mechanism in bi cyclic compounds

![enter image description here](https://i.stack.imgur.com/Q4YYs.jpg)


In both cases ring expansion is happening. What is Carbon tri chloride purpose? 




" What is Carbon tri chloride purpose?" is not answered in the link provided but the answer starts with a carbene attack on furan .


Trichloromethane in presence of a base undergoes 1,1-elimination (or an alpha elimination) because both groups are removed from the same carbon.The electron-withdrawing effect of the halogens makes the hydrogen acidic enough to be removed by a strong base such as hydroxide or alkoxide ion.


A halide ion then acts as a leaving group from the conjugate base, producing the
 carbene.[1](https://i.stack.imgur.com/BNm40.jpg)


[![enter image description here](https://i.stack.imgur.com/BNm40.jpg)](https://i.stack.imgur.com/BNm40.jpg)


Carbenes find a pair of electrons to complete their valence shell of electrons.carbenes attack a lone pair, a C=C double bond (electron-rich or -poor), or even a C–H bond.In its reactions ,


* It is an electrophilic reagent and is known to give ring expansion of aromatic rings.
* Electrophillic attack takes place on pyrrole where the intermediate is most stable.


This leeds to normal Reimer–Tiemann reaction product pyrrole 2-aldehyde.


Carbene attack of a C=C double bond leads to an ring expansion leading to 3-chloropyridine.


[![enter image description here](https://i.stack.imgur.com/qaFUn.png)](https://i.stack.imgur.com/qaFUn.png)


The reaction of bicyclic compounf acts on similar lines of dichloro carbene attack.[2](https://i.stack.imgur.com/qaFUn.png)


[![enter image description here](https://i.stack.imgur.com/S7XZf.jpg)](https://i.stack.imgur.com/S7XZf.jpg)


**References** 


[1](https://i.stack.imgur.com/BNm40.jpg) ORGANIC CHEMISTRY ,JOSEPH M. HORNBACK ,UNIVERSITY OF DENVER


[2](https://i.stack.imgur.com/qaFUn.png) MARCH’S ADVANCED ORGANIC CHEMISTRY REACTIONS, MECHANISMS,AND STRUCTURE ,SIXTH EDITION


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