Alkanes cyclometallation

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. 

Cleavage of C—H Bonds Alkane Activation Cyclometallation Reactions 1199... 

C-H bond, with loss of alkane and formation of novel cyclometalated pyridine A-oxide complexes in greater than 70% yield. The same reactions could be carried out with the analogous thorium(iv) bis(alkyls).129... 

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. 

The initial approach angle (C-H-M) of about 130° was rationalized in terms of a proposed interaction between the C - H o bond and an empty metal d orbital, and back-donation from a filled d orbital to a a orbital of the C-H system, but this angle may also result from geometrical factors. The role of steric and conformational effects in the balance between cyclometallation and attack on an external substrate (such as an alkane), was discussed in the context of the above reaction path. The conclusion was that sterically uncongested metal systems are more likely to activate alkanes, than to undergo cyclometallation. 

The most interesting (from the point of view of alkane activation) cyclometalation reactions of sp -C-H bonds are demonstrated in Scheme IV.3 [7] (in addition to references [7] depicted in this scheme, see also [8]). Scheme IV. 4 gives some examples of the cyclometalation of vinylic C-H bonds, occurring with the splitting of bonds between carbon and some elements, and other special cases [9]. For cyclometalation of the C-H bonds at the C=C bonds, see also [10a,b]. 

Cyclometalation might be expected to have occurred in preference to alkane activation in these PPhs complexes. Recent work on other systems has shown, however, that alkane activation can occur even in PPha complexes. 

This is supported by some important results recently obtained by Bergman in which a variety of alkanes have been activated photochemically. Berg-man has shown that [(C5Me5)IrLH2] on photolysis in cyclohexane gives [(C5Me5)IrL(Cy)H], but that the cyclohexyl hydride formation is competitive with cyclometalation for PPhs, but the major product for PMes. 

The competition between intra- and intermolecular C-H activation has been discussed by Crabtree and his main conclusions are summarized here (i) alkane activation is kinetically favored, while cyclometallation is usually the thermodynamic preferred pathway (ii) higher steric congestion favors cyclometallation, whereas intermolecular activation is more likely in less congested systems. Let us illustrate these points by using the unsaturated complexes IrlCp XPMes) and IrlCp KPPhs). Both activate benzene (reactions (75) and (76)), but the products of the latter are a 1 1 mixture of the phenyl hydride complex and a four-membered metallacycle ... 

CpReL3 (L = PMe3) is also photoreactive with alkanes by loss of The product is CpReL2(R)H (R = methyl, 1-hexyl, cyclopentyl and cyclopropyl). The more sterically hindered substrate, cyclohexane, did not give an alkyl hydride cyclometalation of the coordinated PMe3 or oxidative addition of the C—H bond of the free L takes place instead. This more bulky system has a lower tolerance for steric bulk in the alkane. 

Ibers, DiCosimo and Whitesides showed that the bis-neopentyl platinum complex of equation 16 gave facile cyclometalation. They were surprised that the reverse reaction, which would of course be an alkane activation with neopentane as the substrate, did not take place because, thermodynamically, the bonds formed should compensate for the bonds lost. Presumably, it is the unfavorable entropic term which is the major factor in preventing the reverse reaction, but the substantial steric bulk of the neopentyl group may tend to reduce the Pt—neopentyl bond strength in a bis-neopentyl complex. 

Other reactions such as alkane dehydrogenation [159, 160], decarbonylation reactions [161], cyclization of alkynoic acids [162,163], three-component coupling reactions of boronic acids, allenes and imines [164], fluorenone synthesis by sequential reactions of 2-bromobenzaldehydes with arylboronic acids [165], and hydrosilylation reactions [166, 167] using cyclometalation compounds as their catalysts have also been reported. 

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