Furans and benzannulated forms

C H A P T E R

1 Furans and benzannulated forms The scope is synthesis, coordination modes, and reactivity of the coordinated furan, some of its derivatives, and benzannulated forms. Thiols, amines, Schiff bases, phosphine derivatives, and mixed heterocycles of furans are considered in separate sections. Furan is a planar heteroaromatic ligand, more reactive than benzene due to the electron-donating effects of heteroatom. It is of low aromaticity and chemically resembles 1,3-dienes. It undergoes electrophilic substitution predominantly at the α-position, polymerizes in the presence of electrophiles, and undergoes Diels
Alder reactions. Only seldom is 2,5-functionalization possible. The scope of chemical transformations is apparently narrow despite the high demand for variously derivatized furans in synthetic organic chemistry (natural products, pharmaceuticals, flavor or fragrance compounds). Furan is formally classified as a π-excessive ligand, although it is not prone to π-complex formation with few exceptions (01AHC1). Although benzofuran is considered to be aromatic, its electronic distribution is such that the π-donor ability is lower than in furan, that is, it is less aromatic than furan. The influence of the heteroatom is limited by the five-membered cycle. The oxygen atom is a stronger σ-acceptor and a weaker π-donor than in furan. The furan ring of benzofuran is less π-excessive than the parent heterocycle, and the benzene and furan rings are fairly independent. Dibenzofuran is very stable; its first ionization potential is π in nature. The HOMO
LUMO transition reflects the dienic character of the five-membered ring. This leads to a general view of the electronic distribution in benzannulated five-membered heterocycles. The π-electron delocalization is complete only for the carbocyclic constituent of the molecule. Thus one may expect that coordination of metal carbonyls should occur via the π-conjugated carbocyclic system and the heteronucleus should take part in π-complex formation only with difficulty.

1.1 Coordination modes 1.1.1 η1(C)-mode Cocondensation of furan with lithium atoms in the vapor phase leads to the sequential C
H activation at the α-carbon atoms (Eq. 1.1) (04EJI4525). With calcium atoms, similar Ca
H derivatives are produced (06JOM1110).

Organometallic Chemistry of Five-Membered Heterocycles DOI: https://doi.org/10.1016/B978-0-08-102860-5.00001-8

1

© 2020 Elsevier Ltd. All rights reserved.

2

1. Furans and benzannulated forms

Li( g)

Li( g) O

O

ð1:1Þ

O

Li

Li

Li

Dibromodifuran with trimethylsilyl groups at the 2- and 6-positions of the adjacent five-membered rings with p-block (silicon, germanium, or phosphorus) dichlorides give stable difurans, bridged by diphenylsilyl, diphenylgermyl, or, and phenyl phosphinoxide groups η1(C)-coordinated with respect to the furan rings (Eq. 1.2) (17OM2565). Ph 2 Si Ph 2 SiCl 2 Br

Me3 Si O

SiMe3

O

Me3 Si

Bu n Li

O

O

SiMe3

Ph 2 Ge

ð1:2Þ

Ph 2 GeCl 2 Me3 Si

Br

O

O

SiMe3

( O) Ph P

PhPCl2 H 2 O2 Me3 Si

O

O

SiMe3

3,30 -Diiodobi(benzofuran) with n-butyl lithium followed by dialkylchlorosilane or -germane afforded the η1(C)-coordinated polycycle (Eq. 1.3) (16OM2327). I O

O

Bu n Li, R2 MCl2 O

ð1:3Þ

M = Si, Ge; R = Ph M = Ge, R = 2- EtC6H1 2n

I

O

M R2

2-Lithiofuran with tin tetrachloride as well as tetrakis(2-benzofuryl) tin with potassium give homoleptic six-coordinate with respect to tin and two-coordinate with respect to an alkali metal trinuclear 2-furyl- and 2-benzofuryl tin(IV) where tin is η1(C)-coordinated and lithium or potassium η1(O)-coordinated (Eq. 1.4) (17D8279). 2-Lithiofuran (Eq. 1.5) and 2benzofuran (Eq. 1.6) with tetrakis(2-furyl)tin or tetrakis(2-benzofuryl)tin give homoleptic pentacoordinate tin anions, in which cations are alkali metals coordinated by solvents (Eq. 1.7). O SnCl4 Et 2 O Li

O

O

O Sn

( Et 2O) Li

Li( OEt) 2

O

O O

ð1:4Þ

3

1.1 Coordination modes

O Sn( 2- C4 H3O) 4 TMEDA Li

O

ð1:5Þ

Sn

Li( TMEDA) 2 O

O

O O

Sn( 2- C8 H5O) 4 THF Li

O

ð1:6Þ

O Li( THF) 4

Sn

O

O

O O

O O Sn( 2- C8 H 5O) 4 + K + THF

O Sn

( THF)n K

K( THF)n

O

ð1:7Þ

O O

Furan is coordinated to the ansa-molybdocene in an η1(C)-manner forming C2- and C3coordinated isomers (Eq. 1.8), while benzofuran is solely η1(C3)-coordinated (Eq. 1.9) (06POL499). η1(C2)-coordination is realized in [(η5-Cp)Mo(CO)2(PPh3)(2-C4H3O)], which is the result of coupling between coordinated CO of Na[(η5-Cp)Mo(CO)3] and epibromohydrin (BrCH2CHOCH2) in the presence of an excess PPh3 (87OM1821). H [ ( η5 : η5- Me 4C5 Si( Me 2) C 5Me4 ) Mo( H) 2 ] , hν

( η5 : η5- Me 4C5 Si( Me 2) C 5Me4 ) Mo

O

O

ð1:8Þ

H 5

5

+ ( η : η - Me4 C5 Si( Me2 ) C5 Me 4 ) Mo O

[ ( η5 : η5- Me 4C5 Si( Me 2) C 5Me4 ) Mo( H) 2 ] , hν

H ( η5 : η5- Me 4C5 Si( Me 2) C 5Me4 ) Mo

ð1:9Þ

O O

4

1. Furans and benzannulated forms

Mercuriated furan with Fe2(CO)6-based compound gives the product, in which furan performs the η1:η2 bridging function between two iron sites (Eq. 1.10) (92OM3262). ( CO) 3 Fe ( Et 3NH) [ Fe 2( CO) 6( μ- CO) ( μ- RS) ] O

ð1:10Þ

SR

R = Ph, Bu t

HgCl

O

Fe ( CO) 3

Facile C
H activation of furan and 2-methylfuran taken as triphenylphosphine adducts occurs with the Fe(II) organometallic precursor (Eq. 1.11) (13OM1797). PPh 3 O

5 * [ ( η - Cp ) Fe( CO) ( AN) Ph]

R = H, Me

R

O

R

ð1:11Þ Fe( CO) ( PPh 3)

Deprotonation of the ruthenium carbene leads to the cyclization and generation of the 2-furyl (Eq. 1.12) (10OM38). In the process of ruthenium-catalyzed cyclization of 1,3butyne-2-diols leading to substituted furans, the η1(C2)-coordinated furan is postulated (08OM3614). The whole series of such reactions has been reviewed (13CRV3084). 1

OR O

n

Cp( Ph 3 P) 2Ru

Bu 4NOH

C

Cp( Ph 3 P) 2Ru

O

1

R = Ph, Me, R = Et 1 R = R = Me R

R

1

OR

ð1:12Þ

R R

Deprotonation of the ruthenium α,β-unsaturated propargyl oxycarbene gives the η1(C)coordinated benzannulated furan (Eq. 1.13) (00OM4). Cp( CO) ( Pr i 3P) Ru

O

Cp( CO) Ru( PPr i3 ) BF4 CPh 2 = CH

OCH2C

Al2 O3

Ph

ð1:13Þ

CH

Furans (Eq. 1.14) and benzo[b]furan (Eq. 1.15) enter into the C
H bond cleavage accompanied by the coordination mode change of cyclooctadiene in the ruthenium(0) precursor (03ICA160).

5

1.1 Coordination modes 8

4

[ ( η - COT) Ru( η - cod) ] , PEt 3 or 4 [ ( η - 1,4- COT) Ru( PEt 3 ) 3 ] O

R

R

R = H, COMe

O

Ru( PEt 3 ) 2

ð1:14Þ

Ru( PEt 3 ) 2

ð1:15Þ

[ ( η8- COT) Ru( η4 - cod) ] , PEt 3 or [ ( η4 - 1,4- COT) Ru( PEt 3 ) 3 ] R = H, COMe

O

O

The process below (Eq. 1.16) represents the C
H activation of the furan molecule (04OM5514). N N + O

HB

N N Ru( AN) ( CO) Me

N N

HB

Ru N N

N N

O

( CO) ( AN)

ð1:16Þ

N N

Furan oxidatively adds to the triosmium cluster; it is metalated at position 2 of the heteroring and leads to the hydrido cluster presented as a mixture of the exo- and endoisomers (Eq. 1.17) where the ligand plays the role of an η1:η2 bridge (85JOM(297)141, 89OM1408, 90CC1568, 91JOM(412)177). The product of more deep interaction, the triosmium furyne (85JOM(297)141) reacts further and gives the bis-triosmium containing a bridging furyenyl ligand formed by the route of C
H activation at the uncoordinated C
C double bond (Eq. 1.18) (18CC3464). Thermolysis leads to further C
H activation to yield the furdiyne C4O, which subsequently ring-opens and decarbonylates to yield products containing bridging C3 and CHCCHCC 5 O ligands. O [ Os3 ( CO) 1 0( AN) 2 ] O

( OC) 4 Os

O Os( CO) 3

H Os ( CO) 3

( OC) 4 Os

Os( CO) 3 H Os ( CO) 3

ð1:17Þ

6

1. Furans and benzannulated forms

H

( CO) 4 Os

(OC) 3 Os

H

Os( CO) 3 ( CO) 3

Os( CO) 3

(OC) 3 Os

H

Os

H

Os ( CO) 3

H

[ Os3 ( CO) 1 0( AN) 2 ]

H

Os ( CO) 3

(OC) 3 Os

Os( CO) 3

(OC) 3 Os

+

H

Os( CO) 3

(OC) 3 Os

( CO) 4 Os

H (OC) 3 Os

Os( CO) 3

(OC) 3 Os H

Os ( CO) 3

+ Os( CO) 3

H

H ( CO) 4 Os

H

C

O

O

O

ð1:18Þ

H (OC) 3 Os Os( CO) 3

Os( CO) 3

H

Os ( CO) 3

C O

C C H

H- C C

C

Os ( CO) 3

Os ( CO) 3

H Os( CO) 3

(OC) 3 Os

C

+

+

C

C (OC) 3 Os

H Os( CO) 3

(OC) 3 Os

H

Os( CO) 3

H

H Os ( CO) 3

H

2-Formylfuran oxidatively adds to the triosmium cluster (Eq. 1.19) (86JOM(311)371). Decarbonylation leads the μ3-furan-2,3-diyl bridge. O O

O [ Os3 ( CO) 1 0( AN) 2 ] O

( OC) 4 Os

Os( CO) 3

CHO

Δ

( OC) 3 Os

H

H

Os ( CO) 3

ð1:19Þ

Os( CO) 3 H Os ( CO) 3

Substituted furan is η1(C)-coordinated as far as the heteroring is concerned and is subjected to the carbene-type activation in aldol condensation (Eq. 1.20) (02D827). X

Y O

( OC) 3 Os

O

CHO

Os( CO) 3 H Os ( CO) 4

X

Y

( OC) 4 Os

X = Y = H; X = H, Y = NO 2 X = NO 2 , Y = H; X = H, Y = OH X = OH, Y = H

Os( CO) 3 Os H ( CO) 3

ð1:20Þ

7

1.1 Coordination modes

Furan oxidatively adds to the iridium(I) compound with elimination, which is accompanied by the C
H activation of the heterocycle (Eq. 1.21) (93OM3800, 12JOM163). Same type of reaction is observed for furan and 2-methylfuran with [(η5-Cp*)Rh(PMe3)(Ph)H] (95OM855). [(η4-cod)Ir(PMe3)3]Cl O

O

Ir(H)(Cl)(PMe3)3

ð1:21Þ

The C
H activation of furan in organoiridium chemistry is shown in Eq. (1.22) (08JOM3375). Me2 P

PMe 2

[ ( η5- Cp * ) ( H) Ir ( μ-dm pm)( μ- H) I r ( Ph) ( η5- Cp * ) ] OTf Ir Cp * ( H)

O

H

IrCp* OTf

ð1:22Þ

O

Furan oxidatively adds on [Pt(PEt3)4] at the α-position of the heteroring (Eq. 1.23) (04JOM1315). [ Pt(PEt 3 ) 4 ] O

O

ð1:23Þ Pt(PEt 3 ) 2 H

Tin
palladium transmetalation is another route to the C-coordinated furan (Eq. 1.24) (98JA11016). C3-palladation is the feature of Pd-catalyzed tandem intramolecular oxypalladation/Heck-type coupling (09OL1083). 2-Furfuryl chloride with [Pd(PPh3)4] gives [Pd(η1CH2-2-C4H3O)] (81JOM(209)123). Ph 2P

Pd( C6 H 3( CH 2PPh 2) 2) ( OTf ) O

SnBu 3

n

O

ð1:24Þ

Pd Ph 2P

α-Bromoacetylferrocene with lithium diisopropylamide in THF gives 3-bromo-2,4-bis (ferrocenyl)furan forming η1(C) palladium(II) by oxidative addition of the palladium(0) precursor (Eq. 1.25) (01JOM(637)258). Br

Br ( Ph 3P) 2 Pd

O Fe

Fe

O [ Pd( PPh 3 ) 4 ]

Fe

Fe

ð1:25Þ

8

1. Furans and benzannulated forms

The 2- and 3-furylmercury derivatives are described (79D2037). Theoretical aspects of cycloauration in the process of gold-catalyzed derivatization of furans were presented (09OM741, 10JA7645). Metalation of furan by ytterbocenes, O-coordination in heteronuclear complex, as well as η2(C,O)-coordination are illustrated in Eqs. (1.26)
(1.28) (02OM1759). Cl

Li

O

Cp

Li

O

5

*

Li( THF) 2

2Y

ð1:26Þ

O

*

[ ( η - Cp ) 2YCl 2Li( OEt 2) 2 THF

O [ ( η5- Cp * ) 2 YCl2 Li( OEt 2 ) 2 ]

Cp

TMEDA

*

ð1:27Þ

Li( TMEDA) 2Y

O

O [ ( η5- Cp * ) 2 YH] 2

Cp

–H2

O

*

2Y

O

THF

Cp

*

ð1:28Þ

2Y

THF

C
H bond activation of furan is achieved by a labile yttrium half-sandwich (Eq. 1.29) (01EJI73). 2-Methylfuran is similarly metalated (02JOM(647)158). 5

1

[ ( η : η - C5Me4 SiMe2 NCMe3 ) Y( CH2 SiMe3 ) ( THF) ] + O THF O ( C5 Me4 SiMe 2 NBu t ) Y

t

Y( NBu SiMe2 C5 Me 4)

THF ( C5 Me4 SiMe 2 NCMe 3) Y

ð1:29Þ

O O

1.1.2 η2(C2)-mode Photolysis of [(η5-Cp)Mn(CO)3] in the presence of furan gives the η2-coordinated [(η5Cp)Mn(CO)2L] (07IC7787). A number of the η2-coordinated derivatives of furan whose synthetic schemes are shown in Eqs. (1.30) (89JA5969), (1.31) (01OM3661), (1.32) (99JA6499, 00OM728), (1.33) (06OM435), and (1.34) (03JA2024) serve the purpose of the study of coordinated furan reactivity pattern.

9

1.1 Coordination modes

[ Os( NH 3 ) 5 ] ( OTf) 3

Os( NH 3 ) 5 ( OTf) 2

O

ð1:30Þ

O

N

N

N

N

L CO

+ HB

N N

Re

CO HB

t

O

N N

L = Bu NC, Py, 1- MeIm

N N

ð1:31Þ

O

N

N N

PMe 3 CO

N N

O

Re

N

N N + HB

L

AgOTf, Na/ Hg

HB

Re

PMe 3 CO

N N

N

O

N

N

ð1:32Þ

Re

N

N N [ TpW( NO) ( PMe3 ) Br ] , Na 2

O

R

R1

HB

R1 = R2 = H, Me; R1 = Me, R2 = H

PMe 3 NO

N N

N

Me N

R

O

N NO

HB

N N N

R

Mo

R = H, Me

R

ð1:33Þ

O

R2

N N

N

[ TpMo( NO) ( 1- MeI m ) Br ]

R1

W

R

ð1:34Þ

O

N

Furan is η2:η2-coordinated (Eq. 1.35) in iron carbenes (01OM2387).

( CO) 3 Fe

( CO) 2 Fe

Ar Li/ Et 2O, Et 3OBF4

NPh

NPh O

OEt

O Ar = Ph, p- Tol, o- MeOC6 H 4 , p- MeOC6 H4, p- CF3 C6 H4

Ar

ð1:35Þ

10

1. Furans and benzannulated forms

Maleic anhydride forms the η2-pentacoordinated adduct with rhodium(I) precursor (Eq. 1.36) (05OM5634). Rh( CNC6 H 3Me2 - 2, 6) 2 ( PPh 3) ( CF3 )

ð1:36Þ

[ Rh( CNC6 H 3Me2 - 2, 6) 2 ( PPh 3) ( CF3 ) ] O

O

O

O

O

O

The presence of the η6-coordinating tricarbonyl manganese at the benzofuran ring makes possible the formation of two types of adducts with organoplatinum precursor (99AGE2206). One of them is η2-coordinated via the double bond of the furan heteroring, whereas another is formed via two carbonyl groups hereby becoming bridging groups. Standing allows the preparation of the product of insertion into the relatively strong C
O bond of the five-membered heteroring (Eq. 1.37). ( OC) 3 Mn

(OC) 3 Mn BF4

Pt( PF3 ) 2

[ ( η2- C2H4 ) Pt( PPh 3) 2]

BF4

O

O

[ ( η2-C2H4)Pt(PPh3)2]

ð1:37Þ CO

O C Mn

( Ph 3 P) 2 Pt C O

O

BF4

Pt( PPh3) 2

BF4

O

Dipalladium(I) terphenyl diphosphine bridges furan and 2-methylfuran in an μ-η2:η2 manner (Eq. 1.38) (13JA15830).

R

+ O

Pr i 2P

Pd

Pd

Pr i 2P

i PPr 2 (BF4 ) 2

O

Pd

Pd

PPr i2 (BF4 ) 2

ð1:38Þ

R = H, Me

R

1.1.3 η5-Coordination Furan forms the η5-ruthenium(II) (Eq. 1.39) (88CC711). [ ( η5- Cp * ) RuCl] n , KPF6 or

Cp * Ru

[ ( η5- Cp * ) Ru( Me 2CO) ( H2O) 2 ] PF6 O

PF6 O

ð1:39Þ

11

1.1 Coordination modes

1.1.4 η6-Coordination of benzannulated furans Chromium tricarbonyl complexes with benzofuran (Eq. 1.40), dibenzofuran (Eq. 1.41), and benzo[b]naphtho[2,3-d]furan (Eq. 1.42) contain the η6-coordinated Cr(CO)3 fragment via the benzene ring (68JOM(14)359, 75ADOC47). [ Cr ( CO) 6]

( OC) 3 Cr

ð1:40Þ

O

O

[ Cr( CO) 6]

(OC) 3 Cr

ð1:41Þ

O

O

ð1:42Þ

[ Cr ( CO) 3( AN) 3] ( OC) 3 Cr O

O

The η6-coordinated benzofuran follows from the AlCl3-catalyzed exchange between heteroaromatic benzannulated ligands and ferrocene (Eq. 1.43) (80JOM(186)265). Similar reactions were described for other heterocycles (76ZN(B)525). 5

[ ( η - Cp) 2 Fe] , Al, AlCl3 , NH4 PF6

PF6 O

O

Fe Cp Cp Fe

[ ( η5- Cp) 2Fe] , Al, AlCl3 , NH4 PF6

ð1:43Þ ( PF6 ) 2

O Fe Cp

1.1.5 O-coordination Weak and short-living O-coordinated products are formed in Eq. (1.44) (99D115, 00JPC (A)10587, 01OM3314). ½WðCOÞ5 ðCyHÞ 1 L-½WðCOÞ5 L L 5 2-methylfuran,2,5-dimethylfuran

(1.44)

12

1. Furans and benzannulated forms

Organozinc compound of 2-phenylfuran is the source of the heteroleptic triscyclometalated iridium(III) containing 1-(2,4-difluorophenyl)pyrazole or 2-phenylpyridine (Eq. 1.45) (09OM6079).

n

O Br

[(2,4-F2C6H2-1-Pz)2Ir(μ-Cl)]2 or [(2-PhC5H4N)2Ir(μ-Cl)]2

O

Bu Li ZnCl 2

ZnCl

F N N O F F

Ir

ð1:45Þ

N or

O

Ir N

N N F

1.1.6 η4(C4)-mode Photochemical decarbonylation of the 2,5-dimethylfuran tungsten pentacarbonyl leads to the η4-coordinated complex (Eq. 1.46) (03AGE2179). hν

W( CO) 4

ð1:46Þ

O

O W( CO) 5

1.1.7 Peripheral coordination In 2-furyl-substituted bis(indenyl)zirconium, η5-coordination is via the five-membered carbocyclic ring of the indenyl moiety (00OM4095). The same situation is observed for 2(2-furyl)indene (Eq. 1.47) (01JOM(622)143, 01OM5067) and 1-(5-methyl-2-furyl)indene (Eq. 1.48) (01JOM(621)197). O

Zr Cl4 Li

O

MeLi

Zr Cl2

ð1:47Þ

Zr Me 2

O O

O

13

1.1 Coordination modes

O

O Zr Cl4

ð1:48Þ

Zr Cl2

Li

O

2-Lithiofuran forms stable Fischer carbenes (Eq. 1.49) (92JA2985). [ M( CO) 6 ] O

ð1:49Þ

M( CO) 5

E

M = Cr , W X = H, Li

Li

XO

Lithiated furan precursors and chromium and tungsten hexacarbonyls yield carbenes with a bridging furan substituent and binuclear bis-carbenes (Eq. 1.50) (05D1649). Benzannulation of the monocarbenes is achieved using 3-hexyne. [ M( CO) 6 ] O

Li

LiNPr i2

LiO

M = Cr , W

[ M( CO) 6 ]

LiO

O

O

M( CO) 5

M( CO) 5

Et 3 OBF4

[ M'( CO) 6 ]

EtO

M = M' = Cr , W M = Cr , M' = W

LiO O

M( CO) 5

M( CO) 5

OEt O M( CO) 5

EtC CEt M = Cr , M' = Cr , W

[ M( CO) 6 ]

O

Li

M = Cr , W OLi

O M'( CO) 5

M( CO) 5 HO

Et 3 OBF4 EtO

Li

LiO

OLi

O

Li

M = Cr , W

M( CO) 5

Et

EtO

ð1:50Þ Et

O

M'( CO) 5

Cr OEt M( CO) 5 ( CO) 3 M = Cr, W HO

Et

EtO

Et O M( CO) 5

M = Cr, W OEt

14

1. Furans and benzannulated forms

The range of such carbenes may be extended to ethoxy and amino compounds (Eq. 1.51) (13OM5491, 14JOM(752)171, 15JCC2388). Bu n Li [ M( CO) 6 ] Et 3 OBF4

M( CO) 5 O

O

M = Cr , W

O

Cy NH2

M( CO) 5

Cy NH

EtO NH3

dppe M = Cr

EDA

ð1:51Þ

Y= OEt, NHCy

M( CO) 5 M(CO) 5

O

M( CO) 3 ( dppe)

O

NH2

O

NH

Y NH2

Carbene formation is observed for dibenzofuran (Eq. 1.52), but the product enters into further transformations: [3 1 2 1 1] benzannulation with alkynes to generate hydroquinoid naphthobenzofurans, and haptotropic chromium migration resulting in naphthofuran complexes (02JOM(641)185). MeO n

Cr ( CO) 5

Br 2, Bu Li, Cr ( CO) 6 , Me 3OBF4 O

O

!

R

2

R

Cr ( CO) 3

3

R O

OMe 1

2

R 1

Bu n 2O

t

R , Bu Me 2 SiX, Et 3 N 2

t

3

O

R = R = Et, Ph, R = SiMe 2Bu , X = Cl 1

2

1

3

R = R = Ph, R = H, X = Cl 1 n 2 3 t R = Pr , R = H, R = SiMe 2Bu , X = OTf 1

t

2

2

3

R = R = Et, R = SiMe 2Bu 1

2

t

ð1:52Þ

3

R = R = Ph, R = H

3

R = Bu , R = R = H, X = Cl

!

R

2

R

3

R O OMe

( OC) 3 Cr

O

Lithiated benzofuran with tricarbonyl manganese arenes give η5-dienyls (Eq. 1.53) (17JOM218).

15

1.1 Coordination modes 1

R

OMe

R1 n

R2

+

OMe

Bu Li

R2

ð1:53Þ

O

O

R3

R1 R2 3 R R2

OMe Mn( CO) 3

= = = =

R3 R3 H, H,

= H, = H, 1 R = R1 =

R2 R1 2 R R3

= = = =

Me Me Me Me

3

R

OMe Mn( CO) 3

Furan interacts with the tricarbonyl(cyclohexadienyl)iron cation to yield the product of electrophilic substitution, which is η4-coordinated (Eq. 1.54) (74JOM(71)C11). Fe( CO) 3 4

[ ( η - C6 H7 ) Fe( CO) 3 ]

ð1:54Þ

+

O

O

The copper-catalyzed reaction of azibenzil with tricarbonyl(cycloheptatriene)iron gives the η2-coordinated annulated furan (Eq. 1.55) (84JOM(260)105).

N+ N O-

[ ( η3- cy cloheptatr iene) Fe( CO) 3]

ð1:55Þ Fe ( CO) 3

O

2,5-Bis-(trimethylsilylethynyl)-functionalized furan gives rise to the alkynyl halfsandwich (Eq. 1.56) (15OM2826). 5

[ ( η - Cp) M( dppe) Cl] , KOBu

t

M = Fe, Ru

O SiMe3

Me3 Si

ð1:56Þ

O

Cp(dppe)M

M(dppe)Cp

The furan carboxylate ligand forms the η3-furfuryl palladium of the allyl type (Eq. 1.57) (10OM4431, 12OM5599) in the ruthenium-catalyzed 5-hydroxymethylfurfural, coordination of the CH2OH groups plays a role in the catalytic cycle (14D10224). [Pd(PPh3)4],AgBF4

O

O Cl

ð1:57Þ

O

O OMe

Pd ( PPh 3) 2

OMe

BF4

16

1. Furans and benzannulated forms

1-Dibenzofuranyl-3-methylbenzimidazol-2-ylidene forms cyclometalated platinum(II) containing dimesitylacetylacetonate with phosphorescent properties (Eq. 1.58) (15CEJ12881, 16ACR2680). Me N

Me N

Mes

I N

N

t

4

Ag 2 O, [ ( η - cod) PtCl2 ] , Mes( acac) , Bu OK

O

O

ð1:58Þ

Pt O

O Mes

1.1.8 Coordination with ring-opening Aluminum hydrides cause the splitting of the C
O bond and formation of the sixmembered O
Al containing ring (13OM5260). Furan enters into the ring-opening reaction (Eq. 1.59) leading to the η5-oxapentadienyl (83CC813). O H

[(Ph3P)2ReH(CO)] O

ð1:59Þ

(Ph3P)2Re(CO)

The following example combines peripheral and O-coordination, as well as ringopening. Furyl-derived cyclopentadienyls may be coordinated by rare earth elements either via exclusively the π-system of the cyclopentadienyl or form the chelates where the oxygen heteroatom is involved (Eq. 1.60) (03OM775, 07OM3227). However, both types of products undergo the ring-opening to yield dinuclear yne-enolates.

SiMe2

SiMe2 O Si Me2

[ Ln( CH 2SiMe 3) 3 ( THF) x ]

or

Ln ( CH 2 SiMe3 ) 2 THF

O

R R Ln = Sc, Lu, x = 2; Ln = Sc, R = H, Me; Ln = Y, x = 3 Ln = Lu, R = Me

O Ln ( CH 2 SiMe3 ) 2 THF R Ln = Lu, Y, R = H

ð1:60Þ

–SiMe 4 SiMe2 SiMe2 Ln CH 2 SiMe3

+ THF R

O O

Ln

SiMe2 CH 2 SiMe3

– THF

Ln O

( CH 2 SiMe3 ) ( THF) x R R

17

1.2 Reactivity of coordinated furan

1.2 Reactivity of coordinated furan The overall impression of the poor donor ability of furan was changed somewhat when its organoosmium and later organomolybdenum, tungsten, and rhenium compounds were studied. The reactivity of the η2-coordinated species appeared to be so diverse that many synthetic problems of derivatization were successfully solved. We give a detailed account of the transformations of organometallic species without paying attention to the decomplexation reactions and preparation of the corresponding substituted furans. This information can be easily found elsewhere (97CRV1953).

1.2.1 Electrophilic addition For furans, electrophilic attack is directed predominantly to position 2. Only under special conditions does the three orientation become possible. The η2-complexation enhances the nucleophilic character of C3 atom, eventually enabling facile β-electrophilic attacks. In addition, derivatization of furan is possible with some carbene complexes (99JA3065). η2Coordinated furan is the source of interesting derivatizations and transformations of the furan heteroring (00CCR3). Thus protonation in DMF leads to the cleavage of the O
C2 bond to afford 3-oxopropylcarbyne (Eq. 1.61) (94JA5499). Application of the catalytic amounts of the protonating agent in methanol leads to acetal. Electrophilic addition of methylacetonitrilium triflate goes to the β-position of the heteroring and forms iminium derivative of 4-acetylfuran. Benzaldehyde dimethyl acetal or methyl vinyl ketone adds to the position 4 in the presence of the Lewis acid and the process is accompanied by formation of 4H-furanium, and subsequent nucleophilic attack of MeO on C5 (95OM2861, 96JA5672). Hydrogenation goes to the 4,5-positions. Os( NH3 ) 5 (OTf) 2 O H2 / Rh Os( NH3 ) 5 (OTf) 2 O

( MeCNMe) OTf Ph( OMe) 2 BF3 OEt 2 H N

HOTf

OMe

MeOH Os( NH3 ) 5

.

OMe MeO Os( NH3 ) 5 (OTf) 3

MeCOCH= CH2 BF3 MeOH

.

(OTf) 2

ð1:61Þ

O Ph

O MeO

Os( NH3 ) 5 (OTf) 2 Os( NH3 ) 5 (OTf) 2 MeO

MeO

O

O

In contrast, the coordinated 5-methylfuran with benzaldehyde dimethyl acetal generates the ring-opened product (Eq. 1.62).

18

1. Furans and benzannulated forms

Ph

Os( NH3 ) 5 ( OTf) 2 O

PhCH( OMe) 2 BF3 OEt 2

.

OMe

Os( NH3 ) 5 ( OTf) 2 O

ð1:62Þ

OMe

With other aldehydes, the reaction takes a similar course, although in some special cases, cyclization of the initial aldol product gives bicyclic diacetal or ketal (Eq. 1.63). R2 O Os( NH3 ) 5 ( OTf) 2

H R1

R3

O

1

R = H, R = Me, R3 = CCPh, Ph R1 = H, R2 = R3 = Me R1 = R2 = Me, R3 = Ph

Os( NH3 ) 5 ( OTf) 2 1

ð1:63Þ

R3 CHO R3 H

R2

O

R

2

O Os( NH3 ) 5 ( OTf) 2

3

R

O

R1 R2 R1 = R2 = H, R3 = Me R1 = H, R2 = R3 = Me O

Acetone or benzophenone is not incorporated in this type of reaction, but the protonassisted dimerization occurs (Eq. 1.64). Os( NH 3 ) 5

Os( NH 3 ) 5 ( OTf) 2

H+

Os( NH 3 ) 5 ( OTf) 4

O

O

ð1:64Þ

O

One-electron oxidation agents catalyze an intramolecular condensation of the acetyl group and coordinated ammine and formation of the metallacycle annulated to the dihydrofuran ring (Eq. 1.65). O

H N

H

R

O

Os( NH 3 ) 4

DDQ

Os( NH 3 ) 5 ( OTf) 2

R = Me, Ph, CCPh

H R

H N Os( NH 3 ) 4 H R

( OTf) 2 O

( OTf) 3 O

ð1:65Þ

19

1.2 Reactivity of coordinated furan

Activated furan and methyl furans (most often 2-methylfuran) enter a number of original derivatizations (98JA509). Cβ imination with N-methylacetonitrilium triflate (Eq. 1.66): H CMe) OTf, Et 2 O

( MeN 2

NHMe ( OTf) 3

( H 3 N) 5 Os

R = Me

O

3

R

( OTf) 2

( H3 N) 5 Os R 2

R 2 R = 2 R = 2 R =

2

O

5 3

R

H

5

= R = R = H 3 5 Me, R = R = H 5 3 R = Me, R = H 5 3 R = H, R = Me

CMe) OTf, AN

( MeN 3

ð1:66Þ

NHMe ( OTf) 3

( H 3 N) 5 Os

5

R = R = H

2

O

R

H2 O H

O

( H3 N) 5 Os

( OTf) 2 2

O

R

β-Vinylation (Eq. 1.67):

( H 3 N) 5 Os

( OTf) 2 O HOTf

MeOCH= CHCOMe Bu t Me 2SiOTf

O ( OTf) 2

( H 3 N) 5 Os

ð1:67Þ

O OH ( OTf) 3

( H 3 N) 5 Os O

Vicinal difunctionalization for coordinated furan (Eq. 1.68) and 2-methylfuran (Eq. 1.69): MeO PhCH( OMe) 2 BF3

( H 3 N) 5 Os

. OEt 2

Ph ( OTf) 2

( H 3 N) 5 Os O

OMe

ð1:68Þ

( OTf) 2 O

O

CH 2 = CHCOMe BF3

. OEt 2

( OTf) 2

( H 3 N) 5 Os O

OMe

20

1. Furans and benzannulated forms

HOTf Me2 C= C( OSiMe3 ) ( OMe)

( OTf) 2

( H 3 N) 5 Os O

OMe

O ( H 3 N) 5 Os

( OTf) 2

ð1:69Þ

O

HOTf H 2C= C( OSiMe3 ) ( Me)

( H 3 N) 5 Os

( OTf) 2 O

O

Aldol ring closure (Eq. 1.70):

O ( H3 N) 5 Os

( OTf) 2 O

CH2 = CHCOMe BF3

. OEt 2

( OTf) 2

( H3 N) 5 Os

ð1:70Þ

O O

Acid-catalyzed alcoholysis leading to the ring-opened products (Eq. 1.71): MeO OMe

ð1:71Þ

+

( OTf) 2

( H 3 N) 5 Os

H , MeOH

( H 3 N) 5 Os

O

( OTf) 3 OMe

α-Nucleophilic substitution initiated by electrophilic addition (Eq. 1.72): Ph

H PhCH( OMe) 2 BF3

. OEt 2

( H 3 N) 5 Os MeO

OMe ( OTf) 2 O

ð1:72Þ ( H 3 N) 5 Os

( OTf) 2 O O RCHO BF3 . OEt 2 R = Me, Ph

H ( OTf) 2

( H 3 N) 5 Os O

R

21

1.2 Reactivity of coordinated furan

β-Electrophilic addition (Eq. 1.73): ( H 3 N) 5 Os

H + , AN

( OTf) 2

( H 3 N) 5 Os

O

( OTf) 3

MeCN

MeOH

O

ð1:73Þ ( H 3 N) 5 Os

( OTf) 3

Me( ONe) CN H

O

Acid-catalyzed dimerization (Eq. 1.74): ( H 3 N) 5 Os H ( H 3 N) 5 Os

( OTf) 2 O

ð1:74Þ

HOTf or BF3 . OEt 2

( H 3 N) 5 Os

( OTf) 4

O O

And acid
base initiated formation of the carbyne and carbene structures (Eq. 1.75): O ( H3 N) 5 Os

( OTf) 2

HOTf

( H3 N) 5 Os

( OTf) 3

R = H, Me

O

R R

O

i

Pr 2Et N

ð1:75Þ

OMe

( H3 N) 5 Os

( OTf) 2

HOTf

The η2-coordinated furan acquires the properties of vinyl ether, and the preferential direction of the electrophilic addition becomes the position 3 of a heteroring. 3H-furanium systems often formed enter into nucleophilic reactions at α-positions leading to dihydrofurans or ring-opened vinyl ethers as in Eq. (1.76) (01JA8967). N N

HB

N N

N CNBu t CO Re

N N

N H+ , MeOH

HB

N N

O

N N

MeO H MeO t

CNBu CO Re

ð1:76Þ

MeO

1.2.2 Dipolar cycloaddition The η2-coordinated furans of tungsten readily enter into the [3 1 2] dipolar cycloaddition with such dipolarophiles as maleimides and acrylonitrile (Eq. 1.77), the reaction requiring creation of special conditions for the uncoordinated furans (06OM435).

22

1. Furans and benzannulated forms

N N

HB

N N

R1

W O

2

N

R

N

R1

PMe 3 NO

PMe 3 NO 1

HB

2

R = R = H, Me; R1 = Me, R2 = H

N N

O+

W

-

N N

R2

R = Me, Ph R N

O O

N N

HB

N

N N

O HB

N N

O

N

W

1

PMe 3 R NO

N N

R2

N

R

CN

W

N

2

N

R

H

N

H HB O CN

N

2

N

1

PMe 3 R NO

NR

W

ð1:77Þ

N

N

1

PMe 3 R NO

CN

O

O

With aldehydes, dihydrofuran complexes follow (Eq. 1.78) in an 1,3-dipolar cycloaddition.

N

N N

N CO

HB

N N

N

N N

HB

N N

O

N

ð1:78Þ

H

Re

R O

N

R = Ph, C4H3O, Me, n-hexyl, CH2 CH( Me) ( CH2 ) 2 CH= CMe2

N

N CO

HC( O) R, BF3 OEt 2

Re

O

N

Dipolar cycloaddition with TCNE gives coordinated 7-oxabicyclopheptenes with furan (Eq. 1.79) and 2-methylfuran (02JA7395).

N

N

N N

N

L

N

L

HB

N N

HB

Re

N N

N t

N N L

N N N

Re

O

CN CN CN CN

O

L CO

O2

O

ð1:79Þ

L = PMe 3, Bu NC

CO

Re

N

N

N

N N

N TCNE

HB

CO HB

O+

Re

N

N N

N

L

CO –

CO O

HB

N N

Re

H H

N N O

23

1.2 Reactivity of coordinated furan

Dimethylacetylene dicarboxylate enters into the [2 1 2] dipolar cycloaddition with the uncoordinated double bond of 2-methylfuran (Eq. 1.80).

N

N N

N

N N

N CO

HB

N N

COOMe

H

N

ð1:80Þ

CO MeOOCC

Re

CCOOMe

HB

N N

O

N

Re

COOMe

N

N

N

Furan and 2-methylfuran in the coordination sphere of rhenium(I) react with aldehydes to generate the η2-dihydrofurans or furodioxine (Eqs. 1.81 and 1.82) (03OM4966). N N 2MeCHO, BF3 . OEt 2

HB

N N

HB

O O

Re O

N

N N

t

CNBu H CO

H

N

t

CNBu CO

N N

ð1:81Þ

Re O

N

N

N

N MeCHO, BF3 . OEt 2

HB

O t

CNBu H CO

N N

H

Re O

N N

N N

HB

N N N

N N

t

CNBu CO Re O

N

RCHO, BF3 . OEt 2

HB

N N

R = Me, Ph, 3-C4H3O

N

O t

CNBu H CO Re

Me O

ð1:82Þ

R

N

1.2.3 Cyclopentannulation The η2-coordinated 2-methylfuran enters into the cyclopentannulation reactions with 3penten-2-one (Eq. 1.83), methyl vinyl ketone, cyclopentenone, 2,4-hexadienal, methacrolein, and croton aldehyde (03JA14980, 05OM2903).

24

1. Furans and benzannulated forms

N

N

N N

O

N

N

ð1:83Þ

CO

CO O HB

N

N N

N

MeCH=CHCOMe

Re

HB

N N

BF3 OEt 2

N

Re

N N

N

O

Usage of the η2-coordinated 2,5-dimethylfuran in such reactions increases their stereoselectivity. This refers to cyclopentannulation with 3-methylene-2-norbornanone (Eq. 1.84), methyl vinyl ketone, ethyl vinyl ketone, and methacrolein.

N

N N

N

N N

O

N

N

CO HB

N N

CO HB

Re

O

N

N

ð1:84Þ

Re

N N

BF3 OEt 2

O

N

O H

N

1.2.4 Displacement of the η2-furans The η2-furan complexes of molybdenum (04OM3772) are characterized by the displacement of the η2-ligands by carbon dioxide (13OM2505) and forming of the η2-CO2 (Eq. 1.85). Me N

N N

N NO

HB

N N

R= H

O

R

N

CO 2, DME

R

Mo

N Me N

N N

N N

N

N NO

NO HB

N N

O

Mo

+

HB

N N N

N N

O

ð1:85Þ

Me N

N

O

Mo O

25

1.3 Derivatives of furan

1.3 Derivatives of furan 1.3.1 Furyl thiolate and carbothioamides Furyl thiolate is coordinated via the exocyclic sulfur atom (Eq. 1.86) (08ICA2957). [(η5-Cp)Ru(PPh3)2Cl] O

–

S Li

+

dppm or dppe

LL = dppm , dppe

O

SRuCp(PPh3)2

O

ð1:86Þ

SRuCp( LL)

Furan carbothioamides give dinuclear palladacycles (Eqs. 1.87 and 1.88), catalysts for Heck and Suzuki coupling reactions (04OL3337). NR2

O

S Pd Li2 [ PdCl4 ]

S

R = Me, Cy NR2 = NC4 H 8- cy clo, NC5 H 10 - cyclo

O NR2

Cl

Cl

ð1:87Þ

Pd S O

NR2

N

O

S Pd S

Li2 [ PdCl4 ]

Cl

O

Cl Pd S

N O

N

ð1:88Þ

26

1. Furans and benzannulated forms

1.3.2 Furyl amines N-(2-furylmethyl)benzylamines readily afford palladacycles where the Pd atoms connect to the phenyl ring rather than the furyl ring (Eq. 1.89) (14POL30). Cl( PPh 3 ) Pd

R

O

Na 2[ PdCl 4] , NaOAc, PPh 3

N Me

R = H, Me, OMe

R

ð1:89Þ

N Me

O

2-(Benzofuran-3-yl)ethanaminium triflate gives the C,N-cyclopalladated cationic complex, which with sodium chloride gives the dinuclear palladium(II) with chloride bridges and with 4-methylpyridine or tert-butylisocyanide the products of ligand exchange (Eq. 1.90) (18OM4648). The chloride bridges in the dimer are split under triphenylphosphine.

NH 3 OTf

Pd( OAc) 2 . 2HOAc AN

NH 2 O

O NaCl

Pd ( AN) 2

L = 4- MeC5 H 4N, Bu t NC

(OTf)

2L

NH 2 O

Pd

Cl O

NH 2 Cl

O

Pd

Pd L2

(OTf)

ð1:90Þ

NH 2

PPh 3

NH 2 O Ph 3P

Pd Cl

1.3.3 Furyl Schiff bases Furan carboxaldehyde thiosemicarbazone forms bis-chelates with trimethyl aluminum and gallium where the oxygen heteroatom is one of the donor centers (Eq. 1.91) (97OM5522).

O S MMe 3

N O

N H

NHMe

ð1:91Þ

N

Me2 M

N

M = Al, Ga N Me

MMe 2 S

27

1.3 Derivatives of furan

Hydrazone furan-2-yl-(5-hydroxy-3-methyl-5-phenyl-4,5-dihydro-1H-pyrazol-1-yl)methanone is derived from furan-2-carbohydrazide and benzoylacetone. In a solution and in the solid state, it exists in the cyclic form (15JOM223). Coordination to tin is accompanied by ring-opening and double deprotonation so that hydrazone performs the role of a tridentate ligand coordinating via imine nitrogen and enolic oxygens (Eq. 1.92). N

R2 SnCl2

N

R = Me, Ph

O O HO

R2 Sn

O O

ð1:92Þ

O

Ph

N

N

Ph

Rhenium(I) and technetium(I) chelates of nitrofuryl thiosemicarbazones are described by the N(azomethine),S-coordination units (Eq. 1.93) (15D16136). O 2N crystallization

N

Re ( CO)

S

O

O

N

RHN

NO 2

3

N

R = Me, Et, Ph

N N H

S

( OC) 3 Re

S RHN

NHR

N

O

[ Re( CO) 5 Br ]

NO 2

( CO) 3 ( H 2 O) Tc S

[ Tc( CO) 3 ( H2O) 3 ] Cl

Cl

ð1:93Þ

N

S

[ Re( CO) 5 Br ] slow evaporation ( R = Et)

O

N H

RHN

NO 2

( CO) 3 Br Re N O

N H

EtHN

NO 2

Furyl imine ligand is C
H activated by organoiron precursor, and metalation at the ortho-carbon atom with respect to methyl group is accompanied by the released hydrogen atom transfer to the imino carbon atom (Eq. 1.94) (05JOM3886). Azaferracyclopentadiene structure is formed and it is coordinated by tricarbonyl iron. ( CO) 3 Fe NCy O

[Fe2(CO)9]

Fe( CO) 3

NCy

ð1:94Þ

O H

H

Cycloruthenation of furan and benzannulated form is a common feature (Eqs. 1.95
1.97) (12CEJ15178). Interaction of the monocycloruthenated forms with 3hexyne is a coupling of the azomethine groups and alkyne, which leads to a fused hydropyridine unit, which coordinates the ruthenium moiety in an η5-manner. N,Ndimethyl-3-furancarbothioamide can be cyclometalated using Li2PdCl4, K2PtCl4, RuCl2(CO)3, and Rh(Cl)(PBun3)2 to yield [Pd(Cl)(L)], [Pt(Cl)(L)], [Ru(Cl)(L)(CO)2], and

28

1. Furans and benzannulated forms

[RhCl2(L)(Pbun3)2] containing metallacycles with the C2,S-coordination mode (89ICA(157) 9, 90TMC366, 94ICA91). (p-cymene)Cl Ru [(η6-p-cymene)Ru(μ-Cl)]2,Cu(OAc)2

NPh O

NPh O

Et

Et

Et

ð1:95Þ

NPh

Et O

Ru (p-cymene)

(p-cymene)Cl Ru [(η6-p-cymene)Ru(μ-Cl)]2,Cu(OAc)2 NPh

Et

O Et

NPh Et

O

ð1:96Þ

NPh

Et O

Ru (p-cymene)

(p-cymene)Cl (p-cymene)Cl Ru Ru

ð1:97Þ

[(η6-p-cymene)Ru(μ-Cl)]2,Cu(OAc)2 PhN

NPh O

PhN

NPh O

N,N-dimethyl-2 (or -3)-furancarboselenoamide is cyclopalladated by Li2PdCl4 to form a five-membered Pd,Se,C-metallacycle (89POL2517, 91POL2265). Furan-containing Schiff bases coordinate PtMe2 in an N,N-mode, but in stringent conditions they may form bischelates where α-carbon of the heteroring participates (Eq. 1.98). Such a bis-chelate withstands oxidative addition of methyl iodide. Remarkably, attempt to coordinate additional phosphine ligands leads to exclusion of the N,N-donor function and coordination of the platinum moiety via the C2-atom (04JOM1496).

29

1.3 Derivatives of furan

NMe2

N

N

NMe2

[ Me2 Pt( μ- SMe 2) 2 PtMe 2 ]

Pt Me2

O

O

N MeI

Pt Me2 I

O N Δ

NMe2

ð1:98Þ

NMe2 Pt Me

O

NMe2

N PPh3 O

Pt(PPh3)2Me

Furyl thiosemicarbazones form the C,N,S-coordinated tetranuclear complexes, which are converted to the mononuclear by phosphines and diphosphines (Eq. 1.99) (07ZAAC1875). Bis(diphenylphosphino)methane serves as a bridge between bis-chelate platinum and tungsten in the resultant heterodinuclear arrangement. O

O

cis-[(η4-cod)PtMe2]

N NHR 4

NHR

HN

Ph2PCR2PPh2 S

PPh3 Pt

S

R = Me, Et

N

O

PPh3

Pt

Ph 2 PCX2 PPh 2

O Pt

S

N

S

NHR

NHR

ð1:99Þ

Ph 2 PCH 2 PPh 2

O Pt

[ W( CO) 5 THF] X= H

N

[W(CO)5(dppm)]

X= H X2 = CH 2

S

W(CO)6

N NHR

1.3.4 Furyl phosphines Furyl ligand containing phosphino and Schiff base functionalities is coordinated by tungsten pentacarbonyl via the phosphorus atom (Eq. 1.100) (95T11271). Alkylation of the product and acid hydrolysis proceed via the hydrazone moiety.

30

1. Furans and benzannulated forms

[ W( CO) 5 ( PhNH 2 ) ] O

Ph 2P

Me2 SO4 Ph 2P

N- NMe2

O

W( CO) 5

O

Ph 2P N- NMe2

N- NMe3 MeSO 4

W( CO) 5

ð1:100Þ

H 3 O+

O

Ph 2P

O

W( CO) 5

Tri(2-furyl)phosphine gives rise to chromium and tungsten mono- and multiethoxycarbenes of the Fischer type (Eq. 1.101) (15OM696). O O

P O EtO

M( CO) 5 O O P

O

M( CO) 5

O

P

EtO

n

Bu Li, [ M( CO) 6 ] , Et 3 OBF4

O

ð1:101Þ

M = Cr , W O

EtO OEt

M( CO) 5

(OC) 5 M O P

M( CO) 5

O EtO

O EtO M( CO) 5

Furyl-2-phosphines are popular ligands in transition metal-mediated organic synthesis (01CRV997, 14EJI6126). Tri(2-furyl)phosphine apart from the dinuclear complexes of the Pcoordinated ligand forms the mononuclear product with the C-coordinated furan, which has been eliminated from the starting ligand (Eq. 1.102) (12ICA199). Tri(2-furylphosphine) (L) and [Re2(CO)9(AN)] give the P-coordinated [Re2(CO)9(L)] and [Re2(CO)9L2] (09JOM2941). With [Re2(CO)10], along with these products [(OC)4Re(μ-P(C4H3O)2)(μ-H)Re (CO)4], [(OC)3LRe(μ-P(C4H3O)2)(μ-H)Re(CO)4], [(OC)3LRe(μ-P(C4H3O)2)(μ-H)Re(CO)3L], and [(OC)3LRe(μ-P(C4H3O)2)(μ-Cl)Re(CO)3L] are formed. The products with the hydride bridge contain the rhenium
rhenium bond. In chlorobenzene, an additional product (Eq. 1.103) contains two terminal η1(C)-furanyl moieties.

31

1.3 Derivatives of furan

O [ Mn 2 ( CO) 1 0] O

3

ð1:102Þ

Mn 2( CO) 8L2 + MnL2( CO) 3

P

L O O P [ Re 2 ( CO) 1 0] O

3

( OC) 3 Re

O

ð1:103Þ

Re( CO) 3 P

P

O

O

O

Tri(2-furyl)phosphine with [Fe(CO)5] forms a dinuclear product with two P-coordinated di(2-furyl)phosphine groups and one terminal parent ligand (95JOM91). Tri(2-furylphosphine) with [Fe2(CO)6] splits one of the P
C bonds and forms μ-η1:η2 dinuclear complexes (Eq. 1.104) (13JOM123).

[ Fe 3( CO) 12 ] O

O

O

( OC) 3 Fe

Fe( CO) 3 + ( OC) 3 Fe

P

ð1:104Þ

P ( C4 H3 O) 2

P ( C4 H3 O) 2

3

Fe( CO) 2 ( P( C4 H3O) 3 )

The reaction of the Fe2(CO)6 product with phosphine is a simple CO/PPh3 substitution, whereas with diphosphines carbonyl substitution is accompanied by CO-insertion and formation of the coordinated furyl acyl, where diphosphine is bridging (dppm) or chelating (dppn) (Eq. 1.105) (14JOM(751)326, 14JOM(751)399). O

O Fe 3( CO) 12 O

(OC) 3 Fe

P

Fe(CO)3

PPh 3

( Ph 3 P)( OC) 2 Fe

P ( C4 H3 O) 2

P ( C4 H3 O) 2

3

dppm

dppn

O

ð1:105Þ

O O

( OC) 2 Fe

O Fe(CO)2

P ( C4 H3 O) 2 Ph 2P

Fe(CO)3

( OC) 3 Fe P ( C4 H3 O) 2

PPh 2

CO Fe

PPh 2

P Ph 2

Along with the diadduct [(η1(P)-(L)2Ru3(CO)10], tri(2-furylphosphine) with [Ru3(CO)12] gives the dinuclear product of oxidative addition (Eq. 1.106), in which there

32

1. Furans and benzannulated forms

is μ-η2(CC):η1(C)-coordinated furyl (01D2981). With alkynes, the coupling products follow with allyl σ- and π-bonding as well as η2-bonded furyl ring (08JCL231). Protonation of the dinuclear products leads to the appearance of the bridging hydride, heating of the products affords the trinuclear cluster (08JOM1645). O

O O

P [ Ru 3 ( CO) 1 2 ] O

3

( OC) 3 Ru

O

P HC

Ru(CO)3

CR

( OC) 3 Ru R

P

Ru( CO) 3 H

O R = Ph, p-Tol, p-C6H4NO2, (C4H2S)C

H

CH, ( C4 H2S) 2 C

CH

Δ

H CF3 COOH

O

ð1:106Þ

O O

P

( OC) 3 Ru

( OC) 3 Ru

( OC) 3 Ru

O H Ru( CO) 3 CF3 COO

P

O

P

O

O

Ru( CO) 3 O O

O

There is a variety of reactivity modes with chelating phosphines (Eq. 1.107) (02EJI2103). It may be a simple substitution reaction, orthometalation of the phenyl group of the entering ligand with elimination of the coordinated furyl moiety, cyclometalation of both ends of the entering ligand, which becomes a bridge in a tetranuclear structure, with retention of the furyl group, and formation of the polymeric products, which is not shown. O P

Ph2P(E)PPh2 E = CH2, NH, NMe

( OC) 2 Ru

O Ru( CO) 2

O

Ph 2P

PPh 2 E

O

Ph2P(CH2)nPPh2

O P ( OC) 3 Ru

O

n = 2, 3

P ( OC) 3 Ru

O ( CO) 2 Ru PPh 2 ( CH2 ) n

C6 H4P( Ph) Ru( CO) 3

ð1:107Þ

O O

O

Ph2P(CH2)nPPh2

P ( OC) 3 Ru

n = 4, 5

O ( CO) 2 Ph 2 RuP( CH2 ) n PRu Ph 2 ( CO) 2 O

Ru( CO) 3 P

O O O O P dppf

( OC) 3 Ru

O ( CO) 2 RuP Ph 2

Fe

Ph 2 PRu ( CO) 2 O

Ru( CO) 3 P

O O

33

1.3 Derivatives of furan

The first result of interaction of tri(2-furyl) phosphine is a simple P-coordinated product (08D6219). Thermolysis followed by decarbonylation leads to the μ3-η2 furyne ligand formed as a result of cleavage of both C
P and C
H bonds (Eq. 1.108). Thermolysis of furyne species involves P
C bond activation, elimination of furan, and formation of the μ3-phosphinidene cluster. The reaction with triphenylphosphine is a simple CO/PPh3 ligand substitution. Hydrogen bromide adds oxidatively accompanied by the formation of the Fischer carbene. O

O O ( CO) 2 Ru

P ( Ph 3 P) ( OC) 2 Ru

O ( CO) 2 Ru

P ( Br ) ( OC) 2Ru

PPh2

Ru ( CO) 2

PPh2

Br H

H

Ru ( CO) 2

PPh2

PPh2

O

O PPh3

HBr O O ( CO) 2 Ru

P [ ( Ru 3( CO) 10 ) 2 ( μ- dppm ) ] , Δ O

3

( OC) 3 Ru

Me3 NO

P

PPh2

ð1:108Þ

H Ru ( CO) 2

PPh2

O Δ

P ( OC) 3 Ru

O ( CO) 2 Ru

Ru ( CO) 2

PPh2

PPh2

O

Diphenyl ditellurium oxidatively adds to the furyne cluster forming along with the product of elimination of the trifuryl phosphine, the furanyl cluster accompanied by the C
H bond formation, and on thermolysis clusters containing unsymmetrical furynes (Eq. 1.109) (11JOM1982). O

O O ( CO) 2 Ru

P ( OC) 3 Ru

PPh2

( OC) 2 Ru

H Ru ( CO) 2

Te Ph

PPh2

O Ph 2 P

Ru CO

PPh2 PPh2

O

O

Δ

O ( CO) 2 Ru

P PhTeTePh

P

CO Ru Te Ph

P Ph 2

O

ð1:109Þ

O O ( CO) 2 Ru

Ru ( CO) 2

Ph 2 P Δ TePh H

P

CO Ru Te Ph

P Ph 2

O

O ( CO) 2 Ru

Ru CO

TePh H P

O

3

34

1. Furans and benzannulated forms

Ruthenium orthometalated cluster containing diphosphine causes the carbon
phosphorus bond cleavage of tri(2-furyl)phosphine and coordination of the dissociated furyl fragment in a σ,π-vinyl manner (Eq. 1.110) (09JOM3312).

PhP ( OC) 3 Ru

[Ru3(CO)9(μ3-η1,η1,η2-PhP(C6H4)CH2PPh)] O

3

PhP

Ru O ( CO) 2

P

P

Ru( CO) 2

ð1:110Þ

O

O

Tri(2-furyl)phosphine with [Ru3(CO)10(μ-dppf)] forms P-coordinated [Ru3(CO)9(μ-dppf) (L)] (14JOM(760)231). In contrast to the above dppm-complexes, cycloruthenated μ3-ligand is formed as a result of thermolysis (Eq. 1.111). O

[ Ru 3 ( CO) 9 ( μ- dppf) ( η1( P) - ( ( C4H3 O) 3P) ]

( OC4 H3) 2P ( CO) 3 Ru Ph 2P

Δ

Fc

Ru( CO) 3

ð1:111Þ

Ru ( CO) 3

P Ph 2 H

With [Os3(CO)10(μ-H)2] at room temperature, a simple adduct is formed, whereas at elevated temperatures the product of orthometalation of the furan ring follows (Eq. 1.112) (11JOM607). More reactions, including those leading to the loss of coordinated heterocycle are known (12OM2546). RT

[Os3(CO)10(μ-H)(H)(L)] O

[ Os3 ( CO) 1 0( μ- H) 2 ]

L

P 3

Δ

( OC) 3 Os ( OC) 3 Os

ð1:112Þ

O

P O

O H Os( CO) 3

Tri(2-furyl)phosphine reveals rich reactivity (Eq. 1.113) with tetraruthenium cluster giving away simple substitution products, the μ-phosphido, μ3- and μ4-phosphinidene, furyl and furyne mixed ligand clusters (03OM5100, 08OIC50). Furfuryl-2-(N-diphenylphosphino) methylamine is coordinated with respect to ruthenium-cymene via the terminal phosphorus atom (12POL142). The P-coordinated tri(2-furylphosphine) of the [(η5-Cp)Ru(η4-cod)Cl] is applied as a catalyst for cycloisomerization
oxidation (99JA11680).

35

1.3 Derivatives of furan

H

[ Ru 4 ( CO) 1 2 ( μ- H) 4] O

[ Ru 4 ( CO) 1 0 L2( μ- H) 4 ] +

( OC) 2 Ru

P

3

L

( CO) 2 L Ru H

O

CO Ru ( CO) L Ru ( CO) 2 P

O O

+ ( OC) 2 LRu

O

O

P

P

ð1:113Þ ( CO) 3

( OC) 2

Ru( CO) 2L +

Ru

Ru Ru( CO) 2L

OC

H

( OC) 2 Ru

H H

Ru

H

Ru ( CO) 2 L

( CO) 2 L

O

Formation of bis O,P-chelate by the diphosphino derivative of benzofuran is shown in Eq. (1.114) (11OM2468). [ OsCl 2( DMSO) 4 ] , H2, PhCH2 OH O

i

O

i

PPr i2

Pr 2P

i

Pr 2P

ð1:114Þ

PPr 2 Os H( CO) Ph

2,3-Bis(diphenylphosphino)maleic anhydride coordinates the orgacobalt ethynyl precursor initially via phosphorus atoms, but later one of the phosphine moieties nucleophilically adds to the terminal carbon of the alkyne moiety, and zwitterion results (Eq. 1.115) (94OM3767, 94OM3788, 96JOM(516)65, 07POL3737). In the (OC)2Co(μ-CO)2Co(CO)2, cluster of 4,6-bis(diphenylphosphino)dibenzofuran is P,P-coordinated (96JOM(520)249). H

Ph

( OC) 2 Co Ph 2P

Ph 2P

PPh 2 [ Co 2 ( CO) 6 ( μ- PhC

O

O

Co( CO) 2

Δ

CH) ] O

O H ( OC) 2

Ph

Ph 2P Co P Ph 2

O O O

PPh 2

Co( CO) 2

O

O

ð1:115Þ

36

1. Furans and benzannulated forms

Phosphinimine dibenzofuran forms the O,N-chelate, which is a route to the catalysts of ring-opening polymerization (Eq. 1.116) (11IC8063).

B( C6 F5 ) 4

O

ZnEt 2 , PhBr i

B( C6 F5 ) 4

O

Ar = 2,6- Pr 2 C6 H3, Mes EtZn

P( Ph) Me

P( Ph) Me

ð1:116Þ

N H( Ar )

H( Ar ) N

1.3.5 Mixed heterocycles 2-Furyl-substituted aminopyridine along with purely N-coordinated complex forms the cyclometalated C,N,N-pincer (Eq. 1.117) (12CEJ671). Zr ( NMe2 ) 4

O

O

Δ

N

N

i

N( H) ( 2,6- Pr i2 C6 H3)

N( H) ( 2,6- Pr 2 C6 H3)

ð1:117Þ

Zr ( NMe2 ) 2 ( NMe2 H)

2-(2-Furyl)-1,8-naphthyridine, [(η6-arene)Ru(μ-Cl)Cl]2, and ammonium hexafluorophosphate give cationic products with an unusual for a furyl heteroring N,O-coordination (Eq. 1.118) (08JOM3049). (arene)Cl Ru O N

O

N

N

6

[ ( η - ar ene) Ru( μ- Cl) Cl] 2 , NH4PF6

ð1:118Þ

N

PF6

arene = benzene, p-cymene

The product of interaction of 2-(2,5-dimethyl-3-furyl)-1,8-naphthyridine and diruthenium carbonyl is different (Eq. 1.119) (06OM6054, 14MI1). Two ligands participate in coordination. One of them is classically cyclometalated, while another is N,N-coordinated and participates in agostic interactions.

N

N

N

O

N

H

[ Ru 2 ( CO) 4 ( AN) 6] ( BF4 ) 2 O

Ru Ru ( CO) 2 ( CO) 2 N

N

O BF4

ð1:119Þ

37

1.3 Derivatives of furan

Homoleptic cyclometalated iridium(III) of dibenzofuryl-appended imidazol-2-ylidene prepared in a traditional way exhibits blue phosphorescence (Eq. 1.120) (16ICC26). Et N

Et N N O

1. Ir Cl 3 2. Hacac 3. L

N O

ð1:120Þ

Ir

3 L

2-(2-Pyridyl)benzofuran can be cyclorhodated (Eq. 1.121) and cyclopalladated (Eq. 1.122) (98POL533).

[ RhCl 3( PBu

N

n

Cl2 ( PBu Rh 3 ) 2]

O

n

3) 2

ð1:121Þ

N O ( NO 2 ) ( AN) Pd

Pd( OAc) 2 , O 2, AN

N O

ð1:122Þ

N O

N-dibenzofuranyl-N0 -methylimidazolium chloride gives iridium(III) tris-cyclometalated carbene, a blue-emitting substance, component of valuable materials (Eq. 1.123) (10AM5003).

O

Cl

[(η4-cod)Ir(μ-Cl)]2

Ir

O

N

N

N Me

N Me

ð1:123Þ

3

Heteroleptic cyclometalated platinum(II) complexes are based on a 4- and 5substituted 2-(dibenzo[b,d]furan-4-yl)pyridines and acetylacetonate (Eq. 1.124) and exhibit green (parent, fluoro-, and methyl-substituted) and yellow (trifluoromethylsubstituted) photo- and electroluminescence (16DP165).

38

1. Furans and benzannulated forms X

N O N

Hacac Na 2CO 3

O

K2 [ PtCl4 ]

Pt

O

X = H, 4( 5) - F, 4( 5 ) - Me, 4( 5) - CF3

N

Cl

ð1:124Þ X

X O Pt

O N

O

X

Cyclometalated platinum(II) bezofuryl-pyridine containing various ancillary ligands are halogen-bond acceptors with respect to iodofluorobenzenes, which improves their photophysical performance (Eq. 1.125) (18CEJ11475). O

O

O [ Pt( DMSO) Cl 2]

Pt(DMSO)Cl

PPh 3

Pt( PPh 3) Cl N

N

N

ð1:125Þ

AgNO 3 AN O Pt( PPh 3) CN

PPh 3 AN NaCN

O Pt( AN) 2

N

Cl

N

6-(2-Furyl)-2,20 -bipyridine and K2[PtCl4] form cyclometalated [(η3(N,N,C)-L)Pt(Cl)] (04JA4958). Under MCCR (M 5 H, Cu), CuI, and Pri2NH, the products transform into light-emitting acetylides [(η3(N,N,C)-L)Pt(CCR)] (R 5 Ph, p-Tol, C6F5). 6-Furylpurine is C-metalated with Pt21, Pd21, and Hg21, for example, Eq. (1.126) (15IC4183) binding the metal ions in a bidentate manner, involving the pyridine type nitrogen of the fivemembered counterpart of purine and one of the furyl carbon atoms. O

4

[ ( η - cod) PtCl 2] AgClO4 N

N N

N Me

O Pt( cod) ClO4 N

N N

N Me

ð1:126Þ

39

1.3 Derivatives of furan

Cyclometalated C,C-coordinated platinum(II) contains a combination of imidazol-2ylidene and benzofuran heterocycles (Eq. 1.127) (10AGE10214). It is characterized by emission in the green
blue part of the visible spectrum. Analogs with substituted acetylacetonates (CF3, But, Mes, Ph instead of the methyl group) supplement the range (14EJI256). O

4

Ag 2 O, [ ( η - cod) PtCl2 ] KOBu

O

N

Pt

t

O

N

ð1:127Þ

O

N N

Green-phosphorescent (dibenzo[b,d]furan-4-yl)-pyridinato-N,C30 platinum(II) 1,3-diketonates contain 1,3-bis(3,4-dibutoxyphenyl)-propane-1,3-dionate and dipivaloylmethanate (Eq. 1.128) (10JL217, 13JPC(C)532). R

K2 [ PtCl4 ] , RC( O) CH 2C( O) R Et OCH 2 CH 2 OH, Ag 2O R = 3 ,4 - ( Bu n O) 2C 6H 3 , Bu t

O N

O

ð1:128Þ

Pt

O N

O R

Efficient luminescent compounds can be obtained by a more facile synthetic procedure (Eq. 1.129) (12OL1700). [ PtMe2 ( SMe2 ) ] 2 HOTf Na( acac)

O

O

ð1:129Þ

N

N O

Pt

O

Platinum(II) heteroleptic complexes containing 2-dibenzofuranylpyridine cyclometalating and 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dione ancillary ligand (Eq. 1.130) possess pure red electroluminescence and serve as components of the polymer-based light-emitting diodes (10SM615). OBu

n

OBu Cl

O Pt N

O

O

HO

N OBu

OBu

n

OBu

n

O

O Pt N

Na 2CO 3

+

Pt Cl

n

O OBu OBu

n

n

OBu

n

n

ð1:130Þ

40

1. Furans and benzannulated forms

2-(Dibenzofuran-4-yl)pyridine (Eq. 1.131) and 2-(dibenzofuran-2-yl)-4-(dimethylamino) pyridine (Eq. 1.132) are cyclometalating and 5,50 -(1-methylethylidene)bis(3-trifluoromethyl)-1H-pyrazole is ancillary ligand in the gold(III) complexes, components of OLEDs of improved thermal stability (19CEJ3627). N

N

microwave or AgBF4

AuCl3 Na[ AuCl4 ] . H2O

O

O

CF3 N

ð1:131Þ

HN CF3 HN N

N N

N

N

CF3

AuCl2

Au N

O

O

N CF3

Me2 N

Me2 N

N

N

AuCl3 Na[ AuCl4 ] . H 2O

microwave or AgBF4

O

O CF3 N

ð1:132Þ

HN CF3

HN Me2 N

Me2 N

N

N N AuCl2

N

CF3

N Au N N

O

O

CF3

References

41

1.4 Conclusion 1. Organometallic compounds of furan and derivatives are scarce, and their preparation and studies require special efforts and conditions. A few coordination modes have been studied. a. η1(C) coordination of furan is mostly realized for the nontransition, late transition metals, or lanthanides. As a rule, it involves oxidative addition accompanied by C
H activation. Sometimes it is revealed in combination with the η2(CC) mode. b. η2(CC) coordination via the double bond in the heteroring may be autonomous or occur in combination with the other η2(CC) mode in dinuclear structures. c. η5(C4O) mode is a rarity whereas η6(C6) coordination in benzannulated furans occurs more often. Also η1(O) and η4(C4) modes are scarce. d. Cases when furan ring is not involved in coordination include furyl substituted indenyl and indene, Fischer carbenes, arenes, side alkynyl, or allyl groups. e. Ring-opening may be accompanied by peripheral and η1(O) coordination. 2. Reactivity of coordinated furans has been tested mainly for the η2 mode. a. Electrophilic attack occurs preferentially at the position 3 or 4 of the heteroring, although 4,5-hydrogenation is not precluded. The process may be complicated by ring-opening and cyclization as well as carbene formation. b. η2 coordination is a condition for facile [1,3]-dipolar cycloaddition as well as stereospecific cyclopentannulation and displacement. 3. Additional opportunities are open for the organometallic compounds of functionally substituted furans. a. Furyl thiol, amines, and imines often coordinate via the side functional groups, although heterocycle may also contribute as η1(C) or even as η1(O). b. Furyl phosphines reveal a diversity of coordination situations starting from η1(P) mode and including η1(C) function for the heteroring; splitting of the ligand when one part is P-coordinated and another (furan ring) acquiring η1(C) or η2(CC) modes, or forming the products of insertion. The η3(POP) mode for a pincer is unique. c. Mixed heterocycles in the majority of cases involve the C
H activated furyl ring, although a unique case of O-coordination in combination should be noted.

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1. Furans and benzannulated forms

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44 07POL3737 07ZAAC1875 08D6219 08ICA2957 08JCL231 08JOM1645 08JOM3049 08JOM3375 08OIC50 08OM3614 09JOM2941 09JOM3312 09OL1083 09OM741 09OM6079 10AGE10214 10AM5003 10JA7645 10JL217 10OM38 10OM4431 10SM615 11IC8063 11JOM607 11JOM1982 11OM2468 12CEJ671 12ICA199 12CEJ15178 12JOM163 12OL1700 12OM2546 12OM5599 12POL142 13CRV3084

1. Furans and benzannulated forms

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References

13JA15830 13JOM123 13JPC(C)532 13OM1797 13OM2505 13OM5260 13OM5491 14D10224 14EJI256 14EJI6126 14JOM(751)326 14JOM(751)399 14JOM(752)171 14JOM(760)231

14MI1 14POL30 15CEJ12881 15D16136 15IC4183 15JCC2388 15JOM223 15OM696 15OM2826 16ACR2680 16DP165 16ICC26 16OM2327 17D8279 17JOM218 17OM2565 18CC3464 18CEJ11475 18OM4648 19CEJ3627

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