Ketene cycloadditions

Holder, Richard W.

doi:10.1021/ed053p81

Cited by 30 publications

(5 citation statements)

References 58 publications

(78 reference statements)

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“…While thermal [2+2] cycloadditions are normally forbidden based on the Woodward−Hoffmann rules, alkyne 13 does possess sets of orthogonally disposed p orbitals, which could participate in a concerted [2+2] pathway. The thermal [2+2] cycloadditions of ketenes are thought to proceed through a similar pathway . The fact that variation in exposure to light on the initial reaction between 11 and 13 has no impact on the product distribution does indirectly support the concerted [2+2] pathway.…”

Section: Resultsmentioning

confidence: 99%

Diels−Alder Approach to Biaryls: Elucidation of Competing Tandem [2+2] Cycloaddition/[1,3] Sigmatropic Shift Pathway

2008

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Reaction of 2-halo-6-nitrophenylacetylene with an electron deficient diene gives rise to a [4+2] cycloaddition/cycloreversion biaryl product and a bicyclo[4.2.0]octadiene resulting from a competing [2+2] cycloaddition pathway. The cyclobutene can be opened to give a mixture of cyclooctatriene and biaryl in varying amounts depending on heat and light exposure. The conversion of the cyclobutene into biaryl occurs through a [1,3] sigmatropic carbon shift followed by [4+2] cycloextrusion of ethylene gas.

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Section: Resultsmentioning

confidence: 99%

Diels−Alder Approach to Biaryls: Elucidation of Competing Tandem [2+2] Cycloaddition/[1,3] Sigmatropic Shift Pathway

2008

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“…Addition of 1 equiv of butyllithium to a solution of these o ethers transformed them into lithium enolates. 4 Further metalation of these enolates was performed by an excess of butyllithium in the presence of TMEDA.…”

Section: Resultsmentioning

confidence: 99%

“…When irons-2-butenoyl chloride was treated with 0.95 molar equiv of dry triethylamine in the presence of 6.0 molar equiv of 1,3-cyclopentadiene and worked up after 3 h a product mixture of 60% 7-vinylbicyclo[3.2.0]hept-2-en-6-one (2),18 1% CE)-7-ethylidenebicyclo[3.2.0]hept-2-en-6-one (3), and 39% (Z)-7-ethylidenebicyclo[3.2.0]hept-2-en-6-one (4) resulted (Scheme I). The adduct isomer 2 could be detected by NMR or rapid analytical vapor phase chromatography (VPC) at temperatures below 100 °C, but could not be isolated by preparative VPC since at temperatures above 100 °C or with long retention times it suffered apparent cycloreversion.…”

Section: Methodsmentioning

confidence: 99%

“…(Z)-7-Ethylidenebicyclo[3.2.0]hept-2-en-6-one (4). The third-eluting component, which constituted 23.7% of the mixture, was assigned structure 4: mass spectrum m/e (rel intensity) 134 (32), 106 (13), 105 (15), 91 (41), 69 (16), 68 (11), 66 (100), 65 (14), 51 (13), 41 (17), 40 (17), 39 (28); ir 3060,2940,2860,1741,1661,1605,1440,1170,1047, 896,776,741, 696 cm-1; NMR (0.0392 g in 350 µ\ of CDC13) 5.8, m, 2 H (H2, H3); 5.60, q (J = 7 Hz) further split into ad (J = 1.5 Hz), 1 H (Hs); 3.8, m, 2 H; 2.6, m, 2 ; 2.05, d of d (J = 7.0, 1.5 Hz), 3 H (-CH3).…”

Section: Methodsmentioning

confidence: 99%

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Preparation of vinylketene by 1,4-elimination. Cycloaddition and isomerization to form .alpha.-ethylidenecyclobutanones

Holder¹,

Freiman²,

STEFANCHIK³

1976

J. Org. Chem.

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Vinylketene (1) was shown to result from triethylamine initiated 1,4-dehydrochlorination of trans-2-butenoyl chloride. In the presence of 1,3-cyclopentadiene a ,2 + T2 cycloaddition occurred to form adduct 2. With a trace of excess triethylamine 2 isomerized chiefly to a 73:27 mixture of the E and Z isomers 3 and 4, whose structures were securely assigned using lanthanide induced shift nuclear magnetic resonance techniques. The possible participation of ethylideneketene (CHaCH=C=C=0) was judged remote since triethylamine, 3-butenoyl chloride, and 1,3-cyclopentadiene gave an identical reaction mixture. With either isomeric acid chloride if less than 1.0 molar equiv of triethylamine was used a 60:01:39 mixture of 2:3:4 was formed, which upon addition of triethylamine equilibrated to the 73:27 mixture of 3 and 4. Ketenes (RR'C=C=0) react with conjugated dienes in a highly perispecific and regiospecific manner to form 3-• vinylcyclobutanones.4 Since the olefinic component reacts suprafacially, the fusion geometry is consistently cis. When the two ketene substituents differ in steric bulk the larger one tends to assume the more hindered position in the product.5,6 Ketene cycloadditions represent one of the few synthetic approaches to four-membered rings,7 and also serve as sensitive mechanistic probes of the rare and difficult T2S + T2a allowed concerted8 pathway. A review of the available evi-dence4 and recent theoretical analyses9,10 indicate that both concerted and nonconcerted routes may be traversed, depending on the steric and electronic characteristics of ketene and ketenophile.

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“…Laboratory experiments based on the Diels-Alder reactions and other cycloadditions have been extensively discussed (1,(3)(4)(5)(6)(7)(8)(9)(10), as have the Woodward-Hoffman rules (11)(12)(13)(14). It is not our goal to simply add to this literature, but rather to use this bit of chemistry as part of a larger project (15), in which students are introduced to more advanced concepts.…”

Section: In the Laboratorymentioning

confidence: 99%