Allylic triflones

The ability to conduct radical reactions without the use of tin reagents is important. Allylic triflones have been used to conduct allylation reactions on a range of substrates (39) as a replacement for allyltributylstannane (Scheme 28). The main limitation was that unactivated or trisubstituted triflones failed to undergo reactions. In other nontin radical methods, arenesulfonyl halides have been used as functional initiators in the CuCl/4,4 -dinonyl-2, 2 -bipyridine-catalysed living atom-transfer polymerization of styrenes, methacrylates, and acrylates.The kinetics of initiation and propagation were examined with a range of substituted arylsulfonyl halides with initiator efficiency measured at 100%. 

The ability to conduct radical reactions without the use of tin reagents is important. Allylic triflones have been used to conduct allylation reactions on a range of substrates... 

Triflylmethylalkenes. The radical addition of triflylmethyl iodide to 1-alkenes gives y-iodo triflones. On oxidation with DMD, the allylic triflones are formed (3 examples, 58-93%). 

Synthesis of Allyl Triflone. Alkyl triflones are formed in a clean, but slow, displacement reaction by nucleophUic substitution of primary halides by potassium triflinate with Iodide catalysis in boiling acetonitrile (eq 1). ... 

An alternative synthesis of allyl triflones is the triflination of allyl alcohols, which affords trifllnates such as 4, followed by thermal rearrangement in acetonitrile to give aUyl triflone (5) (eq 7) ... 

Hendrickson synthesized allyl triflones using tetrabutylam-monium triflinate. The quaternary ammonium system is more soluble and 20-40 times more reactive than the conventional potassium triflinate. Tetra-n-butylammonlum azide (6) prepared from tetra-/3-butylammonium hydroxide and sodium azide reacts with triflic anhydride in chloroform at —78°C to give a 1 1 mixture of tetrabutylammonium triflinate (7) and tetrabutyl-ammonium triflate (8). Treatment of this mixture with allyl bromide gives the corresponding allyl triflone (5) in almost quantitative yield. The water-soluble triflate coproduct (8) in the reaction mixture does not interfere with the formation of (5), which is readily Isolated (eq 4). 

Synthesis of Functionalized Allyl Triflones. The Hendrick son tetrabutylammonium triflinate reagent (7/8) reported was used by Fuchs and Curran to prepare functionalized allyl triflones (9-13). ... 

Fuchs and co-workers used radical-mediated atom-transfer addition of iodomethyl trlflone (14) [158530-86 0] to substituted alkynes to afford functionalized allyl triflones. The reaction was complete within 5 10 h In most cases. For example, heating a... 

Allyl Triflone as an Allylating Agent. Frejd reported a low yield method using allyl triflone for aromatic allylation through a dlazotlzatlon/allylatlon process (eq 7). ... 

Curran and Fuchs successfully reacted allyl triflones with THF and cyclohexane to give good to excellent yields of various aUyl products through radical-mediated C-H bond functionalization (eq8).5... 

It has also been reported" " that rearrangements of both a- and y-propylallyl triflinates on heating in acetonitrile yield the same sulfone y-propylallyl triflone. Although the possibility of an ion-pair mechanism may be responsible for the lack of allylic rearrangement in one case as a result of the better leaving group ability of the triflinate anion as compared with the arenesulfmate anion, it is just as likely a consequence of the unbuffered conditions in which these reactions were performed. In this respect this is reminiscent of the results observed by Cope and coworkers" mentioned above which were also performed under nonbuffered conditions, and which could be simply corrected by the addition of 2,6-lutidine . 

Allyl halides also react well with sulfinates and give good yields in this reaction under comparable conditions to those employed for the triflones." "... 

Simple sulfonyl carbanions which do not contain additional carbanion-stabilising groups, e.g. carbonyl groups or heteroatoms, can be readily alkylated in high yield by modern techniques with the use of alkyllithiums and lithium amide bases. A number of allylic halides have been successfully used. In allylic halides, the halogen directly attached to the double-bonded carbon is relatively inert towards nucleophilic attack (Scheme 41). In this way, sulfones (96) can be transformed via desulfonation into vinyl halides (97) or into ketones (98) by hydrolysis (Scheme 41). In contrast to ordinary alkyl sulfones, triflones (99) can be alkylated under mildly basic conditions (potassium carbonate in boiling acetonitrile) (Scheme 42). The ease of carbanion formation from triflones (99) arises from the additional electron-withdrawing (-1) effect of the trifluoromethyl moiety. 

Fuchs has examined a number of additional alkynes. One in particular, silyl-substituted triflone 104, may prove most useful, as it provides silylated alkyne products upon reaction with suitable substrates [62c]. In general, attempts to functionalize triflones with other groups at the alkyne carbon or at propargylic positions were unsuccessful. Triflones with more remote functionality, including bis-acetylenes, gave useful reagents. Fuchs triflone methodology can also be extended to vinylation and allylation reactions (Scheme 23) [63, 64). 

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...

Hartgraves GA, Burton DJ. Regiochemistry, mechanism and synthesis of allylic trifluoromethylsulfones (triflones). Chi-mica Oggi 2009 27(3 Suppl) 27-30. 

---