Alkyl hydride complexes

The catalytic cycle, which is supported by stoichiometric and labeling experiments, is shown in Scheme 38. Loss of 2 equiv. of N2 from 5 affords the active species a. Reaction of a with the 1,6-enyne gives the metallacycle complex b. Subsequently, b reacts with H2 to give the alkenyl hydride complex c or the alkyl hydride complex d. Finally, reductive elimination constructs the C-H bond in the cyclization product and regenerates intermediate a to complete the catalytic cycle. 

In 1978, Schwartz and Gell found that CO would induce reductive elimination of alkane in various zirconocene alkyl hydride complexes with concurrent formation of Cp2Zr(CO)2 (2) (52,53). It was postulated that CO initially coordinates to the 6-e complex 23 forming the coordina-tively saturated species 24 which can then reductively eliminate alkane and/or rearrange to a zirconocene acyl hydride intermediate. When R = cyclohexylmethyl, methylcyclohexane reductively eliminated and Cp2Zr(CO)2 was isolated in 25% yield. 

An important study using cyelopentadienyl (Cp) molybdenum species (196) has shown that reductive elimination of saturated product from an alkyl-hydride complex occurs with retention of configuration at the... 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

Until now, for most of the systems described here it has been accepted that alkane activation occurred through oxidative addition to the 14-electron intermediate complexes. Yet, Belli and Jensen [26] showed, for the first time, evidence for an alternative reaction path for the catalytic dehydrogenation of COA with complex [lrClH2(P Pr3)2] (22) which invoked an Ir(V) species. Catalytic and labeling experiments led these authors to propose an active mechanism (Scheme 13.12), on the basis of which they concluded that the dehydrogenation of COA by compound 22 did not involve an intermediate 14-electron complex [17-21], but rather the association of COA to an intermediate alkyl-hydride complex (Scheme 13.12). 

Table 12.5 Thorium and uranium alkyl/hydride complexes on various supports, including respective C CP MAS NMR data and assignments (5 in ppm) .

The polymerization of ethylene was also qualitahvely inveshgated by pulse injec-hons of ethylene into helium flowing over thorium (67) and uranium (86) metallocene hydrocarbyl complexes supported on 7-AI2O3.950 at 25 °C, both revealing similar achvihes [171, 173]. Supported thorium half-sandwich complexes 65 exhibited higher achvity than surface species, resulhng from coordinatively more saturated tris(cyclopentadienyl) and metallocene U/Th-alkyl/hydride complexes, that is, 77, 79, 82, 90 and 91 [171]. C CP MAS NMR spectra revealed no clear evidence of ethylene insertion into [Th-CHs] or [AL5-CH3] moiehes of material... 

Three years ago, while we were considering possible reasons for the general inability of transition metals to insert into and activate C-H bonds, our attention turned to the question of the instability of transition metal alkyl hydride complexes. We have listed the few alkyl hydride complexes of which we are aware (i) (one additional case (2) recently came to our attention) as well as some of the only slightly more numerous cases of substituted alkyl hydrides stabilized by chelation (3). In contrast, there are enormous numbers of polyalkyls (4, 5) and poly hydrides (6). While rarity does not logically imply instability, it does suggest it, so we considered possible mechanistic explanations for the assumed rapid decomposition of ci -MLn(R)(H) relative to cis-MLnR2 and cis-MLnH2. We have focused on octahedral complexes since they are both more important and more numerous. 

Both normal and inverse KIEs have played a major role in unraveling the mechanisms of alkane activation with transition metal complexes. Alkyl hydride complexes are typical intermediates in such reactions. The loss of alkane in well-defined alkyl hydrides frequently exhibits an inverse KIE and involves a O-alkane complex.86 87 As a result, an inverse isotope effect is now taken as evidence for the intermediacy of o-alkane complexes in reductive eliminations (Scheme 8.14). 

Rh complexes having a cyclopentadienyl ligand shows high reactivity in C—H bond activation. Irradiation of RhCp (PMe3)(H)2 (Cp = C Mcg) in alkane solvent produced a coordinatively unsaturated [RhCp (PMe3)j species, which undergoes activation of the sp C—H bond of the alkane to give an alkyl hydride complex (eq (50)) [59,60]. 

Chart 11.6 Equilibrium favoring metal aryl hydride and alkane over the alkyl hydride complexes and free aromatic substrates likely indicates that the difference of M—Ar and M—R BDEs is greater than difference in BDE for Ar H and R—H (assuming that AS 0 and that the two M—H BDEs are approximately equivalent). 

M—H bond energies are approximately constant and negligible difference in AS, the thermodynamic preference of metal aryl hydride complexes and free alkane over metal alkyl hydride complexes and free aromatic substrate suggests that the ABDE for M Ar versus M—R is greater than the ABDE for Ar—H versus R—H (Chart 11.6). 

For the equilibrium between an alkyl hydride complex and a C—H coordinated substrate, consideration of the differences in zero-point energies suggests an inverse equilibrium isotope effect, which has been used to explain the inverse KIEs observed for C—H(D) reductive elimination processes. Thus, the observation of an inverse KIE... 

As for RE of H2, observation of similar inverse KIE (0.5-0.8) for alkane elimination from alkyl hydride complexes is evidence for unobserved [Pg.239]

The oxidative addition of alkanes with formation of alkyl hydride complexes was first demonstrated directly in studies using indium complexes [14], Thus the iridium dihydride derivative Cp lr(H)jPMe3 (Cp = pentamethylcyclopenta-dienyl) after irradiation in solution in cyclohexane or neopentane, produces the complexes Cp (PMe3)Ir(H)(C6Hn) and Cp (PMe3)Ir(H)CH2CMe3 in a satisfactory yield. Other saturated hydrocarbons and benzene also readily add to an iridium complex. 

By treatment with CHBrs at -60 °C, alkyl hydride complexes were converted into the more stable derivatives Cp (PMe3)Ir(Br)R. Irradiation of Cp lr(H)2PMe3 in CMe4+C6D,3 mixture led to the formation of Cp (PMe3)-lr(H)CH2CMe3 and Cp (PMe2)lr(D)C6Dit with very small admixtures of crossaddition products. 

A second (meth)acrylate polymerization system based on neutral lanthanocenes, particularly (C5-Mes)2SmR (R = alkyl, hydride) complexes, has been developed by Yasuda et al. In this case, the large and highly electropositive organosamarium center can serve simultaneously as both the initiator (Insertion) and catalyst (monomer activation) components of the GTP and a second Lewis acid equivalent is not needed (eq 2). 

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