1.3- dipolar cycloaddition reactions base-catalyzed formation

Within this chapter, two sections are devoted to rhodium and ruthenium. The two main procedures using rhodium are first, the formation of 1,3-dipoles from diazocompounds followed by a 1,3-dipolar cycloaddition [10] and second, hy-droformylation [11], The ruthenium-catalyzed domino reactions are mostly based on metathesis [12], with the overwhelming use of Grubbs I and Grubbs 11 catalysts. 

As 1,4-disubstituted 1,2,3-triazoles are usually prepared through copper-catalyzed 1,3-dipolar cycloadditions of terminal alkynes with organic azides, the use of a single copper complex for a direct arylation-based sequential catalysis was probed. Thereby, a modular chemo- and regioselective synthesis of fully-substituted 1,2,3-triazoles was achieved (Scheme 9.43). Notably, the overall reaction involved the selective coupUng of four components through the formation of one C—C- and three C—N-bonds [58]. 

Tennant [204], Smalley et al. [205], and Jones et al. [206]. For example, l,2,3-triazolo[l,5-a]qninazolines 70 were obtained in moderate yields by base-catalyzed cycloaddition/condensation reaction (Scheme 14) [205]. Synthesis of 4(3H)-qninazolinones and -imines via 1,3-dipolar cycloaddition has been reported also [207,208]. Novel formation of quinazoUnes via thermal ring contraction of 3H-1,4-benzodiazepines is found by Sashida et al. [209]. Synthesis of quinazoline alkaloids by cycloadditions is discussed in Sect. 3. 

The mechanism of the CuAAC reaction was rst proposed by Meldal (Tomoe et al., 2002) and Sharpless (Rostovtsev et al., 2002) and later veri ed by computational methods by Sharpless (Himo et al., 2005) in a series of papers. The proposed catalytic cycle based on a concerted mechanism via a Cu-acetylide intermediate is shown in Fig. 12.7. The most effective variant of the catalyzed 1,3-dipolar azide-alkyne cycloaddition system uses terminal alkynes in combination with copper sulfate and sodium ascorbate. The sodium ascorbate reduces copper sulfate to Cu(I), which forms a Cu-acetylide by reaction with the terminal alkyne via an initial r-complex formation. The copper acetyhde formed is considerably more reactive toward the azide so that a rate enhancement of the 1,3-dipolar cycloaddition results (Englert et al., 2005). 

Use of an imine in conjunction with diazoacetates allows the formation of an azomethine yUde intermediate that can undergo a 1,3-dipolar cycloaddition with a suitable dipolarophile. Based on this strategy, CuOTf efficiently catalyzed a three-coir onent assembly reaction for the synthesis of 2,5-frans-pyrrolidines... 

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