Insertion into C-H bond

The insertion of CO2 into C-H bonds (4.36), despite its undoubtedly great industrial interest for 

Such reaction has been re-examined more recently [183] and some elements of knowledge have been added to its otherwise not yet completely elucidated reaction mechanism. 

A case related to the above is represented by the reaction with CO2 of o-PhO-Mn (II) and n (PhO-)Mn2 systems. The former gives insertion into the Mn-O bond as expected to afford Mn-0C(0)0Ph 

CHsCOCHjCOCHs- CH3C0CH(C00H)HC0CH3 + CH3COCH2COCH2-COOH Ru ZnS + CO2 + light— C02  

CH3COCH2COCH3 + CO2 + PhONa CH3COCH(COONa)COCH3 + PhOH 

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Fluorinated alkenes can insert into C-H bonds at elevated temperatures, a relatively unusual example of simultaneous C-C and C-H bond formation [20, 21] (equations 16 and 17)... 

An unusual reaction of carbenes is that of insertion into C—H bonds (12-19). Thus. CH2 reacts with... 

The highly reactive species methylene inserts into C—H bonds,both aliphatic and aromatic,though with aromatic compounds ring expansion is also possible (see 15-62). This version of the reaction is useless for synthetic purposes because of its nonselectivity (see p. 248). This contrasts with the metal carbene insertion reaction, which can be highly selective, and is very useful in synthesis. Alkylcarbenes usually rearrange rather than give insertion (p. 249), but, when this is impossible. 

Carbenes and carbenoids can add to double bonds to form cyclopropanes or insert into C-H bonds. 

Methylene insertion into C—H bonds is believed to be concerted for the singlet species and stepwise for the triplet.<164,156) The C—H insertion of methylene into the 14C-labeled isobutylene shown below results in 92% unrearranged isopentenes and 8% rearranged isopentene [Eq. (11.22)]. Assuming that an additional 8% of the unrearranged isopentene arises from the stepwise addition, it is clear that 84% of the insertion products result from insertion by singlet methylene and 16% by triplet methylene ... 

For consistency, an initial well associated with the formation of an adduct of the metal ion with the alkane should be included in Figure 11. The chemical activation associated with the formation of such an adduct is likely to be essential in overcoming intrinsic barriers associated with insertion into C-H bonds. In comparison to larger hydrocarbons, the weaker interaction of ethane with first row group 8-10 metal ions may be insufficient to overcome intrinsic barriers for insertion. This would explain the failure to observe dehydrogenation of ethane by these metal ions, even though the process is known to be exothermic. The well depths could be determined from high pressure equilibria. Studies in our laboratory and elsewhere have indicated the ease with which many of... 

Table 18. Regioselectivity of ketocarbenoid insertion into C—H bonds of N-methylpyrrole.

Metal-oxenoid (oxo metal) species and metal-nitrenoid (imino metal) species are isoelectronic and show similar reactivity both species can add to olefins and be inserted into C—H bonds. Naturally, the study of nitrene transfer reactions began with metalloporphyrins, which were originally used as the catalysts for oxene transfer reactions. 

Perez and co-workers reported the electron-deficient copper homoscorpionate catalyst TpBr3Cu(NCMe)-catalyzed nitrene insertion into C-H bonds of toluene, mesitylene, and cyclohexane, which are very unreactive substrates (Equations (100)—(102)). In contrast to the former reports, they obtained very high yields for these products. 

Green and coworkers (5-8) have shown in a series of studies that photogenerated tungstenocene behaves as. an organometallic car-bene and readily inserts into C-H bonds of a variety of solvent molecules, e.g., eq 3 (5). 

H2, it does readily occur upon photolysis to generate [ReH-(dppe) ] as a very reactive photoproduct (13). As detailed below, this species rapidly adds substrate molecules (N, CO, C2H2, C H, and CO ) and inserts into C-H bonds of benzene solvent and the phenyl groups of the dppe ligands. 

Few examples of preparatively useful intermolecular C-H insertions of electrophilic carbene complexes have been reported. Because of the high reactivity of complexes capable of inserting into C-H bonds, the intermolecular reaction is limited to simple substrates (Table 4.9). From the results reported to date it seems that cycloalkanes and electron-rich heteroaromatics are suitable substrates for intermolecular alkylation by carbene complexes [1165]. The examples in Table 4.9 show that intermolecular C-H insertion enables highly convergent syntheses. Elaborate structures can be constructed in a single step from readily available starting materials. Enantioselective, intermolecular C-H insertions with simple cycloalkenes can be realized with up to 93% ee by use of enantiomerically pure rhodium(II) carboxylates [1093]. 

Functionalization of C-H bonds by metal carbenoid or nitrenoid insertions represents a promising alternative to the more traditional approach of direct activation by a metal center. Carbenoids and nitrenoids show unusual regio- and stereoselectivity in insertions into C-H bonds, and unlike insertions of metal centers, these are intrinsically functionalizations rather than activations, which must be followed by functionalization (although in either case, loss of the functionalized group, to regenerate the active metal complex, is still required for catalysis) [129]. The use of dimeric Rh(n) complexes in this area has been extensively developed [129]. 

Carbenes and carbenoids can add to double bonds to form cyclopropanes or insert into C—H bonds. These reactions have very low activation energies when the intermediate is a free carbene. Intermolecular insertion reactions are inherently nonselective. The course of intramolecular reactions is frequently controlled by the proximity of the reacting groups.53... 

Other differences between singlet (concerted) insertion and triplet (abstraction-recombination) carbene insertion are seen in selectivity, stereochemistry, and the kinetic deuterium isotope effect. The triplet states are more selective in C—H insertion than the singlets. For example, the triplet shows higher tertiary to primary selectivity than the singlet in the insertion reaction with 2,3-dimethylbutane. Singlet carbene is shown to insert into C—H bond with retention of configuration, while racemization is expected for triplet insertion reaction from the abstraction-recombination mechanism. For example, the ratios of diastereomeric insertion product in the reaction of phenylcarbene with roc- and mcTO-2,3-dimethylbutanes are 98.5 1.5 and 3.5 96.5, respectively. ... 

Chemically induced dynamic nuclear polarization (CIDNP) is a very powerful tool for establishing the existence of radical pair intermediates and their spin. CIDNP has reinforced the view that singlet carbene undergoes direct insertion into C—H bonds and that the triplet abstracts hydrogen. 

Singlet 2,6-difluorophenylnitrene (16d) and singlet perfluorophenylnitrene (16e) react with hydrocarbon solvents by insertion into C—H bonds. ° In the case of nitrenes 16d and 16e in hydrocarbon solvent, koss is actually... 

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