PALLADIUM(II) IODIDE 1

Palladium(II) Iodide

PdI2

[7790-38-7] PdI2 (MW 360.23)InChI = 1/2HI.Pd/h2*1H;/q;;+2/p-2/f2I.Pd/h2*1h;/q2*-1;mInChIKey = HNNUTDROYPGBMR-YQUJXAAFCB

(reagent used as catalyst in C–C and C-heteroatom bond formationreactions)

Alternative Name: palladium diiodide or palladous iodide.Physical Data: mp 350 ◦C (decomposes); d 6.0 g cm−3.Solubility: insoluble in H2O and organic solvents such as EtOH,

Et2O, insoluble in diluted nitric acid and hydrochloric acid;slightly soluble in hydriodic acid and hot concentrated nitricacid; soluble in ammonia and methyl acetate. Easily solublein a solution (H2O or polar or coordinating organic solvents,such as DMF, DMA, DMSO, DME, 1,4-dioxane) containingan excess of potassium iodide (owing to the formation of thecomplex PdI4

2−).Form Supplied in: black powder; readily available.Preparative Methods: the compound is prepared by the addition

of potassium iodide to a solution of a palladous salt in slightexcess. When the salt is dried in air, the monohydrate,PdI2·H2O, is formed. The anhydrous compound is obtainedin vacuo or in oven at 120 ◦C overnight.

Handling, Storage, and Precautions: eyes, lungs, and skin irri-tant. Light sensitive.

Oxidative Carbonylation Reactions. PdI2 can be solubilizedin water as well as in highly polar and/or coordinating organicsolvents (such as MeOH, DMF, DMA, acetonitrile, DME, 1,4-dioxane) in the presence of an excess of iodide anions. For exam-ple, PdI2 in conjunction with 10 equiv of KI (or more) is readilysoluble in the above-cited solvents even at room temperature andin the absence of additional ligands. Such systems (correspondingto the formation of the PdI4

2− anion, stabilized by the additionaliodide ligands) are superior catalysts for realizing a variety ofoxidative carbonylation processes using oxygen as the oxidant(eq 1, anionic iodide ligands are omitted for clarity).1–3

SH2 + x CO + 1/2 O2 S(CO)x + H2OPdI2 cat.

(1)

The reason for this peculiar activity of the PdI2-KI system inoxidative carbonylation reactions lies in the very efficient reoxi-dation of the Pd0 species ensuing from substrate carbonylation tothe catalytically active PdII species, which occurs through oxida-tion of HI (also derived from substrate carbonylation) to iodinefollowed by oxidative addition of the latter to Pd0 (eq 2; in thisequation and in all subsequent ones, unreactive ligands are omit-ted for clarity). Accordingly, PdBr2-KBr and PdCl2-KCl systemsare unreactive under the same reaction conditions. The excess ofiodide anions not only favors the dissolution of PdI2 in the reac-tion medium, but also increases the catalyst activity and productselectivity. This effect can be connected with the stabilizing effectof iodide anions on palladium intermediates, including Pd0, with

the possible formation of PdInn− species, which are more suscep-tible to the oxidative addition of I2 (itself mostly be present in theanionic form I3

−).

PdI2

SH2 + x CO + PdI2 S(CO)x + Pd0 + 2 HI

2 HI + 1/2 O2 I2 + H2O

Pd0 + I2

(2)

A variety of substrates undergo the PdI2-KI-catalyzed oxida-tive carbonylation, allowing the one-step and atom-economicalsynthesis of important carbonyl derivatives under mild conditionsand with high catalytic efficiency. Depending on reaction con-ditions, terminal alkynes can selectively lead to maleic diesters(eq 3, R′ = alkyl),4,5 maleic acids (eq 3, R′ = H),5,6 or maleicanhydrides (eq 4).6 Thus, maleic diesters are selectively obtainedworking in alcoholic solvents at 25–80 ◦C under 15–25 atm ofCO and 4–9 atm of air (as the source of oxygen), in the pres-ence of 0.025–0.500 mol % of PdI2 in conjunction with 10 equivof KI. Under analogous conditions (20 atm of a 3:1 mixture ofCO-air at 80 ◦C, in the presence of 0.05 mol % of PdI2 + 10 KI),but using a 3:1 mixture of DME:H2O as the solvent, maleic acidsare formed. On the other hand, maleic anhydrides are obtainedworking in aqueous dioxane at 60–80 ◦C under 60 atm of a4:1:10 mixture of CO-air-CO2, in the presence of 0.5–1 mol % ofPdI2 + 10 KI. In this latter case, the presence of added CO2 in thereaction mixture is important to curtail undesired side reactionsassociated with the formation of palladium hydride species, whoseconcentration is indeed lowered in the presence of CO2, which isable to temporarily insert into the Pd–H bond.

RC CH

R′O2C CO2R′

R+ 2 CO + 2 R′OH + 1/2 O2

PdI2 cat

− H2O

20–89%

(3)

R = alkyl, aryl; R' = H, alkyl

RC CHOO O

R

+ 2 CO + 1/2 O2PdI2 cat (4)

44–90%

The PdI2/KI-catalyzed oxidative dicarbonylation reaction ofthe triple bond has been named “the Gabriele dicarbonylationreaction” and has been successfully employed for the prepara-tion of intermediates useful as precursors for the synthesis of thecarbocyclic core of CP-263,114 (phomoidridre B) (eqs 5 and 6).7,8

AcO

O CO, O2, MeOH

AcO

OCO2Me

CO2MePdI2 cat

60 °C

89%

(5)

CO, O2, MeOH110 °C

CO2Me

O O

MeO2C

CO2Me

CO2Me

PdI2 cat

66%

(6)

Avoid Skin Contact with All Reagents

2 PALLADIUM(II) IODIDE

In the presence of the PdI2-KI catalytic system under theappropriate conditions, alkynes bearing suitably placed nucleo-philic groups afford functionalized heterocycles by an oxidativecyclocarbonylation-alkoxycarbonylation process (with incorpora-tion of CO into the cycle) or by oxidative cyclization-alkoxycar-bonylation (without incorporation of CO into the cycle), as, forexample, shown in eqs 7–9 and 10–12, respectively.9−23 In somecases, increasing the excess of KI with respect to PdI2 (from 10to 50–500) has proved useful either for curtailing undesired sidereactions (occurring through mechanisms involving eliminationof iodide anions) or, in the case of basic substrates, to enhance theconcentration of the free HI necessary for Pd0 reoxidation.

HY

R2

R12 CO R3OH O2

YO

CO2R3R2

R1

−H2O+ + + 1/2

Y = O, NR

PdI2 cat

40−80%

(7)

OHR1

R2 R3

2 CO O2 −H2O

OO

CO2R4

R1

R2 R3

+ + R4OH + 1/2PdI2 cat

73−98%

(8)

R2

R1

NHR

2 CO R3OH O2−H2O

R2

R1

NO

R3O2C

R

+ + + 1/2PdI2 cat

42–58%

(9)

YHR4

R1

R2 R3

CO R5OH O2−H2O

Y

R3R2

R1

CO2R5

R4

+ + + 1/2PdI2 cat

Y = O, NR

45−75%

(10)

Interestingly, a peculiar promoting effect by an additional pres-sure of CO2 has been observed in the case of the carbonyla-tion of (Z)-(2-en-4-ynyl)amines to pyrrole-2-acetic esters (eq 10,

Y = NR).19 Indeed, CO2 acts by buffering the basicity of the sub-strate (thus allowing a higher concentration of HI necessary forPd0 reoxidation) without hampering its nucleophilicity, since CO2

can be eliminated during the cyclization process. On the otherhand, prop-2-ynyl carbamates, formed in situ by carboxylationof prop-2-ynylamines, undergo cyclization with incorportation ofCO2, followed by alkoxycarbonylation.12,15 This reaction (eq 13)allows a one-step synthesis of functionalized oxazolidin-2-ones bysequential activation of several simple molecules and representsthe only example known so far for the catalytic incorporation ofboth carbon oxides into an organic substrate.

N

OR1

R2R3H

CO R4OH O2−H2O

O

N

R1

R2

R3

CO2R5

+ + + 1/2PdI2 cat

50−77%

R1 = alkyl, aryl, NHR, COR (11)

R1

R2

OH

R3

CO R4OH O2−H2O

R1 = H, alkyl, aryl

R2 = H, alkyl, alkoxy

R3 = H or alkyl

O

C(R1)CO2R4

R2 R3

O

CO2R4

R2 R3

R1

+ + + 1/2PdI2 cat

(12)+

up to 48% up to 70%

RHN

R1

R2 CO2 N

R1

R2R

OOH+

N

O CHCO2R′O

R1

R2R

PdI2 cat

CO, R′OH, O2

83–89%

(13)

In the case of dipropynyl sulfide, dialkoxycarbonylativecyclization without incorporation of CO into the cycle occurs, withformation of stereoisomeric heterocyclic compounds that are eas-ily aromatized into the corresponding thiophenediacetic derivative(eq 14).24

A list of General Abbreviations appears on the front Endpapers

PALLADIUM(II) IODIDE 3

The PdI2-KI system is also able to catalyze the oxidative mono-aminocarbonylation of the triple bond, with selective formationof 2-ynamides starting from terminal alkynes and dialkylamines(eq 15).25

Suitably functionalized alkynes afford heterocyclic deriva-tives under these conditions, through oxidative monoaminocar-bonylation followed by in situ conjugate addition, as shown byeqs 16 and 17.26–28

SCO, O2, ROH

S

MeO2C CO2Me

Et3N

S

MeO2C CO2Me

PdI2 cat

40% overall

(14)

RC CH CO−H2O

RNR′2

O+ R′2NH + 1/2 O2

PdI2 cat+

R = alkyl, aryl; R = alkyl 36−73%

(15)

′

OHR1

R2

R3 R4

R5

OHR1

R2

R3 R4

R5

R2NO O

R3 R4

R5

R2

R1 NR2

O

PdI2 cat

CO, R2NH, O2

64–79%

(16)

R1

HY

R2

YH O

NR2

R2N

R2

R1−R2NH

HY

R2

R1

NR2

O

Y O

R2N

R2

R1

R′2NHPdI2 cat

CO, R2NH, O2

57–85%

(17)

Y= O, NR′

On the other hand, primary amines readily afford symmetricallydisubstituted ureas (eq 18), through the intermediate formation ofan isocyanate, while a mixture of a primary and a secondary amineselectively gives trisubstituted ureas, through trapping of theisocyanate by the secondary amine (eq 19).29,30 In several cases,working in the presence of an excess of CO2 under pressure hasa beneficial effect on both substrate reactivity and product select-ivity, owing to the “buffering” effect exerted by CO2 on substratebasicity (allowing a higher concentration in solution of free HInecessary for Pd0 reoxidation) and to the stabilizing effect by CO2

as ligand on the palladium intermediates leading to isocyanates.The best solvent for carrying out these reactions is DME, due to itscoordinating ability, which allows the solublization of the PdI4

2−anion, associated with a low dielectric constant, which decreasessubstrate basicity compared with highly polar solvents. Increasingthe excess of KI up to 200 mol per mol of PdI2 can also be usefulfor increasing the concentration of free HI, since in the presence ofa large excess of iodide anions the acid-base equilibrium betweenthe amine and HI to give an ammonium iodide is shifted towardsthe amine and HI.

O

RHN NHR2 RNH2 + CO + 1/2 O2

PdI2 cat

(18)

68–98%

−H2O

O

RHN NR′R′′RNH2 + R′R′′NH + CO + 1/2 O2

PdI2 cat(19)

− H2O

63–95%

Intramolecular trapping of the isocyanate by another aminogroup or by a hydroxyl group allows an easy synthesis of cyclicureas30 and oxazolidin-2-ones,31,32 respectively (eqs 20 and 21).

H2N NH2HN NH

O

+ CO + 1/2 O2

PdI2 cat(20)

− H2O

91–99%

R1 R2

HO NH2

O NH

O

R1 R2

+ CO + 1/2 O2

PdI2 cat

− H2O(21)

PdI2 in association with iodide ligands is also an efficient cata-lyst for the synthesis of acyclic carbamates starting from primaryamines and alcohols (eq 22).33,34

O

RHN OR′RNH2 + R′OH + CO + 1/2 O2

PdI2 cat(22)

− H2O

63–95%

Additive and Reductive Carbonylation Reactions. In thepresence of catalytic amounts of PdI2 or PdCl2 in conjunction withthiourea as ligand (≥4 mol per mol of Pd) and in the absence ofexternal oxidants, 1,6-heptadiynes, dipropynylamines, and dipro-pynyl ethers (preferably α,α-disubstituted) undergo cyclization-additive carbonylation at 25 ◦C and atmospheric pressure of COin MeOH as the solvent, with formation of five-membered ringsbearing a methylene and a methoxycarbonylmethylene group(eq 23).35 The role of thiourea is to stabilize the intermediatepalladium complexes, thus preventing decomposition to Pd metal.In fact, the sulfur atom of the thiocarbonyl group possess a strongaffinity for a soft metal center such as palladium.

Avoid Skin Contact with All Reagents

4 PALLADIUM(II) IODIDE

Y Y

CO2Me

Y = CH2, O, NR

(23)+ CO + MeOHPdI2 cat

Quite interestingly, under similar conditions, and using PdI2

along with 3.0–3.5 equiv of thiourea as the catalytic system, termi-nal alkynes undergo oxidative carbonylation to maleic diesters andreductive carbonylation to furan-2(5H)-ones simultaneously, thenet transformation formally corresponding to additive carbonyla-tion (eq 24).36

RC CH

MeO2C CO2Me

R

O O

R

O O

R

+ 4 CO + 2 MeOHPdI2 cat

34–47% 1–10%R = alkyl, aryl

2

+

30–32%

+ (24)

The role played by the thiourea ligand relates to the stabiliza-tion of the H-Pd-I species resulting from the oxidative process(eq 25) and of the other organometallic intermediates involved inthe subsequent reductive cyclocarbonylation process, at the end ofwhich palladium is reconverted to its original state (eq 26). Indeed,using iodide anions as the ligands in place of thiourea, only thestoichiometric oxidative process takes place. PdCl2 can be usedin place of PdI2, but its activity is considerably lower. This effectcan be connected with a higher tendency of the Pd–I bond to beconverted into the key intermediate Pd-CO2R′ in alcoholic media(R′OH) in the presence of CO and to a higher capability of iodideas ligand to promote a reductive carbonylation process.

R′O2C CO2R′

R

+ 2 CO + 2 R′OH + PdI2

+ HI + H-Pd-I (25)

RC CH

O O

R

+ 2 CO + HI + H-Pd-I + PdI2 (26)RC CH

The application of this methodology to alkynes bearing poten-tially reducible functions allows the selective synthesis of onlyone carbonyl derivative resulting from the combination betweenoxidative carbonylation and reduction within the same substrate.Thus, the PdI2/thiourea-catalyzed carbonylation of 1-[2-(1-hydro-xyprop-2-ynyl)phenyl]prop-2-yn-1-ol, carried out in MeOH at25 ◦C and under 30 atm of CO, gives a naphthofuranoneaceticmethyl ester in 58% yield (eq 27).37 In a similar manner, thereaction of 3,3-diethoxypropyne, carried out in EtOH at 25 ◦Cand under 7 atm of CO, affords (E)-2-ethoxymethylenesuccinicacid diethyl ester in 65% yield (eq 28).38

HO

HO O

CO2Me

O

+ 2 CO + MeOHPdI2 cat

(27)

58%

–H2O

EtO

EtOEtO

EtO2C CO2Et+ 2 CO + EtOH

PdI2 cat

65%

(28)

The PdI2-thiourea system (3–6 equiv of thiourea with respectto PdI2) also promotes the substitutive carbonylation of allylalcohols, carried out at 80 ◦C and 50–100 atm in DMA or MeOH-DMA mixtures, to give β,γ-unsaturated acids or esters (eq 29).39

In this case, the key intermediate is a π-allylpalladium complexobtained from the reaction between an H-Pd-I species (formed,in turn, from the reaction between PdI2, CO, and H2O followedby loss of CO2) and the substrate with concomitant eliminationof H2O. The thiourea ligands then favor the carbonylation of theπ-allylpalladium complex, which otherwise (in the absence ofthiourea and in the presence of iodides only as ligands) preferen-tially undergoes direct nucleophilic attack by MeOH with forma-tion of an allyl methyl ether as the main reaction product.

RCH CHCH2OH RCH CHCH2CO2R′+ CO + R′OHPdI2 cat

− H2O

R = alkyl, aryl; R′ = H, alkyl

(29)40–92%

Very interestingly, working in an aprotic and non-nucleophilicsolvent, such as dioxane, in the presence of small amounts ofH2O, at 80 ◦C and under 10 atm of CO, alk-1-ynes undergoreductive carbonylation to furan-2(5H)-ones in the presence ofthe PdI2-KI system (KI:PdI2 = 10) and in the absence of otheradditional ligands. The process is catalytic in palladium since itoccurs with simultaneous oxidation of CO to CO2 (eq 30).40 Thereaction does not occur using PdCl2-KCl in place of PdI2-KI, andinvolves a palladium hydride species as the key intermediate, asdemonstrated by NMR studies.41 This result confirms the muchhigher ability of iodide with respect to chloride as ligand to pro-mote a reductive carbonylation process.

RC CHO O

R

+ 3 CO + H2O + CO2 (30)PdI2 cat

45–65%

An interesting effect in this process has been observed by sat-urating the reaction mixture with additional CO2 under pressure.Under these conditions, the product distribution changes, withformation of maleic anhydrides and furanones in ca. 1:1 molarratio.41,42

The H-Pd-I species, obtained from PdI2 as described above, isable to promote, under appropriate conditions, the reduction ofsome allylic alcohol moieties through the intermediate formation

A list of General Abbreviations appears on the front Endpapers

PALLADIUM(II) IODIDE 5

of a π-allylpalladium complex followed by protonolysis.41 Thiskind of reactivity constitutes a key mechanistic step in the recentlyreported synthesis of benzofuran-2-acetic esters starting from1-(2-allyloxyphenyl)-2-yn-1-ols (eq 31).43

O

R2

R1 OH

CO2Me

H2O

O

R1

CO2Me

R2

+ 2 CO + 2 MeOH

Pd(0) catPd(II) cat

80−91%

(31)

Formation of benzofurans in fact occurs through an unprece-dented catalytic sequence involving Pd(0)-catalyzedcarbonylativedeallylation followed by PdI2-catalyzed cyclization-alkoxy-carbonylation-reduction, as shown in eq 32. This processrepresents the first example of what has been named “sequen-tial homobimetallic catalysis,” i.e., a process in which the metalin a certain oxidation state catalyzes the deprotection of a func-tional group, which in situ undergoes a subsequent transformationcatalyzed by another complex of the same metal but in a differentoxidation state.

O

R1 OH

R2

Pd

I Pd

O

PdI

MeOH

CO2Me

CO

PdI2

O

R1OH

PdI

R2

O

R1OH

R2

O

PdI

O

R1OH

CO2Me

R2

O

R1

CO2Me

R2

PdI

CO

MeOHH-Pd-I

H-Pd-I

H2O

HI

HI

HI

O

R2

R1 OH

OH

R2

R1 OH

O

R1

CO2Me

R2

Pd(0)

HI +

(32)

Cycloisomerization Reactions. PdI2 in conjunction with2 equiv of KI is an excellent catalyst for realizing somecycloisomerization reactions leading to heterocyclic derivatives.Thus, (Z)-2-en-4-yn-1-ols, (Z)-2-en-4-yne-1-thiols, and (Z)-(2-en-4-ynyl)amines are conveniently converted into the correspond-ing substituted furans,44,45 thiophenes,46 and pyrroles47,48 work-ing under mild and essentially neutral conditions (25–100 ◦C,DMA as the solvent or solventless) (eq 33). Catalytic efficienciesup to 500 mol of product per mol of substrate can be attained.

YHR1

R2 R3

R4YR1

R2 R3

R4PdI2 cat

(33)

Y = O, S, NR36–90%

This reaction has been successfully applied to the synth-esis of fragile, biologically active heterocycles such as rosefuran(eq 34).44,45

OH O

PdI2 cat(34)

rosefuran, 77%

In most cases, the PdI2-KI system shows a greater activitywith respect to PdCl2-KCl, owing to the higher electron-releasingpower of I− compared with Cl−, which favors the final protonol-ysis step (eq 35).

YHR1

R2 R3

R4

PdI2− HI

YR1

R2 R3

PdI

R4

YR1

R2 R3

R4HI

YR1

R2 R3

R4− PdI2

(35)

Under analogous conditions, 2-(1-alkynyl)benzyl alcoholsafford dihydroisobenzofuran and isochromene derivatives (deri-ving from 5-exo-dig or 6-endo-dig cyclization modes, respec-tively) (eq 36).49 The reaction selectivity towards the five- or six-membered rings is mainly dependent on the substitution patternof the substrate and on reaction conditions. In some cases, thesame substrate can be selectively converted into either the dihy-droisobenzofuran or the isochromene derivative by simply chang-ing the reaction solvent and the concentration of the substrate, asshown by eq 37.

R3

OH

R2R1

O

R3

R1 R2

O

R3

R1 R2

PdI2 cat+

up to 89% up to 74%

(36)

Avoid Skin Contact with All Reagents

6 PALLADIUM(II) IODIDE

Bu

OH

EtEt

O

Bu

Et Et

O

Bu

Et Et

PdI2 cat+

(37)

solvent = dioxane 15% 73%solvent = MeOH 77% 0%

In the case of 4-yn-1-ols, the PdI2/KI-catalyzed cycloisomeri-zation, carried out in the presence of an equimolar amountof MeOH at 25 ◦C and in the absence of additional solvents,is followed by MeOH addition to the vinyl ether bond, with for-mation of 2-methoxy-2-methyltetrahydrofurans (eq 38).17

OHR1

R2

R3

R4

O OMe

MeR2

R1

R3

R4

+ MeOHPdI2 cat

62–78%

(38)

PdI2 (with dibenzylideneacetone as ligand or even in the ab-sence of ligands) is also an efficient catalyst for the intramolecularhydroamination of 3-vinyloxypropylamine to 2-methyl-[1,3]-oxazinane, carried out at room temperature in toluene(eq 39).50

O

NH2

O

NH

PdI2 cat(39)

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A list of General Abbreviations appears on the front Endpapers

PALLADIUM(II) IODIDE 7

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Bartolo Gabriele & Giuseppe SalernoUniversity of Calabria, Cosenza, Italy

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