Metal countercations

Particularly important is the influence of the metal countercation e.g., cleavage of the achiral lactone l,3-dimethyl-6//-benzo[ ,]naphtho[1.2-d]pyran-6-one in toluene at — 40 CC, using different metal alkoxides, e.g., (+)-menthol as inexpensive chiral 0-nucleophiles, gives atropisomer ratios of 81 19 for lithium, 66 34 for sodium, and 63 37 for potassium. In other solvents (e.g., THF), this selectivity order may be reversed14. 

Until recently, the Pd- or Ni-catalyzed cyanation had been performed most frequently with metal cyanides containing alkali metals, such as Na and K21a. More recently, however, the use of Zn(CN)2 in conjugation with DMF has been reported to be a useful alternative216 (Scheme 83). Further delineation of the relative merits and demerits among various metal countercations appears to be desirable. 

In contrast to radical polymerizations in which there is only one type of propagating species, ionic polymerizations may involve several active species, each with different reactivities and/or lifetimes. As outlined in Scheme 2, ionic polymerizations may potentially involve equilibria between covalent dormant species, contact ion pairs, aggregates, solvent separated ion pairs, and free ions. Although ion pairs involving alkali metal countercations can not collapse to form covalent species, group transfer polymerization apparently operates by this mechanism. In anionic polymerizations, free ions are much more reactive than ion pairs although the dissociation constants are quite small = 10 ) [5]. 

Despite the general superiority of Zn over Mg, it is important to retain Mg as an option, especially in those cases where arylzincs are generated via arylmagnesium derivatives, and the use of Zn as the second metal countercation must be experimentally justified. It is also important to consider both Ni and Pd as catalyst components. Despite its shortcomings, Ni is significantly less expensive than Pd. So, it is very desirable to experimentally justily the use of Pd in preference to Ni. ... 

In general, critical comparisons among various metal countercations is lacking. Since many of them are comparably effective, vinylmagnesium halides that can readily be obtained as the first-generation vinylmetals should be considered first. Only if it is unsatisfactory for one reason or another, should one then consider other metals such as Zn, Si, and Sn. For yet unclear reasons, vinylboron derivatives have rarely been used. 

Rigorous comparisons of metal countercations are still relatively rare. In the synthesis of (Z)-tamoxifen and related compounds, Mg and Zn appear to be superior to Sn in terms of stereospecificity and operational simplicityt (Scheme 40). In cases where chemoselectivity is critically important, as in the synthesis shown in Scheme Zn would have to be chosen over Mg. 

A.i. Scope of the Pd-Catalyzed Alkynylation with Respect to Metal Countercations... 

As shown in Table 1, a wide variety of metal countercations have been shown to participate in this reaction. Under the same reaction conditions, alkynylmetals containing Zn exhibit the highest reactivity (Entry 6), which is followed by those of Mg (Entries 4 and 5), Al (Entries 12-14), and Sn (Entries 16 and 17). Although the product yield observed with 1-heptynylmagnesium bromide was modest (49%), alkynytmagnesium reagents have since been shown to be very satisfactory in many cases. ... 

TABLE 2. Comparison of Metal Countercations in the Direct Synthesis of Terminal Alkynes hy Pd-Catalyzed Ethynylation with Ethynylmetals Containing Mg, Zn, and Sn... 

As in some other cases of Pd-catalyzed cross-coupling, nearly ten or a dozen metals including Li, Cu, Mg, Zn, B, Al, and Sn have been used as the metal countercations, and trends similar to several other cases have been observed. 

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