Other Alkenyl Radicals

A conclusion that may be reached from Section V.2.A, is that 4-pentenyl 

Reports on the behavior of stabilized radicals bearing a heteroatom in the unsaturated chain are scarce. With the reservations made in Section VI. 1 concerning the occurrence of competitive processes, the results parallel those obtained with purely aliphatic chains although the yields of cyclized products are generally poorer.  

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The more stable (E) alkenyl radical m which the alkyl groups R and R are trans to each other is formed faster than its Z stereoisomer Steps 3 and 4 which follow are fast and the product distribution is determined by the E-Z ratio of radicals produced m step 2... 

The fact that the cyclization is directed to an acetylenic group and leads to formation of an alkenyl radical is significant. Formation of a saturated iodide would be expected to lead to a more complex product mixture because the cyclized product could undergo iodine abstraction and proceed to add to a second unsaturated center. Vinyl iodides are much less reactive, and the reaction product is stable to iodine-atom abstraction. Because of the potential for competition from reduction by the stannane, other reaction conditions have been developed to promote cyclization. Hexabutylditin is frequently used.238... 

As mentioned in the preceding section, when vinyl or other alkenyl groups are present as substituents, photolysis leads to cross-linking, which dominates over chain scission. Probably the cross-linking results when silyl radicals formed in the initial photolysis add to vinyl carbon atoms on neighboring chains, but other mechanisms are also possible. 

An interesting observation was made in the case of the 5-exo dig radical cyclization of the glucosyl pyridyl sulfone derivative 6 (Fig. 9b). An ( ) (Z)-stereoselectivity of greater than 50 1 was observed in the product, which was a fourfold increase in selectivity, compared with the previously reported tin hydride method. Closer examination of this reaction revealed that the stabilized vinylic radical was reduced to an alkenyl organosamarium species, as shown by a remarkably high incorporation of deuterium at this position (91%) when the reaction was performed in the presence of MeOD (Fig. 12). Compared with other studies on similar reductions of stabilized alkenyl radicals, this... 

Other fragmentation approaches have involved the intramolecular addition of alkenyl radicals to furans. The resulting spiro radical intermediate then opens to a new o, 5-unsaturated carbonyl radical intermediate [102]. An interesting cascade process based on 2-cyanophenyl isothiocyanate, promoted by Mn(OAc)3 and featuring the previously unknown addition of an iminyl radical to a thioureido group has been disclosed by Calestani etal. [103]. 

Radical carbonylations of 4-alkenyl halides are distinguished from those of other alkenyl halides, since an acyl radical resulting from a 4-alkenyl radical and CO is ready to undergo rapid 5-exo cyclization to form 3-oxocyclopentylcarbinyl radical. 

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... 

Alkynyl substituents stabilize a radical center by the same 12 kcal/mol that on average is achieved by alkenyl and aryl substituents. From the point of view of the VB model this is due to the fact that propargyl radicals exhibit the same type of resonance stabilization as formulated for allyl and benzyl radicals in the right column of Table 1.1. In the MO model, the stability of propargyl radicals rests on the overlap between the one correctly oriented n system of the C=C triple bond and the 2 pz AO of the radical center, just as outlined for allyl and benzyl radicals in Figure 1.5 (the other 7t system of the C=C triple bond is orthogonal to the 2pz AO of the radical center, thus excluding an overlap that is associated with stabilization). 

Visible light irradiation of l,2-dihydro-2-thioxo-l-pyridinyl jV-(4-alkenyl)-A-alkylcarbamates (PTOC carbamates) produces substituted pyrrolidines by a radical chain reaction (see Section 7.2.5.1). In the absence of hydrogen donors, the intermediate pyrrolidinylmethyl radical reacts with the PTOC carbamate itself to afford 2-[(2-pyridinylthio)methyl]pyrrolidines, e.g., 1, 2, 4 and 521,22. On the other hand, in the presence of a good transfer reagent, another functionality can be introduced. In the presence of diphenyl diselenide, the phenylseleno-sub-stituted products were obtained in good yield, e.g., 3 and 622. In every case, however, a low degree of diastereoselectivity of the cyclization products is observed. 

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