

Intramolecular Heck Reactiono Construction of carbocyclic and heterocyclic ring systemso Can be performed enantioselectively using chiral auxiliaries or with chiral ligands

• Larock Annulation (Larock Indole Synthesis)

o

Useful pharmaceutical process



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Pd-Cross Coupling with Organometallics and Borates

• Similar to Heck except alkenyl and aryl organometallics and borates are used instead ofalkenes

• Role of organometallic:

o

Transfer of alkenyl or aryl group onto R-PdL2X in exchange for halide or triflate• Reagents containing Zn, Al, Zr (Negishi); Boron (Suzuki); and Sn (Stille) are the most

widely used

• Reagents are more compatible with fiunctional groups like esters, amides, nitriles, nitro

groups than R-Li or R-MgX

• General catalytic cycle:

Negishi Reactions (Couplings with Organo Zn, Al and Zr)

• Organozinc reagents

o the C-Zn bond is highly covalent and less reactive toward electrophileso Preparation of organozinc compounds

! Alkylzinc halides

• Prepared by direct insertion of Zn into alkyl halides or treating

alkyl halides with Rieke zinc (reduction of zinc halides with

potassium)

• High functional group tolerance! Dialkylzincs

• Obtained by transmetalation of zinc halides with R-Li or R-MgXreagents or I-Zn exchange catalyzed by CuI

! Zinc carbenoids



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• Oxidative addition of zinc metal to ICH2I gives [ICH2ZnI]

(Simmons-Smith reagent)

o Reactions

! Transmetalations

•

Coupling of alkenyl, aryl and alkynyl halides with unsaturatedorganozincs in presence of Pd(0)

o Preparation of stereodefined aryl alkenes, aryl alkynes,conjugated dienes and enynes

• Advantages:

o Efficient transmetalation to Pd

o

Readily available or easily prepared

o Functional group tolerance

• Organo Al reagentso Coupling of alkenyl, aryl and alkynyl halides with unsaturated organoaluminumso

Prepared in two ways:

1. Hydroalumination

• cis addition of Al-H to C&C bond to product stereodefined alanes



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• Pd-catalyzed cross-coupling of alkenyl alanes with (E)- or (Z)-

alkenyl halides

o Stereoselective synthesis of 1,3-dienes

2. Carboalumination

•

Treatment of alkynes with AlMe3 in presence of Cp2ZrCl (cat) o Regioselective cis-addition of Al-Me to C&C bond

o Attempts to introduce other 1˚ alkyl group (from AlR 3) gives

mixtures of regioisomers



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• Organo Zr reagents

o Produced by treatment of alkynes with Cp2Zr(H)Cl (Schwartz reagent)

o Regioselective cis-addition of Zr-H to C&C bond! Zr occupies less substituted carbon

o Unsymmetrical alkynes give mixtures of regioisomers

! Use excess Cp2Zr(H)Cl- isomerizes to preferred form with Zr @ sterically lesshindered carbon

o Alkenyl Zr compounds couple efficiently with alkenyl and aryl halides in presence of Pd(0) catalysts

Suzuki Reaction (Couplings with Organoboron compounds)

• General method for stereo- and regiospecific synthesis of conjugated dienes, enyne, arylsubstituted alkenes, biaryls



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• Pd-catalyzed cross couplings of vinyl or aryl halides with vinyl, aryl, or alkynyl boron

reagents

• Preserves the alkene geometry of both haloalkene and alkenyl boron in product

• Tolerance of wide range of functional groups

• Readily available or easily synthesized starting materials

• Base required (hydroxypalladation or alkoxypalladation)

o K 2CO3, hydroxide, alkoxide

o Coordination with boron to form complex

!

Enhances nucleophilicity of organic group and facilitates its transfer to Pd! R"OM activates Pd by forming R-Pd-OR"

• Classic Reaction: Use of organoboranes: 1-alkenyl- and arylboroic acids (RB(OH)2) or boronate esters (RB(OR')2)

o Prepared by treatment of R-MgX or R-Li with B(OR)3

o

Acid hydrolysis of boronate ester gives boronic acid

o

Often problems with preparing boron reagents! Purification (low yields), polymerization, lack of atom economy

• To alleviate issues: use of potassium organotrifluoroborates (crystalline solids: readilyisolated, air and moisture-stable)

• (E)- and (Z)-alkenyl boranes and boronic esters or K-trifluoroborateso

Can also be prepared by hydroboration of terminal alkynes or 1-halo-1-alkynes



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•

Reaction scopeo

Conjugated dienes and enynes are easily synthesized

o Facile synthesis of aryl-substituted alkenes



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• Allows for alkyl-alkenyl or alkyl-aryl coupling

o Use B-alkyl-9-BBN derivatives

o Key feature: no #-hydride elimination



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• Allows for carbonylative coupling in presence of CO

o Valuable procedure for synthesizing unsymmetrical ketones

Stille Reaction (couplings with organotin compounds)

• Coupling of organotin compounds with electrophiles

• E+ = acid chlorides, R-X, R-OTf

• Stereospecific and regioselective

• Mild conditions

• High functional group tolerance

• Major disadvantage: organotin reagents are highly toxic (especially alkyltin derivatives)

•

Reactions performed in polar solvents (THF, DMF, etc.)• Vinyl iodides react very fast (work at low temp)

• Use 1-2 mol % Pd catalyst (Pd(II) reduced to Pd(0) by organotin compound)



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• Transfer of R' group from R'SnBu3 occurs as follows:

•

Organotin reagents prepared in two primary ways:1. Radical Sn-H addition to alkynes to give vinyl stannane2. Transmetalation with other organometallics

• Cross coupling Reactivity

o Gives high yields

o Retention of alkene stereochemical integrity

o For allylic systems --> attack at least substituted terminus



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• When carried out in CO atmosphere --> carbonylation to give ketones

Sonagashira Reaction

• Coupling of aryl or vinyl halides or triflates with terminal alkynes

• Use of catalytic amount of Pd(0) or Pd(II) precatalysts

• Often in present of cocatalyst (usually CuI)

•

Often performed in amine solvent



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• Catalytic cycle:

• Use of Cu: Castro-Stephens Reaction

•

Sonagashira reaction is milder and safer alternative

• Preparation of conjugated enynes



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• Preparation of conjugated enediynes

• Preparation of conjugated diynes



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Tsuji-Trost Reaction

• Allylic substrates with good leaving groups are excellent reagents for joining an allylmoiety with a nucleophile

• Reactions are limited- competition between S N2 and S N2'

•

Pd-catalyzed nucleophilic substitution of allylic substrates allows for formation of newC-C and C-heteroatom bonds

• Control of both regio- and stereoselectivity

•

Use of chiral ligands for asymmetric synthesis

• Most effective leaving groups: esters, carbonates, phosphates, OPh

•

Soft nucleophiles give best results for C-C bond formation (malonates, enolates, etc.)

• Nucleophilic addition usually occurs at least hindered (less substituted) site

• Examples:



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• Stereochemical outcome: retention of stereochemistry

Buchwald-Hartwig Amination

• Pd-catalyzed cross-coupling of amine with aryl halides

• Extremely powerful and efficient approach to nitrogen containing compounds and C-N bonds

• Has been applied to C-O bond formation as well



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• Very robust

• Intramolecular reactions



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Olefin Metathesis

• 2005 Nobel Prize in chemistry (Chauvin, Shrock and Grubbs)

• Allows exchange of substituents between different olefins (transalkylidenation)

• General mechanism:

• Grubbs's catalysts most often used in synthesis

•

Ring-closing methathesis (RCM) o Efficient formation of small-, medium- and large-sized rings



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• Ring opening metathesis

o

Reverse of RCM (relief of ring strain)o Can be used in tandem with other reactions



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• Cross metathesis

o Intermolecular olefin metathesis reaction (forms acyclic substituted alkene)

• Alkyne metathesis

o Inter- or intramolecular metathesis reaction of alkynes

o Often used to install cis- or trans-alkene



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• Enyne metathesis

o Metathesis reaction between alkene and alkyne (generates a diene moiety)

Synthesis Using Organometallics

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