Phosphinoyl-elimination

Another new example using titanocene as catalyst has been revealed by Malacria and coworkers. Here, a previously unknown combination of radical cyclizahon involving an epoxide-opening of 3-154 and a 3-phosphinoyl-elimination takes place to furnish various pyrrolidines 3-155, bearing a tetrasubstituted exo-double bond, in good yields (Scheme 3.41) [66]. 

Palacios et al. utilized the modified Neber reaction for the preparation of 2ff-azirine-2-phosphonates 33 as shown in Scheme 18 [28 a]. The use of quini-dine and dihydroquinidine as the chiral base resulted in moderate chirality transfer (20-52% ee). Similarly, 2-phosphinoyl-2H-azirines could be obtained by the Neber 1,3-elimination reaction [28bj. 

Malacria and coworkers reported a vinylation sequence of epoxides by employing vinyl phosphine oxides as a radical trap. The overall sequence relies on the facile elimination of phosphinoyl radicals. With vinyl phosphonates the THF derivatives were obtained (Scheme 22). The reaction works equally well under stoichiometric or catalytic conditions [102,103],... 

An NMR kinetic study of a phosphine-catalysed aza-Baylis-Hillman reaction of but-3-enone with arylidene-tosylamides showed rate-limiting proton transfer in the absence of added protic species, but no autocatalysis.175 Brpnsted acids accelerate the elimination step. Study of the effects of BINOL-phosphinoyl catalysts sheds light not only on the potential for enantioselection with such bifunctional catalysis, but also on their scope for catalysing racemization. 

Scheme 31 Intramolecular radical vinylation based on the /3-phosphinoyl radical elimination...

En route to the synthesis of modified ohgonucleotides through radical addition of hypophosphorous substrate 177 to sugar 178, Hubert et al. reported the unforeseen /1-ehmination of a phosphinoyl radical from a phosphinate (Scheme 58) [167]. This example came after a 1,5-H translocation giving a more stabilized oj-oxy radical (from 179), and it demonstrates further that j6-phosphinoyl radicals are prone to undergo elimination (see Scheme 31), even when the substituents are all alkyl groups. 

Cp2TiCl-mediated intramolecular radical vinylations based on the elimination of P-phosphinoyl radicals have recently been reported by Leca... 

Diesters of phosphinic acids have also received attention. When both phosphonic and phosphinic ester moieties are present in the same substrate molecule, the question is naturally raised as to which of the two phosphinoyl moieties will be expelled and which will remain attached to carbon. Perhaps the simplest system to be studied is also the most recently reported. Thus, in reactions between 171 and aliphatic aldehydes or benzaldehyde in diethyl ether with MgBr2 and Et3N, it is the phosphonate moiety which is preferentially eliminated to leave the methylphosphinic esters 1V although with other bases such as NaH, KOBu or BuLi, all in thf, the balance of this reaction and that giving the alternative 173 is more equitable. For the slightly more complex case of 174 with RCHO (R = Ph or Pr ), the preferential fission occurs to liberate the phosphinate moiety... 

The titanium-mediated intramolecular radical vinylations of oxirane was first reported to yield alkylidene pyrrolidines 488 via the radical (3-elimination of phosphinoyl radical. The reactions of phosphine oxide precursors 487 with a stoichiometric amount of Cp2TiQ2 were carried out at room temperature using powdered manganese as reductant, from which pyrrolidines 488 were obtained in 57-82% yields (Scheme 4.146). The procedure for the synthesis of phosphine oxide precursors 487 is also shown in Scheme 4.146. 

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