Hydrogen four-centered

Hydroboration is a stereospecific syn addition that occurs through a four-center TS with simultaneous bonding to boron and hydrogen. The new C—B and C—H bonds are thus both formed from the same face of the double bond. In molecular orbital terms, the addition is viewed as taking place by interaction of the filled alkene it orbital with the empty p orbital on boron, accompanied by concerted C—H bond formation.158... 

Scheme 17.6 Transition states for the transfer of a hydrogen atom from a molecular hydrogen ligand to a formate ligand a) four-centered b) four-centered and base-assisted [66] and c) six-centered [59].

Scheme 3.28 depicts the proposed reaction mechanism that may happen on these alloys. The first step is the coordination of the ester to the alloy via its oxygen atoms (it is likely that hn favors this coordination due to its electrophilicity). In a second step there is a four-center mechanism where the hydridic hydrogen makes a nucleophilic attack at the electrophilic carbon atom of the carbonyl with forma-... 

CHA-CH=CH-CHB-CHA-CH=CH-CHB-. The hydrogenated polymers may analogously be considered —CHA—CH2 CH2—CHB—CHA— CH2—CH2—CHB—. In these cases the complexity of the disubstituted polymer adds to that of the polymers in which the two chain directions are different. In principle 4 two-center, 8 three-center, and 16 four-center sequences must be considered. 

Catalytic hydrogenation and hydroformyliuion are just two of the many important processes in which these reactions are fundamental (see page 711). The first step in the reaction is coordination of the alkenc to the metal, followed by rapid insertion into the M—H bond. The transition state involves a four-center planar structure. 

If the concerted four-center mechanism for formation of chloromethane and hydrogen chloride from chlorine and methane is discarded, all the remaining possibilities are stepwise reaction mechanisms. A slow stepwise reaction is dynamically analogous to the flow of sand through a succession of funnels with different stem diameters. The funnel with the smallest stem will be the most important bottleneck and, if its stem diameter is much smaller than the others, it alone will determine the flow rate. Generally, a multistep chemical reaction will have a slow rate-determining step (analogous to the funnel with the small stem) and other relatively fast steps, which may occur either before or after the slow step. 

The attack of a chlorine atom on a methane hydrogen is not expected to require a precisely oriented collision. Moreover, the interatomic repulsions should be considerably smaller than in the four-center mechanism discussed previously for the reaction of molecular chlorine with methane because only two centers have to come close together (Figure 4-8). The methyl radical resulting from the attack of atomic chlorine on a hydrogen of methane then can remove a chlorine atom from molecular chlorine and form chloromethane and a new chlorine atom ... 

On the other hand, in the uncatalyzed cleavage by hydrogen chloride, electrophilic attack of a proton on the silicon-silicon bond may play an important role in the four-centered transition state. 

Banks and Bailey concluded that disproportionation occurs when two molecules are adsorbed with the breakage of two opposite bonds without hydrogen shift. Polymerization occurs when two molecules are adsorbed with the formation of a four-center complex and then desorb with the breakage of one bond and hydrogen shift. Skeletal isomerization occurs when one molecule is adsorbed with the formation of a four-center complex and then desorbed With the breakage of one bond and hydrogen shift. 

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