Sigma bonds reactions

Antisymmetric matrix, non-adiabatic coupling, vector potential, Yang-Mills field, 94-95 Aromaticity, phase-change rule, chemical reaction, 446-453 pericyclic reactions, 447-450 pi-bond reactions, 452-453 sigma bond reactions, 452 Aromatic transition state (ATS), phase-change rule, permutational mechanism, 451-453... 

The H3 and H4 systems were discussed above. Another type of sigma bonds involves a p orbital lying along the reaction coordinate, as, for example, in reaction (15) (Fig. 8). 

A similar situation holds foi a molecule containing a tetrahedral carbon is shown in (Figure 16). The reaction converting one enantiomer to another, is formally equivalent to the exchange of two sigma-bond electr on pair s, and... 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state. 

A new synthesis for sigma-bonded, metal alkyls and similar compounds, involving a reaction between metal vapor and free radicals generated in a radio-frequency glow-discharge, has been reported (35). 

As in the case of ethylene and propylene, these reactions lead to increased NO removal, however, NO removal remains almost unaffected, because NO is largely converted to NOz. In addition, it was found that propane is less reactive as compared to propylene [81,88], due to their stable molecular structure with stronger sigma bonds of C-C and C-H. 

If hydrogen gas is added to the reaction mixture of J, and 11 the hydrogenolysis reaction of thorium-to-carbon sigma bonds (J-1 22) allows interception of species 13 and thus, catalytic hydrogenation of the inserted carbon monoxide functionality. At 35 C under 0.75 atm initial H2 pressure with [JJ =9.0 x 10" M and [ 1JJ = 6.5 x 10" M, hydrogenation and isomerization are competitive and both the enolate and the alkoxide reduction product 14 are produced (eq.(13)). Under these conditions, turnover fre-... 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

In contrast to the severe difficulty of cracking a sigma bond, insertion of a transition metal into a pi bond can proceed in facile fashion. This can be illustrated by the attack of Ti on the pi bond of ethylene, which leads to metallacycle formation in the reaction... 

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