Homopropargyl-alkenyl coupling

C. SYNTHESIS OF NATURAL PRODUCTS VIA Pd-CATALYZED HOMOALLYL-ALKENYL AND HOMOPROPARGYL-ALKENYL COUPLING REACTIONS... 

Dienes and 1,5-Enynes via Pd-Catalyzed Homoallyl-Alkenyl Coupling and Homopropargyl-Alkenyl Coupling... 

Scheme 3.56 Synthesis of (2E,6Z,10 )-geranylgeraniol and coenzyme Q,o by Pd-catalyzed homoallyl-alkenyl and homopropargyl-alkenyl coupling reactions [158].

There are six discrete combinations for Pd-catalyzed cross-coupling between homoallyl-, homopropargyl-, or homobenzyhnetals and alkenyl or aryl electrophiles (Scheme 4). In addition to these processes, those involving alkyl, allyl, benzyl, propargyl, alkynyl, and acyl electrophiles are conceivable, and some have indeed been observed. However, these reactions are discussed in other pertinent sections. 

The intramolecular hydrosilation of homopropargyl alcohols also proceeds in a 5-exo manner to form five-membered cyclic vinylsilanes exclusively. Subsequent oxidation affords a p hydroxy ketone (eq 6). The vinylsilane also undergoes a Pd-catalyzed cross-coupling reaction with aryl or alkenyl halides stereoselectively (eq 6). The intramolecular hydrosilation thus provides an efficient methodology for the regio- and/or stereoselective functionalization and carbon-carbon bond formation of the alkyne moiety in homopropargyl alcohol. 

Negishi and his co-workers have described a new synthetic route to 1,5-dienes (or 1,5-enynes) which is particularly suitable for the construction of head-to-tail coupled isoprene units. The key step involves a regio- and stereo-selective palladium-catalysed cross-coupling of homoallylic (or homopropargylic) organozinc compounds with alkenyl halides (e.g. Scheme 95) homo-coupling... 

Homoallylic alcohols can also be prepared using a one-pot transformation of homopropargyl alcohols. Intramolecular hy-drosilylation followed by a palladium-catalyzed coupling of the in situ generated alkenoxysilane with an aryl or alkenyl halide, in the presence of fluoride ions, affords the alcohol product. This process has also been applied to the preparation of 1,3-dienes. 

---