Negishi E. 2002 Handbook of organopalladium chemistry for organic synthesis / 0147 187

II.3.2 STOICHIOMETRIC SYNTHESIS AND SOME NOTABLE PROPERTIES

157

A recent application of the Pd(II)-induced activation of double or triple bonds toward nucleophilic attack is the development of an interesting one-pot synthesis of substituted tetrahydrofurans and pyrrolidines.[66]–[68]

The ease with which nucleophiles add to alkenes and alkynes coordinated to Pd(II) combined with the tendency of Pd to form -allyl complexes accounts for the limited number of Pd(II) – alkene, Pd(II) – diene, or Pd(II) – alkyne complexes. Indeed, as detailed in the next section, depending on the conditions or their structural features, alkenes, dienes, and alkynes often lead to the formation of -allylpalladium complexes instead of 2 or 4 -complexes. This is particularly true in the case of alkenes bearing hydrogens to the double bond, 1,3-dienes, such as 1,3-butadiene, and alkynes of low steric hindrance. In marked contrast, Pd(1,5-hexadiene)Cl2 is obtained from the reaction of Cl2Pd(PhCN)2 with allyl chloride.[35]

C. Pd COMPLEXES CONTAINING ANIONIC CARBON LIGANDS

The majority of known organopalladium complexes contain anionic carbon groups, such as alkyl, benzyl, aryl, alkenyl, and alkynyl groups. Acylpalladium complexes also belong to this category. These organyl ligands form bonds with Pd. With allyl and cyclopentadienyl groups, Pd in most cases is simultaneously linked to an anionic carbon group through a bond and to one or two allylic C"C bonds through -complexation. In such cases, 3-allylpalladium and 5-cyclopentadienylpalladium complexes are formed rather than the corresponding 1-allyl- or cyclopentadienylpalladium complexes.

C.i. - and -Bonded Complexes of Palladium: -Allyl

and Cyclopentadienyl Complexes

A large variety of -allylpalladium complexes have been isolated. Some representative examples of such complexes are shown in Table 6.

A large number of allylic compounds, such as allyl halides, allyl alcohols, allyl acetates, allyl trifluoroacetates, and also allyl Grignard reagents, react with palladium salts, such as PdCl2 or Na2PdCl4, to give -allylpalladium complexes. The reaction of Na2PdCl4 with allyl chloride in MeOH in the presence of CO leads to the formation of the dimeric complex Pd2( 3-C3H5)2( -Cl)2.[84] The role of CO in this reaction is to ensure the reduction of the Pd(II) species into a Pd(0) species (see Sect. II.2.3) necessary for the oxidative addition into the C — Cl bond (Scheme 14). In contrast, allylmagnesium chloride reacts with Pd(II)Cl2 to produce homoleptic Pd( 3-C3H5)2 via transmetallation (Scheme 14). It is interesting to note that this latter type of complexes exist in solution as a mixture of the cis and trans isomers, while the trans isomer is favored in the solid state (Scheme 15).[96] These reactions can be applied to the formation of substituted 3-allylpalladium complexes.

Once Pd2( 3-C3H5)2( -Cl)2 and Pd( 3-C3H5)2 are prepared, they can be transformed into a variety of other allylpalladium species, as exemplified in Scheme 16. Alternative direct synthetic routes starting from Pd(0) or Pd(II) species have also been utilized (Scheme 17).

Cationic allylpalladium complexes represented by [( 3-C3H5)PdL2]X can be generated by the reaction of the dimeric Pd2( 3-C3H5)2( -X)2, where X is Cl or Br, with salts of noncoordinating anions, for example, NaBPh4 or AgBF4, in the presence of ligands such as

158

II

Pd-COMPOUNDS: STOICHIOMETRIC PREPARATION

TABLE 6. Allylpalladium Complexes

Carbon

References

Number a

Pd Complex

Comments

for Preparation

MONOMERS

3

PdX2

Ph4P

X Cl, Br

[69]

6

Pd

[70]

8

Pd

Readily decomposes in solution

[71]

9

Pd

Unstable above 0 C

[72]

PMe3

Br

[73]

Pd

PEt3

11

Air-sensitive; decomposes

[74],[75]

Me3P Pd

in solution above 25 C

12

Pd

Air-sensitive; decomposes

b

above 20 C

[76]

13

Pd

N

X

X Br, BF4

[77]

N

15

PEt3

X Br, BPh4

[78],[79]

Pd

X

PEt3

21

X

X Br, AcO

[80],[81]

Pd

PCy3

Cl

[82]

Pd

PPh3

DIMERS

3

X

X Cl, Br, I, OAc;

[83]–[88]

Pd

Pd

X

commercially available for X = Cl

	II.3.2 STOICHIOMETRIC SYNTHESIS AND SOME NOTABLE PROPERTIES			159
TABLE 6. (Continued )
Carbon			References
Number a	Pd Complex	Comments	for Preparation

4			X		X Cl, Br, I
		Pd	Pd

X

X

7 Pd Pd X Cl, Br

X

Ph

	9		Cl
		Pd	Pd

Cl

Ph

aNumber of carbons in a monomer unit.

b[( 3-C3H5)Pd(bipy)]X.

[84],[87]

[89]–[91]

[92]–[94]

[90],[95]

phosphines[79] or bidentate amines, for example, bipy. Alternatively, this class of complexes can be obtained by the oxidative addition of allylic derivatives in the presence of an excess of ligand or chelating ligands. An interesting example is the preparation of [( 3- C3H5)Pd(bipy)]Br from allyl bromide and Me2Pd(bipy), demonstrating that dialkylpalladium species, which can be converted to Pd(0) species via reductive elimination (vide infra), can also be used to generate -allyl complexes.[77] With smaller amounts of ligands neutral ( 3-C3H5)Pd(L)X are obtained. It should be noted, however, that while the treatment of allyl acetate with Pd(PCy3)2 does provide the expected ( 3-C3H5)Pd(PCy3)OAc complex, the use of Pd – PPh3 complexes, for example, Pd (PPh3)4, does not lead to the desired 3- allyl(acetato)palladium complexes.[80],[81] This latter result is in agreement with a recent study, which provided evidence for the reversibility of the formation of -allylpalladium complexes from the oxidative addition of Pd(0) – PPh3 complexes into allyl acetate.[97]

Some of these allylpalladium complexes, Pd2( 3-C3H5)2( -Cl)2 in particular, are important not only as precursors to a large variety of allylpalladium complexes, but also as Pd(0) sources in a variety of Pd-catalyzed processes.[98] It is actually commercially available.

It is worth noting that in essentially all of the examples listed in Table 6 the isolated species correspond to the 3- -allylpalladium complexes. The structure of Pd2( 3- C3H5)2( -Cl)2 has been determined by X-ray crystallography,[99],[100] which has confirmed that the allyl group is bonded in an 3-mode occupying two coordination sites at an angle of about 110° relative to the plane of the Pd2Cl2 unit. It is indeed quite rare to isolate the-allylpalladium complexes, as they are usually less stable than the corresponding -allyl derivatives. Mixed / -allyl complexes could nonetheless be isolated by adding phosphines to bis( -allyl)palladium complexes.[72],[74],[75] It is important to keep in mind, however, that the two species can in principle interconvert in a dynamic equilibrium. This dynamic equilibrium has been clearly established for species such as ( 3-allyl)PdX(PR3) in which interconversion between syn and anti substituents is rationalized by invoking the intermediacy of 1-allyl species.[96]

160

II Pd-COMPOUNDS: STOICHIOMETRIC PREPARATION

X

1

X

Pd(0)

PdX

Pd

Pd

2

X

PdCl2

2

MgCl

Pd

Pd

−2 MgCl2

Scheme 14

Pd

Pd

trans

cis

Scheme 15

NaI or

X

AgOAc

Pd

Pd

X = I, OAc

[85],[87]

X

KCl/PPh4Cl

PdCl2− PPh4+

Cl

[69]

Pd

Cl

Pd

2 PPh3

Pd

Cl

[82]

PPh3

4 PEt3

+

PEt3

NaBPh4

Pd

BPh4−

[79]

PEt3

PMe3

Pd

PMe3

Pd

PMe3

Me3P

Pd

2

Scheme 16

Alkenes can also react with Pd(II) salts to give -allylpalladium complexes. One key requirement is that the alkene has one or more hydrogens to the double bond that can be abstracted to give a -allyl complex. Heat[89],[90] or the presence of a base (Scheme 18),[101] such as sodium carbonate or sodium acetate, favors the transformation of the initially formed 2-alkene complex into the -allyl species.

II.3.2 STOICHIOMETRIC SYNTHESIS AND SOME NOTABLE PROPERTIES

161

X

X

Pd black

Pd

Pd

X = Br, I

[88]

X

Br2Pd(PEt3)2

Br

Pd

Br

potassium slurry

PEt3

[73]

Br

+

PEt3

Br−

Pd(PEt3)3

Pd

[78]

PEt3

Pd(dba)2

3

Pd

[76]

Scheme 17

n-C3H7

Cl

Cl

Na2CO3

Cl

Pd

Pd

n-C3H7

Pd

Pd

C3H7-n + HCl

Cl

100%

Cl

Cl

C3H7-n

Scheme 18

The use of highly electrophilic Pd(OCOCF3)2 has proved to be useful in cases where alkenes are less reactive. 2-Alkenes usually form the more substituted -allylpalladium complexes. When an electron-withdrawing group, such as a keto or a carboxyl group, is present at an allylic position, -allylpalladium formation is favored, and its regiochemistry is well defined.[102]

Allenes, 1,3-dienes, cyclopropanes, cyclopropenes, and even acetylenes can also serve as the starting materials for -allylpalladium complexes. Some representative examples are given in Scheme 19. It is interesting to note that while sterically hindered t-BuC#CBu-t displaces the two ethylene ligands from [Pd(CH2"CH2)Cl2]2 to give the corresponding alkyne – Pd complex (Sect. B.ii, Table 5), sterically less hindered PhC#CBu-t reacts with the same Pd complex to generate a -allylpalladium species (Scheme 19).[46]

The reactions of -allylpalladium complexes are dominated by the nucleophilic attack on the -allyl ligand. A well-known example is the stoichiometric version of the Tsuji– Trost reaction (Scheme 20).[111]

Another interesting reaction of -allylpalladium species is their reaction with carbon monoxide. -Allylpalladium chloride, for instance, reacts with CO in EtOH to give carbonylation products (Scheme 21).[112]

-Allylpalladium species are implied in many organic transformations catalyzed by Pd species. A notable example is provided by the oligomerization and telomerization of 1,3- butadiene in which the intermediacy of -allylpalladium species was clearly established when the -allylpalladium intermediates of dimerized or trimerized 1,3-butadiene were actually prepared and isolated (Scheme 22).[74]–[76]

Recent studies have shown that allylpalladium complexes can also react as nucleophiles. Bis( -allyl)palladium, for instance, was found to be a key intermediate in the Pd-catalyzed allylation of aldehydes and imines by allylstannane.[113] This complex has

162

II Pd-COMPOUNDS: STOICHIOMETRIC PREPARATION

Allenes[103]−[106]

Cl

Cl

Pd

Pd

Cl

Cl

obtained in nonpolar solvents

PdCl2

+ H2C

C

CH2

Cl

Cl

Pd

Pd

Cl

Cl

obtained in polar solvents

1,3-Dienes[107]−[109]

Nu

PdCl2

+

Nu = Cl, OR, OAc

Cl

Pd

Pd

Cl

Cyclopropanes[110]

Nu

Cl

[Pd(CH2

CH2)Cl2]2

+

Cl(CH2)2

Pd

Pd

(CH2)2Cl

Cl

Alkynes[46]

Bu-t

Cl

Cl

Ph

[Pd(CH2

CH2)Cl2]2

+

Ph

C

C

Bu-t

Ph

Pd

Pd

Cl Cl

Bu-t

Scheme 19

1

Cl

NaCH(COOEt)2

Pd

Pd

CH(COOEt)2

+ Pd(0) + HCl

2

Cl

Scheme 20

1

Cl

Pd

CO, EtOH

Pd

COOEt

2

Cl

Scheme 21

	Pd	PR3	Pd	R3P Pd
		2
	Pd(dba)2	3	Pd

Scheme 22

II.3.2 STOICHIOMETRIC SYNTHESIS AND SOME NOTABLE PROPERTIES

163

also been found to react as an amphiphilic allylating agent, that is, a reagent that can simultaneously act as a nucleophile and an electrophile, in the Pd-catalyzed double allylation of activated olefins by allylstannane.[114]

Like allylic electrophiles, propargylic halides or acetates oxidatively add to Pd(0) derivatives, such as Pd(PPh3)4. In this case, however, either 1-propargylpalladium or allenylpalladium complexes are obtained depending on the size of the C3 substituent. These complexes display some interesting reactivity.[115] A recent report clearly demonstrates the possibility of3 bonding in the case of propargyl chloride having a bulky group at C3, when the propargylpalladium species is generated from Pd2(dba)3 in the presence of 1 equiv of PPh3 per Pd atom. If a second equivalent of PPh3 is added, however, the 1-propargylpalladium is formed.[116]

Closely related to the -allylpalladium complexes are the cyclopentadienylpalladium complexes. Although Cp2Pd, where Cp is 5-C5H5, remains unknown, several monocyclopentadienyl – Pd complexes have been prepared and characterized. Some representative examples of such complexes are summarized in Table 7.

With the exception of CpPdNO, a Pd(0) species prepared from Pd(Cl)NO and TlCp, all of the Cp – Pd complexes shown in Table 7 are Pd(II) complexes.

Cyclopentadienylpalladium complexes are generally obtained by the reaction of appropriate Pd complexes with MCp, where M is Na or Tl. Complexes of the CpPd(L)X type, with X Cl, Br, or I, and L PR3, are obtained by the reaction of TlCp or NaCp with dimeric complexes Pd2X4L2 or Pd2X2L2( -OAc)2.[119],[127] These complexes of the general formula CpPd(L)X can in turn react with another ligand L , where L can be PR3, CO, or C2H4, to give [CpPdLL ]X complexes. When X is not a halogen but ClO4 or PF6 , AgClO4 or KPF6, respectively, is required. In special cases where L L PEt3 or olefin, an alternative synthesis of complexes of the general formula [CpPdL2]X is possible from X2PdL2 and TlCp or FeCpBr(CO)2. This is the case with [CpPd(PEt3)2]Br,[119] [CpPd(1,5-cod)](FeBr4),[120] and [CpPd( 4-C4Ph4)](FeBr4).[120] One such example is shown in Scheme 23.

When Pd2X2L2( -OAc)2 is treated with 2 equiv of TlCp per Pd instead of one, ( 5- Cp)( 1-Cp)PdL is obtained.[123] These complexes exhibit a fluxional behavior in solution, that is, / (or 1/ 5) exchange of the two Cp ligands.[123] This is rather interesting, since NMR studies on a variety of Cp – Pd complexes have shown that the Cp ligand is bound symmetrically with no fluxional behavior.[130]

Other types of cyclopentadienylpalladium complexes have been generated by similar approaches. One such example is CpPd( 3-C3H5), which is prepared from Pd2( 3- C3H5)2( -Cl)2 (Scheme 24).[118]

Pd complexes containing substituted cyclopentadienyl ligands, such as C5Me5 and C5Ph5, have also been prepared using various methods discussed above.

Cyclopentadienylpalladium complexes have not yet been shown to be very useful in organic synthesis. This may be due to the fact that the Cp ligand is very labile. It is indeed easily cleaved under a variety of conditions, as exemplified in Scheme 25.[130]

One interesting feature of the Cp – Pd complexes is that they are intensely colored. This characteristic could potentially be interesting, as it provides a direct and straightforward way of monitoring their formation or disappearance.

C.ii. -Bonded Complexes: Alkyl, Benzyl, Aryl, Alkenyl, Alkynyl, and Acyl Complexes

As mentioned previously, -bonded organopalladium complexes may contain carbon groups, such as alkyl, benzyl, aryl, alkenyl, alkynyl, and acyl groups. Representative

164

II

Pd-COMPOUNDS: STOICHIOMETRIC PREPARATION

TABLE 7. Cyclopentadienylpalladium Complexes

Carbon

References

Number

Pd Complex

Comments

for Preparation

5

PdNO

[117]

8

Pd

[118]

11

X Cl, Br, I

[119]

Pd(PEt3)X

13

X BF4, FeBr4

[120]–[122]

Pd

X

16

Pd

[123]

17

PEt3

X Br

[119]

Pd(PEt3)2

X

23

X Cl, Br

[119],[124]

Pd(PPh3)X

24

Pd

CO

X

X ClO4;

[125],[126]

polentially explosive

PPh3

25

Pd

X

X ClO4;

[125],[126]

potentially explosive

PPh3

28

Pd(PPh3)Cl

[127]

Pd

[123]

PPh3

Ph

Ph

P

31

Pd

X

X PF6’ OTf

[128],[129]

P

Ph

Ph

Ph

Ph

33

Pd

X

X FeBr4, Br;

[120]

Ph

Ph

air-stable

II.3.2

STOICHIOMETRIC SYNTHESIS AND SOME NOTABLE PROPERTIES

165

Ph

Ph

2 FeCpBr(CO)2

+ Ph

Ph

FeBr −

Br

Pd

Pd

2

4

Ph

Ph

Ph

Ph

Scheme 23

Cl

CpLi

Pd

Pd

Pd

+

LiCl

Cl

Scheme 24

HCl

Cl

+

C5H6

Pd

Pd

Cl

Pd

PPh3

Pd(PPh3)4

Scheme 25

examples of alkyland benzylpalladium complexes are listed in Table 8. Aryl-, alkenyl-, and alkynylpalladium complexes are exemplified in Table 9, and acylpalladium complexes in Table 10. In cases where palladium is bonded to chelating ligands, such species may be considered as palladacycles. Those palladacycles containing at least one C — Pd bond in the ring are shown in Table 11. Finally, some polymeric carbon – Pd complexes containing Pd in their backbones are given in Table 12.

C.ii.a. Alkyl-, Benzyl-, Aryl-, Alkenyl-, and Alkynylpalladium Complexes. Alkyland benzylpalladium complexes (Table 8) can be obtained by a variety of synthetic methods.

Oxidative addition of Pd(0) species into carbon – halogen bonds leading to monoalkylor benzylpalladium complexes of the type “RPdX,” where X is a halogen atom, is certainly one of the most general methods for the synthesis of -bonded organopalladiums. Pd(0) complexes, such as Pd2(dba)3 and Pd(PPh3)4, have been used for this purpose. Metallic palladium generated by metal vapor technique has also been used in the cases of perfluorinated alkyls, such as CF3I and CF3CF2I.[15] Although this method is not widely applicable and is low-yielding, it has been used to generate remarkably stable CF3(CF2)7PdBr (vide infra) from the corresponding bromide.[133] Most often, however, monoalkylor benzylpalladium complexes are isolated as complexes of a variety of stabilizing phosphorus or nitrogen ligands and are usually prepared either from a Pd(0) complex of the desired ligand or from Pd2(dba)3 in the presence of the desired ligand (Scheme 26). Pd(0) complexes can also be generated in situ from Pd(II) species in the presence of an appropriate reducing agent (see Sect. II.2.3). Unless bidentate ligands are involved, the oxidative addition generally leads to the formation of trans-palladium complexes.

Pd(II) complexes can also serve as precursors to monoalkylpalladium complexes via transmetallation if 1 equiv of an organometallic reagent per Pd complex is used. A large

166

II

Pd-COMPOUNDS: STOICHIOMETRIC PREPARATION

TABLE 8. Alkylpalladium and Benzylpalladium Complexes

Carbon

References

Number a

Pd Complex

Comments

for Preparation

Monorganyl Complexes

MONOMERS

7

MePdI(PMe3)2

trans

[131],[132]

8

CF3(CF2)7PdBr

[133]

9

MePdCl(1,5-cod) b

[134]

10

[MePd(PMe3)3]X

X BPh4

[131],[132]

11

MePdX(bipy) c

X Cl, Br, I

[135],[136]

MePdI(CNBu-t)2

trans

[22]

13

MePdX(PEt3)2

X Cl, Br, I; trans

[78],[137]–[139]

CF3PdBr(PEt3)2

trans

[15],[140]

PhCH2PdCl(PMe3)2

trans

[141]

27

CF3PdI(Ph2PCH2CH2PPh2) d

[142]

37

MePdX(PPh3)2

X Cl, Br, I; trans

[143]–[145]

CF3PdI(PPh3)2

trans

[142]

43

PhCH2PdX(PPh3)2

X Cl, Br; trans

[144]–[146]

DIMERS

3

(MePdX.SMe2)2

X Cl, Br, I

[147]

19

Me

Cl

PPh3

[134],[148]

Pd

Pd

Ph3P

Cl

Me

25

PhCH2

Cl

PPh3

[149]

Ph3P

Pd

Pd

CH2Ph

Cl

Diorganyl Complexes

MONOMERS

S

8

Me2Pd

S

[150]

S

Me Pd(Me

NCH

CH

NMe

2

) e

[151]

2

2

2

2

Me Pd(Me

PCH

CH

PMe

) f

[151]

2

2

2

2

2

Me2Pd(PMe3)2

cis and trans

[152],[153]

10

Me2Pd(1,5-cod)b

[137],[154]

Et2Pd(PMe3)2

trans

[153]

12

Me2Pd(bipy) c

[135],[137]

14

Me2Pd(phen) g

[135]

Et2Pd(bipy) c

[155]

Me2Pd(PEt3)2

cis and trans

[137],[153]

22

PdMe(PPh3)

[156]

24

[157]

PdMe(PPh3)