Ring opening Metathesis [olefin ring] Ring opening metathesis is often used in p

Question

Ring opening Metathesis [olefin ring]

Ring opening metathesis is often used in polymerization to design unique monomers. The ring opening often depends on the particular character of the monomer ring. Order the following compounds in terms of least favorable to the most favorability to promote a metathesis polymerization reaction.

Please give me explanation.

The following structures have only one double bond and plenty single bonds.

triangular ring, square ring, pentagonal ring, hexagonal ring, heptagonal ring, octagonal ring

Explanation / Answer

Acc to metathesis olefinic reaction,

the breaking of bond involves [2+2]cycloaddition reactions in which one of the pi bond and sigma bond breaks and another one forms.

It works on ring strain property of the following olefins such that the cycloalkene with less number of carbons and hydrogen shows maximum strain and also due the bond angle it shows more strain whereas cyclohepentane shows minimum amount of strain.

However these metathesis polymerization reaction are accompanied by some catalyst like RuCl3/alcohol mixture by forming metal-carbene complex as Ruthenium metal easily breaks the double bond present in the compound and form complexes.The carbene of the metal carbene complex attacks the double bond in the ring structure forming a highly strained metallacyclobutane intermediate. The ring then opens giving the beginning of the polymer i.e. a linear chain double bonded to the metal with a terminal double bond. The new carbene reacts with the double bond on the next monomer, thus propagating the reaction.

The initial formation of carbene complex depends on various factors:

1.Solvent Effect

2.Substituent Effect

Therefore the order of formation of the monomers will be:

Cyclopropene>cyclobutene>cyclopentene>cyclohexene>cycloheptene>cyclooctene

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