Carbon-hydrogen bonds cyclopropanation

Whether carbene, germylene, and silylene are justifiable terms for stabilized versions of the reactive species is a very debatable question [28], However, for synthetic chemists, the most important issue is to know whether these stable species feature the reactivity of transient carbenes. Formation of azaphospholidines upon thermolysis (intramolecular carbene insertion into a carbon-hydrogen bond), cyclopropanation reactions, and [l + l]-addition to isocyanides giving keteneimines... 

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 above results, which are not strongly dependent on the particular value chosen for s, show that about 10% of the trimethylene diradicals lead to propylene, and that the limiting yield of cyclopropane is about 90%. Additional propylene may of course be formed by direct insertion into carbon-hydrogen bonds. 

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. 

Methylcarbomethoxycarbene. Alkylcarbenes are generally useless as cyclopropane precursors since their singlet states rapidly undergo hydride shifts the prevailing reaction is insertion into carbon hydrogen bonds. For example, direct... 

Cyclopropane belies its simple appearance. Hybridization of the carbon-hydrogen bonding orbitals in cyclopropane is close to sp (actually sp ) and that of the orbitals forming the carbon-carbon bonds is sp It was proposed that these latter orbitals lie outside the line connecting adjacent carbon atoms such that the angle between the axes of the sp hybridized orbitals is 104°. The CCC internuclear angles are clearly 60° and so the carbon-carbon bonds are bent by 22°. 

The hybridization of the carbon-hydrogen bond orbitals in cyclopropane is clearly similar to those in alkenes. However, the chemical shifts of these two types of proton are appreciably different and factors other than hybridization must be responsible. 

In addition to these exchange reactions, a number of alkane/alkane and al-kane/arene exchange reactions could be studied as equilibria (benzene, toluene, cyclopropane, methane, ethane, neopentane, cyclohexane). Determination of equilibrium constants allowed calculation of AG° values and estimation of relative metal-carbon bond energies. Wolczanski concluded that the differences between metal-carbon bond energies and the corresponding carbon-hydrogen bond energies were essentially the same [82]. 

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. 

To examine the structural a.spects of the.se molecules, you will find your set of models to be indispensable. Cyclopropane is the only flat cycloalkane ring. All larger cycloalkanes are nonplanar. Ring distortion away from a planar structure reduces eclipsing interactions between neighboring carbon-hydrogen bonds. 

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 ". 

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