The Bredt rule, which proved useful in earlier decades mainly as an exclusion criterion in the assignment of structures, has in recent years become the stimulus for efforts to synthesize strained bicyclic systems with a bridgehead double bond. On the basis of the ring-strain forces operating in such systems, an extended Bredt rule is discussed. The "Bredt compounds" to which this rule applies, which include bicyclic systems not previously considered from this point of view (e. g. bridged methylenecyclopropanes), form a clearly circumscribed, interesting field within the chemistry of highly-strained hydrocarbons with ring unsaturation.

Organolithium compounds are thermally quite stable, despite their extremely high reactivity. Thermolability, i. e. the property of undergoing irreversible changes at low temperatures without the participation of molecules of a different kind, is associated with additional functional groups, which may be readily eliminated together with the metal cation, may cause intramolecular rearrangements, or may lead to intermolecular additions and substitutions involving several molecules of the same type. Thermolabile organolithium compounds have frequently been postulated or detected as intermediates of important reactions, and are therefore of theoretical and practical interest. If certain precautions are taken, it is occasionally possible (mainly at low temperatures) to prepare them intact, and hence to release them from the constraint of the fate assigned to them as short-lived intermediates. Investigations with this aim have for some time occupied a place of considerable importance in the chemistry of organometallic compounds.

Some years ago we found that a-halogenoorganolithium compounds (carbenoids) previously postulated as transient intermediates in organolithium-initiated a-eliminations can be obtained in a stable form. The present paper is a review of the methods for their preparation and of their reactivity. They possess both nucleophilic and electrophilic properties, which can be utilized singly or combined for the synthesis of substances of very different types. Their thermolability, which is considerably reduced (with only one known exception) by the solvent tetrahydrofuran, is due to electrophilic secondary reactions, in which carbenes evidently do not occur as intermediates. A mechanism is proposed which fits the experimental data for various carbenoid reactions.

I-Chlor-, trans-I .2-Dichlor-und Trichlorvinyllithium werden aus Halogenkohlenwasserstoffen durch Metallierung oder Halogen-Metall-Austausch bei tiefer Temperatur dargestellt und als Carbonsauren charakterisiert. Die Stabilitat der Titelverbindungen erlaubt Riickschliisse auf den Mechanismus ihres zu Acetylen-Derivaten fuhrenden Zerfalls. Seit grundlegenden Arbeiten von Wittig und Mitarbb.3) iiber das Verhalten von Halogenkohlenwasserstoffen gegeniiber Phenyllithium ist bekannt, dab Vinylhalogenide mit Alkalimetallorganylen in Acetylen-Derivate ubergehen4-7) (GI. 1). R'-CH=CH-X 3 R'-C-C-Li + LiX (1) -2 R--H Mit Ausnahme des auf Umwegen zuganglichen Perfluorvinyllithiurns 8) konnten metallierte Halogenolefine bisher weder als Zwischenstufen nachgewiesen noch dargestellt werden. Die vorliegende Arbeit beschreibt die Ubertragung einer von Kob-A r 2 C = d rich und Trapp angegebenen Methodeg) zur Gewinnung von 1-Chlor-2.2-diaryl-vinyllithiurn-Verbindungen 1 auf Chlorderivate des Vinyllithiums und kommt zu Aussagen iiber die Mechanismen metall-Li h 1 organischer Dehydrohalogenierungen 10). 1) 16. Mitteil. iiber Kohlenstoffe mit Halogen-und Alkalirnetallsubstituenten. -15. Mitteil.2).

Nitrobenzol wird durch Lithiumorganyle nicht metalliert. Dagegen oxydiert es Phenyllithium in Tetrahydrofuran (THF) schon bei tiefer Temperatur quantitativ zu Phenol. Der Brom-Lithium-Austausch an 2-Brom-I-nitro-benzol und seinen Methylhomologen fiihrt in hohen Ausbeuten zu den unterhalb von ~ 100' stabilen, tieffarbigen Titelverbindungen, die man durch Carboxylierung nachweist. Die mit dem Halogen-Metall-Austausch konkurrierende Reduktion der Nitrogruppe tritt bei hiiherer Temperatur und in weniger polaren Losungsmitteln starker hervor; sie uberwiegt bei 3-und 4-Brom-I-nitro-benzol auch unter optimalen Bedingungen.