Strained double four-membered

The reason why zeolite A has a resonance corresponding to Si(AAl) at -88.5+1.5 stems from the unique feature of the structure strained double four-membered rings, with T-O-T angles of 129°, 152°, 152° and 177°. We believe that the presence of linear or nearly linear T-O-T linkages modifies the bonding characteristic for a central Si atom in a Si(0A1) (OSi). unit (10, 32). It is very likely that there ar other0... 

The formation of strained three- and four-membered rings by electrophile-initiated cyclization requires that the reaction be conducted under conditions which minimize the possibility of simple addition of the activating reagent across the double bond or reversal of the cyclization product to intermediates that can be trapped by external nucleophiles. 

There are nonetheless just a few isolated examples of three- or four-membered ring forming anionic cyclisations curiously it was in fact a three- and a four-membered ring forming reaction which provided the first two pieces of evidence that organolithiums do indeed cyclise onto unactivated double bonds. In 1960, Wittig107 showed that n-BuLi not only added to one of the double bonds of norbornadiene but that the major product of the reaction arose from organolithium cyclisation of the intermediate 201 onto the other double bond. This odd reaction is presumably made possible by the strain already inherent in the bicyclic system. After protonation, the major product was 202. 

If two of the carbons that underwent change are obviously those pertaining to the double bond of I, the two other active sites must be derived from the heterolytic separation of a C-C bond in the strained four-membered ring. The isopropanol chain clearly suggests the position where the positive center to be is located. 

Selectivity may be facilitated by steric strain. Hydrogenation of the Na salt of the diolefin, 5, proceeds stepwise over Pd, the double bond in the strained four-membered ring being reduced first ... 

It is not that this cycloaddition is energetically unprofitable—in spite of the ring strain, the cyclobutane is lower in energy than the two alkenes—so there must be ahighkinetic barrier to the cycloaddition of one alkene to another. This is a deeply importantpoint, and it is just as well that it is true—if alkenes and other double-bonded compounds could readily dimerise to form four-membered rings, there would be few stable alkenes, and life would be impossible. 

A double bond in a six-membered ring is less strained than a double bond in a four-membered... 

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