Magnesium alkene complexes

For a recent example of enantioselective intra- and intermolecular hydroaminations of terminal amino-alkenes and styrene derivatives with heteroleptic chiral magnesium-phenolate complexes, see Emge, T. J. Hultzsch, K. C. Angew. Chem. Int. Ed 2012, 51, 394. 

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. 

Finally, C—C bond formation occurs through regio- and stereospecific reaction between allyl alcohols and ethyl magnesium chloride, in the presence of complex zirconium catalysts, followed by treatment with oxygen (equation 111)434. In this reaction the alkene functionality is lost. 

To our knowledge there is no example in the literature of a stable alkene (or alkyne) complex with magnesium as central atom, n Complexes have, however, been postulated as intermediates in the rearrangement reactions of some alkenyl Grignard reagents (86). 

A variety of new ligand designs and ligand combinations were used in attempts to mimic some properties of the ubiquitous bent metallocene environment at the early metal centers consequently, some of these systems were used in the further development of butadiene zirconium chemistry. The pyridine based chelate zirconium dichloride complex 43 cleanly formed the butadiene complex 44 upon treatment with butadiene-magnesium. Its structure shows that the C4H6 is arranged perpendicular to the chelate ligand plane. Complex 44 inserts one equivalent of an alkene or alkyne to form the metallacyclic 7i-allyl system 4545 (Scheme 13). 

Dienyl acetate (256) (readily prepared l-hepten-6-one and lacking the disulfone moiety) was also easily transformed to ( )-1,4-diene (259 87%) (Scheme 53). This regiochemistry parallels that of stoichiometric additions of allylpalladum complexes to notbomadiene, but was subject to uncertainty in view of the relatively fast 1,3-Pd migration within the allyl component. Therefore the conversion (256) -> (259) demonstrates a new stereocontrolled access to exocyclic trisubstituted alkenes which implies C—C bond formation at the less-substituted allylpalladium terminal and thus shows a regio- and stereo-chemistry both opposite to the type II magnesium-ene process (c/. Section 1.2.3.2.2 81 -> 83). 

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