A simple cobalt catalyst system for the efficient and regioselective cyclotrimerisation of alkynes

Hilt, Gerhard; Vogler, Thomas; Hess, Wilfried; Galbiati, Fabrizio

doi:10.1039/b417832g

Cited by 124 publications

(66 citation statements)

References 23 publications

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“…In this context it should be noted that hexaphenyl benzene displays only a single multiplet in its 1 H NMR spectrum at d = 6.83 ppm. [21] The assignments made here for the new derivatives 8-15 were aided by heteronuclear CH COSY (HMQC) spectra and the clear observation of vicinal coupling in homonuclear HH COSY (TOCSY) experiments between H7 and H10 and between H8 and H11. HMBC NMR experiments were used to determine the 13 C NMR signals of the quaternary carbons C13, C14, and C15 and the aromatic protons H8 and H11 by virtue of their coupling (see Table 1).…”

Section: Resultsmentioning

confidence: 99%

Structurally Characterized Hetero‐Oligopolyphenylenes: Synthetic Advances Toward Next‐Generation Heterosuperbenzenes

Gregg

Ollagnier

Fitchett

et al. 2006

Chemistry A European J

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The successful Diels-Alder [2+4] cycloaddition of dipyrimidyl acetylene and suitably substituted 2,3,4,5-tetraarylcyclopenta-2,4-dien-1-ones (3-7) generates a series of selectively functionalized hexaarylbenzenes. Each has two pairs of peripheral functional groups (R' and R=tert-butyl 8 and R=methyl 9, methoxy 10, bromo 11, triisopropylsilylethynyl 12) and four ortho-imine nitrogen atoms. The dibromo derivative 11 is a useful precursor for the formation of a mono ethynyl 13 and diethynyl 14 substituted polyphenylene. Changing the dienophile to di(2-thienyl)acetylene gives an S-heteroatom polyphenylene 15. The compounds were fully characterized by using (1)H, (13)C and a range of 2 D NMR spectroscopic techniques, elemental analysis, and mass spectrometry. Oxidative cyclodehydrogenation of dimethoxy hexaphenylbenzene 10 by using iron(III) chloride results in the formation of a spirocyclic dienone 16, which in a separate reaction undergoes dienone/phenol rearrangement to give the first 4-fused-ring, N-heterosuperbenzene (HSB) 17. Six single crystal molecular structures reveal the commonality of unidirectional twisting of the external aromatic rings in these heteroatom polyphenylenes. The twist angles and any H-bonding or interdigitation in these structures are discussed.

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Section: Resultsmentioning

confidence: 99%

Structurally Characterized Hetero‐Oligopolyphenylenes: Synthetic Advances Toward Next‐Generation Heterosuperbenzenes

Gregg

Ollagnier

Fitchett

et al. 2006

Chemistry A European J

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“…We can not rationalize the effect of the iron powder in preventing the formation of the [2+2+2]-cyclotrimerization side product. [8] When one component of the catalyst system is omitted, reduced chemoselectivities or reduced conversions are encountered. In some cases the formation of the 1,3,6-cyclooctatriene side product is observed.…”

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confidence: 99%

meta‐Directing Cobalt‐Catalyzed Diels–Alder Reactions

2006

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The Diels-Alder reaction has belonged to the arsenal of synthetic organic chemists for decades as can be seen in the large number of applications in natural-product synthesis and of biologically active compounds.[1] The usefulness of the Diels-Alder reaction is based on the cyclic transition state which allows the transfer of the stereochemical information in the starting material E-or Z-selectively into cis-or transconfigured products. Furthermore, the regiochemistry of the intermolecular Diels-Alder reaction can be predicted by the Woodward-Hoffmann rules and the therein implied ortho/ para rules.[2] While the ortho/para-selectivity of the thermal Diels-Alder reaction allows access to ortho-or para-substituted products (such as 1, Scheme 1) the intermolecular reaction is limited to the generation of para-substituted products when starting from isoprene, the meta-substituted products of type 2 (Scheme 1) can not be accessed. Only the thermal Diels-Alder reaction of alkenyl boron compounds, such as the vinyl-9-borabicyclo[3.3.1]nonane, with unsymmetrical 1,3-dienes, such as isoprene, poses an exception. In this case the regioselective meta-directing Diels-Alder reaction generates the product of type 2 in a ratio of meta:para = 90:10.[3]To our knowledge an analogous meta-directing DielsAlder reaction to produce 4 starting from a 1,3-diene and an alkyne has not been described. Nevertheless, the metasubstituted dihydroaromatic systems 4 have been detected and isolated as side products in transition-metal-catalyzed cycloaddition reactions.[4] In these transition-metal-catalyzed reactions the para-substituted product 3 is formed as the major product.In our investigations of cobalt-catalyzed cyclizations we reported the use of a simple cobalt catalyst system consisting of [CoBr 2 dppe] (dppe = 1,2-bis(diphenylphosphino)ethane), ZnI 2 , Zn, or Bu 4 NBH 4 as reducing agent for the Diels-Alder reaction of unactivated starting materials under mild conditions for the production of the para-substituted Diels-Alder adducts 3.[5] Recently, we also identified a simple cobalt catalyst system consisting of [CoBr 2 (diimine)] (5 Mol %), ZnI 2 , Zn, and Fe powder (each 10 Mol %) which performs the cobalt catalyzed Diels-Alder reaction between phenyl acetylene and isoprene, gives very good yields, and forms the meta-substituted isomer 5 in excellent regioselectivity. The corresponding aromatic product 6 was then generated by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and its identity determined by NMR spectroscopy. Among several ligands tested the best chemo-and regioselectivity was obtained for cobalt complexes with pyridine donor ligands, such as 7 (Scheme 2) or 2,2'-bipyridine (8, Scheme 3) or 1,10-phenanthroline.[6] Also symmetrical aliphatic diimine ligands (9) as well as mixed pyridine imine ligands (10 and 11) gave good results. In addition, by using a combination of zinc-and iron powder as the reducing agent, the formation of the [2+2+2]-cyclotrimerization product from the alkyne was completely suppressed...

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“…A recent theoretical study [12] showed that the flyover (C 6 R 6 ) Co 2 (CO) 4 compounds are the lowest energy structures only for systems with large R groups such as those with multiple CF 3 and/or tert-butyl substituents. For systems with smaller R groups, including (C 6 H 6 )Co 2 (CO) 4 and (C 6 Me 6 )Co 2 (CO) 4 , isomeric structures with a bent benzene ring bridging the two cobalt atoms were found to be lower energy structures than the corresponding ''flyover'' compounds ( Fig.…”

Section: Introductionmentioning

confidence: 99%

“…Such complexes have been isolated and characterized structurally in systems with large R substituents, e.g. [1,3,6-tBu 3 C 6 H 3 ]Co 2 (CO) 4 .…”

Section: Introductionmentioning

confidence: 99%

“…The [2+2+2] Reppe cyclotrimerization of alkynes leading to benzene derivatives using a variety of transition metal catalysts [1][2][3][4] has been an effective method for the synthesis of substituted arenes [5][6][7][8]. Cobalt carbonyls such as Co 2 (CO) 8 have been among the most frequently used catalyst precursors for such reactions [9][10][11].…”

Section: Introductionmentioning

confidence: 99%

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Cyclic versus acyclic structures of six-carbon ligands in binuclear cobalt carbonyl derivatives: Some thermochemical observations

Zeng

Feng

et al. 2015

Inorganica Chimica Acta

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A simple cobalt catalyst system for the efficient and regioselective cyclotrimerisation of alkynes

Abstract: The intermolecular cyclotrimerisation of terminal and internal alkynes can be catalysed by simple cobalt complexes such as a CoBr2(diimine) under mild reaction conditions when treated with zinc and zinc iodide with high regioselectivity in excellent yields.

Cited by 124 publications

References 23 publications

Structurally Characterized Hetero‐Oligopolyphenylenes: Synthetic Advances Toward Next‐Generation Heterosuperbenzenes

Structurally Characterized Hetero‐Oligopolyphenylenes: Synthetic Advances Toward Next‐Generation Heterosuperbenzenes

meta‐Directing Cobalt‐Catalyzed Diels–Alder Reactions

Cyclic versus acyclic structures of six-carbon ligands in binuclear cobalt carbonyl derivatives: Some thermochemical observations

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