Hydroamination transition metal catalyst

Organometallic complexes of the /-elements have been reported that will perform both intra-and intermolecular hydroamination reactions of alkenes and alkynes, although these lie outside of the scope of this review.149-155 Early transition metal catalysts are not very common, although a number of organometallic systems exist.156-158 In these and other cases, the intermediacy of a metal imido complex LnM=NR was proposed.159,160 Such a species has recently been isolated (53) and used as a direct catalyst precursor for N-H addition to alkynes and allenes (Scheme 35).161,162... 

Transition metal catalysts from across the periodic table have been investigated for this transformation. [56b, 57] Early transition metal catalysts [58] are of particular interest due to their high reactivities, with reduced air and moisture sensitivity compared with the rare earth metal systems, and lower cost and toxicity compared with the late transition metal catalysts. The A,0-ligands generating tight four-membered metallacycles described above have been studied as precatalysts for hydroamination methodologies that display promising substrate scope and reactivity. 

Catalytic asymmetric hydroamination of alkenes can be achieved using early and late transition metal catalysts and lanthanide-based catalytic... 

Early Transition Metal Catalysts 11147 Table 15.6 Group 4 hydroamination of gem-disubstituted aminoalkene substrates. 

Beller took advantage of simple Zn salts and used Zn(OTf)2 for the hydroamination of terminal aUcynes with arylamines (218). These reactions require similar temperatures as has been reported for other late transition metal catalysts (100-120 °C) and as is most commonly observed for this combination of substrates, the Markovnikov product is preferred (Scheme 15.37). 

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

Key contributions in the development of late transition metal catalysts toward alkene hydroamination, which precede the 2008 comprehensive review [10], focus on contributions using group 9 and 10 metals. Preferred substrates for these transformations include aminoalkenes [230] for intramolecular reactivity or the use of activated alkenes such as styrene [93, 109, 113, 245] or alkenes substituted with electron-withdrawing substituents to generate hydroamination products via aza-Michael-type reactions [246-249]. Au has also been applied to the hydrofunctionalization of alkenes, although these reactions have demanded the use of protected amine substrates such as ureas [250], tosylamides [251], and carbamates [252]. 

Protected nitrogen substrates in combination with late transition metal catalysts have proven exceptionally useful for addressing the aforementioned substrate scope problems when trying to mediate hydroamination with unactivated alkene substrates. In asymmetric variants of this reaction, early work by Yamamoto showed that protected aminoalkynes could be used as cydohydroamination substrates to yield chiral heterocydes with vinyl substituents [108, 225). Here the chiral chelating phosphine ligand 66 ((J ,J )-RENOPHOS) in combination with a Pd(0) precursor and benzoic acid yielded the desired products in good yield with up to 91% ee (Table 15.26). Unfortunately, to obtain these optimized enantiomeric excesses,... 

A variety of catalyst systems have been developed for the facile intermolecular hydroamination of alkynes, in particular employing early transition metal catalysts based on group 4 metals (Fig. 13). Important issues such as reactivity, reaction... 

Thus, early transition metal catalyst systems have yet to reach the nearly perfect degree of stereoselectivity (up to 99% ee) achieved with late transition metal catalysts [263-266] and dithiophosphoric acids [267]. However, it should be noted that these systems are confined to 77-protected (tosylates, ureas, carbamates) amines with reduced nucleophilicity, and the highly selective asymmetric hydroamination of aminoallenes with simple amino groups remains a challenge. 

Consequently, the late transition metal-catalyzed hydroamination is focused in this chapter. In general, the late transition metal catalysts are relatively stable in air and tolerant of most of the polar functional groups. Accordingly, the catalysts are convenient to handle and perhaps applicable to many industrial syntheses. 

The first transition metal-catalyzed hydroamination of an olefin was reported in 1971 by Coulson who used rhodium(I), rhodium(III) or iridium(III) catalysts (Eq. 4.8) [105,106]. 

The hydroamination of 1,3-dienes has been attempted with catalyst precursors of nearly all transition metals. The most active are those based on Group 9 and 10 transition metals. 

Due to its marked atom economy, the intramolecular hydroamination of alkenes represents an attractive process for the catalytic synthesis of nitrogen-containing organic compounds. Moreover, the nitrogen heterocycles obtained by hydroamination/cyclisation processes are frequently found in numerous pharmacologically active products. The pioneering work in this area was reported by Marks et al. who have used lanthanocenes to perform hydroamination/cyclisation reactions in 1992. These reactions can be performed in an intermolecular fashion and transition metals are by far the more efficient catalysts for promotion of these transformations via activation of the... 

Late transition-metal hydroamination is the method of choice for the atom economical and functional group-tolerant construction of C—N bonds, and in this context Ir plays a central role (indeed, homogenous transition-metal-catalyzed OHA was discovered with Rh and Ir). However, there is a strong need for the development of better OHA catalyst systems that are applicable to a wider range of substrates and conditions. The characteristics of current Ir based catalyst systems to function via N—H bond activation, though, is a potential handicap to achieve this goal, since it implies highly reactive Ir intermediates that are prone... 

The excellent ability of late transition metal complexes to activate alkynes to nucleophilic attack has made them effective catalysts in hydroamination reactions. The gold(l)-catalyzed cyclizations of trichloroacetimidates 438, derived from homopropargyl alcohols, furnished 2-(trichloromethyl)-5,6-dihydro-4f/-l,3-oxazines 439 under exceptionally mild conditions (Equation 48). This method was successfully applied to compounds possessing aliphatic and aromatic groups R. With R = Ph, cyclization resulted in formation of 439 with complete (Z)-stereoselectivity <2006OL3537>. 

There is a rich patent literature about the use of transition metal complexes in photochemical hydroamination, and the application of heterogeneous catalysts and the ammonium ion as catalyst.290... 

Summaries of results of hydroamination mediated with Rh(I) amide complexes584 and comprehensive reviews giving detailed information of the field are available.585-587 Therefore, only the more important relatively new findings are presented here. In most of the transformations reported transition metals are applied as catalysts. The feasibility of the use of tcrt-BuOK was demonstrated in the base-catalyzed amination of styrenes with aniline.588... 

As the first transition metal-based homogeneous catalysis of hydroamination, in the early 1970s Coulson from the Du Pont laboratories had described the addition of secondary aliphatic amines to ethylene in the presence of various rhodium compounds [15, 16]. Definite results were reported with RhCl3 3 H2O as pre-catalyst in tetrahydrofuran as solvent under starting ethylene pressures of 5-14 MPa at 180-200 °C for different secondary amines (Table 3). 

Earher mechanistic studies by Milstein on a achiral Ir catalyst system indicated that the iridium catalyzed norbornene hydroamination involves amine activation as a key step in the catalytic cycle [27] rather than alkene activation, which is observed for most other late transition metal catalyzed hydroamination reactions [28]. Thus, the iridium catalyzed hydroamination of norbornene with aniline is initiated by an oxidative addition of aniline to the metal center, followed by insertion of the strained olefin into the iridium amido bond (Scheme 11.4). Subsequent reductive elimina tion completes the catalytic cycle and gives the hydroamination product 11. Unfor tunately, this catalyst system seems to be limited to highly strained olefins. 

---