Re · • (re)discovered in 1925 by Noddack, Tacke, and Berg in Germany--named "Rhenus" for the river Rhine •predominantly used for manufacturing of jet engines Basic Properties

Oxorhenium Species in Catalysis Joe Derosa3/9/17

Re75

Rhenium

186.207

[Xe]4f145d56s2

Rhenium Facts:•third-row transition metal (group VII)•can access oxidation states from -3 to +7•estimated average concentration of 1 ppb (part per billion) in Earth's crust•only obtained as a byproduct of refinement of molybdenum and copper ore•costs $2,750/kg compared to $24,701/kg for palladium (April 2013)

History• originally discovered by Masataka Ogawa in 1908 and named Nipponium-- discredited due to controversial analysis• (re)discovered in 1925 by Noddack, Tacke, and Berg in Germany--named"Rhenus" for the river Rhine•predominantly used for manufacturing of jet enginesBasic Properties•third highest melting point (3186 ºC) and highestboiling point (5630 ºC)•electropositive•hexagonal close-packed crystal structure•fourth densest element at 21.02 g/cm3

Re

Re

ClClCl

Cl

ClCl ClCl

2-

octachlorodirhenate

quadruplebond

3dσg

3dπu

3dδq

3dδq

3dπg

3dσu

3d 3d

2 e-

• Rhenium complexes are often dimeric and contain quadruple bonding between metal centers, i.e. Re2Cl82- •Re2Cl82- maintains a sterically unfavorable eclipsed geometry due tothe rigidity of the δ-bond formed from the overlap of d orbitals•It is also possible for rhenium trimers to exist, but relatively unexplored


ReO

ReO

O O

O

OORe0

O2

∆Me3Sn Me Me

ReO

OOmethyl trioxorhenium = MTO

•metallic rhenium can be heated in the presence of O2 to form a bridged rhenium (VII) oxo dimer that can be used as a supply of starting material for the synthesis of various oxorhenium species•treatment of Re2O7 with HCl, TMSCl, SOCl2, and a variety of temperaturesproduces chlorooxorhenium in a wide range of oxidation states

Oxygen Atom Transfer with MTO

R2 OH

R4R3

R1 1.2 eq. H2O2 0.1 mol% MTO

0.1 eq. 3-methylpyrazoleDCM, 10 ºC, 8h

R2 OH

R4R3

R1O

Yamazaki, J. Org. Chem. 2012, 77, 9884-9888.

NR

2 eq. Na2CO3•H2O20.2 eq. AcOH, 1 mol% MTO

MeCN, 50 ºC, 1h NR

O

Sain and coworkers, Synlett 2006, 2661-2663.

R1

N R3

R2

R2N

R1

R3 O

"

•For examples involving imines, see Goti, Org. Lett. 2007, 9, 473-476.

Inorganic Trivia

Introduction to Rhenium-oxo SpeciesDue to time contraints, the focus of this presentation will be rhenium-oxo species in catalysis. The following classes of transformations will not be detailed herein: (1) olefin disproportionation, (2) olefin metathesis, (3) enynecyclizations, (4) electrochemical oxidations.

SOCl2

ReO

Cl

Cl

ClCl

Girolami, Dalt. Trans. 2007, 675.

Re2O3Cl6•(H2O)2ReO3Cl•(THF)2

PhS

R

1.0 eq. H2O2 0.1 mol% MTO

PhS

R

O

Yamazaki, Bull. Chem. Japan 1996, 69.

MeRe

O

OO

H2O2 MeRe

O

OO

O


Selected Hydrogenation Examples•The degree of selectivity depends on the substrate and oxidation state ofthe oxorhenium species employed: (1) higher oxides are generally less effective in reducing aromatics, (2) lower oxides react faster with alkenes

NO2

Br

NH2

Br

Br Br

O2N H2N

S S

S S

N Ar HN Ar

Re2O3

ReO2

ReO

ReO2

ReO3

NO2 > CO > CH=N > C=C > C–C >> C–S >>C–Xgeneral order of sensitivity

Broadbent, "Rhenium and its Compounds as Hydrogenation Catalysts" 1967.

H2

H2

H2

H2

H2

Evolution of Oxorhenium Catalysts

RCN

RNH2

2 eq. Ph3SiH10 mol% ReIO2(PPh3)2

ReOOH

IL

NCR Re

OIL

NCR

HOSiR3

ReOO

IL

N

R

SiR3Re

OIL

N HOSiR3

R

ReOO

IL

N

R

SiR3

R3Si

R3Si

ReOO

IL

Ph3SiH

Ph3SiH

C NR

NSiR3

SiR3

R

Fernandes, Tet. Lett., 2007, 67, 8183.


•For oxorhenium-catalyzed reductions of sulfoxides, alkenes, nitroarenes,and reductive amination of aldehydes: Fernandes, J. Org. Chem., 2009, 74; Fernandes, Tetrahedrom Lett., 2010, 51.; Fernandes, Adv. Synth. Catal.,2009, 352.

•In 2003, Toste and coworkers set out to "reverse the role" of metal-oxo π-bonds for chemoselective catalytic reductions of aldehydes using a rhenium(V)-dioxo complex

Toste, J. Am. Chem. Soc. 2003, 125, 4056.

H SiR3+

H SiR3+(PPh3)2ReVO2I

(PPh3)3RhICl RhIIIH

SiR3

PPh3

Ph3PPh3P

Cl

ReVPPh3

PPh3OR3SiI O

H

•low valent hydrosilyation involves an oxidativeaddition step

via:

ReVPPh3

PPh3OR3HSi

IOH •"role reversal" of strongly oxidizing

species to a reducing metal hydride

R1

O

R2 R1

OSiMe2Ph

R2H

2 mol% (PPh3)2ReO2IMe2PhSiH

benzene

OSiMe2PhH

Me

O

O

MeO

I

OSiMe2Ph

H

OMe66% 68%

ORe

SMe2ClClCl

OPPh3N

OCN

HN

O

R R

R= 4-tBu-Ph

DCM, rt

NO

NCN

O

ReO

ClCl

OPPh3

(CNbox)ReV-oxo$1.83/mg, Sigma Aldrich

Short Re-O bond length of 1.66 Å

•Toste and coworkers performed mechanistic studies on previous work,carbonyl reduction, and observed necessity for neutral ligand dissociation•Sought to make a chiral ligand capable of asymmetric transformations, monodeprotonated BOX ligands were tested


R1

N

R2

P(O)Ph2

R1

HN

R2

P(O)Ph23 mol% (CNbox)ReV-oxoMe2PhSiH

DCM, rt

Ph

HN

O

OEt

P(O)Ph2

Et

HN

MePh

P(O)Ph2

O

HN

Me

P(O)Ph2

83%, >99% ee 71%, >99% ee 76%, 99% ee

•Interestingly, compounds whose equilibrium favor the enamine tautomerundergo the reaction in high yield and ee•p-methoxyphenyl (PMP) imines could also serve as competent substrates•aliphatic imine substrates proceeded with poor enantiocontrol


Other oxorhenium complexes:

O

O NRe

NO

O

O

L

O

NN Me

OReO Solv

Abu-Omar, J. Am. Chem. Soc. 2005, 127, 15374.

•For asymmetric reduction of ketones using chiral (CNbox) ligands, seeToste, Chem.– Eur. J., 2010, 16, 9555.

C–C, C–O and C–N Bond Forming Reactions

R1

OH

R3

R1

OR

R3

1 mol% (dppm)Re(O)Cl3ROH

MeCN, 65 ºC

Ph2P

Ph2P ReO

ClClCl

(dppm)Re(O)Cl3

Mechanism (Meyer-Schuster-esque):

R1

OH

R3

LnRe(O)

R1

O

R3

ReO

L

R1

O

R3

[O][3,3]

R1 •

R2

OReO

H+

R1

O

R2

ROH

R1

OR

R3



•Based on the proposed mechanism, the chiral allene intermedaite washypothesized to be indicative of a stereospecific process; however, the process seemed to erode enantiopurity

OH

nBu

1 mol% (dppm)Re(O)Cl3MeOCH2CH2OH

MeCN, 65 ºC

86% ee

O

nBu

OMe

0% ee

Potential Rationalization (1) - 1,3 oxygen shift catalyzed by oxorhenium

R1 •

R2

OReO

OH

R2R1

OH

R2R1

Gordon, Organometallics 1998, 17, 1835.

Potential Rationalization (2) - Rhenium-alkyne complexation

OH

R2R1 Re

OH+

R1 •

R2

ReO

ROHOH

R2R1

diyne-oxorhenium complex

Norton, Inorg. Chem. 2001, 40, 2942.

R1

OH

R3

R1

Ar

R3

5 mol% (dppm)Re(O)Cl3ArH, 5 mol% KPF6

MeNO2, 65 ºC

•Aryl nucleophiles and other carbon nucleophiles required the use of catalytic hexafluorophosphate to sequester chloride ligand from rhenium•Hypothesized to ease chelation of propargyl alcohol and favor allene intermediate•Methodology also extended to include allylation via allyltrimethylsilane, see: Toste, J. Am. Chem. Soc. 2003, 125, 15761.Synthesis of mimosifoliol:

OH

TMS

OHOMe

MeO

1. 2,5-dimethoxyphenol5 mol% (dppm)Re(O)Cl3

5 mol% KPF6

2. TBAF, THF 0ºC 3. Pd/BaSO4

58% overall

R1

OH

R3

R1

NR1R2

R3

5 mol% (dppm)Re(O)Cl3R1R2NH, 5 mol% NH4PF6

MeNO2, 65 ºC

•Also applicable to nitrogen nucleophiles:

BocHN NH

O

MeO CbzNH2

Toste, Org. Lett. 2005, 7, 2503.


Performance of other metals:

OH

nBu

O

nBu

Cl

5 mol% catalyst

MeCN, 14h, 65 ºC

HOCl

entry catalyst % enone % ketone %6

6

12345

V(O)(acac)2[Mo2O7(BINOL)2](NBu4)2

MoO2(acac)2(catechol)ReOCl3

(dppm)ReOCl3

002077

trace

2910

trace0

trace

1915772596

•MoO2(acac)2 was found to only perform well for 1º alcohols

Tandem Meyer-Schuster-hydrosilyation:

R1

OH

R2

Ln*Re(O)

PhMe2SiH R1

OH

R2

via:

R1 •

R2

OReO

H+

R1

O

R2

•Initial results showed that traditional (CNbox) chiral oxorhenium complexes gave a mixture of products

Selected examples of new chiral ligands:

N

O

CN

N

O

Ph Ph

N

O

CN

N

O

Cl

Cl

MeO Cl OMe

Cl1 2

ROH

NH2

EtO

NH

OEt

NH21.

2. nBuLi, THF3. TsCN

N

O

CN

N

O

R R

Ph

OH

Me

Synthesis of (CNbox) ligands

catalyst

Ph Me

O

Me

OPh

Ph

O

Me

Ph

Me

•It was found that Re-oxo complex with1 and 2 gave a mixture of 1:0.1:0.1 and 1:2:0.6, respectively


OHPhPh

R

Ph

Ph R

OH

ORe

SMe2ClClCl

OPPh3

2.5 mol%

then 3 mol% 2,2 eq. Me2PhSiH Yields up to 85%

ee's up to 92%•Reaction also applied to allenyl alcohols and realized with similar successDeoxygenation of Diols and Epoxides

R2R1

O10 mol% MTO

H2 (80-300 psi), 150 ºC

R2R1

R1

HO OH

R2

10 mol% MTOH2 (80-300 psi), 150 ºC

R2R1

Abu-Omar, Inorg. Chem. 2009, 48, 9998.•Methodology mainly limited to alkyl chains

mechanism involves a deoxydehydration (DODH) step!

HO OH

HO OH

2.5 mol% MTO

3-octanol, N2, 170 ºC

Toste, ACIEE 2012, 51, 8082.

Not observed:O

HO OH

HO OH

OH

HO

89%

MeReVII

O OO

MeReVII

OO

MeReV

O O

MeReV

OO O

O

RR

H

H

O

RR

R RR R

OH

O

RRR R

HO OH

H2O


C–H Functionalization•In nearly every C–H activation/functionalization, optimization tables showthat oxorhenium species are not competent catalysts for the intended transformation•This section will focus only on C(sp2)–H bonds, for C(sp3)–H bonds see:Hartwig, ACIEE 1999, 38, 3390-3392.

RH

NtBu R

Ph

R

HNtBu

R

Ph3 mol% [ReBr(CO)3(thf)]2

toluene, reflux, 24 h

Yields as high as 99%

•Takai and coworkers observed low yield when using a pentacarbonylrhenium species and no reactivity under CO pressure - why?

Takai, J. Am. Chem. Soc. 2005, 127, 13498-13499.

OMe

NnC8H17 iPr

NCNiPr

2.5 mol% Re2(CO)10

chlorobenzene, 170 ºC 24 h

N

NnC8H17

NiPr

iPr

85%

•The reaction did not proceed when the catalyst was substituted with Pd(OAc)2, RhCl(PPh3)3, Cu(OAc)2, and only proceed in 16% with [Cp*IrCl2]2

Kuninobu, Org. Lett. 2016, 18, 2459-2462.

•Attempts have been made toward involving pincer ligands in oxorheniumcatalysis using PNP-connectivity for DHDO reactions, for pincer-rhenium complexes competent in electrochemical oxidation see:Peruzzini, Inorg. Chim. Acta 2002, 327, 140.

•Ultimately, both PNP-oxorhenium species failed as competent catalysts for oxygen atom transfer under both oxidizing conditions in the presence of an alkene and N-oxide abstraction -- thought to be noninnocent when deprotonated for dehydration reactions


Gebbink, Organometallics 2014, 33, 2201-2209.

Gebbink, ACS Catal. 2015, 5, 281-300.

C–O Bond Formation from Rhenium-Oxo Alkyl Complexes•For the full paper, see Goddard, III Organometallics 2010 29, 2026-2033.

•In the case of phenyl substituted complex, the final phenoxide is thoughtto form by an aryl migration mechanism -- however, an analgous ethylsubstitution yielded acetaldehyde

Possible Pathway (1) - Ethyl 1,2-migration•This would be the parallel mechanism to the initial phenoxide result•DFT calculations suggested that the reaction barrier of the migration was only 22.1 kcal/mol -- only 4.2 kcal/mol higher than phenyl case!Experiment: Independent synthesis of (HBpz3)ReO(OEt)OTf and exposure toreaction conditions -- generates acetaldehyde AND ethanol!

indicates that there is another lower energy pathway!

Possible Pathway (2) - α− or β−Hydrogen Abstraction by Oxo(a) α−Hydrogen Abstraction by Oxo•The formation of a Re-ethylidene would require 44.9 kcal/mol to overcome, making it thermally inaccesible Explanation: Molecular orbital analysis throughout the course of the reactionsuggests that no molecular orbitals remain both bonding and orthogonal intransition state

"2+2 forbidden"

(b) β−Hydrogen Abstraction by Oxo•Reaction barrier of the process is only 8.0 kcal/mol lower than (a)

indicates that there is another lower energy pathway!

Triflate assisted H-abstractionfollowed by DMSO oxygentransfer!

Documents

Re · • (re)discovered in 1925 by Noddack, Tacke, and Berg in Germany--named "Rhenus" for the river Rhine •predominantly used for manufacturing of jet engines Basic Properties