Tungsten reactions with alkanes

Aspects of the apparatus for the synthesis using metal atoms are described. The reactions of the atoms of rhenium, tungsten, and osmium with hydrocarbons including alkanes are described. It is shown that metal atom reactions with alkanes can give isolable organometallic compounds including l-alkylidene compounds. 

Certain dinuclear Rh(II) carbene complexes react with alkanes to generate products from insertion of the carbene imit into the alkane C-H bond with high diastereo- and enantioselectivity (Equation 6.59). ° These reactions occur by mechanisms distinct from those of the reactions of C-H bonds witti the tungsten alkylidene and alkylid5me complexes just described. The reactions of the dinuclear Rh(II) carbene complexes appear to occur by a mechanism that involves direct reaction of the carbene at the C-H bond without coordination of the alkane and addition across the M=C bond of the carbene. Such rhodium carbene complexes have not been isolated, but the absence of an open coordination site cis to the carbene ligand in the accepted carbene intermediate is thought to preclude initial reaction of tire substrate at the metal center to form a new metal-carbon bond. The catalytic chemistry that occurs via these carbene complexes is presented in more detail in Chapter 18 (catalytic C-H bond functionalization). 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

It is noteworthy that the double-bond isomerization step is faster than the overall elementary steps of alkane metathesis. Formation of lower alkanes is due to the tungsten hydride intermediate, favoring chain walking with double-bond migratirHi followed by fast cross metathesis with coordinated ethylene leading to lower alkenes in turn giving lower alkanes on hydrogenation. This intramolecular reaction pathway, without formation of the free olefin, probably is the difference between alkane and olefin metathesis. 

As an initial exploratory experiment dodecafluoro n-pentane was chosen as a model compound for P.T.F.E. to simulate part of the perfluorinated molecular chain and allowed to adsorb at 0.13 pPa pressure and ambient temperature on a freshly prepared iron film produced by an evaporation process in situ from an iron wire wrapped around a tungsten filament. The events inside the vacuum chamber were followed with an MS 10 mass spectrometer and (where appropriate) a Pi rani gauge. These experiments suggest that the clean iron acts as a catalyst for the breakdown of the perf1uorinated alkane producing a variety of transiently reactive fragments, such as CF2, that in turn may react with the borosilicate (Pyrex) glass walls of the reaction chamber. 

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