Alkenyl, hydrogen abstraction

Sakamoto et al. reported the X-ray crystallographic data and the solid-state photoreaction of eight Ar,AT-disubstituted a,p-unsaturated thioamides 23a-h, which involves hydrogen abstraction by the alkenyl carbon atom conjugated with thiocarbonyls (Fig. 9) [44-46]. 

The proposed mechanism (Scheme 1) involves the mixed-valence compounds [Rh2" " ( Ji-cap)4(OH)] and [Rh2 (p.-cap)4(OOt-Bu)] formed from the homolytic cleavage of t-BuOOH. The t-BuOO radicals in the medium promote a selective hydrogen abstraction from the alkene to give the allylic alkenyl radical. This species traps the peroxide in [Rh2 (p.-cap)4 (OOt-Bu)] to produce the alkenyl hydroperoxide, which rapidly decomposes to the isolated products, thus regenerating the catalyst. 

The UV irradiation of allyloxy-ketones and -aldehydes has led, by 5-hydrogen abstraction, to a useful high-yield synthesis of the 2-alkenyl-3-hydroxytetrahydrofurans (80CC657). This substitution pattern occurs in the natural mycotoxins citreoviridin and asteltoxin (Scheme 56). 

Intramolecular (5-hydrogen abstraction (1,6-hydrogen transfer) is often a competing process and is preferred in the absence of a suitably positioned y-hydrogen atom. 2-Alkenyl-3-hydroxytetrahydrofurans (458) and (459)... 

The formation of rings with more than seven atoms has unfavorable rates because the addition step is often too slow to allow it to compete successfully with other pathways open to the radical intermediate. In stannane based chemistry for example, premature hydrogen abstraction from the organotin hydride is difficult to avoid. However, Baylis-Hillman adducts 111 derived from enantiopure 1-alkenyl (or alkynyl)-4-azetidinone-2-carbaldehydes are used for the stereoselective and divergent preparation of highly functionalized bicycles 112 and 113 fused to medium-sized heterocycles (Scheme 38) [80, 81]. The Baylis-Hillman reaction using nonracemic protected a-amino aldehydes has been attempted with limited success due to partial racemization of the chiral aldehyde by DABCO after... 

Molecules of 43c adopt chiral packing (space group P2 ) and a helical molecular conformation, and crystallize in (E,Z) conformation which is unfavorable for the oxetane formation. The solid-state irradiation of 43c was found to give the oxetane 44c and a 3-lactam derivative 45c. The (3-lactam 45c was revealed to be enantiomerically enriched to 88% ee, whereas the other photoproduct 44c was racemic. The occurrence and the mechanism of transformation of 43c to 45c involve hydrogen abstraction by the alkenyl carbon atom. 

Scheme 14 Asymmetric synthesis involving hydrogen abstraction by alkenyl carbon.

Scheme 19 Asymmetric synthesis via hydrogen abstraction by the alkenyl carbon atom.

Terminal alkyne complexes of zirconium, while challenging to isolate, have been implicated in a number of organometallic transformations. Mixing an alkenyl zirconocene 218 with a transient dialkyl zirconocene 111 furnishes a yu-acetylide complex 219 (Scheme 33).112 This reaction is believed to proceed by initial transmetallation to form a zirconocene alkenyl alkyl 220, which undergoes subsequent /3-hydrogen abstraction to generate the terminal alkyne complex 221. This proposed intermediate can be trapped with PMe3 222 from the alkylation of the zirconocene alkenyl bromide with butyllithium. Comparison of the spectroscopic features of this product to the... 

Irradiation of asymmetrically substituted A -benzyl-A -isopropyl-aP-unsatu-rated thioamides (102) in benzene solution induces hydrogen abstraction by the alkenyl carbon from the benzyl and isopropyl groups to give a P-thiolactam (103) and 1,3,5-dithiazinane (104) as products. In the solid state, however, photolysis causes hydrogen abstraction from only the isopropyl group to give the isomeric P-thiolactam (105). 

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]. 

Generation of non-stabilized alkyl radicals. Malacria has developed several new cascade reactions taking advantage of selective hydrogen atom abstractions by alkenyl radicals [145]. Recently, he achieved the preparation of a linear triquinane from an acyclic precursor. The last steps of the reaction sequence are a hydrogen abstraction by a vinyl radical at the /i-position of the sulfone followed by the elimination of a sulfonyl radical, affording the final product in 50% yield (Scheme 44)... 

Photolysis of crystals of 67 gave the spiro (3-thiolactam 68 with high ce and is the first example of an absolute asymmetric synthesis involving hydrogen abstraction by an alkenyl carbon atom <96JAI0664>. 

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