Dichlorocyclopropanation Followed by Rearrangement

In cases where the dichlorocarbene adduct of an olefin is either strained or otherwise unstable, the adduct can ionize. The cation thus generated can rearrange and then add back chloride or it can eliminate. In the latter case, if a source of dichlorocarbene is also present, further addition to the system can occur. The addition of dichlorocarbene to norbornadiene, for example, affords only monoaddition products, but each of these results from rearrangement [36—38]. The reaction is formulated in equation 2.17. 

Norbornene [5] and methylnorbornene [39] both undergo phase transfer dichlorocyclopropanation with rearrangement (see Eqs. 2.18 and 2.19). In the former case, a single product was isolated and in the latter case, products of both ring expansion and ion pair collapse and ring expansion followed by dehydrochlorination were isolated. 

A closely related example is found in the dichlorocyclopropanation of 7-r-butoxynor-bornadiene [36]. One aspect of this reaction seems surprising. It appears that addition of dichlorocarbene occurs almost exclusively syn to the f-butoxy group. This 

Reaction of e J 9-5-hydroxymethyl-2-norbornene with dichlorocarbene generated under phase transfer conditions leads to 3-chloro-5-oxatricyclo-[5.2.1.0 ]-dec-2-ene as the major product (see Eq. 2.21). Formation of this product probably involves initial addition of dichlorocarbene to the carbon-carbon double bond to yield a 1,1-dichlorocyclopropane which ionizes and ring-opens to form a chloro-substituted allylic carbonium ion. This cation is then trapped by the intramolecular nucleophilic alcohol [40]. 

The addition of dichlorocarbene to hexamethyl Dewar benzene has been studied (see Eq. 2.22) [37, 41]. The initial mono-dichlorocarbene adduct undergoes ionization and opening of the cyclopropane ring to yield an allylic carbonium ion which loses a proton from a methyl group to yield adduct 1. 

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