Carbenes and Carbanions

Most known carbenes are very unstable with lifetimes 1 s. A few phospha-carbenes (6.464) have been prepared. 

Phosphonate and phosphinoxy carbanions (6.465) can be formed by the action of strong bases on appropriate phosphonate and phosphine oxides, respectively. For example, acetonyl diethylphospho-nate reacts with sodium in benzene to form a white crystalline material which is very hygroscopic but is also soluble in several organic solvents (6.466). Similarly diphenylmethyl phosphine oxide reacts with sodamide to form a salt (6.467). 

The bis methylene carbanion is formed by the action of strong bases on methylene phosphoranes. 

In the product of (6.466) and similar metal salts which can be made, the metal atoms are beheved to be coordinated by both carbonyl and phosphoryl oxygen (6.469). Analogous salts with only phosphoryl coordination but with similar physical properties can be obtained from bis(diethoxyphosphonyl) methane by reacting it with potassium in THF (6.468). Various canonical forms probably contribute to these bidentate hgands, and both types of salt are analogous to acetylacetonate ions (6.469c). 

At least some of the alkali metal salts of phosphonate and phosphinoxy carbanions can be alkylated, or reacted with ketones, to give alkenes in a manner analogous to the phosphonium ylids. 

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Concurrent cyclopropanation by carbenes and carbanions has been investigated.163 It has been demonstrated that the deliberate addition of halide ions afforded concurrent cyclopropanation of electron-poor alkenes by an equilibrating mixture of phenyl-halocarbenes and phenyldihalomethide carbanions, permitting smooth modulation of selectivity between electron-rich and electron-poor alkenes. 

Carbocations, Carbanions, Free Radicals, Carbenes, and Nitrenes... 

CARBOCATIONS, CARBANIONS, FREE RADICALS, CARBENES, AND NITRENES... 

There have been ab initio studies of the bonding for a series of imino and amino phosphorus molecules,15 and for the phosphonium ylides (8).14 The anion character of the ylidic carbon is intermediate between that of olefinic and substituted carbanions. Polarisation of the H3P group has a stabilising effect. The influence of the substituent X on the inversion of the carbanion and also the tendency of the molecule to dissociate to carbene and phosphine is also discussed.14 CNDO/S MO calculations on the... 

Organic halides play a fundamental role in organic chemistry. These compounds are important precursors for carbocations, carbanions, radicals, and carbenes and thus serve as an important platform for organic functional group transformations. Many classical reactions involve the reactions of organic halides. Examples of these reactions include the nucleophilic substitution reactions, elimination reactions, Grignard-type reactions, various transition-metal catalyzed coupling reactions, carbene-related cyclopropanations reactions, and radical cyclization reactions. All these reactions can be carried out in aqueous media. 

These three types, radicals, carbocations and carbanions, by no means exhaust the possibilities of transient intermediates in which carbon is the active centre others include the electron-deficient species carbenes, R2C (p. 266), nitrenes, RN (p. 122) and also arynes (p. 174). 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

Nanosecond techniques have now been superceded by picosecond and femtosecond techniques, allowing detection in time domains as short as 10 s. Yet, nanosecond techniques remain powerful tools in the arsenal of the physical organic chemist quite simply, many radical, carbene, carbocation, carbanion reactions take place in the nano- and microsecond time scales. 

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