Alkynyl triflone

Xiang et al. added Bu SnH to an alkynyl triflone [176] in the presence of Pd(PPh3)4. Casson and Kocienski obtained a-alkoxyalkenyltins by addition of Bu3SnH to the corresponding alkynyl ethers [199]. 

Alkynylation. The introduction of an alkynyl group to an unactivated position in a carbon skeleton is a remarkable achievement. With alkynyl triflones in the presence of a radical initiator, good yields of a-alkynyl ethers or 1-alkynyladamantanes are formed from rne corresponding ethers and adamantane, respectively. Alkenes mainly undergo addition give P-trifluoromethylalkyl alkynes. 

Alkenyl triflones The fZ>alkenyl triflones can be made from alkynyl triflones by the addition of HI followed by StUle coupling. Some other alkenyl triflones are available from organocopper reactions. 

Alkenyl triflones have unusual reactivities. A stereoselective synthesis of trisubstituted alkenyl triflones involves the organocoppers and alkynyl triflones. 

Even unactivated hydrocarbons can be alkynylated to furnish disubstituted alkynes. For example, treatment of cyclohexane with the acetylenic triflone (RC=C-S02CF3)... 

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]. 

An example of allylation is depicted in Scheme 25 (Eq. 25.1). In this reaction, dioxane is used as solvent [104]. The alkenylation reactions (Eqs. 25.2 and 25.3) are run under similar conditions [105]. Very interestingly, these reactions are stereospecific and the geometry of the initial triflone is preserved during the process. Finally, the alkynylation of adamantane is performed in acetonitrile (Eq. 25.4) [110]. The reaction occurs exclusively at the bridgehead position. 

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

The TIPS variant in eq 1 is used not only neat on the cyclic C-H substrates THF, tetrahydrothiophene, and cyclohexane, but also in acetonitrile with adamantane, which substituted exclusively at the tertiary C-H (50% yield). Alkynylation was also successful with distal functionality, i.e. R = (CH2)20SiR3 and (CH2)3C1, but the triflone could not be formed with ether functionality closer to the acetylene. 

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