Cobalt catalyzed tandem

Fig. 56 Cobalt-catalyzed tandem radical addition/cross-coupling reactions...

Oshima provided a number of examples of cobalt-catalyzed tandem radical 5-exo cyclization/cross-coupling reactions (Fig. 59, Table 4). Allyl a-haloaldehyde... 

Fig. 60 Cobalt-catalyzed tandem reactions involving radical cyclizations...

However, in another contribution, Chung and coworkers realized the one-pot synthesis of [ S.5.5.6] fenestranes 42 via intramolecular cobalt-catalyzed tandem Pauson-Khand/Diels-Alder reactions of dienediynes 41 [44] (Scheme 21). 

It is also worthy of note that the construction of tetracyclic compounds 44 by cobalt-catalyzed tandem Pauson-Khand/[2 + 2 + 2] cycloaddition reactions of 1,6-diynes 43 was also reported by Chung et al. [45,46]. It was experimentally shown that the [2 + 2 + 2] cycloaddition reaction occurs after the [2 + 2 + 1] cycloaddition between CO and the substrate (Scheme 22). 

Wakabayashi et al. introduced a very powerful mixed cobalt-catalyzed tandem reaction based on a radical cyclization followed by cross-couplings (Scheme 69) [187]. The reactive species is believed to be a 17-electron cobalt ate-complex, which would imdergo SET to yield an anion radical derived from 222, and thus a radical upon loss of bromide. The cobalt(l) complex gen-... 

Although cobalt catalysis is somewhat less developed compared to other transition metal catalysis for cross-coupUngs, but has shown rather unconventional, novel reactivity profiles [50]. For example, Oshima reported a cobalt-catalyzed tandem radical cyclization/cross-coupling reaction between an aryl Grignard reagent and an alkyl halide bearing an (o-alkenyl group (Equation 5.44) [51]. 

Scheme 6.125 Cobalt-catalyzed tandem hydroformylation-hydrogenation reaction of paraformaldehyde.

A tandem radical 5-exo cyclization/radical addition/allylic substitution reaction was subsequently described [292]. Allylic ot-bromo acetal 242b cyclized cobalt-catalyzed. Addition to diene 245 and subsequent coupling with coformed organocobalt(I) species generates an allylcobalt complex, which undergoes reductive elimination to cyclic product 246 in 93% yield (cf. Fig. 56). 

Table 5.5 Cobalt-catalyzed cross-coupling (and tandem cyclization/arylation) reactions between unactIvated alkyl halides and aryl metal reagents.

Cobalt-Catalyzed Hydroformylatlon-Hydrogenation Tandem Reaction... 

A breakthrough was achieved by Slaugh and MuUineaux at Shell, who discovered the beneficial effect of trialkylphosphines such as PEtj, P( Bu)3, or P(Cy)3 on the cobalt-catalyzed hydroformylation-hydrogenation tandem reaction with several olefins as substrates (1- and 2-pentene, 1-butene, propylene, methyl-pentenes, cyclohexenes, dimethyl-butenes, and higher olefins) [31, 36]. Large substituents at the phosphorus, as present in tris(2-ethylhexyl)phosphine, reduce... 

Waser J, Nambu H, Carreira EM (2005) Cobalt-catalyzed hydroazidation of olefins convenient access to alkyl azides. J Am Chem Soc 127(23) 8294-8295 Shyam PK, Jang HY (2014) Metal-organocatalytic tandem azide addition/oxyamination of aldehydes for the enantioselective synthesis of P-amino a-hydtoxy esters. Eur J Org Chem... 

More recently, van der Donk and coworkers reported radical cyclizations catalyzed by vitamin B12 using titanium(III) citrate as a stoichiometric reducing agent (Fig. 69, entry 17) [330]. Here /V-allylic 2-(isopropenyl) pyrroles 290 or allyl 2-phenylallyl ethers serve as the starting material in tandem hydrocobaltation/ radical 5-exo cyclization sequences giving dihydropyrrolizine derivatives 291 or 292. The mechanistic course is not completely clear. However, it is assumed that the reactions start with an initial hydrocobaltation of the isopropenyl unit in the presence of the allylic alkene (see Sect. 5.7). The benzylic cobalt intermediate is subject to homolysis of the very weak cobalt-carbon bond and initiates the radical 5-exo cyclization. Interestingly, the fate of the cyclized radical is dependent on the... 

Synthesis of substituted stilbenes has been reported through a tandem process involving Diels-Alder reaction/Wittig reaction/DDQ oxidation sequence (Scheme 16.21) [22]. The Diels-Alder step is catalyzed by cobalt(II) and Zn(II) salts. 

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