Abstract:Transition metal complex catalyzed cocyclotrimerization of 6-alkynylpurines 1 with various diynes enables the preparation of a plethora of substituted 6-arylpurines 3 in good yields. The most general catalyst for the reaction is a user-friendly system based on a nickel-phosphine complex and reductant (NiBr(2)(dppe)/Zn) in MeCN. The reaction conditions are compatible with various protective groups on the purine moiety (Bn, THP). As far as other potential catalysts were concerned, only CoBr(PPh(3))(3) showed rea… Show more
“…The complementary reactivity of dipimp and dppe ligands in the reaction of 5 and 2c might be synthetically useful. The known catalysts such as ClRhA C H T U N G T R E N N U N G (PPh 3 ) 3 [9] and X 2 NiA C H T U N G T R E N N U N G (dppe)/Zn [9] were utilized for the reaction under the appropriate reaction conditions but product 7 instead of 6 was obtained or the reaction gave a complex mixture. These results demonstrate the high reactivity of the dipimp/ CoCl 2 ·6 H 2 O/AgOTf/Zn catalyst.…”
The addition of silver triflate (AgOTf) or silver hexafluoroantimonate (AgSbF 6 ) significantly increased the activity of the 2-(arylimino)methylpyridine/cobalt(II) chloride hexahydrate (CoCl 2 ·6 H 2 O)/zinc catalyst in alkyne cyclotrimerizations thereby accelerating the reaction and enabling the use of unactivated, simple internal alkynes as the monoyne substrate: The rate of reaction was found to be highly dependent on the nature of the counter anion (X À ) and the ligand (L) in the postulated cationic cobalt(I) complex [LCo(I)] + X À .
“…The complementary reactivity of dipimp and dppe ligands in the reaction of 5 and 2c might be synthetically useful. The known catalysts such as ClRhA C H T U N G T R E N N U N G (PPh 3 ) 3 [9] and X 2 NiA C H T U N G T R E N N U N G (dppe)/Zn [9] were utilized for the reaction under the appropriate reaction conditions but product 7 instead of 6 was obtained or the reaction gave a complex mixture. These results demonstrate the high reactivity of the dipimp/ CoCl 2 ·6 H 2 O/AgOTf/Zn catalyst.…”
The addition of silver triflate (AgOTf) or silver hexafluoroantimonate (AgSbF 6 ) significantly increased the activity of the 2-(arylimino)methylpyridine/cobalt(II) chloride hexahydrate (CoCl 2 ·6 H 2 O)/zinc catalyst in alkyne cyclotrimerizations thereby accelerating the reaction and enabling the use of unactivated, simple internal alkynes as the monoyne substrate: The rate of reaction was found to be highly dependent on the nature of the counter anion (X À ) and the ligand (L) in the postulated cationic cobalt(I) complex [LCo(I)] + X À .
“…[45] Although Co is also an effective catalyst in this reaction, optimal yields were obtained in the presence of the a,w-diynes were effective in this reaction. The synthesis of axially chiral arylpurines should be possible utilizing the same protocol and a chiral ligand-Ni complex.…”
Section: Synthesis Of Substituted Benzene Derivativesmentioning
Recent progress in the synthesis of benzene and 1,3-cyclohexadiene derivatives, and heterocyclic compounds such as pyridines, pyridones, pyrans, pyrimidine diones, etc, has been reviewed. The general mechanistic aspects of the [2 + 2 + 2] cycloaddition reaction are discussed. The asymmetric variants of these reactions are also discussed along with the proposed models of asymmetric induction. Keywords: arenes; catalysis; cycloaddition; heterocycles; metallacycles; transition metals
IntroductionThe significance and increasing popularity of [2 + 2 + 2] cycloaddition reactions is evident from the number of reviews that have recently appeared in the literature. [1][2][3] The [2 + 2 + 2] cycloaddition reaction is remarkable in terms of its ability to utilize various unsaturated substrates such as alkynes, diynes, alkenes, imines, isocyanates, isothiocyanates, and CO 2 in the synthesis of a broad variety of highly substituted cyclic molecules such as benzenes, pyridines, pyridones, 1,3-cyclohexadienes, pyrones, thiopyridones and cyclohexanes. Multisubstituted benzenes and pyridines have traditionally been synthesized by aromatic electrophilic substitution (AES) reactions and a variety of metal-mediated coupling reactions. Although, these reactions are extremely efficient, they generally involve multistep syntheses. The application of AES reactions in the synthesis of polysubstituted aromatic rings is limited by the effect of the substituent groups and, hence, it may be very difficult or even impossible to add various functionalities at a specific position on the aromatic ring. On the other hand, [2 + 2 + 2] cycloaddition reactions are extremely atom-efficient and involve the formation of several C À C bonds in a single step. Another important feature of the [2 + 2 + 2] cycloaddition reaction is tolerance of a myriad of functional groups such as alcohols, amines, alkenes, ethers, esters, halogens, and nitriles. Moreover, the availability of numerous catalytic systems that have Adv. Synth. Catal. 2006, 348, 2307 -2327 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2307 REVIEWS been efficient in tedious syntheses highlights the applicability of the [2 + 2 + 2] cycloaddition reaction. These are important requirements for the [2 + 2 + 2] cycloaddition reaction to become a universal synthetic tool for the synthesis of benzene, pyridine, and other cyclic derivatives. An important problem with the [2 + 2 + 2] cycloaddition reaction is the lack of chemoand regioselectivity observed in earlier reported reactions. However, significant effort has been focused on attaining a high degree of chemo-, regio-and even enantioselectivity with considerable success as evident from recent reports. Excellent reviews on [2 + 2 + 2] cycloaddition reactions are available. Kotha and co-workers have reviewed the synthesis of benzene derivatives by the cyclotrimerization of three alkyne functionalities utilizing transition metal systems as catalysts.[1] Their review is divided into: (a) intermolecular reactionsin which the three pi (alkyn...
“…Most recently, Hilt et al have described that a CoBr 2 /2PR 3 , disulfide, or diimine/Zn/ZnI 2 reagent can effectively catalyze intermolecular cyclotrimerization of alkynes . However, it is somewhat surprising that no reaction has been investigated for partially intramolecular reactions (type II), , which are synthetically useful as can be seen in applications to synthesis of complex molecules 2,4 and have often been used as a benchmark reaction to evaluate the synthetic potential of the method. 1 …”
[reaction: see text] A 2-(2,6-diisopropylphenyl)iminomethylpyridine (1a)/CoCl(2).6H(2)O/Zn reagent has been developed as an effective instant catalyst for the intra- and intermolecular cyclotrimerization of alkynes to substituted benzenes, making the method extremely practical since the reagent, 1a/CoCl(2).6H(2)O/Zn, is inexpensive and easy to handle and the reaction is less sensitive to moisture and is reasonably general.
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