Alkenyl carbenoids

Scheme 3.9. Insertion of an alkenyl carbenoid into an organozirconocene chloride.

Scheme 3.10. Insertion of 2,2-disubstituted alkenyl carbenoids into organozirconocene chlorides.

The only other alkenyl carbenoid with a proton trans to the halide that can readily be generated by deprotonation is the parent 1-lithio-l-chloroethene 57 [43] (Scheme 3.13). Insertion into organozirconocenes arising from hydrozirconation of alkenes and alkynes, followed by protonation, affords terminal alkenes and ( )-dienes 59, respectively [38]. The latter provides a useful complement to the synthesis of 54 in Scheme 3.12 since the stereocontrol is >99%. 

A key feature of the elaboration of organozirconocenes by insertion of a carbenoid is that the product retains the carbon—zirconium bond functionality of the substrate. Several useful elaborations are shown in Scheme 3.16 for the case of the organozirconium product of the insertion of alkenyl carbenoids 52 or 60 [38],... 

Symmetrical 2,2-disubstituted-l-lithio-l-chloroalkenes mono-insert cleanly into zirconacycles to give trisubstituted olefin products in high yield following protic work-up of the ring-expanded six-membered zirconacycle (Scheme 3.18 R1 = R2 = alkyl) [48]. The insertion of non-symmetrical 2,2-disubstituted alkenyl carbenoids has not yet been reported. A wide range of 2-monosubstituted alkenyl carbenoids (51, 52, 60, 62) arising from in situ deprotonation of alkenyl chlorides can be successfully inserted into zirconacyclopen-tanes and -pentenes with the expected inversion of configuration at the carbenoid carbon [49,50],... 

Scheme 3.18. Insertion of alkenyl carbenoids into zirconacycles.

The stereospecific insertion of 2-monosubstituted alkenyl carbenoids was successfully employed in the preparation of 1-alkyl-1-zircono-dienes. The Z and E carbenoids of 1-chloro-l-lithio-l,3-butadiene (69 and 70, respectively) are generated in situ fromE- andZ-l,4-dichloro-2-butene [53] (Scheme 25). Inversion of configuration at the carbenoid carbon during the 1,2-metalate rearrangement stereospecifically yields terminal dienyl zirconocenes 71 and 72 [54] (Scheme 25). As the carbenoid-derived double bond is formed in 9 1=Z E for 69 and >20 1=E Z isomeric mixtures for 70, the metalated dienes 71 and 72 are expected to be formed with the same isomeric ratio. Carbon-carbon bond formation was achieved by palladium-catalyzed cross-coupling with allyl or vinyl halides to give the functionalized products with >95 5 stereopurity [55-57]. 

Unlike the insertion of 2-monosubstituted alkenyl carbenoids (69, 70, and 73), the reaction of 2,2-disubstituted alkenyl carbenoids with alkenyl zirconocene chlorides afforded the expected products as a mixture of stereoisomers. Thus, when 77, derived from the deprotonation of the stereodefined E-l-chloro-2-methyl-l-octene 76, was reacted with -l-hexenylzirconocene chloride 78 at low temperature, a partial inversion of configuration at the alkenyl carbenoid center occurred before or during the rearrangement to afford the expected metalated diene 79 with an E Z isomeric ratio of 58 42 after hydrolysis (see 80, Scheme 27) [53]. The poor stereocontrol was attributed to the metal-assisted ionization [58-60], which promotes the interconversion of the E- to the Z-alkenyl carbenoids 77. The latter occurs at a rate comparable with that of the insertion into organozirconocene chloride, and hence this is responsible for the loss of stereochemistry. 

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