Allylpotassiums

In the case of allylpotassium, the metal complex exists as a symmetric n structure. No temperature dependence was shown by either 13C NMR for A5[C(i) —C(3)] or by 1H NMR for substitution with deuterium at Cp). Thompson and Ford measured experimentally a variety of allylalkali metal compounds using variable-temperature NMR in THF-dx45. Addends such as TMEDA, hexamethylphosphoric triamide (HMPA), 15-crown-5-ether, [2.1.1]cryptand and n-butyllithium showed either no change in the spectrum or rapid decomposition of the complexing agent. Measurement of the populations of E (17) and Z (18) isomers of 1-isopropylallylpotassium showed the Z isomer to be more stable (Table 11). 

Further investigation of allylpotassium complexes have shown that 2-isopropylallyl potassium does not show diastereotopism of the methyl groups at temperatures as low as —155 °C54,59. Therefore, the activation barrier for interconversion is on the order of 4 kcal mol 1 or lower. Both crotyl (19) and prenyl (20) potassium complexes are further examples of the preference for allylpotassium compounds to exist as symmetric it species. The potassium has the appropriate atomic radius to reach both C(i) and C(3). No increase in stabilization is gained upon addition of solvent. Allylcesium behaves in the same manner. In general, the theoretically calculated rotational barriers (Table 9) are higher... 

From Hard Allylic Organometallics. The most common preparation of allylic boranes and boronates is the addition of a reactive allylic metal species to a borinic or boric ester, respectively (Eqs. 10 and 11). Preparations from allyllithium, " " allylmagnesium, and allylpotassium " ° reagents are all well known. These methods are popular because the required allylic anions are quite easy to prepare, and because they generally lead to high yields of products. 

Allylpotassium derivatives exist in two stereoisomeric (endo or exo) forms.239,240 Although the endo isomer is more stable, the equilibration is slow enough to allow the synthesis of stereoisomeric heterosubstituted derivatives [Eqs. (10.42) and (10.43)]. On the other hand, even the E-alkenes after appropriate equilibration can be transformed to Z-boron compounds 10 ... 

Trapping of Allylpotassium Reagents with Carbon Electrophiles... 

Dialkylallyl boranes have also been prepared119 by metallation of alkenes with trimethylsilylmethylpotassium followed by trapping of the allylpotassium species with dialkylchloroboranes. Such allylboranes have been then used... 

Allylic alcohols may be derived from alkenes by metallation to give the allylpotassium species, followed by treatment with fluorodimethoxyborane. Oxidation of the resultant boronic ester with hydrogen peroxide gives the allylic alcohol (Scheme 15). ° - ° Some allylic rearrangement may be observed for example, metallation of a-pinene with potassium r-butoxide in petroleum ether solution and subsequent boration and oxidation gave myrtenol (42%) and /rons-pinocarveol (1%) (equation 24), while treatment of the allylpotassium with oxirane gave the alkylated prc ucts in a ratio of ca. 2 1. ° ... 

Further investigation of allylpotassium complexes have shown that 2-isopropylallyl potassium does not show diastereotopism of the methyl groups at temperatures as low as — Therefore, the activation barrier for in ter conversion is on the order of... 

Even a hydrocarbon as weakly acidic as propylene reacts with potassium (example 9) however, the conditions are severe and the yield of allylpotassium moderate. Catalysts induce Li to react with propylene in THE at 0°C to give either (E)-propenyllithium or allyllithium, depending on the choice of catalyst. ... 

The rotational barriers of allylpotassium 23c (16.7 kcal/mol) and allylcesium 23d (18.0 kcal/mol) are much higher than those of 23 a and 23b. This observation and the question ofwhether aggregates are involved in the rotational process, clearly emphasize once more the important influence of the gegenion in carbanion reactions. The experimental results with 23a-d thus lead only to a lower limit (18.0 kcal/mol) for the rotational barrier of the allyl anion in solution. Incidently, this value comes close to the lowest calculated (MP2/4-31 + G//4-31 + G) value of the allyl anion rotational barrier (22.2 kcal/mol)33). 

The formation of C—C bonds by way of allylpotassium compounds has been reviewed. ... 

AUyl phenyl selenides, 338 Allylpotassium compounds, 638 Allyrselenides, 338... 

Allylic alcohols may be derived from alkenes by metallaticHi to give the allylpotassium species, followed by treatment with fluorodimethoxyborane. Oxidation of the resultant boronic ester with hydrogen peroxide gives the allylic alcohol (Scheme Some allylic rearrangement may be observed for... 

Among the most common preparations of allylboronates (1) is the addition of a reactive allylmetal (6) to a borate ester (Equation 1). Preparations from allyllithitim reagents [10-14], allyl Grignards [10,16] and allylpotassium [9,14,15,17-21] species are all well known. This method is popular because the required anions are quite easy to prepare, and because it generally leads to high product yields. 

The Hiickel-aromatic dipotassium cyclooctatetraenediide is formed by spontaneous cleavage of the central bond in bicyclo[3.3.0]octa-3,7-diene-2,6-diyldipotas-sium (387, Scheme 1-302). " The bicyclic allylpotassium 388 (Scheme 1-302) equally gets rid of ring strain by scission of the central carbon-carbon bond. Accidental protonation by the solvent gives a mixture of regioisomeric dienes. 

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