Alkynes actinide complexes

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Phosphorus—selenium bonds, alkyne additions, 10, 782 Phosphorus—sulfur bonds, alkyne additions, 10, 781 Phosphorus ylides with gold(I), 2, 275 with trinuclear Os clusters, 6, 740 Phosphoylides, actinide complexes, 4, 203 Photoacoustic calorimetry, in thermochemistry, 1, 612-613 Photoactive molecular devices, and -[RefCO) ,] fragment, 5, 886... 

The actinide complexes Cp 2AnMe2 (An = Th, U) have been found to effectively catalyze the coupling reaction of terminal alkynes and /-butylisonitrile, B NC. The catalytic conversion of the isonitrile and alkyne to l-aza-1,3-enynes was achieved in toluene or benzene at 90-100 °G, while no reaction was observed in the absence of a catalyst. Scheme 97 illustrates a plausible mechanism for the catalytic coupling of BuNC and terminal alkynes mediated by Cp 2AnMe2.193... 

The remainder of this section will focus on true SBMs, which have been the subject of vigorous research. Despite the electron deficiency of early transition metal, lanthanide, and actinide complexes, several groups reported that some of these d f" complexes do react with the H-H bond from dihydrogen and C-H bonds from alkanes, alkenes, arenes, and alkynes in a type of exchange reaction shown in equation 11.32. So many examples of SBM involving early, middle, and late transition metal complexes have appeared in the chemical literature over the past 20 years that chemists now consider this reaction to be another fundamental type of organometallic transformation along with oxidative addition, reductive elimination, and others that we have already discussed. 

We have shown in this review that neutral and cationic organoactinide complexes have been extensively studied, in the last decade, as catalysts for several organic transformations [9-12, 111]. Polymerization of alkenes[112,113], oligomerization of terminal alkynes [55, 114], hydrosilylation of terminal alkynes [41], and 1,1-insertion of isonitriles into terminal alkynes [28] comprise some other studied processes not presented here. However, due to the high oxophilicity of the actinide complexes (as mentioned above), substrates containing oxygen have been excluded because of the expected and predictable oxygen—actinide interaction. 

Hydroamination of Alkynes Catalyzed by Lanthanide and Actinide Complexes... 

Some of the most active catalysts for the hydroamination of alkynes are based on lanthanides and actinides. The turnover frequencies for the additions are higher than those for lanthanide-catalyzed additions to alkenes by one or two orders of magnitude. Thus, intermolecular addition occurs with acceptable rates. Examples of both intermolecular and intramolecular reactions have been reported (Equations 16.87 and 16.88). Tandem processes initiated by hydroamination have also been reported. As shown in Equation 16.89, intramolecular hydroamination of an alk5me, followed by cyclization with the remaining olefin, generates a pyrrolizidine skeleton. Hydroaminations of aminoalkynes have also been conducted with the metallocenes of the actinides uranium and thorium. - These hydroaminations catalyzed by lanthanide and actinide complexes occur by insertion of the alkyne into a metal-amido intermediate. 

The hydroamination of olefins has been shown to occur by the sequence of oxidative addition, migratory insertion, and reductive elimination in only one case. Because amines are nucleophilic, pathways are available for the additions of amines to olefins and alkynes that are unavailable for the additions of HCN, silanes, and boranes. For example, hydroaminations catalyzed by late transition metals are thought to occur in many cases by nucleophilic attack on coordinated alkenes and alkynes or by nucleophilic attack on ir-allyl, iT-benzyl, or TT-arene complexes. Hydroaminations catalyzed by lanthanide and actinide complexes occur by insertion of an olefin into a metal-amide bond. Finally, hydroamination catalyzed by dP group 4 metals have been shown to occur through imido complexes. In this case, a [2+2] cycloaddition forms the C-N bond, and protonolysis of the resulting metallacycle releases the organic product. 

A number of actinide complexes have been investigated with respect to their catalytic activity in the intermolecular hydroamination of terminal alkynes with primary ahphatic and aromatic amines [98, 206-209]. Secondary amines generally do not react and the reaction is believed to proceed via an metal-imido species similar to that of group 4 metal complexes. The reaction of Cp 2UMc2 with sterically less-demanding aliphatic amines leads exclusively to the anti-Markovnikov adduct in form of the -imine (31) [207] however, sterically more demanding amines, e.g., t-BuNH2, result in exclusive alkyne dimerization. The ferrocene-diamido uranium complex 12 (Fig. 4) catalyzes the addition of aromatic amines very efficiently (32) [98]. 

Although zirconium is only one out of over 50 potentially usable metals in this class (including the lanthanides and actinides), virtually all synthetic applications of hydrometallation with transition metals involve zirconium Why is this so The primary reason derives from the near requirement of a d -metal center for hydrometallation of a general alkene or alkyne. For later transition metals, hydrometallation to give a stable organometallic product can usually be achieved only for special cases—conjugated dienes, alkenes with electronegative substituents, etc. This is due to the relative stability of the ti -complex, as discussed previously. 

As discussed in Chapter 9, the insertion of olefins and alk)nes into metal-amido complexes is limited to a few examples. Such insertion reactions are proposed to occur as part of the mechanism of the hydroamination of norbomene catalyzed by an iridium(I) complex and as part of the hydroamination of alkenes and alkynes catalyzed by lanthanide and actinide metal complexes. This reaction was clearly shown to occur with the iridium(I) amido complex formed by oxidative addition of aniline, and this insertion process is presented in Chapter 9. The mechanism of the most active Ir(I) catalyst system for this process involving added fluoride is imknown. 

Uranium (III) redox reactivity with small molecules always consists in the preliminary double reduction of the latter by two U(III) complexes. Thus, the reactivity concerns only U(IV) bimetallic complexes. DFT calculations can be used for this problem thanks to the recent development of 5f-in-core ECPs by Moritz et al. and a methodology to compute the preliminary redox step via the combination of small core and 5f-in-core calculations. Recent theoretical studies have mainly concerned the reduction of CO2 and CO, but attention has also to be paid to the reduction of CS2, COS, PhNCO, and PhNs, the C-C coupling of terminal bis-alkynes to form U(IV) vinyl complexes and the reduction of arenes. However, in order to have more elements to discuss about actinide properties, one must perform calculations at a multireference post Hartree-Fock level to take into account the important effect of electronic correlation in these systems however, there is the obstacle of the computational power to perform calculations of real systems at this level that stands still. 

Only a limited number of organoactinide catalysts have been investigated for the hydroamination/cyclization of aminoalkenes (Fig. 4, Table 2) [55, 96-98]. The constrained geometry catalysts 11-An (An = Th, U) show high activity comparable to the corresponding rare earth metal complexes and can be applied for a broad range of substrates [55, 96, 97]. The ferrocene-diamido uranium complex 12 was also catalytically active for aminoalkene cyclization, but at a somewhat reduced rate [98]. Mechanistic studies suggest that the actinide-catalyzed reaction occurs via a lanthanide-like metal-amido insertion mechanism and not via an imido mechanism (as proposed for alkyne hydroaminations), because also secondary aminoalkenes can be cyclized [55, 98]. 

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