Alkene derivatives leaving groups

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

Active methylene and methine compounds bearing a leaving group (X) on the y-carbon atom can afford cyclopropyl derivatives via 1,3-elimination of HX. 1,2-Elimination to give alkenes and direct nucleophilic substitution by base may compete with the 1,3-elimination, particularly in the preparation of excessively strained cyclopropyl derivatives. The preferred reaction course is, however, highly dependent on reaction conditions, especially on the nature of the base and solvent employed, as exemplified by the reactions of 4 (equation 7) 4. 

Cyclopropanations are known for several other carbanionic intermediates of the general type (7), in which the substituent G is ultimately lost as an anionic leaving group in the last step of the ring-forming pathway (see Scheme 3 above). The substituent G is most often a functional group based upon sulfur, selenium or nitrogen. Halide-substituted derivatives probably react via the a-elimination pathway in most cases (see Section 4.6.3.1), but in some reactions with electron deficient alkenes as substrates, the normal order of steps may be altered (e.g. Table 10, ref. 162). 

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