The dicobalt hexacarbonyl(alkyne) moiety as a stereocontrol element in intramolecular Friedel–Crafts alkylations<sup>1</sup>

Grove, D. D.; Corte, James R.; Spencer, Roxanne P.; Pauly, Malinda E.; Rath, Nigam P.

doi:10.1039/c39940000049

Cited by 13 publications

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“…We note that Melikyan and Nicholas have rationalized the Mn(OAc) 3 -mediated oxidative cycloaddition of β-dicarbonyl compounds with Co 2 (CO) 6 -complexed 1-alken-3-ynes in terms of a transition state whereby the bulky cluster unit is disposed pseudoequatorially; this favors a pseudoaxial approach by the incoming nucleophile . Similarly, the stereospecificity of the Friedel−Crafts cyclization of cobalt-stabilized propargyl cations onto suitably activated arenes has been explained by invoking a transition state in which the (−C≡CH)Co 2 (CO) 6 fragment adopts the less hindered pseudoequatorial site …”

Section: Introductionmentioning

confidence: 94%

Alkynylcyclohexanol Chairs and Twist-Boats: Co₂(CO)₆as a Conformational Switch

et al. 2001

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Treatment of 1-[axial]-(trimethylsilylethynyl)cyclohexan-1-ol with dicobalt octacarbonyl results in a conformational ring flip such that the bulky dicobalt-alkyne cluster moiety now occupies the favored equatorial site. However, when a 4-tert-butyl substituent is present, the coordinated alkynyl group retains its original axial or equatorial position. Complexation of trans-[diaxial]-1,4-bis(triphenylsilylethynyl)cyclohexane-1,4-diol brings about a chair-to-chair conformational inversion such that both cluster fragments now occupy equatorial sites. In contrast, cis-1,4-bis(triphenylsilylethynyl)cyclohexane-1,4-diol reacts with Co(2)(CO)(8) to yield the twist-boat conformer in which the two axial hydroxy substituents exhibit intra-molecular hydrogen bonding. Likewise, the corresponding reaction of cis-1,4-bis(trimethylsilylethynyl)cyclohexane-1,4-diol with Co(2)(CO)(8) leads to a twist-boat, but in this case, the molecules are linked through inter-molecular hydrogen bonds. Eight of these cobalt clusters have been characterized by X-ray crystallography, and the potential use of twist-boats in synthesis is discussed.

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Section: Introductionmentioning

confidence: 94%

Alkynylcyclohexanol Chairs and Twist-Boats: Co₂(CO)₆as a Conformational Switch

et al. 2001

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“…6 In the same year the rapid racemisation of a cobalt-stabilised cation was exploited to afford a stereoselective synthesis of fused-ring systems. 7 Chiral propargyl alcohols may be prepared via the asymmetric reduction of ynones 8 or from the asymmetric addition of alkynes to aromatic aldehydes. 9 Although the catalytic enantioselective addition reaction of dialkyl and alkenyl zinc reagents to aldehydes may be carried out efficiently, using a wide range of catalysts, advancements with the corresponding asymmetric alkynylation reaction appears to be far less developed.…”

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confidence: 99%

Enantioselective Alkynylation Reactions to Aldehydes: The Effects of Aromatic Substituents upon the Enantioselectivity

Tyrrell¹,

Tesfa²,

Millet³

et al. 2006

Synthesis

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Asymmetric alkynylation reactions to linear alkyl and substituted aromatic aldehydes have been accomplished in good yields and with a range of selectivities. For aromatic aldehydes we observed that the selectivity of the alkynylation reaction appears to depend upon the substituents on the aromatic ring. Thus with electron-withdrawing substituents both the yield and enantioselectivities were good to excellent. In contrast to this, the presence of electron-donating groups provided excellent conversions; however, these were coupled with poor enantioselectivities.In a recent paper 1 we revealed a diastereoselective cobaltmediated synthesis of benzopyrans using a novel variation of an intramolecular Nicholas 2 reaction in the key step. Using this chemistry we were able to access a number of functionalised benzopyran derivatives 2, which were then subsequently screened against cromakalim 3 3, a known modulator of potassium channels (Scheme 1).In our efforts to achieve an enantioselective Nicholas reaction 4 we have focussed upon the synthesis of optically active propargyl alcohols, based upon 1, for use in benzopyran synthesis. The ability to transfer the stereochemical information, contained within a dicobalt hexacarbonyl complexed chiral propargyl alcohol, into the resulting product still remains a challenge to the synthetic chemist. This limitation has been explained by the fluxional nature of a cobalt-stabilised carbocation that exposes both faces to the incoming nucleophile 5 thus leading to racemisation. One solution to this racemisation process lies in the ability to rapidly quench the cation before delocalisation effects take place. The first example of an enantiospecific Nicholas reaction, a process which provided chiral products from chiral substrates, was disseminated in 1994. 6 In the same year the rapid racemisation of a cobalt-stabilised cation was exploited to afford a stereoselective synthesis of fused-ring systems. 7 Chiral propargyl alcohols may be prepared via the asymmetric reduction of ynones 8 or from the asymmetric addition of alkynes to aromatic aldehydes. 9 Although the catalytic enantioselective addition reaction of dialkyl and alkenyl zinc reagents to aldehydes may be carried out efficiently, using a wide range of catalysts, advancements with the corresponding asymmetric alkynylation reaction appears to be far less developed. 10 The major limitations seem to be based upon a combination of factors such as the need to employ stoichiometric amounts of catalyst/ ligand, limitations in the availability of appropriate reagents such as chiral ligands and, in some examples, the production of significant quantities of by-products. 11From our inspection of the existing literature, in this interesting area of asymmetric synthesis, we were able to corroborate the inconsistencies in the percentage yields and/ or enantioselectivities for asymmetric alkynylation reactions. For instance with aldehydes such as 4 (Figure 1), with R = alkyl, good to excellent yields for the alkynylation reaction have been repor...

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Propargylation of Aromatic Compounds with Propargylic Alcohols Catalyzed by a Cationic Diruthenium Complex

et al. 2003

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The dicobalt hexacarbonyl(alkyne) moiety as a stereocontrol element in intramolecular Friedel–Crafts alkylations¹

Abstract: A dicobalt hexacarbonyl(a1kyne) moiety is used as a stereocontrol element in Friedel-Crafts cycloalkylation reactions that deliver cis-4a-ethynyl-1,2,3,4,4a,9,10,1 Oa-octa hydrop henanth renes exclusively.

Cited by 13 publications

References 8 publications

Alkynylcyclohexanol Chairs and Twist-Boats: Co₂(CO)₆as a Conformational Switch

Alkynylcyclohexanol Chairs and Twist-Boats: Co₂(CO)₆as a Conformational Switch

Enantioselective Alkynylation Reactions to Aldehydes: The Effects of Aromatic Substituents upon the Enantioselectivity

Propargylation of Aromatic Compounds with Propargylic Alcohols Catalyzed by a Cationic Diruthenium Complex

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The dicobalt hexacarbonyl(alkyne) moiety as a stereocontrol element in intramolecular Friedel–Crafts alkylations1

Abstract: A dicobalt hexacarbonyl(a1kyne) moiety is used as a stereocontrol element in Friedel-Crafts cycloalkylation reactions that deliver cis-4a-ethynyl-1,2,3,4,4a,9,10,1 Oa-octa hydrop henanth renes exclusively.

Cited by 13 publications

References 8 publications

Alkynylcyclohexanol Chairs and Twist-Boats: Co2(CO)6as a Conformational Switch

Alkynylcyclohexanol Chairs and Twist-Boats: Co2(CO)6as a Conformational Switch

Enantioselective Alkynylation Reactions to Aldehydes: The Effects of Aromatic Substituents upon the Enantioselectivity

Propargylation of Aromatic Compounds with Propargylic Alcohols Catalyzed by a Cationic Diruthenium Complex

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The dicobalt hexacarbonyl(alkyne) moiety as a stereocontrol element in intramolecular Friedel–Crafts alkylations¹

Alkynylcyclohexanol Chairs and Twist-Boats: Co₂(CO)₆as a Conformational Switch

Alkynylcyclohexanol Chairs and Twist-Boats: Co₂(CO)₆as a Conformational Switch