Polar organometallic compounds

Organohthium and organomagnesium compounds are stable species when prepared m suitable solvents such as diethyl ether They are strongly basic however and react instantly with proton donors even as weakly acidic as water and alcohols A proton is transferred from the hydroxyl group to the negatively polarized carbon of the organometallic compound to form a hydrocarbon... 

You have already had considerable experience with carbanionic compounds and their applications in synthetic organic chemistry The first was acetyhde ion m Chapter 9 followed m Chapter 14 by organometallic compounds—Grignard reagents for example—that act as sources of negatively polarized carbon In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic and that this property can be used to advantage as a method for carbon-carbon bond formation... 

Organomercury compounds, reaction with NaBH4, 222 Organometallic compound, 345 polarity of, 143... 

The first example of chemically induced multiplet polarization was observed on treatment of a solution of n-butyl bromide and n-butyl lithium in hexane with a little ether to initiate reaction by depolymerizing the organometallic compound (Ward and Lawler, 1967). Polarization (E/A) of the protons on carbon atoms 1 and 2 in the 1-butene produced was observed and taken as evidence of the correctness of an earlier suggestion (Bryce-Smith, 1956) that radical intermediates are involved in this elimination. Similar observations were made in the reaction of t-butyl lithium with n-butyl bromide when both 1-butene and isobutene were found to be polarized. The observations were particularly significant because multiplet polarization could not be explained by the electron-nuclear cross-relaxation theory of CIDNP then being advanced to explain net polarization (Lawler, 1967 Bargon and Fischer, 1967). 

It might be mentioned that matters are much simpler for organometallic compounds with less-polar bonds. Thus Et2Hg and EtHgCl are both definite compounds, the former is a liquid and the latter is a solid. Organocalcium reagents are also known, and they are formed from alkyl halides via a single electron transfer (SET) mechanism with free-radical intermediates. "... 

Organometallic compounds that are not ionic but polar-covalent behave very much as if they were ionic and give similar reactions. 

Applications RPLC-ICP-AES was used for specia-tion and quantification of polar, low-MW silanols [686]. Cr(III, VI) can be determined by IC-ICP-OES at the ppt level. However, many HPLC applications for organometallic compounds demand the use of gradient elution, and a high flow-rate combined with gradient elution results in unstable plasma conditions. 

In organometallic compounds of the form RR R"C—M, pretty well the whole spectrum of bonding is known from the essentially covalent, via the polar-covalent, RR R"C, —M4+, to the essentially ionic, RR R"CeM . In their reactions, predominant retention, racemisation, and inversion of configuration have all been observed the outcome in a particular case depending not only on the alkyl residue, but also on the metal, and particularly on the solvent. Even with the most ionic examples it seems unlikely that we are dealing with a simple carbanion thus in the reaction of EtI with [PhCOCHMe]6 M , the relative rates under analogous conditions are found to differ over a range of 104 for M = Li, Na and K. 

In the preceding chapter the formation of the less stable of two isomeric acids on rapid neutralization of an organometallic compound was explained in terms of a free ion or ion pair as the reagent. The compounds involved were the salts of comparatively strong acids, however, and the media were highly polar. The ion or ion pair would be less likely to be the reagent in the case of the salt of a very weak acid, such as an alkyl lithium compound, in a non-polar solvent. Salts of unsaturated but weak acids might exist in either of two partly covalent forms this possibility provides an alternative explanation for the formation of the less stable of the two isomeric acids on neutralization. Presumably the bulk of the covalent or partly covalent... 

The success of these reactions was intriguing because, under normal reaction conditions, group 14 organometallic compounds serve as carbanion synthons. The anodic oxidation reaction successfully reversed this polarity thereby expanding the overall synthetic utihty of group 14 organometallic intermediates. 

The polarity of carbon-halogen bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds. Nucleophilic substitution reactions are categorised into and on the basis of their kinetic properties. Chirality has a profound role in understanding the reaction mechanisms of Sj l and Sj 2 reactions. Sj 2 reactions of chiral all l halides are characterised by the inversion of configuration while Sj l reactions are characterised by racemisation. 

SCHEME 22. Stereochemical outcome of the addition of polar organometallic compounds R"M to a-alkoxy aldehydes and ketones (G = protecting group)... 

It might be mentioned that matters are much simpler for organometallic compounds with less-polar bonds. Thus Et2Hg and EtHgCI are both definite compounds, the former a liquid and the latter a solid. 

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