Aryl, cyclization

Arylative cyclization of alkynals with arylboronic acids is catalyzed by rhodium-diene complexes and even proceeds enantioselectively in the presence of a chiral diene (Equation (48)).399... 

Simiraly, alkynones undergo arylative cyclization with arylboronic acids in the presence of a rhodium catalyst (Equation (49)).400 When acetylenic /3-keto esters are employed as shown in Equation (50), arylative cyclization (formation of cyclobutanols) and subsequent, facile acid-catalyzed bond cleavage take place to give <5-keto esters.401 Ring expansions of cyclic [3-keto esters are also possible according to this reaction. 

Recently, addition of organorhodium species to nitriles has been reported.420 4203 4201 Intermolecular reaction of benzonitrile with phenylborate (accompanied with r//w-aryiation) (Equation (65)), arylative cyclization of acetylenic nitriles (Equation (66)), and cyclization of 2-cyanophenylboronic acid with alkynes or strained alkenes (Equation (67)) are proposed to proceed via this process. 

Scheme 16.18 Formation of a heterocycle via arylative cyclization using indium.

Figure 3.46. Scope of Rh/48-catalyzed asymmetric arylative cyclization of alkynals with arylboronic acids.

Whereas 1-alkyl substituted 5-aminopyrazoles 12 (R = alkyl or aryl) cyclize on treatment with -keto esters into pyrazolo[3,4-h] pyrimidines, cyclocondensation of 12 (R = Ts Ts, p-toluenesulfonyl) with / -keto esters affords pyrazolo[l,5-a]pyrimidines (71JPR969). 

A microwave-assisted one-pot two-step approach towards 1-arylindazoles relied on a copper-catalyzed intramolecular N-arylation-cyclization of in situ formed arylhydrazones [72], Traditionally, the Cul-diamine-complex catalyzed N-arylation reactions occurred at elevated temperatures (up to 110 °C) and required prolonged time (up to 24 h) to go to completion [73]. The com-... 

When the presumed intermediate allylic ion 139 (Scheme 33) carries an aryl group on a terminal car n, e.g. = aryl, cyclization occurs to yield the indene(s) 141. Examples include the formation of indenes 141a, 1416, " 141c, and from acid-... 

Indoles and Carbazoles. - Formation. 2-Arylindoles (132) are formed by intramolecular Wittig reaction of the phosphonium salts (131). The hydroxamic acids PhN(OH)COCH2COR (R = alkyl or aryl) cyclize in boiling toluene to mixtures of indoles (133) and 3-isoxazolones (134). Irradiation of a solution of o-iodoaniline and the potassium enolate of acetone affords 2-methylindole. The enamino-ketone (135) cyclizes photochemically to 1,2-dimethylindole (136) with elimination of acetaldehyde/ The styrene derivative (137), obtained by the action of Meerwein s acetal, Me2NCH(OMe)2, on o-nitrotoluene, yields 1-hydroxyindole on treatment with zinc/ Azidobenzocyclobutanes (138 R = Me, Ph, or CH2Ph) are converted into indoles (133) by the action of concentrated sulphuric acid/ ... 

Shintani et al. have also applied similar reaction conditions to those used for 1,4-conjugate additions ofboronic acids to a-P-unsaturated ketones to the arylative cyclization of alkynals, e.g., 275. Using a variety of boronic acids, the enantioselectivity was found to be generally high (75-96% ee) with good-to-excellent yields.t J... 

Scheme 8.43 Rhodium/diene-catalyzed arylative cyclization of alkynals 155.

Arylative cyclization of propargyl-substituted malonate esters RC=CCH2CH-(C02Me)2 with aryl halides, catalysed by the Pd-xantphos complex, has been reported to afford alkylidenecyclopropanes. The reaction proceeds via an initial flnft-carbopalladation of the C C bond, which controls the configuration of the resulting C=C in the product. ° ... 

The tandem arylrhodation/conjugate addition reaction, rhodium-catalyzed asymmetric arylative cyclization of cyclohexadienone-containing meso-l,6-dienynes was efficiently developed to provide optically pure d5-hydrobenzo[f ]furans with high to excellent yields and excellent enantioselectivities (13AGE5314). 

Figure 2.23 A cross-section of results for the copper-catalyzed sequential one-pot Chan-Lam amidine arylation-cyclization reported by Jieping Zhu s group [103].

MacMillan and co-workers developed a novel catalytic asymmetric cascade arylation-cyclization reaction of indole-based nucleophiles with diaryliodonium salts using Cu(I)-bisoxazoline catalysis under mild conditions (Scheme 39)... 

Scheme 39 Asymmetric cascade arylation-cyclization reaction...

Lin and Tian developed a rhodium-catalyzed asymmetric arylative cyclization of cyclohexadienone-containing prochiral 1,6-dienynes 170 through a tandem arylrhodation/conjugate addition reaction (Scheme 2.44). This reaction could provide optically pure dy-hydrobenzofurans 172 with high to excellent yields (80-99%) and excellent enantioselectivities (95-99% ee). Furthermore, the cyclization products could also be transformed to interesting chiral frameworks of some natural products, thus demonstrating the utility of the products [63]. 

He, Z.-T., Tian, B., Fukui, Y, Tong, X., Tian, R, Lin, G.-Q. (2013). Rhodium-catalyzed asymmetric arylative cyclization of /we5o-l,6-dienynes leading to enantioenriched cw-hydrobenzofurans. Angewandte Chemie International Edition, 52, 5314-5318. 

The products can undergo palladium-catalyzed desilylative arylation with aryl halides and, thus, various dihydrofuran derivatives can be prepared by the sequence of cross-dimerization/arylation/cyclization. 

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