Fuchs alkynylation

Step 1 Wittig olefmation gives the ( )-alkene as the major product (93 7). Step 5 Corey-Fuchs alkynylation. 

Another approach to carrying out tin-free radical fragmentation processes, developed by Fuchs, utilizes trifluoromethyl sulfone, or triflone, derivatives. Fuchs first reported examples of free radical alkynylation reactions using acetylenic triflone 102 [62]. What is most remarkable about these reactions is that the radicals being alkynylated are formed from the cleavage of C-H bonds standard radical precursors are not required. For example, when tetrahydrofuran is mixed with triflone 102 at room temperature, alkynylation occurs a to the ether oxygen in 92% yield (Scheme 21). In this case, the radical chain process is most likely initiated by traces of peroxides in the THF. Similarly, unactivated alkanes such as cyclohexane will react with triflone 102 in good yield (83% for cyclohexane) when heated with a catalytic amount of AIBN. 

In a series of papers, Fuchs has reported allylation [104], alkenylation [105, 106] and alkynylation [107-110] of C-H bonds. All these processes are based on the radical -fragmentation of triflones liberating a trifluroromethyl radical suitable for hydrogen atom abstraction (Scheme 24). The regioselectivity of the hydrogen abstraction has been examined and is governed by polar and thermodynamic effects as shown in Scheme 24 [107]. 

Scheme 25. Fuchs allylation, alkenylation and alkynylation with allyl and vinyl triflones...

Alkenyl triflones have unusual reactivities. A stereoselective synthesis (Xiang and Fuchs 1996) of trisubstituted alkenyl triflones 132 involves the organocoppers and alkynyl triflones 131 (Scheme 7.50). 

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