Carbenes reaction with alkanes

Another example involves the soxurce of acetylene as a major product from the reaction of recoiling carbon atoms and cyclopropane (18). MacKay and Wolfgang attributed the formation of acetylene from carbon-atom reactions with alkanes to an insertion into a C—bond followed by fragmentation of the intermediate carbene (18). The hi yield of labeled acetylene from cyclopropane was in accord with this mechanism, since the intermediate cyclopropylmethylene was believed to cleave into two stable molecules ethylene in addition to acetylene. 

Trifluoromethyl-substituted diazonium betaines [176]. Synthetic routes to trifluoromethyl-substituted diazo alkanes, such as 2,2,2-trifluorodiazoethane [ 177, 7 78, 179] and alkyl 3,3,3-trifluoro-2-diazopropionates [24], have been developed Rhodium-catalyzed decomposition of 3,3,3-tnfluoro-2-diazopropionates offers a simple preparative route to highly reactive carbene complexes, which have an enormous synthetic potential [24] [3-1-2] Cycloaddition reactions were observed on reaction with nitnles to give 5-alkoxy-4-tnfluoromethyloxazoles [750] (equation 41)... 

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

There is some increase in selectivity with functionally substituted carbenes, but it is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with i-propylbenzene, ethylbenzene, and toluene.212 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.213 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.214 Owing to low selectivity, intermolecular insertion reactions are seldom useful in syntheses. Intramolecular insertion reactions are of considerably more value. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene and good yields can frequently be achieved. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

Some Schrock-type carbene complexes, i.e. high-valent, electron-deficient, nucleophilic complexes of early transition metals, can undergo C-H insertion reactions with simple alkanes or arenes. This reaction corresponds to the reversal of the formation of these carbene complexes by elimination of an alkane (Figure 3.36). 

Cohen and Daniewski have shown that thiophenol can act as a leaving group when l,l,3-tris(thiophenyl) alkanes are treated with methyl lithium in THF (Scheme 31, Table 18) The reaction appears to go by a direct S 2 attack of the anion (76a) even though surprisingly high yields are obtained in cases requiring reaction with a bulky nucleophile at the site of a sterically hindered carbon atom (see Table 18). Labeling experiments and stereochemical studies have conclusively ruled out alternative electron transfer and carbene mechanisms Scheme 32 shows that the anionic intermediate 76 can also be... 

Carbene complexes can also react by addition of alkane C-H bonds across the M=Cbond to generate a product containing two metal-carbon single bonds or materials derived from such products. This reaction is the reverse of the a-elimination, which is a common reaction that forms Schrock-t5q5e carbene complexes, as noted in Chapters 3,10, and 13. Such reactions of carbene complexes with the C-H bonds of alkyl groups were first observed to occur intramolecularly, particularly after generation of cationic carbene complexes. " However, intermolecular additions of C-H bonds of arenes ° and alkanes are also known. 

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). 

Fryzuk and coworkers investigated the coordination chemistry of the tridentate diamido-V-heterocyclic carbene ligand 31 with tantalum (V) [72]. Aminoly-sis or alkane elimination reactions with the carbene ligand led to bidentate... 

A review on the CH insertion reactions of carbenes and nitrenes with alkanes has appeared developing the current state of the art of this valuable synthetic method. ... 

Addition of the conjugate acid of the desired ligand to a metal-carbene or -carbjme compound results in protonation of the hydrocarbyl ligand (see also Sections 2.6.1.4 and 2.6.4.4), transforming it into an allcyl or carbene ligand with formation of a new metal— heteroatom bond. Examples are the preparations of compound 19 (Scheme 7) and compound 65 (Scheme 27). The reaction of the carbene substrate 101 with tert-butyl alcohol produces product 102 (Scheme 46) which, upon heating, eliminates alkane and affords a new carbene compound in an overall process which resembles the protonolysis of an al-l 1 The same reaction of 101 with triphenylsilanol at low temperatures leads to the protonolysis product directly. 

Homogeneous reactions of alkanes frequently involve the radical abstraction of hydrogen atoms (chlorination and autoxidation are classical examples) and in many cases, the formation and the reactions of these radicals are mediated by transition metal complexes [2, 3]. Other reagents, such as superacids and carbenes, can react with alkanes, apparently by non-radical mechanisms [4]. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into trans-l,2-diols by acid-catalyzed hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with OSO4. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. In addition, alkenes react with divalent substances called carbenes, R2C , to give cyclopropanes. Nonhalo-genated cyclopropanes are best prepared by treatment of the alkene with CH2I2 and zinc-copper, a process called the Simmons-Smith reaction. 

September 1985) have been published. The 300th volume of J. Organomet. Chem. is commemorated by a number of reviews by well-known organometallic chemists many of these reviews are of relevance to this chapter. Carbene and carbyne complexes of ruthenium, osmium, and iridium,and the role of electrophilic metal carbenes as reaction intermediates in catalytic reactions, have been reviewed. The reactivity of the ions Ru", Rh", and Pd with alkanes in the gas phase has been shown to be markedly different to that of their first row congeners. 

The atom abstracting behavior of the carbene center is shown by reaction with carbon tetrachloride to form the cyclohexadienone 15. Reactions in alkane solvents also lead to hydrogen atom abstraction, and this step is intermediate in the cross polymerization of piperylene oligomers by irradiation in the presence of 1,4-benzenediazo-oxide. ... 

The chemistry of tantalum is different. The tantalum pentaalkyl complex does not exists, because it transforms easily into a carbene complex by a-elimination. This complex reacts also with silica, leading to a supported tantalum complex.262,263 Their reaction proceeds first by the addition of the silanol OH group across the tantalum-carbon double bond followed by elimination of an alkane (Scheme 7.19). 

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