Olefins dichlorocarbene production

German chemists have used the method successfully for preparation of dichloro-cyclopropanes from olefins which yield little or no products when the dichlorocarbene is generaied from chloroform and potassium /-bntoxide. TTiey also generated dibromo-carbene in the same way. Cyclopropcnes are obtained in only low yields from acetylenes owing to side reactions. 

Both benzyltriethylammonium chloride (1) and benzyltriethylammonium hydroxide (2) partition between the aqueous and organic phases. In the aqueous phase, the quaternary ammonium chloride (1) reacts with concentrated hydroxide to give the quaternary ammonium hydroxide (2). In the chloroform phase, 2 reacts with chloroform to give the tri-chloromethyl anion (3), which eliminates chloride ion to give dichlorocarbene, iCClj, (4), and 1. The carbene (4) reacts with the olefin to give product (5). 

Halogen-substituted carbenes attack olefins from their sterically less hindered face, however, exceptions are known for alkenes with oxygen substituents (vide infra). Two of numerous examples demonstrate the directive power of steric effects. Thus, spiro compound 1 and dichlorocarbene combine to provide the product 2 with the dichloromcthylcnc unit exclusively trans to the phenyl group14. 

Duroquinone reacts with diazomethane to give adducts that under the action of acid catalysis, heat, or light lose nitrogen to give products (Scheme 5.36). Addition of carbene to CN double bonds has also been observed. The addition of dichlorocarbene to diazo compounds gives olefins (Scheme 5.37). 

In the presence of more conventional bases carbene production is supressed. The decomposition of the chlorodifluoromethyl anion has been shown to be reversible and the yield of difluorocyclopropane is increased by increasing the alkene concentration. Epoxides upon treatment with dichlorocarbene afford cyclopropanes stereospecific-ally. The reaction proceeds by stereospecific deoxygenation of the epoxide to give olefin (41) which is subsequently trapped by the carbene. 

The enantioselective addition of dichlorocarbene to unsymmetrical olefins in the presence of a chiral phase-transfer catalyst has been further examined and it has been found that the optical rotation of the product is strongly dependent on the structure of the catalyst employed. The first products of di-iodocarbene (generated from iodoform with t-butoxide) addition have been characterized and di-iodonorcarane is stable for long periods at 0 °C, contrary to earlier reports. The base-catalysed decomposition of ethyl trichloroacetate in methanol results in dimethoxycarbene formation and. 

Despite the many advantages of the phase transfer method for generating dichlorocarbene, it should be noted that the reactive species arises from the trichloromethide anion as it does in most other methods. Because of this, attempted reactions with electron poor olefins will yield products arising from Michael addition of ClaC" to the olefin instead of, or in addition to, cyclopropanation products. The thermal decomposition of trihalomethyl metal compounds remains the unique method for generating dihalocarbenes without prior formation of a trihalomethyl anion [8]. 

The dichlorocyclopropanation of simple olefins is characterized by good yields, convenient reaction conditions and inexpensive reagents. When there is more than one isolated double bond in a substrate, products of both mono and multiple cyclopropanation are isolated unless a selective catalyst is used (see above). Examples of the simple dichlorocyclopropanation reaction are presented in Table 2.1. Examples of multiple dichlorocyclopropanation are recorded in Table 2.2. Imines also add dichlorocarbene and are discussed in Sect. 3.4. 

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. 

Dichlorocarbene generated in a two-phase system in the presence of a chiral amine catalyst, reacts with olefins to afford dichlorocyclopropanation products having small optical rotations. The reaction is discussed in terms of steric interactions in the transition state and is analogous to those discussed in section 2.2 and equations 2.9—2.11. 

More tetra-alkylammonium salts have been found to be effective phase-transfer catalysts in the Makos2a method of dichlorocarbene cycloaddition to simple olefins. Yields approaching 100% are now possible by this procedure. It has been shown in other reactions which are subject to phase-transfer catalysis (PTC) that the reaction does not occur at the interface or within the bulk aqueous phase. Rather, the catalysis appears to be caused by the tetra-alkylammonium ion increasing the solubility of the anionic base in the organic phase (as originally proposed). It is possible that the cycloaddition is catalysed in the same way. The procedure has been used for cyclopropanation of simple and steroidaP olefins. With phenanthrene the initial adduct (44) could be isolated whereas with 9-methylphenanthrene and other alkyl-aromatics spiro-products were isolated. 

---