The stereoselectivity of nucleophilic addition on oxocarbenium ions derived from the bicyclic pyranoside model with or without a C-OR group can be understood through the use of the bent-bond and the antiperiplanar hypothesis in conjunction with the concept of hyperconjugation as an alternative interpretive model of structure and reactivity.
L. T. Scott has discovered the 1,2-swapping of carbon and hydrogen atoms which is known to take place on benzenoid aromatics (up to ∼1000 °C range). For example, C-1-naphthalene is specifically converted toC-2-naphthalene, and there is evidence that this occurs through the formation of benzofulvene and a naphthalene-carbene intermediate. Application of the bent bond/antiperiplanar hypothesis leads to the postulate that higher in energy pyramidal singlet diradical intermediates can be used to propose a mechanism that rationalizes various atom rearrangements on benzenoid aromatics and related isomeric compounds.
The glycosylation stereoselectivities for a series of
bicyclic
furanoside models have been carried out in the presence of weak nucleophiles.
These results were analyzed through the bent bond/antiperiplanar hypothesis
(BBAH) orbital model to test its validity. According to the BBAH,
incoming nucleophiles displace one of the two bent bonds of bicyclic
oxocarbenium ion intermediates in an antiperiplanar fashion. The glycosylation
stereoselectivity is then governed by the displacement of the weaker
bent bond as determined by the presence of electron-withdrawing or
-donating substituents at C2. Overall, the BBAH analysis
expands Woerpel’s “inside/outside attack” glycosylation
model by considering the stereoelectronic influence of neighboring
electron-withdrawing and -donating groups on the nucleophilic addition
to oxocarbenium ion intermediates.
Glycosylation reactions were performed
on a series of bicyclic
C2-substituted pyranoside models to isolate and analyze
factors that control the glycosylation stereoselectivities observed
in carbohydrates. The bent bond/antiperiplanar hypothesis (BBAH) orbital
model rationalizes all of these results by considering hyperconjugation
interactions between groups at C2 and the two τ bonds
(bent bonds) of oxocarbenium ion intermediates formed under the glycosylation
conditions. According to the BBAH, nucleophiles add to oxocarbenium
intermediates by SN2-like antiperiplanar displacement of
the weaker of their two τ bonds.
A theoretical study of the C‐H system reveals that, at high temperatures, many polymeric species have to be taken into account. Free energy functions of the species involved have been calculated to permit the evaluation of the equilibrium compositions of the system in both the heterogeneous and homogeneous regions. Pressures ranging from 0.1 to 10 atm. and carbon to hydrogen ratios from 0.5 to 20 were investigated between 2,000 and 6,000°K.
Since the reaction of carbon vapor with hydrogen is highly exothermic, a reaction scheme was also developed whereby the endothermic heat of cracking of various hydrocarbons could be balanced by the former reaction. Several conditions related to a global heat of reaction have been considered.
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