Neopentane, elimination

The most important contributions in this area, however, directly related to bond activation chemistry, and, undoubtedly triggered by theoretical considerations along the lines of Figure 1, were reported by Whitesides and coworkers in 1986 and 1988 [11]. It was shown that the bent, bisphosphine-coordinated platinum chelate complex [(dcpe)Pt(O)] (9) (dcpe = bis(dicyclohexylphosphino)ethane), which could be generated thermally as a "hot" reactive intermediate by reductive elimination of neopentane from its ris-neopentylhydride Pt(II) precursor at around 60-70°C in solution, was able to activate C-H bonds, even of unactivated alkanes. 

The mechanism of this remarkable a-elimination reaction has been scrutinized by several research groups [17,49,51,396-404]. From the experimental data obtained this process is best described as an intramolecular deprotonation of one neopentyl ligand by another, the latter being released as neopentane (Figure 3.4). 

As already discussed (Section 3.1.1) the elimination of, for instance, neopentane from penta(neopentyl)tantalum corresponds to an a-deprotonation of one alkyl ligand by another, the latter being eliminated as neopentane. Hence in the reverse reaction the carbene carbon atom of the (nucleophilic) carbene complex must formally deprotonate the incoming alkane with simultaneous electrophilic attack of the metal at the newly formed, carbanionic alkyl group (Figure 3.36). 

Thermal treatment of (=SiO)Hf(CH2Bu )3 at increasing temperatures leads to the successive evoluhon of neopentane, isobutene and isobutane as well as several alkanes varying from Cj to C5. Polyisobutenes are also formed on the surface. The mechanism by which such decomposition occurs suggests a succession of y-H eliminations with formahon of neopentane followed by P-methyl transfer and formation of isobutene and [Hf]-Me (Scheme 2.14). This isobutene is reinserted into [Hf]-Me with formahon of isopentene and [Hf]-H. 

SiO)MNp3 The supported tris(neopentyl)titanium complex easily undergoes decomposition even at low temperatures [42]. Up to 150 °C, only neopentane is released, which can be explained by either a-H elimination, to give an alkylidene species, or y-H elimination, to give a metaUacyclic species (Scheme 11.1). Nevertheless, no such species could be observed by C CP-MAS or H MAS NMR. The... 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

In contrast to the results obtained with -alkanes and cycloalkanes, H2 elimination is found to be an unimportant process in the photolysis of neopentane [107]. At 7.6 eV the methane elimination and direct C-C bond cleavage to radicals are the predominant processes ... 

Alkane elimination is a basic photodecomposition mode of highly branched alkanes, e.g., most of the excited neopentane molecules split directly to methane and isobutene. 

The mechanism of the formation of the ylide has been studied by deuterium-labeling experiments, and it is assumed that the pentakisneo-pentyl tantalum, which is formed first, is decomposed with elimination of neopentane. This idea parallels the findings for pentamethylarsenic (37), which has also been found to decompose via an ylide intermediate. 

The complex Ta(CH2But)5 has yet to be isolated initial attempts at its preparation led Schrock instead to a remarkable alkylidene (carbene) complex (Figure 4.28). Neopentane was amongst the side products of the reaction and this most likely arises from a concerted a-C-H abstraction by an adjacent neopentyl ligand. In other systems, there is evidence for transfer of the hydrogen to the metal centre and agostic alkyls (Figure 2.24) can be seen as a step en route to the a-M-H elimination transition... 

Thermal a-elimination (see a-Eliminatiori) of neopentane from the dimethylallyl complex, Cp W(NO)(CH2Bu0( -Me2C3H3), generates a reactive 16e ) -allene intermediate that activates hydrocarbon solvent C-H bonds to... 

The photolysis of neopentane has been studied by Lias and Ausloos at 1236 and 1470 A. The two main processes which occur are molecular methane elimination from, and fragmentation of, the excited neopentane. The products of photolysis are methane, isobutene, hydrogen, ethane, and propene, with smaller amounts of isobutane, ethylene, propane, acetylene and 2,2-dimethylbutane. The suggested reactions are... 

Comparison of the results at 1236 and 1470 A shows that at the shorter wavelengths the molecular elimination of methane is less important relative to the fragmentation or dissociative processes which result in increased yields of hydrogen, ethane, propene and excess isobutene. It should be noted that the molecular elimination of both hydrogen and methylene are not found to be important processes for neopentane this is in sharp contrast to methane, ethane and propane, while the butanes show the hydrogen elimination but not the methylene elimination. 

Indeed, the reaction was first observed in the synthesis of the hydrides. As mentioned above, when 5 is heated under dry hydrogen to 150 °C for three hours, (=SiO)3ZrH (14) is formed together with nine equivalents of methane and three equivalents of ethane. The formation of methane and ethane rather than neopentane was clear evidence of hydrogenolysis under the synthesis conditions [5, 15, 16]. It was observed that the reaction of neopentane occurred by stepwise formation of firstly isobutane and methane, then conversion of the former to a second equivalent of methane and propane which is further converted to ethane and a third equivalent of methane. The C-C bond of ethane cannot be cleaved by P-methyl elimination because a surface metal-ethyl fragment has no methyl group in the S-position. 

Surface organometallic species have also been used for the olefin metathesis reaction [10, 13]. In the case of molybdenum, the molecular complex N=Mo[CH2C(CH3)3]3 showed very little activity for the metathesis of simple olefins, presumably because the catalytically active carbene complex did not form under reaction conditions. The reaction of this complex with silica, however, proceeds by the addition of the silanol O-H bond over the Mo N triple bond leading first to a trisalkyl Mo complex which undergoes a-elimination of neopentane to produce a carbene complex which was found to exhibit a significant activity for the metathesis of internal olefins according to eq. (2) [10]. 

One of the earliest reports of alkane C-H activation was made by Shilov in 1969 in which H/D exchange was reported between methane and a D20/CH3C00D solvent in the presence of K2PtCl4 [57]. While the mechanistic details of this exchange were not entirely clear, the work stood as an isolated example of alkane activation for many years. Alkane activation by platinum was not reported again until 1986, when Whitesides found that (Cy2PCH2CH2PCy2)Pt(neopentyl)H lost neopentane and activated a variety of alkanes at 50°C (Eq. 12) [58,59]. These reactions are believed to proceed by way of an initial reductive elimination to... 

Marks has examined the reactivity of thorium metallacycles with hydrocarbons, where ring strain is used to provide the thermodynamic driving force for alkane activation in a reaction with methane (Eq. 17). Reaction with CD4 shows a dramatic kinetic isotope effect, with kH/kD=6, which is typical of the four-centered electrophilic transition state hydrocarbon activations [76]. The metallacy-cle is formed by the elimination of neopentane from the bis-neopentyl derivative [77]. Reaction with cyclopropane and tetramethylsilane gave the bis-cyclopropyl product Cp 2Th(c-propyl)2 and the bis-TMS product Cp 2Th(CH2SiMe3)2, respectively [78]. 

Wolczanski also investigated the chemistry of a tantalum imido system. In this system, elimination of hydrocarbon from the bis-amido imido complex occurs with difficulty at 183°C to give an amido bis-imido complex. The elimination is reversible, with the bis-imido species not being directly observed (Scheme 10). Under methane pressure, the phenyl complex loses benzene and adds methane. Neopentane, benzene, and toluene (benzylic activation) were also found to undergo activation, but not cyclohexane. The authors conclude from their equilibrium studies that the differences in metal-carbon bond strengths are approximately equal to the differences in carbon-hydrogen bond... 

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