Methane bond formation

Carbon can not only be involved in a single two-electron three-center bond formation but also in some carbodications simultaneously participate in two 2e-3c bonds. Diprotonated methane (CH/ ) and ethane... 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

The development of molecular orbital theory (MO theory) in the late 1920s overcame these difficulties. It explains why the electron pair is so important for bond formation and predicts that oxygen is paramagnetic. It accommodates electron-deficient compounds such as the boranes just as naturally as it deals with methane and water. Furthermore, molecular orbital theory can be extended to account for the structures and properties of metals and semiconductors. It can also be used to account for the electronic spectra of molecules, which arise when an electron makes a transition from an occupied molecular orbital to a vacant molecular orbital. 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

Figure 1.12 The hypothetical formation of methane from an sp -hybridized carbon atom. In orbital hybridization we combine orbitals, not electrons. The electrons can then be placed in the hybrid orbitals as necessary for bond formation, but always in accordance with the Pauli principle of no more than two electrons (with opposite spin) in each orbital. In this illustration we have placed one electron...

Relatively soon after the discovery that aqueous solutions containing PtCl - and PtClg- can functionalize methane to form chloromethane and methanol, a mechanistic scheme for this conversion was proposed (16,17). As shown in Scheme 4, a methylplatinum(II) intermediate is formed (step I), and this intermediate is oxidized to a methylplatinum(IV) complex (step II). Either reductive elimination involving the Pt(IV) methyl group and coordinated water or chloride or, alternatively, nucleophilic attack at the carbon by an external nucleophile (H20 or Cl-) was proposed to generate the functionalized product and reduce the Pt center back to Pt(II) (step III) (17). This general mechanism has received convincing support over the last two decades (comprehensive reviews can be found in Refs. (2,14,15)). Carbon-heteroatom bond formation from Pt(IV) (step III) has been shown to occur via nucleophilic attack at a Pt-bonded methyl, as discussed in detail below (Section V. A). 

The carbon-carbon bond formation is accomplished by the reaction of the silicon-stabilized carbanions with electrophiles. Magnus and Roy have reported that methoxy(trimethylsilyl)methane is deprotonated with sec-butyllithium in... 

Trimethylsilyl triflate (McsSiOTf) acts as an even stronger Lewis acid than Sc(OTf)3 in the photoinduced electron-transfer reactions of AcrCO in dichloro-methane. In general, such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C—C bond formation between organosilanes and aromatic carbonyl compounds via the Lewis-acid-catalyzed photoinduced electron transfer. Formation of the radical ion pair in photoinduced electron transfer from PhCHiSiMes to the (l-NA) -Mg(C104)2 complex (Scheme 11) and the AcrCO -Sc(OTf)3 complex (Scheme 12) was confirmed by the laser flash experiments [113]. 

A new mechanism, called the methane-formaldehyde mechanism, has been put forward for the transformation of the equilibrium mixture of methanol and dimethyl ether, that is, for the formation of the first C-C bond.643 This, actually, is a modification of the carbocation mechanism that suggested the formation of ethanol by methanol attaching to the incipient carbocation CH3+ from surface methoxy.460,462 This mechanism (Scheme 3.3) is consistent with experimental observations and indicates that methane is not a byproduct and ethanol is the initial product in the first C-C bond formation. Trimethyloxonium ion, proposed to be an intermediate in the formation of ethyl methyl ether,447 was proposed to be excluded as an intermediate for the C-C bond formation.641 The suggested role of impurities in methanol as the reason for ethylene formation is highly speculative and unsubstantiated. 

Nitrogen attracts three additional electrons and is thus able to form three covalent bonds, as occurs in ammonia, NH3, shown in Figure 6.16. Likewise, a carbon atom can attract four additional electrons and is thus able to form four covalent bonds, as occurs in methane, CH4. Note that the number of covalent bonds formed by these and other nonmetallic elements parallels the type of negative ions they tend to form (see Figure 6.6). This makes sense, because covalent bond formation and negative ion formation are both applications of the same concept nonmetallic atoms tend to gain electrons until their valence shells are filled. 

Multiply Protonated Methane Ions and Their Analogs. In addition to the involvement in a single 2e-3c bond formation, carbon is also capable of simultaneously participating in two 2e-3c bonds in some carbodications. Diproto-nated methane (protiomethonium dication, CH62+) is the parent of such carbodications. 

It is not necessary to assume a complete cleavage of methonium ion [CH5]+38 to a free, energetically unfavorable methyl cation. The carbon-carbon bond formation can indeed be visualized as the C—H bond of methane reacting with the developing methyl cation [Eq. (5.68)]. 

Fig. 15.7. c-Type interactions between the unoccupied Is AO of a proton and the doubly occupied sp3 AO of a CH3 anion influence of the magnitude of the overlap on the stabilization of the transition states of two bond-forming reactions. Left, formation of tetrahedral methane right, formation of a fictitious stereoisomer—an unsymmetrical trigonal bipyramid. 

The largest group of organic molecular compounds, in which hydrogen bond formation plays no part, are the compounds, usually in the ratio 1 1, between on the one hand aliphatic and aromatic nitro compounds (nitromethane, tetranitro-methane, chloropicrin CC13N02, nitrobenzene, s-trinitrobenzene, picric acid), quinones, anhydrides (phthalic acid-and maleic acid anhydride) and ketones with on the other hand especially aliphatic and aromatic amines (aniline, pyridine), unsaturated aliphatic and aromatic hydrocarbons, ethers etc. 

Oxa-di-TT-methane rearrangement A photochemical reaction of a P,y-unsaturated ketone to form a saturated a-cyclopropyl ketone. The rearrangement formally amounts to a 1,2-acyl shift and bond formation between the former a and y carbon atoms. 

The activation of C-H bonds for direct C C bond formation reactions has the potential to become very important especially if it can be accomplished for sp C-H bonds, in methane or alkanes as these are the major feedstocks available. In addition, C-H bond activation of functionalized organic compounds for selective C-C bond formation has been and will continue to be a very important goal of organometallic catalysis. So far the use of transition metal complexes has led to interesting results which however are not yet industrially relevant. 

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