Olefin homologation with titanium methylene compounds

Tebbe, F. N.; Parshall, G. W.; Reddy, G. S.

doi:10.1021/ja00479a061

Cited by 1,040 publications

(481 citation statements)

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“…The literature suggests that g,g-difluorination accelerates [3,3]-Claisen rearrangements in some cases; 5 though the idea has not been tested fully and definitive theoretical work does not exist, a general concensus exists that the rehybridisation of an sp 2 CF 2 centre to sp 3 is favoured as geminal CF 2 -substitution destabilises the alkene. Unambiguous accelerative effects exerted in g,g-difluorinated systems have also been reported in other rearrangement systems including [2,3]-Wittigs, 6 heteroatomic [2,3]-rearrangements 7 and oxyCope 8 reactions.…”

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confidence: 72%

An unusually rapid Claisen rearrangement involving ring expansion

Dimartino

Percy

2000

Chem. Commun.

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A Claisen rearrangement of a partially-fluorinated system involving ring expansion occurred at an unusually low temperature, 100 °C lower than a comparable system from the literature.Paquette and co-workers showed how the Claisen rearrangement could be used to transform readily available vinylic lactones into medium-ring carbocycles during a series of landmark natural product syntheses. 1 The Tebbe reaction 2 fulfils a critical role in providing the vinyl ether component of the allyl vinyl rearrangement precursor; Scheme 1 summarises the sequence.The rearrangement step requires the use of high temperatures and some special experimental precautions; 3 after the Tebbe reaction, the allyl vinyl ether is sealed in base washed tubes to minimise decomposition through enol ether protiolysis and Paquette notes that following minimal enol ether purification, traces of the Tebbe reagent by-products may be present in the rearrangement medium. Given that aluminium reagents are indeed known to cause dramatic accelerations of [3,3]-Claisen rearrangements, 4 some assistance to the rearrangement would not be surprising, though the nature of the active Lewis acid is not clear nor is the effect large given the reaction conditions (23 h at 175 °C in Scheme 1). Double bond migration can also compete with rearrangement forming non-productive allyl vinyl ethers.We were interested to see if halogenation of the allylic fragment would accelerate the ring expansion and lower the reaction temperature. The literature suggests that g,g-difluorination accelerates [3,3]-Claisen rearrangements in some cases; 5 though the idea has not been tested fully and definitive theoretical work does not exist, a general concensus exists that the rehybridisation of an sp 2 CF 2 centre to sp 3 is favoured as geminal CF 2 -substitution destabilises the alkene. Unambiguous accelerative effects exerted in g,g-difluorinated systems have also been reported in other rearrangement systems including [2,3]-Wittigs, 6 heteroatomic [2,3]-rearrangements 7 and oxyCope 8 reactions.Stork enamine chemistry allowed the syntheses of ketoesters 2a and 2b and the addition of 1-chloro-2,2-difluoroethenyllithium 9 generated from 1-chloro-2,2,2-trifluoroethane afforded good yields of alcohol products 3a as a mixture of diastereoisomers (Scheme 2); we assume that the major product has a trans-relationship between the sidechains. 3 No lactone products were detected; what we expected was the collapse of the initial alkoxide onto the ester in at least one of the diastereoisomers. After isolating the hydroxyester, we tried to lactonise it under acidic and basic conditions but failed. Instead of hydrolysing the acid, we synthesised tert-butyl ester 3c as a mixture of diastereoisomers; a single lactone, to which we assigned structure 4a, was isolated in moderate yield after treatment of these hydroxyesters with trifluoroacetic acid in dichloromethane. Presumably, alkyl-O cleavage and carboxy protonation allows closure to the lactone; it seems less likely that the acid intercepts an ally...

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confidence: 72%

An unusually rapid Claisen rearrangement involving ring expansion

Dimartino

Percy

2000

Chem. Commun.

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“…The use of Tebbe´s reagent 7 to transform 10→5a turned out to be interesting because this reagent is used in a non-basic medium. Thus, racemization does not take place on substrates with enolizable chiral centers or other base sensitive groups.…”

Section: Resultsmentioning

confidence: 99%

An easy access to two epimeric N-substituted (2S)-2-(2'-hydroxypropyl)pyrrolidines

Silva

Cottier

Srivastava

et al. 2005

J. Braz. Chem. Soc.

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Uma fácil e eficiente síntese para obtenção de β-aminocetonas 5a, 6 e γ-aminoálcoois 1a, 2a, 3, 4 enantiomericamente puros a partir de L-prolinol 7 e L-prolina 15 é descrita. A etapa principal da reação foi o uso do reagente de Tebbe que permitiu a transformação da função éster a um enol éter que após hidrólise formou a cetona sem nenhuma racemização.An easy and efficient route for the synthesis of enantiomerically pure β-aminoketones 5a, 6 and γ-aminoalcohols 1a, 2a, 3, 4 from L-prolinol 7 and L-proline 15 is described. One of the key steps is the use of Tebbe´s reagent allowing the transformation of the ester function to an enol ether without any racemization.

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“…A small amount of a white precipitate (MgBr 2 ) formed, but NMR analysis of an aliquot revealed that no apparent change had taken place. 12a: 1 .5°], 7430 were unique (R int ϭ 0.029); 4477 reflections with I Ͼ 2.5σ(I) were used in the structure solution and refinement. Application of empirical and analytical absorption correction algorithms on the measured data had no significant effect.…”

Section: Methodsmentioning

confidence: 99%

“…A well-known example is Tebbe's reagent (1), which is conveniently obtained from titanocene dichloride and trimethylaluminium [1] (Scheme 1); it has very crowded due to the trimethylsilyl substituents. Partial metalla-alkene character is indicated by the short zirconium−carbon bonds, and agostic zirconium−hydrogen interactions by the short zirconium−hydrogen distances (Zr···H−C) and small 1 J C,H coupling constants.…”

Section: Introductionmentioning

confidence: 99%

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Investigations on the Reactivity of Bis(bromomagnesio)trimethylsilylmethane − Molecular Structure of a Tebbe‐Type Spiro‐Organomagnesium Compound

et al. 2004

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The reactivity of the 1,1‐di‐Grignard reagent Me3SiCH(MgBr)2 (8) towards transition metal dihalides has been investigated. While most transformations were sluggish and gave no well‐defined products, the reaction of 8 with [Cp2ZrCl2] in a 1:1 ratio gave, in low yield, the novel spiro‐organomagnesium compound [Cp2ZrCHSiMe3(μ‐Br)]2Mg (12), which is stable at room temperature and does not disproportionate on addition of dioxane. An X‐ray crystal structure determination has shown that, in the solid state, 12 is present as the C2 symmetric stereoisomer 12a; it is sterically very crowded due to the trimethylsilyl substituents. Partial metalla‐alkene character is indicated by the short zirconium−carbon bonds, and agostic zirconium−hydrogen interactions by the short zirconium−hydrogen distances (Zr···H−C) and small 1JC,H coupling constants. In benzene solution, equilibrium between 12a and its C1 symmetric stereoisomer 12b was detected by 1H and 13C NMR spectroscopy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

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Olefin homologation with titanium methylene compounds

Cited by 1,040 publications

References 2 publications

An unusually rapid Claisen rearrangement involving ring expansion

An unusually rapid Claisen rearrangement involving ring expansion

An easy access to two epimeric N-substituted (2S)-2-(2'-hydroxypropyl)pyrrolidines

Investigations on the Reactivity of Bis(bromomagnesio)trimethylsilylmethane − Molecular Structure of a Tebbe‐Type Spiro‐Organomagnesium Compound

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