Olefinations carbon-hydrogen bonds

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... 

The various modes of bonding that have been observed for alkenes to the trinuclear osmium clusters are shown in Fig. 7 [see (88)]. The simple 77-bonded structure (a) is relatively unstable and readily converts to (c) the vinyl intermediate (b) is obtained by interaction of alkene with H2Os3(CO)10 and also readily converts to (c) on warming. Direct reaction of ethylene with Os3(CO)12 produces (c), which is considered to be formed via the sequence (a) — (b) — (c) and (d). Both isomers (c) and (d) are observed and involve metal-hydrogen and metal-carbon bond formation at the expense of carbon-hydrogen bonds. In the reaction of Os3(CO)12 with C2H4, the complex 112088(00)902112, (c), is formed in preference to (d). Acyclic internal olefins also react with the carbonyl, with isomerization, to yield a structure related to (c). Structure (c) is... 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

Methylene attacks the double bond and the carbon-hydrogen bonds of olefins. These reactions lead to excited adducts which may undergo isomerization, collisional deactivation, or dissociation ... 

In contrast to the case of allylsilanes, anodic oxidation of disubstituted olefins provides in general four regioisomeric products because all the allylic carbon-hydrogen bonds can be cleaved. In the case of allylsilane, the cleavage of a C—Si bond takes place... 

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. 

A widely exploited procedure for bringing about carbenoid reactions of organic mono- and fifem-dihalides is by use of lithium alkyls. Examples are given in equations (11) and (12). Dimeric olefin formation, stereospecific cyclopropane formation from olefins, and insertion into carbon-hydrogen bonds have all been observed in suitable cases, together with further reactions of these products with excess of the lithium alkyl. 

If on the other hand [434] has the 9-barbaralyl cationic structure [2], the observed shifts of the two signals at — 151°C are as expected the carbon-hydrogen bonds involving C(l), C(2), C(4), C(5), C(6) and C(8) are not equivalent and therefore C-H will preferentially be found in olefinic positions since olefinic C-H-bonds have lower zero-point energy than saturated C-H-bonds. Therefore the C-average chemical shift will be shifted downfield (279). By the same reasoning, only a minor shift is expected for the C(3), C(7)... 

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