Formation of carbenoid intermediates in the reaction of ditertiobutyl ketone with low-valent titanium reagents

Villiers, Claude; Vandais, A.; Ephritikhine, Michel

doi:10.1016/s0022-328x(00)00651-3

Cited by 16 publications

(9 citation statements)

References 16 publications

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“…Not surprisingly, the total yield of 2 and 3 varied with R in the order Me b Et b iPr, reflecting the more difficult coupling of the more sterically hindered ketones. Coupling of tBu 2 CO did not occur, [4] as invariably noted so far with a variety of McMurry reagents. [2,5] For a given ketone, the yield of coupling products was higher with M U; the remarkable efficiency of the uranium system was previously observed in the reductive coupling of aromatic ketones.…”

Section: Resultsmentioning

confidence: 56%

Reactions of Aliphatic Ketones R2CO (R=Me, Et,iPr, andtBu) with the MCl4/Li(Hg) System (M=U or Ti): Mechanistic Analogies between the McMurry, Wittig, and Clemmensen Reactions

Villiers

Ephritikhine

2001

Chem. Eur. J.

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Analysis of the products of the reactions of ketones R2CO (R = Me, Et, iPr, tBu) with the MCl4/Li(Hg) system (M = U, Ti) at 20 degrees C revealed significant differences. For R = Me, the reaction proceeded exclusively (M = U) or preferentially (M = Ti) via a metallopinacol intermediate resulting from dimerization of ketyl radicals. Pinacol was liberated by hydrolysis, and tetramethylethylene was obtained after further reduction at 65 degrees C. For R=iPr, formation of iPr2C=CiPr2 as the only coupling product, the nonproduction of this alkene by reduction of the uranium pinacolate [U]-OCR2CR2O-[U] (R= iPr) at 20 degrees C, and the instability of the corresponding titanium pinacolate towards rupture of the pinacolic C-C bond indicated that reductive coupling of iPr2CO did not proceed by dimerization of ketyl radicals. Formation of 2,4-dimethyl-2-pentene was in favor of a carbenoid intermediate resulting from deoxygenative reduction of the ketyl. These results revealed that for sterically hindered ketones, McMurry reactions can be viewed as Wittig-like olefination reactions. For R=tBu, no coupling product was obtained and the alkane tBu2CH2 was the major product. The involvement of the carbenoid species [M]=CtBu2 was confirmed by its trapping with H2O, leading to tBu2CH2, and with the aldehydes RCHO, giving the cross-coupling products tBu2C=C(R)H (R = Me, tBu). Therefore, in the case of severely congested ketones, McMurry reactions present strong similarities to the Clemmensen reduction of ketones, owing to the involvement in both reactions of carbenoid species which exhibit similar reactivity.

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Section: Resultsmentioning

confidence: 56%

Reactions of Aliphatic Ketones R2CO (R=Me, Et,iPr, andtBu) with the MCl4/Li(Hg) System (M=U or Ti): Mechanistic Analogies between the McMurry, Wittig, and Clemmensen Reactions

Villiers

Ephritikhine

2001

Chem. Eur. J.

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“…36 An alternative mechanism proposed for the McMurry coupling of hindered ketones involves titanium carbene intermediates rather than pinacolates. This hypothesis is supported by the observation that the pinacols are never obtained by the reductive coupling of diisopropyl ketone 37,38 and di-tert-butyl ketone 38,39 with the low-valent titanium species generated from the TiCl 4 -Li/Hg system. Furthermore, it was demonstrated that the titanium pinacolates, Cl 3 TiOC(i-Pr) 2 -(iPr) 2 COTiCl 3 , prepared by transmetalation of the corresponding lithium pinacolate with TiCl 4 , rapidly decomposes into diisopropyl ketone and TiCl 3 without formation of alkene.…”

Section: Methodsmentioning

confidence: 80%

The McMurry Coupling and Related Reactions

Takeda

Tsubouchi

2013

Organic Reactions

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The McMurry coupling reaction has been recognized as one of the most efficient methods for the synthesis of alkenes from carbonyl compounds. This reaction can be applied to the preparation of various alkenes that are otherwise difficult to prepare. For example, sterically congested tetrasubstituted alkenes as well as medium to large membered rings involving natural products may be accessed via this process. This coupling utilizes various low‐valent titanium reagents generated by the reduction of titanium (III or IV) chloride with K, Zn, LiAlH 4 , C 8 K, amongst others. Furthermore, a variety of low‐valent metal species other than titanium, including aluminum, zirconium, niobium, molybdenum, indium, tungsten, and samarium, have also been found to promote the reductive coupling of carbonyl compounds. The scope and limitations of these reagent systems are reviewed in this chapter, together with the stereochemistry and reaction mechanism. This reaction is categorized into four coupling modes: i) homocoupling giving symmetrical alkenes, ii) mixed coupling giving unsymmetrical alkenes, iii) intramolecular coupling giving cycloalkenes, and iv) tandem coupling giving cyclic polyenes. Characteristics of these reaction modes are described briefly. A selection of synthetic applications are reviewed, including examples of the preparation of sterically congested and strained alkenes, medium to large‐ring compounds, biologically active targets, as well as substrates applicable in material science. Experimental conditions used for this versatile process are summarized to assist the choice of suitable conditions.

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“…The involvement of carbenoid species in the McMurry reaction of sterically hindered ketones was further demonstrated by analysis of the products resulting from the reduction of di-tert-butyl ketone with the MCl 4 -Li(Hg) systems (Scheme 6.15) [280,281]. In this case, H migration within the [M]bCtBu 2 species gave the expected cyclopropane compound.…”

Section: Evidence Of Carbenoid Intermediatesmentioning

confidence: 97%