Iodide precursors

J(P1)427>. The regioselectivity of the second radical cyclization depends on the electronic nature of the homoallylic double bond pyrrolizinones 240 which result from a final 5-o -cyclization mode are preferred in the case of electron-poor carbon-carbon double bonds, such as enones or enoates electron-rich double bonds lead to indolizinones via a final 6-f db-cyclization. The best yields of pyrrolizinones were observed with iodide precursors. The cir-isomers of 240 predominate in this 5-f rf6i-5-f3co-cyclization. 

Typically, these are bromide or iodide precursors such as bromoacetylene (CaHBr), vinyl bromide (C2H3Br), ° and propargyl bromide (C3H3Br) ° to yield supersonic beams of helium-seeded ethynyl, vinyl, and propargyl radicals, respectively. Briefly, the radical precursor is entrained in helium at a ratio of typically 1%. The gas mixture is released by a pulsed valve at 700 Torr backing pressure. A teflon extension with a slit located parallel to the expansion direction of the pulsed beam was... 

A frequently used indirect method involves cyclizable (cf. (7)) or other mechanistic probes which should provide evidence for free radical intermediates and thus for SET [19,37,59]. However, Newcomb and Curran have pointed out the pitfalls of such an approach especially if iodide precursors are used [17]. The supposedly radical-indicative reaction may come about albeit slower by a different, nonradical mechanism or the radical formation may occur via a secondary process which is not directly related to the first reaction step. A similar side-route can be made responsible for the appearance of stable radical compounds which may arise via a comproportionation reaction between non-reduced starting material and the doubly reduced species which can be formed from a hydro form (the normal product, Eq. (5)) and the usually strongly basic organometallic or hydridic reagents (Eq. (9)) [58]. The ability of strong bases to produce reduced radical species via complicated electron/proton transfer processes has been known for some time in the chemistry of quinones and quaternary salts [60,61]. 

The first enantioselective total synthesis of (+)-macbecin I was accomplished by R. Baker and co-workers. A key vinyl iodide precursor was prepared stereoselectively using the malonic ester synthesis. Diethyl methylmalonate was treated with in situ generated diiodocarbene in ether at reflux to afford diiodomethylmethylmalonate in good yield. This dialkylated malonic ester then was converted to ( )-3-iodo-2-methyl-2-propenoic acid by reacting it with aqueous KOH. The saponification was accompanied by a concomitant decarboxylation. 

Arylation of butyl vinyl ether with an arylpalladium chloride complex predominantly delivers the terminally arylated product (as expected if a neutral oxidative addition complex is involved). Internal arylation of the highly polarized vinyl ether dominates with the corresponding iodide precursor, suggesting that in this case the cationic intermediate is more plausible as the key intermediate. ... 

The generation of the reduced species 29-32 requires very strong reducing conditions therefore it is not obvious that such type of compounds can arise as intermediates in a catalytic process. However, Cade [86] has recently shown that a pyrrole-based PNP hgand reacts smoothly with Ni(cod)2 (cod, cyclooctadiene) to afford a stable Ni(I) complex 34 (Scheme 2.10). This reaction presumably involves the intermediacy of an unstable hydride [(PNP)Ni-Hj. When this hydride is generated by treating a Ni(II) iodide precursor with superhydride at low temperature, it spontaneously loses H2 to afford 34. Conversely, 34 is cleanly oxidized to the iodide precursor, demonstrating the chemical reversibility of this redox system. 

Arynes are also highly suitable dienophiles for the synthesis of PAHs via Diels-Alder reactions [27, 28]. For example, Kitamura et al. reported a new hypervalent iodide precursor 27 for the in situ generation of naphthyne, which can be trapped in a Diels-Alder reaction to form PAHs, such as 28 and 29 (Scheme 8) [29]. 

PI3 is a useful Hydrogen Iodide precursor in the silica- and alumina-mediated additions of HI to alkenes. PI3 has also found use as a source of electrophilic phosphorus in the synthesis of novel organophosphorus compounds (e.g. (CF3)3P ) and novel phosphorus-containing organometallic compounds. ... 

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