Cobalt-Mediated [2+2+2] Cycloadditions of Pyrimidine Derivatives to Alkynes

Abstract

The scope and limitations of the cobalt-mediated [222] cycloaddition of pyrimidine derivatives to alkynes has been investigated. The 5,6-double bond of these heterocyclic nuclei has been found to participate in an entirely intermolecular fashion to generate chemo-and stereoselectively novel, fused and substituted 5,6-dihydropyrimidine cobalt complexes, which upon oxidative demetallation liberate the correspond-ing new heterocyclic ligand. On the other hand, 1-alkynyl pyrimidines have been found to be suitable partners in the cocyclization with disubstituted alkynes, such as bis(trimethylsilyl)acetylene (BTMSA) or dimethyl 2-butyn-1,4-dioate (DMAD), to allow the direct prep-aration of hitherto unknown dihydropyrido[3,2-ij]quinazoline cobalt complexes. Effects of the substitution on the pyrimidine nucleus, the cocyclization partner, the complex auxiliary, and the reaction conditions were examined, and in some cases competing pathways that lead to [CpCo(cyclobutadienes)], cyclopentadienone complexes, and compounds that arise from a C À H activation-type reaction were observed.

Chemo‐, Regio‐, and Stereoselective Cobalt‐Mediated [2+2+2] Cycloaddition of Alkynyl Boronates to Alkenes: 1,3‐ and 1,4‐Diboryl‐1,3‐cyclohexadienes

“…Stereochemical assignments were based on the characteristic chemical shifts of the tertiary cyclohexadiene hydrogen atoms, which are shielded when located anti to cobalt and deshielded when located syn 11c,e. 18a, 22 The endo diastereomer was favored in all cases, in agreement with the stereochemical preference of cycloadditions of some α,ω‐diynes to pyrimidines23 and indoles 24…”

Cobalt‐Mediated [2+2+2] Cycloaddition versus CH and NH Activation of 2‐Pyridones and Pyrazinones with Alkynes: A Theoretical Study

“…As a result, the isolation of only one product in each case (see, for example, Scheme ) is deemed fortuitous 8. Further support for this notion is found in the occurrence of both types of activation of the same substrate in the indole 4a and pyrimidinone series 7a…”

“…While we had never encountered previously NH activations of the type leading to 1 in this chemistry, C‐dienylation was noted occasionally with other heterocycles, in the case of some furans and thiophenes even catalytically 2. 4a, 7a Moreover, recent work in our laboratories has revealed that even simple alkenes, including ethene, undergo this process 9. A clue with respect to the mechanism of these insertions was provided in early investigations by Yamazaki and Wakatsuki, in which both types were observed during the thermolysis of Cpcobaltacyclopentadienes,10 implicating the latter as intermediates.…”

Hyperbranched Poly(aryleneethynylene)s: Synthesis, Thermal Stability and Optical Properties

“…Polymerizations in toluene at room temperature, however, yielded either low molar mass or insoluble polymers. Disappointed by the results using TaCl 5 Ph 4 Sn, attention was shifted to CpCo(CO) 2 , a well known catalyst for alkyne cyclotrimerization 9–19. Homo‐ and copolymerizations were carried out in THF under UV irradiation and the results obtained are listed in Table 1.…”