Carbenes in the usual form of reimer-tiemann reaction react by forming one bond and getting negatively charged like this but in abnormal reactions of cyclopentadiene(and I suspect all 5 member rings) it forms two bonds in a cyclic manner like this, remains uncharged and causes ring expansion.

Why does that happen? Is this something about singlet or triplet carbenes? But then both situations have different products depending on whether singlet or triplet carbenes are formed. How does the carbene know to react in a specific way?

Moreover, the wiki page on carbenes says they react with non-aromatic alkenes in the latter waycarbene reacting by forming cyclic structure, so is reimer-tiemann reaction of phenol an exception?



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