Metallo-ene-allene reaction

The perfect diastereoselectivity observed in these reactions lead to the assumption that they do not proceed via a simple anionic intramolecular cyclization (i.e. carbozincation) as was proposed in earlier61 publications. Cyclizations of non-propargylic substrates15,62 were shown to proceed with significantly lower diastereoselectivity (cis/trans typically 75/25). Thus, in the case of propargylic compounds, e.g. 114, a simple carbozincation process is unlikely to be operative. It was therefore reasoned that the active species in these reactions is not 115-ZnBr but rather its allenic isomer 120, which undergoes a metallo-ene-allene reaction in a chair-like transition state as depicted in equation 5659a. 

Another facet of the cyclization of secondary organometallic derivatives is the use of d-ethenic allylmetals [61] this has been abundantly reviewed [58] and is considered as a metallo-ene reaction. However, although the ene-allene reaction was disclosed 10 years ago [59] to interpret the formation of a cyclic product by flash pyrolysis of a 2,3,8-nonatriene, the corresponding metallo-ene-allene reaction has received attention only veiy recently in organic synthesis [60]. 

In contrast to the metallo-ene reaction [58], this metallo-ene-allene reaction is not reversible, even at 120 °C in toluene. The unique cis stereoisomer obtained in this... 

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. 

As allenes are about lOkcal/mol less stable than their 1,3-diene isomers, the thermal reaction of SO2 gives corresponding (3,y-unsaturated sulhnic acid, as illustrated in Fig. 22.3. Metallo-ene reactions of SO2 have been reported for aUyltrialkyltin compounds, aUylgermanes, aUyl-... 

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