Reductive cyclization with activated alkenes

The most important feature of organocobalt cyclizations is that a variety of functionalized products can be obtained, depending on the nature of the substrate and the reaction conditions. The most common transformation has been formation of an alkene by cobalt hydride elimination. Alkenes are often formed in situ during the photolysis, and with activated alkene acceptors the formation of these products by cobalt hydride elimination is very facile. Scheme 31 provides a representative example from the work of Baldwin and Li.143 The alkene that is formed by cobalt hydride elimination maintains the correct oxidation state in the product (54) for formation of the pyrimidone ring of acromelic acid. Under acidic conditions, protonation of the cyclic organocobalt compound may compete 144 however, if protonated products are desired, the cyclization can probably be conducted by the reductive method with only catalytic quantities of cobalt (see Section 4.2.2.2.2). 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

Phenyl o-radicals generated by reduction of aryl halides can also interact with an intramolecular alkene bond. Ihe method has been developed for the formation of dihydroindoles by reductive cyclization of N-allyl-2-chloroacetanilides. The results indicate the importance of a time interval between electron addition to give a radical-anion and the fragmentation of this species to give the active a-radical, The time interval allows the radical-anion to diffuse away from the electrode surface so that when the a-radical is foimed, it has time to cyclize before it can be reduced at the surface. 

The substrate 90 has been cyclized using reduced vitamin Bi2a as a catalyst (0.01 eq.). The radical formed by cyclization can be trapped with an activated alkene (added in excess) to form a new radical, which probably undergoes reduction and protonation to give a mixture of diastereomeric products [240] ... 

Reductive dimerization. Alkenes, which are geminally substituted with two activators, undergo dimerization at the (3-position. Cyclization follows. 

Unfortunately, ( PDI)Mn(THF)2 exhibited little activity for the hydrogenation of alkenes or [2 + 2] cyclization of dienes. In an attempt to synthesize an active bis(imino)pyridine manganese precatalyst, alternative reduction conditions were explored. Sodium amalgam reduction of ( PDI)MnCl2 in pentane yielded the red bis(chelate) complex ( PDI)2Mn. X-ray diffraction established a cis-divacant octahedral compound where one imine arm on each of the chelates is dissociated from the metal center. The metrical parameters from X-ray diffraction in conjunction with SQUID magnetic and EPR spectroscopic data established that the overall S = 3/2 compound is best described as a high-spin Mn(II) species (Sy = 5/2) with two bis(imino)pyridine radical anions. 

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