Insertion reactions, of methylene

Sensitizers have also been used to induce inters5retem crossing to the triplet The insertion reaction of methylene into C—H bonds in solution is widely acknowledged to be a concerted process resulting from the singlet state. Photolysis of... 

Insertion reactions of methylene with methyl orthoformate or 2-methoxy-1,3-dioxolane in the gas phase and in solution [211]. 

Doering and Prinzbach20 photolyzed CH2N2 in the presence of 2-methylpropene 1-14C in the liquid phase and in the gas phase at 400 mm. The product ratios (Table II) in the liquid were quite similar to the high pressure values of Frey and Knox et al., although Doering and Prinzbach also report no 3-methylbutene-l. The chief object of this work was to study the mechanism of the insertion reaction of methylene into CH bonds. The product 2-methyl-butene-l, which is formed entirely by insertion and not by isomerization, was separated from the reaction... 

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazo ketones and esters.215 In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into methine hydrogen is preferred to a methylene hydrogen. Intramolecular insertion can be competitive with intramolecular addition. Product ratios can to some extent be controlled by the specific rhodium catalyst that is used.216 In the example shown, insertion is the exclusive reaction with Rh2(02CC4F9)4, whereas only addition occurs with Rh2(caprolactamate)4, which indicates that the more electrophilic carbenoids favor insertion. 

In 1956, Doering et al. reported that methylene (CH2) inserted into the C H bonds of pentane, 2,3-dimethylbutane, and cyclohexene with no discrimination (other than statistical) between chemically different sites CH2 was classed as the most indiscriminate reagent known in organic chemistry. Doering and Kirmse also demonstrated that the C—H insertion reactions of CH2 in solution were direct, single barrier concerted processes with transition states that could be represented as 27 (Fig. 7.12). In particular, they did not proceed via initial H abstraction to give radical pair intermediates that subsequently recombined. (Triplet carbene C H insertions, however, do follow abstraction-recombination, radical pair mechanisms, as demonstrated in classic experiments of Closs and Closs and Roth (see Chapter 9 in this volume). 

The Insertion Reaction. The major reaction of methylene with the C—H bond leads to a product corresponding to insertion of CH2 into the bond ... 

Butene-2. The experimental results for the reaction of methylene with cis and trans butene-2 are summarized in Table III. In the liquid phase and in the gas phase at high pressures (>200 mm.) the major products are 1,2-dimethylcyclopropane (addition to double bond), pentane-2, and 2-methylbutene-2 (insertion products), and the steric configuration of the butene-2 is predominantly retained in the products... 

No trace of any ring expanded products due to C—C insertion was seen in the reactions of methylene with strained spirobicyclic molecules (46) and (47).59 Ab initio calculations on the reaction between quartet methylidyne (CH) with methane predicted a barrier of 3.5 kcal mol-1.60... 

Recently, Kistiakowsky and co-workers8 found that reactions of methylene, formed by photochemical decomposition of ketene or diazomethane, produce hydrocarbon molecules with considerable excess vibrational energy. The C—H insertion reaction is represented by reaction (3) ... 

The formation of tri- and especially tetrasilanes which are already branched (tertiary Si-units) as the first reaction products (described elsewhere [4]) suggests the appearance of intermediate silylene species which could enter in insertion reactions of Si-Si as well as Si-Cl bonds. The tri- and tetrasilanes undergo thermal crosslinking reactions at reaction temperatures of 165-250 °C. In addition dehydrochlorination reactions initiated by acid H-abstraction of methyl groups cause the formation of carbosilane (methylene) units in the polymer framework. Table 1 shows the gross compositions of poly(methylchlorosilanes) which are determined by the reaction temperature. 

Methylene insertion into a silicon sihcon bond is also possible for a reactive Si-Si bond (equation 36). Insertion reactions of divalent silicon species, silylene, also occur to the same substrate. Silylene forms a new silicon-carbon bond with a variety of substrates, which will be described later. 

The following nonplanar methylene-group binding, which is formed by an inter-molecular insertion reaction of the reactive radical chain end into a CH-bond of... 

The application of both criteria to gas-phase reactions is complicated further by the formation of vibrationally excited products. Both the insertion and addition reactions of methylene are exothermic by approximately 93 kcal. mole (based on recent estimates of AH (CH2) = 94 kcal.mole" ). Vibrationally excited alkanes and alkenes may dissociate into free radicals, and excited cyclopropanes may undergo structural and geometrical isomerizations unless collisionally stabilized . The occurrence of hot molecule reactions excludes any reasonable estimation of singlet and triplet methylene fractions. The data presented in the following paragraphs have been taken from experiments at high-pressures", which are thought to ensure complete collisional deactivation of excited reaction products. 

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. 

An electrochemical hybridization sensor can be designed using special intercalator substances. Useful intercalators are electrochemically active reversible redox couples which are able to be inserted (intercalated) into the so-called ir-stack of the dsDNA molecule. They are flat molecules of planar size such as e.g. methylene blue. The redox reaction of methylene blue is reversible. It can be reduced to give its colourless form (leukomethylene blue), which is readily reoxidized (Fig. 7.44). 

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

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