Hydroamination allenes

Allene hydroamination is less commonly explored, even though the thermodynamic profile of the reaction is comparable to alkyne hydroamination [40]. Intermolecular allene hydroamination has been established using group 4 catalysts in combination with reactive arylamine substrates [8, 41]. The more reactive aforementioned alkyne hydroamination catalyst 7 has been shown to be usefiil for allene hydroamination catalysis in an intermolecular manner, even with less reactive, sterically less demanding alkylallene substrates. In this case, only the branched product is observed (Table 15.5). These results show good selectivity for the branched product, and recent results show that even heteroatom-substituted allenes can be tolerated with this precatalyst [42]. 

Most importantly, this reaction demands control of regioselectivity and can also be carried out asymmetrically (Scheme 15.38). Thus, branched imine or linear allylamine products can be selectively prepared. Diastereoselective and enantiose-lective allene hydroamination can also be targeted with advances in enantioselective catalysis using late transition metals being reviewed in Section 15.3.7. 

More recently, reactivity investigations have explored the scope of reactivity of previously developed catalysts. With respect to advances in group-lO-catalyzed allene hydroamination, Widenhoefer [229] explored Pt(II) neutral and cationic species for the intermolecular hydroamination of allenes with secondary alkylamines to make aUylamine products with excellent regioselectivity and diastereoselectivity (E/Z) (Scheme 15.43). This work builds upon a 2005 report for neutral Pt(II) compleiKs for the intermolecular hydroamination of alkenes with secondary amines [230, 231]. In the 2010 report, a variety of cyclic and acyclic alkylamines can be used as substrates, although neither arylamines nor primary amines are disclosed as substrates. This system is also limited to monosubstituted allenes. Consistent with long-standing proposals [231], outer-sphere addition of the amine to a cationic Pt(II) It-allene complex is proposed [229]. 

Scheme 15.44 N,P-chelated Pd-catalyzed allene hydroamination with dihydroindoie.

In gold-catalyzed allene hydroamination, Bertrand has shown that the previously discussed cationic CAAC complexes for Au(I) (Table 15.17) are very important in the hydroamination of allenes with ammonia (Section 15.3.3) [179], and with both primary and secondary amines [235]. Notably, both aryl- and alkylamines can be used, and both 1,1-disubstituted allenes and monosubstituted allenes can be used as substrates to give allylamine products. In the case of monosubstituted allenes, mixtures of the T/Z diastereomers resulted. Depending on the amine substrate... 

In an effort to realize a readily accessible catalyst system capable of mediating allene hydroamination with a variety of substrates, Widenhoefer [239] showed that a commercially available bulky phosphine ligand (P(tBu)2-o-biphenyl) can be complexed to AuCl and activated in situ with AgOTf to generate catalytically active systems. 1,1-Disubstituted allenes react with primary and secondary arylamines to typically give the allylamine products in good to excellent yield at modest reaction temperatures (45 °C) within 24 h (Table 15.18). This same system can also mediate the hydroamination of 1,3-disubstituted allenes with modest to excellent diastereoselectivity (up to >25 1 /Z selectivity). No notable examples of primary or secondary alkylamines have been reported. 

Late Transition Metal Catalysts 11191 Table 15.19 Au-NHC-catalyzed allene hydroamination of carbamates. 

Scheme 15.123 Au-catalyzed allene hydroamination in the formal synthesis of swainsonine.

Allene Hydroamination with Zirconium Ureate Complex (5) [42]... 

NHC-Au catalysts, in line with results obtained in hydration and hydroalkox-ylation reactions vide supra], proved to be highly efficient in this field, and the inter- and intramoleeular hydroamination of various alkenes, allenes, and alkynes were reported with a number of NHC-Au complexes. In 2008, Bertrand and co-workers disclosed an extremely efficient alkyne and allene hydroamination catalyzed by a CAAC-Au eomplex (Scheme 11.15). These peculiar NHCs allowed for the challenging use of ammonia and proved useful in a hydroamination/hydroarylation cascade leading to 1,2-dihydroquinolines. The same authors subsequently employed a similar method to perform the addition of hydrazine on alkynes in an intermoleeular manner. ... 

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