Methylene carbon-hydrogen insertions

The singlet-triplet energy difference of methylene was found to be about 7.5 kcalmob. Direct photolysis of 3H-diazirine in chloroform gives 1,1,2-trichloroethane (92% yield) by carbon chlorine insertion, while in the presence of tetradecane, the carbon-hydrogen insertion product 1,1,1,-trichloro-ethane is formed. These are illustracted in Scheme 8 and identification was carried out using CIDNP NMR measurements to demonstrate the formation of singlet biradicals. ... 

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 methylene fragment also reacts with undecomposed diazomethane to form nitrogenous products. Many other reactions, including insertion in carbon-hydrogen bond (cf. the photochemical reaction of diazomethane with ether and with 2-propanol183 occur. 

It has been known for some years that singlet methylene (CH2) radicals will insert in carbon-hydrogen bonds. Thus methylene plus ethane will give propane. This matter will be discussed later. Oxygen atoms are isoelectronic with methylene... 

With olefins two reactions occur (a) insertion in one of the carbon-hydrogen bonds, and (6) reaction at the double bond to give an episulfide. The insertion reaction is suppressed by addition of carbon dioxide as would be expected. In the case of singlet methylenes, reactions with the double bond are stereospecific while those of the triplet methylenes are not. Here Gunning and his coworkers found the sulfur atoms to behave differently and both singlet and triplet atoms reacted stereo-specifically with double bonds in olefins. 

Another special kind of a-n switch involving a a MO and a it orbital is allowed. Taking singlet methylene as an example—any singlet atom will do—we note that its n LVMO can interact with HOMO of a carbon-hydrogen bond. A typical insertion reaction is given in (100) (Kirmse, 1964) ... 

This reaction constitutes a special type of process in which a hydrogen atom and a nucleophile are added across the diene with fonnation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20] amination [21,22], and addition of active methylene compounds [21 a,23,24], These reactions are initiated by an oxidative addition of H-Nu to the palladium(0) catalyst, which produces a palladium hydride species 1 where the nucleophile is coordinated to the metal (Scheme 8-1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a (jr-allyl)palladium intermediate. Now depending on the nature of the nucleophile (Nu) the attack on the jr-allyl complex may occur either by external trans-aVtBck (path A) or via a cw-migration from palladium to carbon (path B). 

Methylene can insert itself mio every carbon-hydrogen bond of most kinds of molecules. We cannot take time to say more here about this remarkable reaction, except that when addition is the desired reaction, insertion becomes an annoying side-reaction. 

The validity of this assumption has been questioned the total amount of insertion products R-CH was found to exceed the possible contribution from radical recombination, as estimated from the yields of R-R and CH ,-CH ). Ring and Rabinovitch have, therefore, suggested that methylene both in the singlet and triplet states inserts into carbon-hydrogen bonds. The calculations of Ring and Rabinovitch were based on the preposition that under their experimental conditions (800-fold excess of nitrogen, approximately 1.6 atm) methylene was entirely in its triplet state. It should be pointed out. however, that similar conditions did not afford the same relative amounts of stereo-isomeric cyclopropanes starting from either cis-or /rans-2-butene ". 

The oxygen atom of aliphatic ethers has a small promoting effect on methylene insertion into neighboring carbon-hydrogen bonds . By analogy with the behavior of alkanes, discrimination appears to be more pronounced in the... 

Support for the postulate of a hot methylene- C insertion into carbon hydrogen bonds had already been obtained from observations on the yields of labeled hexanes produced in the irradiation of pentane plus 2-methylpyrazine (Table 11). The statistical nature of the product... 

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. 

The work of Bell and Kistiakowsky (1962) provides a possible analogy to this situation. They found the dominant mode of formation of methyl radicals from the reaction between excited methylene and methane to be, first, insertion of methylene to give excited ethane, and then decay to give two methyl radicals. Hydrogen abstraction by methylene to give two methyl radicals was ruled out. The details of methyne formation may involve a very short-lived excited intermediate formed by attack of the carbon atom essentially normal to a carbon-hydrogen bond, followed by rapid scission of this bond before appreciable equilibration of the energy has taken place. 

The silicon-carbon double bond in intramolecularly donor-stabilized silenes is also extremely reactive. This is demonstrated by the reaction of (dichloFometfayl)tris(trimethylsilyl)silane with 8-dimethylaminomethyl-l-naphthyllithium, which deviates from the general reaction pattern. We expected to get another stable silene, but we obtained in a yield of 51 % the 1-sila-acenaphthene 13. Obviously, the silene 12 is unstable, and the silicon-carbon bond is inserted into one carbon-hydrogen bond of the benzylic methylene group (Scheme 5). 

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