Alkyls kinetically stabilize

Kinetic stability of lithium and the lithiated carbons results from film formation which yields protective layers on lithium or on the surfaces of carbonaceous materials, able to conduct lithium ions and to prevent the electrolyte from continuously being reduced film formation at the Li/PC interphase by the reductive decomposition of PC or EC/DMC yielding alkyl-carbonates passivates lithium, in contrast to the situation with DEC where lithium is dissolved to form lithium ethylcarbonate [149]. EMC is superior to DMC as a single solvent, due to better surface film properties at the carbon electrode [151]. However, the quality of films can be increased further by using the mixed solvent EMC/EC, in contrast to the recently proposed solvent methyl propyl carbonate (MPC) which may be used as a single sol-... 

From the relative stabilities of the actinide homoalkyls or -allyls and the tris(cyclopentadienyl) actinide alkyls, it appears that a coordinatively saturated metal center is necessary for kinetic stability. In contrast to f-transition metal alkyls, the absence of hydrogens appears to be of minor importance. In the case of the lanthanide alkyls and the tetrabenzylthorium, where the formal coordination number is only four, the steric bulkiness of the Hgands must be responsible for their observed thermal stability. 

Exps. 9 and 10 are convincing illustrations of the high lrineiic acidity of the ethynyl proton. In principle, there are four reaction pathways if l-bromo-5-hexyne and a strongly basic reagent are allowed to interact abstraction of the acetylenic proton, Br-metal exchange, displacement of Br by the "nucleophilic" part of the base, and elimination of HBr with formation of HCsC(CH2)2CH=CH2- Only the first process takes place under the conditions of this experiment. The kinetic stability of the intermediate LiOC(CH2)4Br is sufficient to allow for successful functionalizations with a number of reagents. For alkylations with most of the alkyl halides, the polarity of the medium will usually be insufficient. 

The different reaction course followed by the silyl- and alkyl-substituted allenes is fully consistent with the hyperconjugation model, which predicts that a /i-silyl group stabilizes carbenium ions better than a /3-alkyl group, and with previous conclusions that a-silyl substitution in carbocations is destabilizing relative to a-methyl substitution. The larger space requirements of the trimethylsilyl groups compared with methyl groups may also contribute to the kinetic stability of 413. 

With regard to the susceptibility of many organometallics to aerial oxidation, it must be remembered that most compounds containing C-H bonds are thermodynamically unstable with respect to oxidation by molecular oxygen to produce water and carbon dioxide. The mechanisms whereby (for example) many alkyls are spontaneously flammable in air are necessarily difficult to determine the mechanisms involved in the combustion of hydrocarbons are still incompletely understood. Suffice it to say that the kinetic stability of ER to air seems to be low where E is of low electronegativity. 

These are generally limited to what are termed kinetically stabilized alkyls, i.e. those devoid of protons p to the metal (Figure 4.5). These also include norbornyl and adamantyl examples since decomposition via p-metal hydride elimination (see below) would require the installation of an alkenic bond between the a and p carbons of the precursor alkyl. This is precluded for these two alkyls because of the prohibitive strain associated with forming a double bond to a bridgehead atom within a small cage structure (Brendt s rules). The tetrakis(norbornyl) complexes are also remarkable because some uncharacteristic oxidations states can be attained, e.g. Cr(IV), Co(IV) (low-spin e4t2°). The second factor which may confer stability is steric bulk bimolecular decomposition routes are thereby discouraged. 

The kinetically stabilized alkyls shown in Figure 4.5 do not have the eliminations 3-M-H elimination route available to them owing to the absence of P-protons. An alternative decomposition route, however, arises when these are bound to highly unsaturated metal centres, involving loss of an cx-C-H which may be either transferred (Figure 4.27) to (a) the metal centre itself (a-elimination), (b) a co-ligand or (c) an external basic or electrophilic reagent (a-abstraction). 

Of these molecules, only compound 27 has actually been isolated and characterized. The others are too reactive for ordinary isolation. Simple alkyl substitutions on the thiophene ring of compound 26 are sufficient to allow isolation of its derivatives <1979JOC2887>, but quite bulky substitutions are necessary to achieve sufficient kinetic stabilization to isolate derivatives of compound 25 <1970JA7610>. 

Erom a formal point of view the reaction of kinetically stabilized phosphaalkynes with 1,3-dienes should furnish 1-phosphacyclohexa-l,4-dienes. However, at 90°C with alkyl-substituted phosphaalkynes the primarly formed Diels-Alder adducts have not been detected, because the initial [4-f 2] cycloaddition was followed by an ene-reaction and a final intramolecular [4-1-2] cycloaddition to give diphosphatricyclooctenes. 

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