Alkane metathesis complex

Note also that (1) d° Ta alkyhdene complexes are alkane metathesis catalyst precursors (2) the cross-metathesis products in the metathesis of propane on Ta are similar to those obtained in the metathesis of propene on Re they differ only by 2 protons and (3) their ratio is similar to that observed for the initiation products in the metathesis of propane on [(=SiO)Ta(= CHfBu)(CH2fBu)2]. Therefore, the key step in alkane metathesis could probably involve the same key step as in olefin metathesis (Scheme 27) [ 101 ]. 

The high activity of iridium PCP pincer complexes in transfer dehydrogenation has been applied in a very elegant approach to devise the first homogeneous alkane metathesis process (Equation 12.5) [3]. 

P and R have similar coordination stractures with an Mo-O-Si bond in terms of both their comparable NMR data and initial TOFs for olefin metathesis. The chemical bonding between the Si02 surface and the Mo imido alkylidene complex is suggested to prevent some deactivation pathways such as dimerization of reactive intermediates, resulting in longer Hfetime under metathesis reaction conditions [57]. The SiO -supported Mo imido alkylidene complex (P) was also efficient for alkane metathesis [58]. 

Using a procedure related to that for the preparation of tantalum catalysts, supported W-polyhydride species were prepared to evaluate their catalytic activity in alkane metathesis. Hydrogenolysis of the grafted, alkylidyne d°-complex W(=fBu)(CH2tBu)3 [52] led to W-polyhydride complexes (Scheme 2.8) [53]. 

Extensive studies on the coordination sphere of the metal using Mo-bisdiphenylamido and dipyrrolyl complexes for olefin metathesis revealed how to improve the activity and selectivity of these catalysts [62-68]. This seminal work led to the synthesis of a new generation of highly active olefin metathesis catalysts [69, 70]. Using a similar strategy, various surface-metal alkylidene and alkylidyne complexes have also been tested in alkane metathesis to determine if they can be employed as catalyst precursors [11, 71]. This section describes the structure-activity relationship of different metal-alkyl complexes with oxide surfaces in alkane metathesis. 

The use of a silica-supported, tantalum alkyhdene as a precursor for alkane metathesis was found to result in a stoichiometric, alkane cross metathesis in which an initial pendant alkyl-alkylidene group was transformed to produce the desired, active species [76, 90]. This reaction was later observed to work with additional, well-defined systems supported on alumina [58] and sihca-alumma [53]. As mentioned previously, this transformation does not occur when the surface organometallic precursor bears no alkyl group. Exposing these supported, metal neopentyl, neopentylidene, and neopentyhdyne complexes to alkane at 150 °C produced alkanes containing a neopentyl fragment (CH jiBu) via cross metathesis. Propane metathesis with these alkyl-alkyhdene surface complexes typically generates stoichiometric amounts of dimethylpropane, 2,2-dimethylbutane, and 2,2-dimethylpentane (Scheme 2.11). 

The alkane metathesis with the W-alkyhdyne/alkyl complexes supported on alumina or sihca-alumina yielded a different distribution of branched, neopentyl alkane derivatives, albeit in lower amounts. These results suggest a different mechanism than the alkyhdene-supported metal complexes depicted above. A chrect, C - H bond activation of propane on the alkyhdyne species [91,92] or, alternatively, the formation of bisalkylidene intermediates [93] have been proposed [71]. 

Building off of seminal work on the development of iridium complexes for dehydrogenation - hydrogenation catalysts, an improved, homogeneous, dual-catalytic system for alkane metathesis has recently been discovered [101, 102]. Goldman and Brookhart found that a combination of an Ir-pincer-based catalyst with Schrock-type, Mo- or W-alkylidene complexes (as the olefin metathesis catalyst) transformed alkanes into lower and higher new alkanes (Figure 2.12). 

The discovery of the (de)hydrogenation steps using transition metals, in particular, the iridium pincer complexes, was essential to the success of this tandem, dualalkane metathesis reaction. As such, efforts made to improve these Ir catalytic systems will first be discussed. Subsequently, the application of these catalysts to the tandem, dual-alkane metathesis reaction will be elaborated. 

Cross-Alkane Metathesis In a parallel study on cross-alkane metathesis using supported, Ta polyhydrides [34], Schrock and coworkers recently reported the dual-catalytic, homogenous, cross-alkane metathesis of n-octane, and ethylbenzene. The authors [136] employed various W-monoaryloxide pyrrolide complexes in combination with several iridium pincer complexes. For example, employing Ir-2(H2) and W-1 with a ratio of n-octane/ethylbenzene (1 1.33, in v/v) produced the best productivity toward alkylbenzenes, with good selectivity over linear alkanes (Scheme 2.20). 

Computational studies have shown that alkane metathesis cannot occur via the o-bond metathesis between the C-C a-bonds and the M-C a-bonds originally proposed [101]. Experimental evidence has also suggested that the reaction mechanism must involve alkene metathesis as the key step and alkylidene hydrido metal complexes as associated intermediates [92, 93, 102]. To date, only few computational studies on alkane metathesis have been reported [103-106]. 

Scheme 6.26 Proposed alkane metathesis pathway for alumina-supported alkylidyne-bisalkyl W(VI) complexes with the dual role of alumina anchoring W and catalyzing the dehydrogenation/hydrogenation steps.

C-H bond activation of a ligand in which a C atom is located in a, p or y position vs. the metal involves a-bond metathesis in many cases, in particular for a and y-elimination. These aspects are dealt with in Chaps 3 (stoichiometric reactions leading to metal-alkylidene complexes section 5), 15 (Ziegler-Natta polymerization section 1) and 20 (silica-supported alkane metathesis catalysts section 5). 

Early transition-metal akyl and hydride complexes grafted on silica catalyze alkane disproportionation by o-bond metathesis. For instance, a metal-hydride reacts with an alkane to yield a metal-alkyl on silica, then combination of C-H a-bond metathesis with a and (3-H eliminations leads to alkane metathesis (disproportionation), the mechanism involving metal-carbenes and metallocyclobutanes as for olefin metathesis. 

The overall conversion, formally described as alkane metathesis, consists of three steps alkane dehydrogenation, alkene metathesis, and hydrogenation. As shown by reaction 7.3.3.3, an iridium pincer complex (see Section 2.3.5) is used in the first step as the precatalyst. Here the thermodynamically unfavorable dehydrogenation of the alkane to alkene is achieved. Notice that in this reaction other isomers of the alkene may also be produced. 

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