Arylation at carbon

For a monograph on the formation of C—C bonds by radical reactions, see Giese, Ref. I. For a review of arylation at carbon, see Abramovitch Barton Finet Tetrahedron 1988, 44. 3039-3071, For a review of aryl-aryl coupling, sec Sainsbury Tetrahedron 1980, 36, 3327-3359. 

Alkynes can also be arylated by the Meerwein procedure, as shown by Muller in 1949. The reaction of buta-l-en-3-yne (Scheme 10-47) was studied by Kheruze and Petrov (1960). Arylation at an sp2-hybridized carbon is obviously considerably faster than the analogous reaction at an sp-hybridized carbon. A mechanistically interesting case of a Meerwein reaction with phenylacetylene will be discussed later in this section. 

Some cyclic thioacetals have an A-SE2 hydrolysis mechanism,206 as do some 2-aryl-2-methyl-l,3-dithianes, except for the 4-NO2 derivative, which looks more A2-like.207 In 10 vol% dioxane/aqueous HC104 mixtures, reactive 2-aryl-2-phenyl-l,3-dithianes are believed to have an A-SE2 hydrolysis mechanism, whereas the least reactive ones have an A2 mechanism.130 Isothiocyanates are believed to hydrolyze by a mechanism that involves simultaneous proton transfer to nitrogen and attack of water at carbon in a cyclic transition... 

Carbon monoxide rapidly inserts into the carbon—zirconium bond of alkyl- and alkenyl-zirconocene chlorides at low temperature with retention of configuration at carbon to give acylzirconocene chlorides 17 (Scheme 3.5). Acylzirconocene chlorides have found utility in synthesis, as described elsewhere in this volume [17]. Lewis acid catalyzed additions to enones, aldehydes, and imines, yielding a-keto allylic alcohols, a-hydroxy ketones, and a-amino ketones, respectively [18], and palladium-catalyzed addition to alkyl/aryl halides and a,[5-ynones [19] are examples. The acyl complex 18 formed by the insertion of carbon monoxide into dialkyl, alkylaryl, or diaryl zirconocenes may rearrange to a r 2-ketone complex 19 either thermally (particularly when R1 = R2 = Ph) or on addition of a Lewis acid [5,20,21]. The rearrangement proceeds through the less stable... 

In the case of 4-(./V,./V -diaryl)guanidines 64, base-induced cyclization occurs at carbon bearing the methylthio to give the corresponding 7-aryl-6-arylamino-2-methyl-777-[l,2,4]triazolo[4,3- ][l,2,4]triazol-3(2//)-thiones 65 (Equation 12) <1986H(24)3363,1987JP(1)2667>. 

According to this correlation model, in which the principles of steric control of asymmetric induction at carbon (40) are applied, the stereoselectivity of oxidation should depend on the balance between one transition state [Scheme 1(a)] and a more hindered transition state [Scheme 1(6)] in which the groups and R at sulfur face the moderately and least hindered regions of the peroxy acid, respectively. Based on this model and on the known absolute configuration of (+)-percamphoric acid and (+)-l-phenylperpropionic acid, the correct chirality at sulfur (+)-/ and (-)-5 was predicted for alkyl aryl sulfoxides, provided asymmetric oxidation is performed in chloroform or carbon tetrachloride solution. Although the correlation model for asymmetric oxidation of sulfides to sulfoxides is oversimplified and has been questioned by Mislow (41), it may be used in a tentative way for predicting the chirality at sulfur in simple sulfoxides. 

Uneyama has described some interesting reactions of the N-aryl imines of ethyl trifluoropyruvate. Tandem alkylation/defluorination occurred (Eq. 97) upon exposure to diethylzinc, via attack at nitrogen and SN2 displacement of fluoride anion [279]. Interestingly, an alkylzinc halide reagent attacked regioselec-tively at carbon, perhaps the more expected outcome. 

Arylation at a Carbon Containing Active Hydrogen Bis(ethoxycarbonyl)methyl-de-halogenation, etc. 

Imidothioesters offer a number of other possibilities. Thus, it was orted that N-phenyl thioimidoesters can be metallated at low perature by LDA or n-butyllithum. Although the resulting enaminates wn in the scheme) were alkylated on nitrogen when R1 was an alkyl up, more interesting alkylation occurred regioselectively at carbon when 1 was a vinyl or aryl group [152] and also with R =H or Me3Si [380]. 

During arylations of carbon nucleophiles with aryl halides in the presence of palladium triarylphosphine complexes products containing the aryl group of the phosphine can result (Scheme8.16). These reactions proceed via reversible arylation of the Pd-bound phosphine, which occurs at temperatures above 50 °C, particularly readily in the presence of iodide [11] (see Section 8.2). Electron-deficient aryl groups usually migrate less readily than electron-rich groups [23, 25],... 

---