Enynes addition reactions

Hie coppetfl) arenetliiolate complexes 19 [ 30], brsl developed and studied by van Rolen s group, can be used as catalysts for a number of copper-niedialed reactions SLidi as 1,4-addilion reactions lo etiones [31] and 1,6-addition reactions lo enynes [32]. 

Scheme 2.21 Regioselectivity in conjugate addition reactions to acceptor-substituted enynes 58.

Additional routes to a-allenic-a-amino acids were described more recently and utilize radical [136] or transition metal-catalyzed [137] allenylations, in addition to copper-promoted Michael additions [15b]. Thus, sterically demanding amino acid derivatives (e.g. 151) are accessible via a 1,6-addition reaction of lithium di-tert-butyl-cyanocuprate with acceptor-substituted enynes of type 150 (Scheme 18.48). 

In this chapter, nucleophilic l,n-additions (n = 4, 6, 8,. ..) to acceptor-substituted dienes, enynes and polyenes are presented2. Addition reactions which obviously proceed via non-nucleophilic pathways (e.g. catalytic reductions, electrophilic or radical additions3), as well as 1,2-additions to the acceptor group, are not covered. 

As in the case of addition reactions of carbon nucleophiles to activated dienes (Section HA), organocopper compounds are the reagents of choice for regio- and stereoselective Michael additions to acceptor-substituted enynes. Substrates bearing an acceptor-substituted triple bond besides one or more conjugated double bonds react with organocuprates under 1,4-addition exclusively (equation 51)138-140 1,6-addition reactions which would provide allenes after electrophilic capture were not observed (cf. Section IV). 

In contrast to nucleophilic addition reactions to activated dienes (Sect. 4.2.1), the mechanism of 1,6-cuprate additions to acceptor-substituted enynes is quite well understood, largely thanks to kinetic and NMR spectroscopic investigations [3oj. 

Enynes are also excellent substrates for tandem addition reactions. Pandey and co-workers have reported a photoinduced electron transfer (PET) promoted reaction of a selenium radical addition to an enyne [95T1483]. The high stereoselectivity observed in this cyclization is noteworthy. 

---