Steric transannular

Onium ions of small and large heterocyclics are usually produced by electrophilic attack on a heteroatom. In three- and four-membered rings nucleophilic attack on an adjacent carbon follows immediately, in most cases, and ring opening stabilizes the molecule. In large rings the onium ion behaves as would its acyclic analog, except where aromaticity or transannular reactions come into play (each with its electronic and steric pre-conditions). A wide diversity of reactions is observed. 

Catalyst 70 is very effective for the reaction of terminal alkenes, however 1,1-disubstituted olefins provide hydrosilylation products presumably, this is due to steric hindrance [45]. When a catalyst with an open geometry (78 or 79) is employed, 1,1-disubstituted alkenes are inserted into C-Y bonds to give quaternary carbon centers with high diastereoselectivities (Scheme 18). As before, initial insertion into the less hindered alkene is followed by cyclic insertion into the more hindered alkene (entry 1) [45]. Catalyst 79 is more active than is 78, operating with shorter reaction times (entries 2 and 3) and reduced temperatures. Transannular cyclization was possible in moderate yield (entry 4), as was formation of spirocyclic or propellane products... 

Sterically strained 1,9-dithiosubstituted dibenzothiophenes 31 are destabilized by repulsion of the lone pairs at the transannular sulfur atoms and can be also transformed to S-S dications by concentrated sulfuric add (Scheme 10).54 56... 

High oxidation state alkylidene complexes in which a heteroatom is bound to the alkylidene carbon atom are extremely rare [41]. Since the approach shown in Eq. 43 failed, the related approach shown in Eq. 44 was taken to prepare the medium-sized ring subunits [222]. The latter product was formed in good yield when n=2, R H, R2=Et, but only poor yield when n=2, R =Et, R2=H, possibly due to unfavorable interactions between the ethyl substituent and transannular groups in the transition state for cyclization of the allyl ether [222]. Ruthenium catalysts either failed or gave low yields, presumably because of the steric hindrance associated with ring-closing dienes of this type. 

The electronic spectra of the [2.2]paracyclophanes provide valuable information regarding the extent and mechanisms of transannular electronic interactions in strained 7t-electron systems. A rigid system like the [2.2]paracyclophane molecule is of great value as a model for checking theoretical data with a view to interpretating U V spectra of sterically hindered molecules ( overcrowded compounds ). 

Chemical reactions have also provided valuable information regarding transannular electronic and steric effects. 

Depending upon substituents, transannular interactions in the [2.2]paracyclophane system are characterized by the steric or electronic effects of one aromatic nucleus on the physicochemical behavior of the other aromatic ring. The transannular reactions themselves, of course, are very dependent upon molecular geometry. 

Most of the pKa values for the phosphonium salts have been determined by potentio-metric measurements in the case of / -ketophosphonium salts562 (Table 13). The acidity of these phosphonium salts depends not only on the R and R groups of the carbonyl chain, but also on the other substituents linked to the phosphorus562 564. Particularly for the cyclic phosphonium salts, the pKa value can vary significantly depending on the different steric and electronic (even transannular) effects on the phosphorus atom564 ... 

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