Reactions with organo-alkali-metal compounds are mostly carried out in ether-type solvents. It has long been known that ether cleavage takes place at the same time. The mechanism of the cleavage, in particular the fact that dialkyl ethers can be decomposed by a variety of mechanisms acting simultaneously, has only become clear in the most recent investigations. The observation that even with purely aliphatic ethers a considerable amount of Wittig ether rearrangement can occur is remarkable. Unusual secondary reactions can also occur by means of which alkyl-and aryllithium compounds in ether or tetrahydrofuran yield new organolithium compounds-sometimes with rearrangement. The reactions of alkyl aryl ethers with alkali metals are also varied and five different mechanisms for them have been discussed in the literature. It is interesting, for example, that the cleavage of anisole can be directed simply by changing the solvent so that in one case only cleavage of the aryl-oxygen bond occurs and in the other almost 100% cleavage of the alkyl-oxygen bond results. The formation of biphenyl as a by-product upon cleavage of the alkyl-oxygen bond in anisole was also puzzling. This was later shown to occur not through the dimerization of phenyl radicals but via 2-methoxybiphenyl. The number of cleavage mechanisms reduces to two if one assumes two different o*-radical anions as being intermediates in the kinetically controlled reaction. Comparison of the reactivity of thioethers and ethers reveals not only gradual but also fundamental differences-the cleavage of the thioethers is thermodynamically controlled.

In 1953 Wittig and Geissler found that reaction of benzophenone with methylenetriphenylphosphorane gave 1,1‐diphenylethylene and triphenylphosphine oxide in almost quantitative yield; the phosphine had been prepared from triphenylmethylphosphonium bromide and phenyl lithium. The discovery led to the development of a new method for the synthesis of olefins, under the name Wittig reaction. One advantage of this new method is that the carbonyl group is replaced specifically by a carbon‐carbon double bond without the formation of isomeric olefins. In contrast, the older method of converting carbonyl compounds to olefins using the Grignard reaction usually give a mixture of isomeric olefins. Another advantage is the Wittig reaction is carried out under mild conditions. Discussions of modern techniques for the preparation of methylene phosphoranes are included in this review.

No abstract

Sowohl die cyclischen Chlor-und Quecksilberverbindlingen 6 und 9 als auch die offenkettige Chlorverbindung 7 liefern bei der Umsetzung mit Lithiumpulver in Diethylether die reine etherfreie Cyclopropylmethyllithitim-Verbindung 10. Nur bei -60°C lassen sich ausgehend von 7 noch 8% der entsprechenden offenkettigen Lithiurnverbindung 11 neben 10 nachweisen. Die Rontgenstrukturanalyse von 10 ergab hexamere Cluster, in denen die Lithiumatome ein trigonales Antiprisma bilden, dessen obere und untere Dreiecksflache unbesetzt ist, wahrend iiber jcder der sechs seitlichen Dreiecksflachen ein 2,2,3,3-Tetramethylcyclopropylmethyl-Rest koordiniert ist. Synthesis and Crystal Structure of 3-(Lithiomethyl)-l,l,2,2-tetramethyIcyclopropane, a Stable Cyclopropylmethyllithium CompoundThe cyclic chlorine and mercuiy compounds 6 and 9 as well as the open-chain chloro compound 7 react with lithium powder in diethyl ether to afford the pure cyclopropylmethyllithium compound 10 completely free of diethyl ether. Only at -6 0°C starting with 7 besides 10 8% of the corresponding open-chain organolithium compound 11 was detected. X-ray analysis of 10 showed solvent-free hexameric clusters with the lithium atoms forming a trigonal antiprism with unoccupied triangular faces on the top and at the bottom while each of the six laterally triangular faces is coordinated to a 2,2,3,3-tetramethylcyclopropylmethyl ligand. ~~~~~ ~Chem. Ber.