Actinide complexes alkenes

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. 

Table 6.17 Hydrogenation of alkenes catalyzed by actinide complexes.

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. 

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. 

Finally, actinide complexes such as [MCp 2R2] (M = Th or U, R = alkyl or H) are very active catalysts for the hydrogenation and polymerization of olefins. The complexes [U(allyl)3X] (X = Cl, Br, I) are excellent initiators for the stereospecific polymerization of butadiene, which produces rubbers that have remarkable mechanical properties. Some other complexes are active for the heterogeneous CO reduction and alkene metathesis. The field of catalysis using organoactinide complexes should considerably expand in the near future. 

Actinide and lanthanoid complexes have been employed for hydrogenation reactions, for which they often generate dramatic rate increases and high numbers of turnovers42. Many of these complexes exhibit good selectivity for preferential reduction of the less hindered alkene in situations where more than one is present in a substrate (Scheme l)43. 

Catalysts based on high-valent compounds of the early transition series (groups 3-5) and the lanthanides and actinides generally form high polymers these electron-deficient see Electron Deficient Compound) metal complexes are poor n-bases and have little ability to bond either into the a orbital on the /3-C-H bond or into the rx of the bound alkene. The later transition metals (groups 9 10) are far more electron-rich ... 

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. 

In Section 3.11.1.4 it was pointed out that salts of certain transition metals, lanthanides and actinides promote the hydroalumination reaction. Since such metal salts are introduced into the reaction in their high oxidation states it can be assumed that the metal ions are rapidly reduced to a lower oxidation state and that this state is the active catalyst. For nickel(II) salts, Wilke has shown conclusively that the active agent is a nickel(0)-alkene complex. Analogously, for titanium(IV) salts, such as TiCU, Ti(OR>4 and Cp2TiCl2, it is most likely that a titanium(III) state is involved. The possible role of such metal centers in accelerating hydroalumination will be considered in the next section. 

The coordination properties of phosphine oxides has been explored with late transition-metal (Ru, Co, Rh, Ir, Pd, Pt, Cu, and Au),301 303 305 306 310 316 early transition-metal,317 lanthanide,304,318,319 and actinide307,320 ions. One interesting complex is the palladium(II) complex (148) (Scheme 10) which is an extremely rare example of a ds metal center with a tetrahedral geometry.313 Phosphine oxides have found uses in the extraction of alkali, alkaline earth, and actinide metals in catalysis (hydroformylation of alkenes and epoxides, carbonylation of methanol324) and as a useful crystallization aid (Ph3PO).325... 

Reactions of the monopositive actinide ions An+ with pentamethylcyclopentadiene, HCp, have been studied by mass spectrometry. This was the first study of the An+/HCp reaction for An+ = Am+, Cm+, Bk+, Cf/ and Es+. Each of the actinide ions reacted with HCp to produce [AnCp ]+ (+H), as well as additional products. Both Cf1-and Es+ have previously been found to be inert toward most alkenes, but efficiently reacted with HCp to induce (i) H-loss and [AnCp]+, (ii) H2-loss and [An(C5Me4CH2)]+, and (iii) CH3-loss and [An(C5Me4H)]+ (An = Cf, Es). These were the first organoeinsteinium complexes derived from activation of an organic substrate. Secondary products included [Cp 2An]+ (An = Am, Cm, Bk, Cf, Es), the compositions of which correspond to the metallocene sandwich complexes.93... 

Most complex chemistry of the actinides is in aqueous solution. However, a number of neutral complexes and complexes between halides and neutral donor ligands such as Ph3PO or Me2SO are known. There are very few complexes formed by n acid ligands, providing a notable contrast to the J-block elements for example, there is no evidence for the bonding of NO or alkenes. However, uranium carbonyls have been trapped in matrixes at4°K.16a... 

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. 

A very thorough study of insertions of alkenes into the M—H bonds in [( -Cp )2M(H)OR] (M = Th or U R = achiral or chiral alkyl groups temperatures = 30-90 °C), has been reported. In these relatively electron-poor actinide systems, c-olefine complexes are not stable and the insertion products are stable by ca 30kcalmol Reactions are first order in [complex] and [alkene] and the rates with cyclohexene show relatively little dependence on solvent (THF versus toluene) or metal (Th versus U). They do depend on R [fc (t-Bu)/A (CH(t-Bu)2) 10 ]. Activation parameters for reaction of cyclohexene with M = Th, R = r-Bu show a low kH (9kcalmol ) and an exceptionally low AS (—47 cal mol ). Coupled with a modest... 

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