Anti-Markovnikov Hydrofunctionalization

Although ruthenium and Group 6 metal catalysts are commonly employed for anti-Markovnikov alkyne hydrofunctionalization (Chapter 10), some interesting rhodium- and iridium-catalyzed methods have also been reported. These can be divided into three groups based on the nature of the incoming functional group  

The Trost group has disclosed two methods of rhodium vinylidene-mediated hydrofunctionalization (i) intramolecular hydroalkoxylation of alkynols and 

Tungsten- and molybdenum-catalyzed methods involving vinylidene intermediates have been described for this transformation (Chapter 5). The use of rhodium provides some advantages in terms of catalyst turnover and selectivity. The catalyst formed in situ from a RhCl source and a fiuormated triarylphosphine promotes the cyclization of a variety of alkynols (Table 9.6). 

Alkynyl anilines are simple and convenient starting materials for the preparation of indoles. Unprotected, unfunctionalized 2-ethynyl aniline can be cycloisomerized to indole via molybdenum vinylidene-mediated catalysis [15]. Unlike (Et3N)Mo(CO)5, 

Hydroboration and hydrosilylation reactions ofalkynes, when they do not proceed spontaneously, can be catalyzed by numerous transition metals [17]. Metal vinyli-dene-mediated processes uniquely provide (Z)-alkenes via trans-addition . In 2000, Miyaura and coworkers discovered that both Rh(I)- and Ir(I)-complexes supported by bulky electron-rich phosphine ligands catalyze the hydroboration of terf-butylacety-lene to give alkenylboronate 41 with 95 5 Z/E selectivity (Equation 9.5) [18]. 

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Radicaloid insertions of olefins into the Rh H bond of [Rh (TPP)(H)] has been used to obtain Rh —CH2—(alkyl)—Nu// species (NuH = OH, NH) using olefins functionalized with end-on —OH and —NH functionalities. Under basic conditions, intramolecular SN2-type attack of the Nu at the a-carbon atom of Rh —CH2—(alkyl)—Nu yields [Rh (TPP)] and cyclic organic products (—CH2—(alkyl)—Nu—) (see Fig. 47). Protonation of [Rh TPP)] then allows regeneration of [Rh° (TPP)(H)]. The combination of these reactions constitutes a new method for selective intramolecular anti-Markovnikov hydrofunctionalization of olefins with O—H and N—H functionalities (150). In this way, three-and five-membered ring compounds (epoxides, furan derivatives, pyrrolidine derivatives) were readily obtained. Formation of four- or six-membered rings... 

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