Carbanion rearrangements

A recent paper [44] shows that the treatment of silyl thioketones 68 with lithium diethylphosphite proceeds via a thiophiUc attack followed by a thio-phosphate mercaptophosphonate (69 70) carbanionic rearrangement and the migration of the silyl group from the carbon to the sulfur atom leading to the S-silylated sulfanylphosphonate carbanion 71. The last step represents the first example of the thia-Brook rearrangement (Scheme 18). 

Just as in its carbonium ion twin, the carbanion rearrangement takes place with retention of configuration of the migrating group. This proves at once that the reaction is intramolecular and that the displacement takes place at the front side.428... 

A reaction related to the pure carbanion rearrangement, but one that eventually involves an external displacing agent as well, is the Favorskii reaction. It has been shown with the aid of isotopic tracers that the mechanism is 429... 

The influence of the halogen on the mechanistic course of carbanionic rearrangement of 3-hexylhalomethylenecyclobutanes (Scheme 11, X = F, Cl, Br, I) to l-halo-4-hexylcyclopentenes has been explored by studying the fate of C-labelled methylene... 

The [1,2]-Wittig Rearrangement is a carbanion rearrangement that proceeds via a radical dissociation-recombination mechanism. The lithiated intermediate forms a ketyl radical and a carbon radical, which give an alcoholate after fast recombination within the solvent cage ... 

These rearrangements involve one pair of electrons therefore, they are thermally allowed. We have already seen that they are very common. In contrast, the corresponding rearrangement of carbanions involves two electron pairs and is thermally forbidden. This is why carbanion rearrangements are rare... 

The n-BuLi cleavage of the axial C-Se bond of bis(phenylseleno)acetals, derived from 4-ferf-butylcyclohexanone, always occurs under kinetic control at -78°C [14]. In the case of a mixed acetal, having respectively the phenylseleno and methylseleno groups in axial and equatorial positions, the cleavage of the equatorial C-SeMe bond was predominantly observed. The resulting stabilized carbanion rearranges almost instantaneously leading to the a-phenylselanyl-cyclohexyllithium with the C-Li bond in the axial position [14] (Scheme 14). 

Ph), or phosphonic amides (97 R = NMe2 or MC HgO) with sodium in dioxane, or with magnesium activated by anthracene in THF, affords the bis(2-hydroxyaryl)phosphinic acid derivative (or phosphine oxide) (98) following a carbanionic rearrangement. 

Drozd, V. N. Carbanion rearrangement of o-methyidiaryl sulfones (the Truce rearrangement). International Journal of Sulfur Chemistry 1973, 8, 443-467. 

Eichinger, P. C. H., Bowie, J. H. Gas-phase carbanion rearrangements. Does the Wittig rearrangement occur for deprotonated vinyl ethers J. Chem. Soc., Perkin Trans. 2 1990, 1763-1768. 

The intra- and intermolecular rearrangements of C2h- and D2-74 clearly indicate the importance of rearrangements hitherto not normally covered in a chapter on carbanion rearrangements. Organometallic chemists, however, are learning more and more about the complex structure(s) of such compounds in solution and in the solid state, as well as their rearrangements, e.g. within aggregates. It is thus predictable that the near future will provide us with more examples of this sort. 

This result was confirmed when semibullvalene 71 was reacted with potassium or Na/K alloy. Even at —78 °C this reaction resulted only in the rearranged 73 56 c>. A common intermediate should be the dipotassium compound 72 (and/or its dimers 74). The different behavior of the lithium and potassium species 72 (and/or 74) illustrates again the problem of carbanion rearrangements changing the gegenion from lithium to potassium leads to species with rather different properties. 

The reader will recognize the heterogeneity of the subject — carbanion rearrangements are not limited by rules especially if one includes carbanion radicals. This, in turn, is responsible for the richness of the chemistry associated with the topic. Because of the intensive world-wide use and investigation of carbanions and their rearrangements, it is foreseeable that this field will remain an active one in the future. 

Due to our engagement in carbanion rearrangements we were especially interested in the homoallylic system 4,4-dilithio-l-butene 33. In the case of homoallyllithium 29 ring-chain equilibrium strongly favors the open-chain lithiumorganic compound and the introduction of four methyl groups into the a- and -positions is necessary... 

Bromoaryloxy(chloromethyl)diorganosilanes (150) react with sodium or n-butyllithium via a metalation, [l,3]-carbanionic rearrangement, cyclization sequence to afford 1,3-benzoxasilolenes (151) in a one-pot-reaction (Equation (28)) <83JOM(243)l,92PS25,9lEGP296280>. 

When a benzyloxysilane is metalated at the alpha position, the resulting carbanion rearranges immediately to give mainly the a-silylcar-binol anion (34) ... 

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