Intramolecular cyclobutadiene

Scheme 79 Snapper s total synthesis of asteriscanolide (116) by sequential intramolecular cyclobutadiene cycloaddition, ring-opening CM (ROCM), and Cope rearrangement [167]...

Limanto, J., Snapper, M. L. Sequential Intramolecular Cyclobutadiene Cycloaddition, Ring-Opening Metathesis, and Cope Rearrangement Total Syntheses of (+)- and (-)-Asteriscanolide. J. Am. Chem. Soc. 2000, 122, 8071-8072. 

Limanto. J. Snapper. M.L. Sequential intramolecular cyclobutadiene cycloaddition, ring-opening metathesis, and cope rearrangement Total syntheses of (-i-)- and... 

Intramolecular cycloadditions between cyclobutadiene and an oxygen-tethered unactivated alkene (alkyne) offers an attractive route to benzo[c]furans (Scheme 26, <96JA9196>). 

Co-free PAE). In PAE-CoCpl, the fluorescence quantum yield is only 18% of that observed for Co-free PAE, even though the quencher substitutes less than 0.1% of the aryleneethynylene units. The fluorescence in solution disappeared in PAE-CoCp4, where every fifth unit is a cyclobutadiene complex. The mechanism by which this quenching occurs is via the cobalt-centered MLCT states [82,83], conferred onto the polymer by the presence of cyclobutadiene complexes. Even in the solid state the polymers PAE-CoCpl-2 are nonemissive. It was therefore shown that incorporation of CpCo-stabilized cyclobutadiene complexes into PPEs even in small amounts leads to an efficient quenching of fluorescence in solution and in the solid state. Quenching occurs by inter- and intramolecular energy transfer [84]. 

Alkvnes do rot dimerize photochemically to give cyclobutadienes, but dimers are formed from arylalkynes under conditions of electron-transfer sensitization (2.105). These dimers arise from a reaction of the alkyne radical cation with ground-slate alkyne, followed by intramolecular electrophilic attack on the benzene ring. 

When the cation radical of this alkyne is generated by y radiolysis in a solid matrix at 77 K and then warmed to 150 K, the ESR spectrum of the 1,2,3,4-tetramethyl-1,3-butadiene cation radical is observed. An analogous intramolecular reaction was also observed even in a rigid matrix at 77 K. The feasibility of the cycloaddition step itself is therefore indicated, but little work has yet been done in respect of the aminium salt or PET induced cycloadditions of alkynes in solution at ambient or near-ambient temperatures. Whether a chain or catalytic alkyne cyclodimerization can be effected is yet unclear, as is the potential fate of the cyclobutadiene products. 

The adduct generated from the benzvalene and butadiene was brominated and dehydrobrominated to a cyclohexadiene which seems to be a suitable precursor of tetrahedrane. However, its photolysis yields (2+2)intramolecular cycloadducts and its thermolysis gives only a cyclobutadiene dimer (99)99). 

The impact of (2 + 2)-cycloaddition and (2 + 2)-cycloreversion reactions of heterocyclic compounds on organic chemistry over the last 10 years is clearly illustrated by several examples. Various members of the important /Hactam antibiotics, penicillin and cephalosporin C, as well as structurally related heterobicyclic compounds have been obtained by (2+ 2)-cycloaddition of heterocycles with ketenes (Section II,D,l).n Intramolecular photochemical (2 + 2)-cycloadditions of 2-pyrones yield 2-oxabicyclo 2.2.01hex-5-en-3-ones, which upon further irradiation afford cyclobutadienes (Section III,D,2).12 Intermolecular (2 + 2)-cyclo-additions of vinylene carbonates with olefins and with acetylenes offer a simple route to cyclobutanes and cyclobutenes, respectively (Sections III,B,3 and 5).13 (2 + 2)-Cycloaddition and (2 + 2)-cycloreversion reactions have contributed substantially to the development of the chemistry... 

Of special interest are the photochemical intramolecular (2 + 2)-cycloadditions of 2-pyrones and 2-pyridones because the resulting 2-oxa- or 2-azabicyclo[2.2.0]hex-5-en-3-ones are precursors of cyclobutadiene and stable complexes of cyclobutadiene.12,238-261 In 1964 Corey and Streith238 reported the photochemical conversion of 2-pyrone (222) and N-methyl-2-pyridone (223) into the corresponding bicyclic isomers (224 and 225) at —20°. More recent studies performed at a temperature as low as 8°K with 2-pyrones in a matrix of argon or nitrogen showed that the intramolecular (2 + 2)-cycloaddition is only a minor photochemical pathway.12,260-262 The major reaction proved to be... 

The reaction of cyclic diynes with CpCo(CO>2 or CpRh(CO)2, or in some cases with Fe(CO)5, leads to the tricyclic cyclobutadiene complexes, as demonstrated for the reaction of 1,7-cyclododecadiyne (3) with CpM(CO)2 in Eq. (24). This reaction was discovered and explored 20 years ago [77, 79]. The availability of cyclic diynes with uneven chains such as 1,6-cyclodecadiyne (100), 1,8-cyclotetradecadiyne (120) and 1,10-cyclooctadecadiyne (168) led to the superphanes 169-171 and to the intramolecular complexes 172-174 [Eq. (25)] [1 c, 80]. 

A related intramolecular cyclization occurs to give the cyclobutadiene complex 41, which on treatment with lithium gave the dilithium species 42 (Scheme 16). The latter could be isolated and characterized by crystallography <2000CL896>. 

In a metallation reminiscent of those described in Section 9.18.4.2.3 (Equation 8), treatment of the biscyclopenta-dienyl compound 124 with (ClMe2Si)2 gives the trisilaheterocycle 125 in 69% yield <1997JOM(541)9>. The cobalt cyclobutadiene complexes 126 are formed via intramolecular cyclizations in 56-70% yield. This demonstrates that silaheterocycles can be formed at a metal center to form complexes as well as being preformed and then added to a suitable metal center to give metal derivatives (Scheme 35) <19970M646>. 

Cyclooctatetraene (COT), a An non-aromatic hydrocarbon, is only one of the many (CH)s compounds, but it is a central character. The other isomers, many of which interconvert with COT by thermal and photochemical pathways, include bicyclo[4.2.0]octa-2,3,7-triene (BOT), semibullvalene (SB), barrelene (B), tricyclo[3.3.0.0 ]octa-3,7-diene (TOD), the cyclobutadiene dimers (CBD), tetracyclo[4.2.0.0. " 0 ]oct-7-ene (TOE), cubane (C), tetracyclo[4.2.0.0. 0 ] octene - the intramolecular Diels-Alder isomer of BOT (IDA), tetracy-... 

Intramolecular trapping of a cyclobutadiene has been accomplished by Ce oxidation of the complex (826), which affords (827) in high yield. ... 

In cases where the diene is unstable, a dihalo starting material may be used with the iron carbonyl acting as a reducing agent. Both the trimethylene methane complex 10.5 (Scheme 10.3) and the cyclobutadiene complex 10.7 (Scheme 10.4) have been made in this way. The cyclobutadiene-iron complex 10.7 is a convenient storable form of this highly unstable diene. It can be liberated by oxidation and, if this is done in the presence of a dienophile, the Diels-Alder product is obtained. The Diels-Alder reaction with 2,5-dibromobenzoquinone gave the expected e do-product 10.8. An intramolecular photochemical [2-1-2] cycloaddition, followed by a Favorskii reaction, gave a cubane dicarboxylic acid 10.10. ... 

Endocyclic dienes ranging in ring size from four to seven have also been used in the intramolecular Diels-Alder reaction. Cyclobutadienes are prepared from the corresponding metal complexes [37]. Cyclopentadienes are most readily prepared by alkylation of cyclopentadienyl anion [38, 39]. They are also available by Michael addition to fulvene derivatives and by aminal exchange [40, 41]. 

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