C-X bonds formation

The activation of C—H bonds and C—C bonds has attracted much attention in both academic and industrial laboratories because of their potential economic and ecological advantages. In the field of asymmetric synthesis, enantioselective catalytic C—X bond formation via the activation of C—H bonds and/or C—C bonds should have a great impact on asymmetric synthesis in both theory and practice. In theory, it is interesting to see how these very unreactive bonds can react preferentially in the presence of more reactive bonds with asymmetric control. In a practical sense, such C—H and C—C bonds are equivalent to the C M bonds in organometaUic reactions and would turn the corresponding stoichiometric amounts of metal into catalytic amounts. Conceptually, there are two fundamental ways to... 

Subsequent ion exchange of the metal cation with the quaternary ammonium ion catalyst provides a lipophilic ion pair (step 2), which either reacts with the requisite alkyl electrophile at the interface (step 3) or is partitioned into the electrophile-containing organic phase, whereupon alkylation occurs and the catalyst is reconstituted. Enantioselective PTC has found apphcation in a vast number of chemical transformations, including alkylations, conjugate additions, aldol reactions, oxidations, reductions, and C-X bond formations." ... 

The first cyclative cleavage of a small molecule was Crowley and Rapo-port s study41 of intramolecular Dieckmann condensations on solid phase (Fig. 13), which was complicated by the reversible nature of the cyclization. Despite the unique opportunities afforded by C-C rather than C-X bond formation during cleavage, there are few examples from the combinatorial age. A modern version of the Claisen-type condensation by Kulkarni and Ganesan42 uses a strongly acidic active methylene group to ensure unidirectional cyclization, and furnishes tetramic acids with three points of diversity. 

Bis(acetoxy)iodo]benzene (7, BAIB) can be used in combination with various reagents for oxidative additions to C,C-double bonds. Recent publications in this area are focused on C-N, C-O, C-S, C-Se, and C-X bond formation with emphasis on the co-introduction of non-equivalent heteroatom groups. The role of BAIB in such reactions is diverse, ranging from addition of the iodane to the double bond to the generation of iodine(III) and non-iodine(III) species that react with the olefinic substrates. 

Whereas the value of this reaction lies in the forging of the N—A bond, the importance of the C—X bond formation cannot be overlooked since it is often intrinsically necessary in heterocycle formation. It is with reactions of type (2) that this chapter deals. The only previous review article pertaining to this subject is that of E. N. Zil berman4 which does not deal with heterocyclic syntheses specifically. Recent advances justify a new review of the latter area. Principally, the use of three types of reactions are discussed (a) the Ritter reaction (b) reactions involving nitrilium salts and (c) a, j-dinitrile cyclizations. All appear to be variations of the above theme. 

Marek I (2004) In de Meijere A, Diederich F (eds) Modern C-C and C-X bond formations by metal-catalyzed cross-coupling reactions. Wiley-VCH,Weinheim (in press)... 

The reductive elimination product from alkyl halide complexes L M(X)yR2 is usually the alkane R—R. In some cases, however, C—X bond formation may compete with C—C elimination, as in (dppe)Pt(I)Me3 101... 

Figure 35. Silver-mediated C X bond formation and preparation of diiodobenzene derivative.

Silver-mediated C—X bond formation was known for many years as the Hunsdiecker reaction, in which substrates bearing carboxylic acids are oxidatively decarboxylated to give alkyl halides in the presence of halogens (142). If olefins are used with silver benzoate and I2, the Prevost reaction occurs to yield diols (143). 

The enantioselective C—C and C—X bond-formation catalyzed by inorganic supported heterogeneous chiral catalysts is summarized in Table 2.3, and selected examples are further described in this section. 

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