Carbenes with methane

Figure 1.4 Stability of some carbenes, as determined by isodesmic calculations for the reaction of each carbene with methane to generate NHCH2 and dihydrocarbene [13].

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

Like many singlet carbenes, nucleogenic, arc generated and chemically generated C atoms react with aliphatic C—H bonds by insertion. In the simplest case, reaction of chemically generated C atoms with methane yields ethylene and acetylene. When a mixture of CH4 and CD4 is used, product analysis indicates that the acetylene results from H abstraction followed by dimerization of the CH, while the ethylene results from C—H insertion followed by H migration in the carbene (Eq. 15). It seems probable that CH is formed in all reactions of carbon with hydrocarbons as acetylene is invariably produced in these reactions. 

An unusual reaction of carbenes is that of insertion into C—H bonds (2-20). Thus CH2 reacts with methane to give ethane and with propane to give n-butane and isobutane. 

Thus, generated nitrogen atoms can react with methane yielding the NH radicals, whereas reactions with carbene (CH2) and methyl radicals yield hydrogen cyanide. 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

Two valleys were found in the potential surface for the reaction of triplet carbene to methane. They correspond to the modes of approach indicated in Fig. 24. The mode leading to 2 CH3 (b in the figure) is preferred with a barrier height of 3.8 kcal/mole. 

Dithiocarbene 159 is obtained when a trifluoromethyl group is reacted with a thiol (Equation (28)). Yields are high and the solution resonance of the carbon bonded to platinum is reported as 306 ppm. On the NMR timescale, the two SMe groups adopt different orientations relative to the Pt-G bond, indicating that there must be significant 7r-bonding between the sulfurs and the carbene carbon. This work was expanded to include the synthesis of 3 -heterocylic carbenes with the use of ethane-1,2-dithiol in place of methane thiol. ... 

The imidazoline denvative cibeiuoline (64) is a class I antiarrhythmic agent which has undergone clinical trials in the United States with apparently satisfactory results It is synthesized by diphenylcyclopropananon of acrylonitrile by thermal carbene generation from diphenyldiazo methane (62) to give 1 cyano 2,2 diphenylc>clopropane (63) Reaction of this with ethylenedia mine tosylate completes the synthesis of ciben/oline (64) [221... 

An unprecedented carbene insertion reaction was observed on reaction of the cationic re-arene ruthenium amidinates with trimethylsilyldiazo-methane (Scheme 145, TFPB = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). 

Insertion of a carbene unit into the N—H bond of primary or secondary amines by copper salt catalyzed decomposition of diazo compounds has been known for a number of years14). The copper chelate promoted reaction of diazodiphenyl-methane 291) or 2-diazo-1,2-diphenyl-1-ethanone 292) with primary benzylamines or... 

It is not known whether there is any carbenoid chemistry of end-on coordinated manganese-diazoalkane complexes 420 410). However, it is known that diaryldiazo-methanes 408,411 and azibenzil411 yield carbene rather than diazoalkane complexes with (n5-C5H5)Mn(CO),THF. 

The existence of such a growth step is consistent with the high proportion of C2 products found in the Ir4(CO)12/NaCl-2AlCl3 system (59). Furthermore, in systems where dimerization is less favored, hydrogenation of the primary carbene fragment could explain the considerable amounts of methane formed in many heterogeneous Fischer-Tropsch systems. 

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

The most important difference between Chauvin s mechanism for olefin metathesis and the mechanism for alkane metathesis is that the latter applies itself to the reverse reaction of cleavage of alkanes by methane (which has no single C-C bond, see below) and, especially, it is based on a metal hydrido-carbene in equi-Ubrium with a metal-alkyl. 

The vapor-phase pyrolysis of 4-hydroxy-1,2,3-triazole and its iV-methyl derivative affords methan-imine and its A-methyl analog. Analysis of the reaction path by the MNDO method shows the presence of two stable or metastable isomers, (liif)-4-hydroxy-l,2,3-triazole and its ketone protomer <89NJC551>. 4-Diazo-1,2,3-triazoles (122) thermolyze or photolyze in benzene to 4//-l,2,3-tri-azolylidenes (123) which convert benzene to 4-phenyl-1,2,3-triazoles and/or isomerize to a-diazo-nitriles (124). Intermediates (124) react with benzene via a carbene to give addition, ring expansion or substitution products (Scheme 17) <82TL5115>. The similar thermolysis of diazotriazoles in substituted benzene gives complex mixtures in which all of the components are sometimes impossible to isolate and identify <90AHC(48)65>. 

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