Cross-coupling, Ni-catalyzed

This method can extend Ni-catalyzed cross coupling to functionalized organometallic reagents. 

Aryl-alkenyl cross-coupling is straightforward. Simple alkylmagnesium reagents (Me, Et, CH2SiMe3, etc.) can be easily involved in Ni-catalyzed cross-coupling (27),139,140 while more complex alkyl halides—particularly branched ones prone to /3-hydride elimination—require Pd catalysts with bidentate phosphines, such as dppf, to achieve good selectivity (Section 9.6.3.4.7). 

More intricate and potentially more attractive are catalytic transmetallation processes involving Zr, which may be classified into three categories shown as Classes 3—5 in Scheme 1.21. This area was mainly initiated and developed by Negishi in the 1970s. The initial discovery of the Ni-catalyzed cross-coupling [10] was soon followed by that of the Pd-cat-... 

Scheme 1.23. Pd- or Ni-catalyzed cross-coupling reactions of alkenylzirconium derivatives by Zr-to-Pd (or Ni) transmetallation.

Scheme 1.24. Synthesis of alkenylalanes by Zr-catalyzed carboalumination of alkynes followed by Pd- or Ni-catalyzed cross-coupling.

We have developed asymmetric syntheses of isocarbacyclin [3] (Scheme 1.3.2) and cicaprost [4] (Scheme 1.3.3) featuring a Cu-mediated allylic alkylation of an allyl sulfoximine [5-7] and a Ni-catalyzed cross-coupling reaction of a vinyl sulf-oximine [8-10], respectively, transformations that were both developed in our laboratories. The facile synthesis of an allyl sulfoximine by the addition-elimination-isomerization route aroused interest in the synthesis of sulfonimidoyl-sub-stituted aiiyititanium complexes of types 1 and 2 (Fig. 1.3.2) and their application as chiral heteroatom-substituted allyl transfer reagents [11]. 

Hydrozincation is not as widely observable as that involving , A1 and Zr. This is one of the main reasons why the indirect hydrometallation-transmetallation procedures shown in Schemes 8 and 12 have been developed and used. It is nevertheless highly desirable to directly generate organozincs to be used for the Pd- or Ni-catalyzed cross-coupling from alkenes and alkynes via hydrozincation. Indeed, such reactions have been developed, as shown in Scheme 17. However, further developments are clearly desirable. 

Although some bulky trialkylphosphines have been known for a long time, their usefulness in many previously difficult cases of the Pd- or Ni-catalyzed cross-coupling, such as those involving organic bromides and chlorides, especially alkyl bromides and chlorides, and alkylmetals, has been demonstrated mainly within the past several years. Some representative examples of alkyl-containing phosphines are shown in Figure 1. 

In most, if not all, of the transition metal-catalyzed organometallic reactions including the Pd- or Ni-catalyzed cross-coupling, solvents are not used merely to dissolve and dilute reactants and reagents. They often serve as cocatalysts or promoters and even reactants in a limited number of cases. More than 20 solvents have been used for the Pd- or Ni-catalyzed cross-coupling (Table 2). In the absence of any specific information, it is not unreasonable to choose first THF. Frequently, it is desirable to use one or more cosolvents typically for an increased level of solvent polarity and/or electron-donating ability. One of the most frequently used solvents for this purpose is DMF, but some others, such as NMP, pyridine and NMI (A-methylimidazole), have also been used frequently. In some extreme cases,... 

III. RECENT ADVANCES IN THE Pd- OR Ni-CATALYZED CROSS-COUPLING WITH ORGANOMETALS CONTAINING Zn, Mg, Al AND Zr... 

Inspection of Table 1 indicates that acylation, cyanation and enolate substitution represent the other classes of Pd- or Ni-catalyzed cross-coupling. The vast topic of Pd-catalyzed enolate -substitution including the Tsuji-Trost reaction and other -substitution reactions22 is not discussed in this chapter, and the readers are referred to pertinent reviews including that cited above. The other topics are very briefly discussed below. 

---