The majority of the Pd–P complexes listed in Tables 1and 2are commercially available.
For various reasons, however, it may be useful to prepare them or know how they are pre- pared. For example, Pd(PPh3)4 is relatively unstable to oxygen. So, in some cases, one may prefer preparing and using it before its degradation. In general, Pd–P compounds may be either prepared as isolable and storable compound or generated in situin the reac- tion system.
Although metallic palladium is the source of Pd for essentially all Pd compounds, Pd(II) salts containing halogens and/or oxygen ligands, such as PdCl2, M2PdCl4
47 Handbook of Organopalladium Chemistry for Organic Synthesis, Edited by Ei-ichi Negishi
ISBN 0-471-31506-0 © 2002 John Wiley & Sons, Inc.
TABLE 1. Some Synthetically Significant and Well-Characterized Palladium–Phosphine Complexes Containing Achiral Monodentate Phosphines
Carbon Commercial References
Number Pd – Phosphine Complex Availability for Preparation
12 Cl2Pd(PEt3)2 [1]
16 Cl2Pd(PPhMe2)2 [2]
24 I2Pd[P(Bu-n)3]2 [3]
Cl2Pd(TFP)2a [4]
26 Cl2Pd(PPh2Me)2 [5]
36 Pd(PCy3)2 [6]
Cl2Pd(PPh3)2 [7]
Br2Pd(PPh3)2 [8]
I2Pd(PPh3)2 [8]
Cl2Pd(PCy3)2 [9]
40 (AcO)2Pd(PPh3)2 [10]
42 Cl2Pd(TTP)2a [11]
52 Pd(PPh2Me)4 [6]
72 Pd(PPh3)4 [12]
aTFP = Tris(2-furyl)phosphine. TTP = Tris(o-tolyl)phosphine
TABLE 2. Some Synthetically Significant and Well-Characterized Palladium–Phosphine Complexes Containing Achiral Bidentate Phosphines
Cabon Commercial References
Number Pd – Phosphine Complex Availability for Preparation
26 Cl2Pd(Ph2PCH2CH2PPh2)a [13]
Br2Pd(Ph2PCH2CH2PPh2)b [14]
27 Cl2Pd[Ph2P(CH2)3PPh2]c [15]
28 Cl2Pd[Ph2P(CH2)4PPh2]d [16]
34 Cl2Pd(Ph2PCpFeCpPPh2)e [17]
42 Cl2Pd(Ph2PC20H14PPh2)f [18]
52 Pd(Ph2PCH2CH2PPh2)2g [19]
aCl2Pd(dppe).
bBr2Pd(dppe).
cCl2Pd(dppp).
dCl2Pd(dppb).
eCl2Pd(dppf).
fTransphos =
gPd(dppe)2.
(MLi, Na, K), and Pd(OAc)2, generally serve as immediate precursors to Pd–P com- plexes (Protocol 1,Scheme 1). In some cases, these halogen- and oxygen-containing Pd complexes are converted first to Pd complexes containing carbon and other types of ligands, such as Pd(dba)2 and Cl2Pd(PhCN)2, which may then be converted to Pd–P complexes (Protocol 2, Scheme 1). It should also be noted that even some phosphorus-containing
Ph2P Pd Ph2P CH2
CH2 Cl2
complexes [e.g., Cl2Pd(PPh3)2and Pd(PPh3)4] as well as various organopalladium complexes [e.g., PhCH2Pd(PPh3)2Cl] can serve as precursors to Pd – P complexes.
More specifically, Pd(II) – phosphine complexes of the Cl2Pd(II)(PR3)2 type, such as Cl2Pd(II)(PPh3)2,[7]and even those containing bidentate ligands, such as Cl2Pd(II)(dppp),[15]
can be most conveniently prepared from M2Pd(II)Cl4 and Cl2Pd(II)(PhCN)2according to the general equations shown in Scheme 2. The direct use of PdCl2can be complicated by its low solubility in most organic solvents, while M2PdCl4 and Cl2Pd(PhCN)2 are soluble in various organic solvents, such as THF and MeOH. The preparation of Pd(0)–phosphine complexes of the Pd(0)(PR3)4type, such as Pd(0)(PPh3)4,[31]and the related complexes con- taining bidentate phosphines, such as Pd(dppe)2,[32],[33]can be most conveniently achieved using Pd(0)(dba)2 and related Pd(0)–dba complexes.[34] Alternatively, Pd(II) complexes, such as PdCl2and M2PdCl4, may be used as Pd sources in conjunction with external reduc- ing agents, such as hydrazine hydrite[12]and n-BuLi,[35]as indicated by several methods for the preparation of Pd(PPh3)4shown in Scheme 3, which are readily adaptable to the synthe- sis of others represented by Pd(0)(PR3)4.
Many different kinds of reducing agents can be used for reducing Pd(II) compounds to Pd(0) compounds. It is, however, advisable to choose reagents that do not produce unde- sirable by-products. The use of external reducing agents is desirable in cases where Pd–P TABLE 3. Some Achiral Phosphines and Phosphites Incorporated in Synthetically Useful Palladium–Phosphorus Complexes
Carbon Acronym or Commercial References
Number Pd–P Complex Abbreviation Availability for Preparation Phosphines
15 (i-Pr)2P(CH2)3P(Pr-i)2 dippp [20]
16 (i-Pr)2P(CH2)4P(Pr-i)2 dippb [21]
17 Ph2PCH2CH2NMe3X [22]
18 DPMSPP [23]
TMSPP [24]
Phosphites
3 P(OMe)3 [25]
6 P(OEt)3 [26]
TMPP [27]
9 P(OPr-i)3 [28]
18 P(OPh)3 [29]
42 P O [30]
3
O OO Et P
SO3H
P 3
SO3H Ph2P
PdCl2
4 PPh3
Li2PdCl4
Cl2Pd(PPh3)2 2 PPh3
Pd(0)(dba)2
[31]
Pd(0)(PPh3)4 + 2 dba 4−5 PPh3, 4 N2H4, 2H2O, Me2SO
Pd(0)(PPh3)4 + N2H5Cl + N2
Pd(0)(PPh3)4 + 4 LiCl
Pd(0)(PPh3)4 + 2 LiCl 2 n-BuLi, 4 PPh3
2 n-BuLi, 2 PPh3
[12]
[36]
[35]
Scheme 3 PdCl2
M2PdCl4 MCl
Pd(OAc)2
PdCl2
M2PdCl4
RCN Cl2Pd(RCN)2
Pd P complexes
Pd P complexes Protocol 1
phosphine
Protocol 2
M = Li, Na, OR K
dba Pd(dba)2
phosphine reducing agent, if needed
Scheme 1
+ 2 LiCl
+ 2 PhCN Li2Pd(II)Cl4
Cl2Pd(II)(PhCN)2
Cl2Pd(II)(PR3)2
Cl2Pd(II)(PR3)2 2 PR3
2 PR3
Scheme 2
complexes are to be prepared and isolated. On the other hand, in cases where Pd–P com- plexes are to be generated in situ as catalysts, external reagents are often unnecessary, since a wide variety of organic compounds including olefins, alcohols, amines, phos- phines, organometals, metal hydrides, and CO are capable of the required reduction.
Some representative examples of such reduction reactions are shown in Scheme 4, and they are further discussed in detail in later parts.