Phosphine ligands, promotion

Scheme 10.14 rationalizes the divergent behavior of the two catalytic systems in these selective transformations of pent-l-yn-ols. The presence of phosphine ligands promotes the formation of ruthenium vinylidene species which are key intermediates in both reactions. The more electron-rich (p-MeOC6Fl4)3P phosphine favors the formation of a cyclic oxacarbene complex which leads to the lactone after attack of the N-hydroxysuccinimide anion on the carbenic carbon. In contrast, the more labile electron-poor (p-FC6H4)3P) phosphine is exchanged with the N-hydroxysuccinimide anion and makes possible the formation of an anionic ruthenium intermediate which liberates the cyclic enol ether after protonation. 

As shown in Eq. (4), parallel screening of ligand libraries has allowed us to establish that a closely related peptide-based phosphine ligand promotes the catalytic asymmetric conjugate addition of alkylzincs to nitroalkenes [14]. Not only are the corresponding alkyl nitrones obtained efficiently and in high diaster-eo- and enantioselectivity, appropriate acid workup can deliver the derived ketone directly. 

In the BASF process the 1,2-diacetate is the substrate for the hydroformylation step. It can be prepared either directly via oxidative acetoxylation of butadiene using a selenium catalyst or via PtCl4-catalyzed isomerization of the 1,4-diacetate (see above). The latter reaction affords the 1,2-diacetate in 95% yield. The hydroformylation step is carried out with a rhodium catalyst without phosphine ligands since the branched aldehyde is the desired product (phosphine ligands promote the formation of linear aldehydes). Relatively high pressures and temperatures are used and the desired branched aldehyde predominates. The product mixture is then treated with sodium acetate in acetic acid to effect selective elimination of acetic acid from the branched aldehyde, giving the desired C5 aldehyde. 

Like the Stille reaction, unsaturated iodides react fastest, although bromides and triflates can be used. Unsaturated chlorides react very slowly and need high temperatures, although electron-rich phosphine ligands promote their coupling. 

Figure 16.1 Chiral phosphine ligands promoting asymmetric Mizoroki-Heck reactions.

Fundamental reactions of Pd are briefly explained in order to understand how reactions either promoted or catalyzed by Pd proceed. In schemes written for the explanation, phosphine ligands are omitted for. simplicity. First, a brief explanation of chemical terms specific to organopalladium chemistry is given. 

Fagnou and co-workers reported on the use of a palladium source in the presence of different phosphine ligands for the intramolecular direct arylation reaction of arenes with bromides [56]. Later, they discovered that new conditions employing palladium complex 27 promoted the direct arylation of a broad range of aryl chlorides to form six- and five-membered ring biaryls including different functionalities as ether, amine, amide and alkyl (Scheme 7.11) [57]. 

Kurosawa et al. have reported that the relative stability of the ti-allyl palladium thi-olate 39 and the allyl sulfide/Pd(0) was highly ligand dependent. In the presence of PPhs or P(OMe)3 the stability was in favor of reductive elimination (Eq. 7.28), while in the presence of olefin or in the absence of any additional ligand the stability was in favor of oxidative addition (Eq. 7.29) [38]. This can explain the reactivity of the n-allyl palladium thiolate 33 and 38 proposed in Eq. (7.24) and path (c) of Scheme 7-10. The complex 33 should react with PhSH, but C-S bond-forming reductive elimination has to be suppressed in order to obtain the desired product 32. On the other hand, the complex 38 requires the phosphine ligand to promote the C-S bond-forming reductive elimination. 

Ligand screening experiments were conducted on the alkenes 1-pentene and pent-4-en-l-ol, because such substrates were inert to 3a-3c (15). Pentene lacks any polar or protic group and pentenol contains the alkene and OH separated by 3 carbons. The preliminary studies involved phosphines with both imidazol-2-yl and pyrid-2-yl substituents on P as well as t-Bu, i-Pr, Ph, and Me groups (16). From the screening, complex 1 derived from the phosphine ligand 4 (17) was identified as the most capable (in terms of both reaction rate and final yield) of promoting isomerization of both 1-pentene and pent-4-en-l-ol. 

Palladium-Catalyzed Arylation of Enolates. Very substantial progress has been made in the use of Pd-catalyzed cross coupling for arylation of enolates and enolate equivalents. This reaction provides an important method for arylation of enolates, which is normally a difficult transformation to accomplish.171 A number of phosphine ligands have been found to promote these reactions. Bulky trialkyl phosphines such as /n. v-(/-butyl)phosphinc with a catalytic amount of Pd(OAc)2 results in phenylation of the enolates of aromatic ketones and diethyl malonate.172... 

The cross-dimerization of various electron-rich 1-alkynes 5 with electron-deficient internal alkynes such as methyl phenylpropiolate 6 was promoted by an [lrCl(cod)]2 combined with bidentate phosphine ligands such as (racj-BlNAP (Equation 10.2) [16]. This reaction produces a 1 1 adduct 7 in high regioselectivity and stereoselectivity. 

Extensive experimentation delineated the crucial features for the catalytic process to be (1) a cationic complex is necessary to promote the reaction efficiently (2) the use of l,2-bis(diphenylphosphino)benzene (dppbe) as the phosphine ligand and triflate as the counterion generates the product in high yield and suppresses isomerization and (3) employment of 10 atm. CO in 1,2-dimethoxyethane (DME) provides the optimum results, which are summarized in Tab. 11.10. 

The cobalt is present as a carbonyl derivative and can be directly active in the hydrocarbonylation steps of the process only when a large excess of cobalt is used in the presence of phosphine ligands and iodide promoters (Co/Ru 10/1). In this case the ruthenium is probably mainly involved in the hydrogenation of the aldehydes and their acetals to alcohols (O. 

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