Phosphine ligands early studies

In contrast to the free-radical polymerizations, there have been relatively few studies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(II)-bisphosphine complexes have been found to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Figure 10.5) as described in Chapter 5. 

It appears from these early studies that modest to rather high yields and enan-tioselectivities can be achieved with structurally very diverse chiral ligands. Furthermore, both relatively hard (amino alcohols) and soft (thiols, phosphines) ligands... 

Asymmetric hydrocarbalkoxylation of alkenes has been studied since the early 1970s, but the number of papers published on this subject is much less than that of asymmetric hydroformylation. This difference is mainly due to the fact that these reactions catalyzed by palladium complexes with chiral phosphine ligands usually require a very high pressure of carbon... 

Pure phosphine-metal atom studies with the early transition metals have been lacking. The only complex prepared this way prior to 1978 was Cr(Pp3)6. King and Chang, with the unique aminophosphine systems Me2NPF2 and MeN(Pp2)2, have generated a variety of new homoleptic M-(L) systems (see Homoleptic Compound). The codeposition of Cr vapor with these ligands yields the complexes shown ... 

Early studies on the palladium-catalyzed asymmetric hydrosilylation of cyclic conjugated dienes employing menthyl- and neomenthyldiphenylphosphine and ferrocenylamino-phosphine ligands gave low enantiomeric excesses of the corresponding allylsilane [13]. 

The Pd-catalyzed carbonylation of aryl halides (cf Section 2.1.2) occurs with high turnover numbers and reaction rates in SCCO2 as the solvent using standard precursor complexes and commercially available phosphine or phosphite ligands [30]. The generally better performance of the phosphite-based catalysts was attributed to their better solubility in the reaction mixture, but the formation of Pd carbonyl complexes was also mentioned as a possibility. The [Ni(cod)2]/dppb system (dppb = l,4-bis(diphenylphosphino)butane) was investigated in an early study as a catalyst for the synthesis of pyrones from alkynes and CO2 under conditions beyond the critical data of carbon dioxide [31]. Replacing dppb with PMcs results in a system with better solubility and catalytic performance, albeit catalyst deactivation remains a problem [3 c, 15]. 

Addition reactions of the Si-Si bonds across carbon-carbon triple bonds have been most extensively studied since the 1970s by means of palladium catalysts. In the early reports, palladium complexes bearing tertiary phosphine ligands, mostly PPh3, were exclusively employed as effective catalysts, enabling the alkyne bis-silylation with activated disilanes, i.e., disilanes with electronegative elements on the silicon atoms such as hydro [36], fluoro [37], chloro [38], and alkoxy-disilanes [39,40] and those with cyclic structure (Scheme 4) [41-44]. The bis-silylation reactions could be successfully applied to terminal alkynes and acetylenedicarboxylates to give (Z)-l,2-bis(silyl)alkenes, which are otherwise difficult to synthesize. 

A relatively new approach involves the functionalization of phosphines or other ligands with weakly basic or acidic functionalities. An early study reports on a modified cobalt catalyst [12]. This cobalt carbonyl complex containing the P(CH2CH2NEt2)3 ligand was claimed to be extractable into dilute carbonic acid and could be re-extracted into an organic phase by simply reducing the C02 pressure. 

A variety of nickel(O) complexes, when treated tvith allyhc electrophiles, afford jt-allyl complexes (see also Houben-Weyl, Vol. E18, pp 64 and 76).l In early studies, tetracarbon-ylnickel(O) was widely employed. However, otving to its extreme toxicity, it is now rarely used. Direct treatment of bis(T] -cycloocta-l,5-diene)nickel(0) (2) with allyl halides such as 20 is now the method of choice for the stoichiometric preparation of nickel-3t-allyl complexes. In the absence of strong donor ligands such as phosphines, halo-bridged dimers (e.g., 21) are t5T3ically obtained (Scheme 12). In the presence of phosphines, monomeric species such as 22 may be obtained. Other less-electrophilic allylic substrates such as allylic ethers and allylic alcohols also serve as precursors to nickel-Jt-allyl complexes in cataljdic procedures. However, these precursors are less widely used than allyl halides in the stoichiometric preparation of the Jt-allyl complexes. 

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