Insertion hydrogen-carbon bond

Intramolecular C-H insertion reactions of ( -cyclo-pentadienyl)dicarbonyliron carbene complexes can be used to prepare complex polycyclic compounds. Carbon-hydrogen bond insertion using an iron carbene was used as a key step in the synthesis of sterpurene andpentalene (Scheme 81). ... 

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. 

Particular interest has been shown in the reactions of photochemically generated arylcarbenes. A study of the photodecomposition of phenyldiazomethane in 2-chloropropane has revealed that carbon-chlorine bond insertion of singlet phenylcarbene predominates at low temperature in solution, whereas carbon-hydrogen bond insertion is preferred in a rigid matrix. The principal products of irradiation of diazoalkane (63) are phenylacetylene (64) and the cyclobutene (65), even in the presence of alkenes. At lower temperatures, however, carbene addition predominates as shown, for example, with isobutene as shown in Scheme 7. 

Rhodium(II) compounds have become the premier choice in catalytic transformation of a-diazo compounds to induce cyclopropanation, aliphatic carbon-hydrogen bond insertion, heteroatom hydrogen bond insertion, aromatic substitution, and ylide formation. 

Whether or not the carbon atom is in an excited state cannot be readily answered in the light of current theory. If these insertion reactions do indeed take place, then we are faced with a question similar to that posed by the reactions of methylene, i.e. how are the carbon-hydrogen bond insertion reactions of photolytically generated methylene related to the internal energy, the excess translation energy and the electronic state. 

This argument means to say that the major fraction of ethylene formed by excited intermediates (in systems which are effectively scavenged so as to eliminate thermal reactions) comes from primary carbon-hydrogen bond insertion by methyne. Small amounts of ethylene may arise from more complex reactions. 

Examples of such reactions are well known. Sloan, Breslow, and Renfrow found that both alkane and arenesulphonyl azides insert into the carbon-hydrogen bonds of saturated hydrocarbons 12>. Thus, 1-pentane,- 2-propane- and -toluene-sulphonyl nitrene inserted into cyclohexane to give 54, 60, and 58% yields of the corresponding IV-cyclohexylamide derivatives 8>. Similarly, 2-phenoxybenzene-, diphenyl sulphide-2-, and... 

Only limited success was achieved in determining the relative reactivity of primary, secondary, and tertiary carbon-hydrogen bonds to sulphonyl nitrenes 8>. Insertion of p-toluenesulphonyl nitrene into 2-methylbutane gave a mixture of products which could not be completely resolved. The ratio of (primary) (secondary + tertiary) = [38 + 39 40 + 41] was 1.53, compared to a ratio of 5.6 for carbethoxynitrene58>, indicating the lowered selectivity of the sulphonyl nitrene relative to the carbethoxynitrene, as might be expected from the possible resonance stabilization of the latter species. 

Benzyne reacts with benzene to give a mixture of products in low yield. The original experiments 38> showed that the 1,4-cyclo-adduct (benzo-barrelene) (19), the valence-bond isomerised 1,2-cyclo-adduct (benzo-cyclo-octatetraene) (20), and the product of insertion into a carbon-hydrogen bond (biphenyl) (21), were obtained in 2,8, and 6% yields respectively. 

Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium- l-arylidene compounds (2 2) (Figure 3). The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction is in accord with statistical attack at all available sp3 C-H bonds. 

The co-condensation reactions described above have led to the formation of interesting new compounds and sometimes very unexpected products. The nature of the products formed for example in the osmium atom experiments indicate high degrees of specificity can be achieved. However, the detailed mechanisms of the co-condensation reactions are not known. It seems most likely that in all cases the initial products formed at the co-condensation temperature are simple ligand-addition products and that the insertion of the metal into the carbon-hydrogen bond occurs at some point during the warming up process. In support of this hypothesis we note the virtual absence of any... 

Among typical carbon-carbon bond (C-C) formation reactions with carbenes, the cyclopropanation reaction with olefins has been well studied including its application to industrial processes. The second typical reaction of carbenes is the insertion reaction into the carbon-hydrogen bond (C-H) which seems to be a direct and efficient C-C bond forming reaction. However, its use for synthetic purpose has often been limited due to low selectivity of the reactions.3... 

A second process that has a central position in the analysis of the chemical properties of carbenes is their reaction with hydrocarbons. As is the case for alcohols, singlet and triplet carbenes react with hydrocarbons in distinctive ways. It has long been held that very electrophilic singlet carbenes can insert directly into carbon-hydrogen bonds (11) (Kirmse, 1971). On the other hand, triplet carbenes are believed to abstract hydrogen atoms to generate radicals that go on to combine and disproportionate in subsequent steps (12)... 

As shown in the manganese- and ruthenium-catalyzed intermolecular nitrene insertions, most of these results supposed the transfer of a nitrene group from iminoiodanes of formula PhI=NR to substrates that contain a somewhat activated carbon-hydrogen bond (Scheme 14). Allylic or benzylic C-H bonds, C-H bonds a to oxygen, and very recently, Q spz)-Y bonds of heterocycles have been the preferred reaction sites for the above catalytic systems, whereas very few examples of the tosylamidation of unactivated C-H bonds have been reported to date. 

The oxidation of alkanes involves what is formally the insertion of an oxygen atom into a carbon-hydrogen bond (Fig. 4.41), although the reality of the mechanism is considerably more complex. 

Elimination of trimethylchlorosilane and nitrogen occurs when the (phos-phino)(silyl)diazomethane la is reacted with para-toluenesulfinyl chloride at low temperature. The formation of the four-membered heterocycle 92, obtained in 87% yield, can be rationalized by a multiple-step mechanism involving the formation of the (phosphino)(sulfinyl)carbene 2v. The insertion of the (phosphoryl)(sulfenyl)carbene 91, resulting from a 1,3-oxygen shift from sulfur to phosphorus in 2v, into a carbon-hydrogen bond of a diisopropylamino group readily accounts for the formation of 92.84... 

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