Allyl anions 1-substituted

If R1 differs from R2. the preparation may lead to both regioisomers. In these cases, a synthetic route which does not rely on allyl anion substitution is often the most advantageous one. Thus, the best results are recorded for allylboronates and -silanes which also possess the required constitutional and configurational stability. 

A more general solution to the problem is to use an allyl anion substituted on both ends with a heteroatom 136. The anion 137 may then react regiospecifically with electrophiles to give products 138 with a masked carbonyl group (vinyl Y in 138) and a leaving group (X in 138) so that hydrolysis gives the enone 139. 

The Julia olefination found its first industrial application for the produdion of retinoic acid. [74, 75] The original design ofthe synthesis, however, concealed a difficulty The allyl anion, substituted on both sides with electron-accepting substituents, does not react regioselectively. 

I.3.3.2.2.I. Reagents Representing a,y-Dicarbon-Substituted Allyl Anions... 

Table 4. Utilization of 1,3-Dicarbon-Substituted Allyl Anions in Enantioselective Carbonyl Addition Reactions...

I.3.3.2.3.I. Reagents Representing a-Hetero- and a,a-Bis(hetero)-Substituted Allyl Anion... 

Table 5. Synthons and Reagents for Carbonyl Addition to x-lIetero-Substituted Allyl Anions...

Allyl anion synthons A and C, bearing one or two electronegative hetero-substituents in the y-position are widely used for the combination of the homoenolate (or / -enolate) moiety B or D with carbonyl compounds by means of allylmetal reagents 1 or 4, since hydrolysis of the addition products 2 or 5 leads to 4-hydroxy-substituted aldehydes or ketones 3, or carboxylic acids, respectively. At present, 1-hetero-substituted allylmetal reagents of type 1, rather than 4, offer the widest opportunity for the variation of the substitution pattern and for the control of the different levels of stereoselectivity. The resulting aldehydes of type 3 (R1 = H) are easily oxidized to form carboxylic acids 6 (or their derivatives). 

Several reviews cover hetero-substituted allyllic anion reagents48-56. For the preparation of allylic anions, stabilized by M-substituents, potassium tm-butoxide57 in THF is recommended, since the liberated alcohol does not interfere with many metal exchange reagents. For the preparation of allylic anions from functionalized olefins of medium acidity (pKa 20-35) lithium diisopropylamide, dicyclohexylamide or bis(trimethylsilyl)amide applied in THF or diethyl ether are the standard bases with which to begin. Butyllithium may be applied advantageously after addition of one mole equivalent of TMEDA or 1,2-dimethoxyethane for activation when the functional groups permit it, and when the presence of secondary amines should be avoided. 

The THF-soluble salt tetrabutylammonium fluoride (TBF) is a common source of fluoride. An alternative reagent is tetrabutylammonium triphenyldifluorosilicate (TBAF).115 Unsymmetrical allylic anions generated in this way react with ketones at their less-substituted terminus. 

In 1989 we reported on the synthesis and structure of the first l,3-diphospha-2-sila-allylic anion 3a [4], mentioning its value as a precursor for phosphino-silaphosphenes. In analogy to 3a we obtained the anions 3b-f [5] by treatment of 4 equivalents of the lithium phosphide 1 with the adequately substituted RSiC, of which 3b and 3c were investigated by X-ray analyses. The very short P-Si bond lengths (2.11-2.13 A) of 3a-c and the almost planar arrangement of Pl-Sil-P2-Lil indicate the cr-character of the Lithium P-Si-P allyl complex. 

Allyl anion is too strongly basic to be studied as the free anion in solution. Bordwell developed an acidity scale based on equation 1 in dimethyl sulfoxide (DMSO) at 25 °C3 and applied the method to a number of more acidic substituted allylic systems. A summary of some results is shown in Table 10. DMSO is sufficiently polar that there is little ion... 

It was considered of interest to utilize dianion 26 <89PHC(1)1> as a potential three carbon allyl anion fragment to react with 6 with a view to develop a new synthesis of 2,3-substituted and fused carbazoies 28 involving protection, activation and deprotection of the... 

Cathodic substitution stands for C,C bond or C, heteroatom bond formation with cathodically generated anions. The question of regioselectivity is encountered in the reaction of such anions with allyl halides (path a) or in the reaction of allyl anions generated in an ECE process from allyl halides (path b). Cathodic reductive silylation of an allyl halide proceeds regioselectively at the less substituted position (Fig. 15) [91]. From the reduction potentials of the halides it is proposed that the reaction follows path b. 

Unsymmetrical allylic anions generated in this way react with ketones at their less substituted terminus. 

The exhaustive controlled-potential reduction of 6-chioro-l-phenylhex-l-yne at — 1.57 V in dimethylformamide containing tetrabutylammonium perchlorate gave a mixture of products. among which was ( >(2-phcnylvinyl)cyclobutane (9).11 It is probable that the mechanism involves initial isomerization of the acetylene to an allene 8 which is reduced at — 1.57 V to the radical anion. Protonation and further onc-clectron reduction then yield the allylic anion. An intramolecular nucleophilic substitution eventually gives the cyclobutane.11... 

Alternatively, the ambident oi-hetero substituted allyl anions have been utilized as homoenolate equivalents. For example, in the presence of HMPA, allyl phenyl sulfides (251),192 allyl phenyl sulfones (252)192b c and allyl phenyl selenides (253)192d e add to a,(3-enones in a l,4(0)-mode, while allyl phenyl sulfoxides (254) and allyl phosphine oxides (255) afford 1 A j-addition exclusively, irrespective of solvent used.193 Hua has shown that additions of either chiral sulfoxide (254 R1 = R2 = R3 = R4 = H, R5 = p-tolyl) or allyl oxazaphospholidine oxide (256) occur with excellent enantioselectivity (>95% ee).194 Similarly, Ahlbrecht reports that the a-azaallyl (257) adds exclusively in a 1 A j-mode to acceptor (59) to afford 1,5-diketones (Scheme 86).195... 

Should an allylic anion be formed as a result of an initial ionisation, attack of an electrophile on the anion could take place either at the (original) a carbon atom or at the (original) y carbon atom, the latter leading to substitution with rearrangement. An illustration for the case of a crotyl compound follows, viz. 

Theoretical calculations of the reactions of CH3SSR (R = H or CH3) with fluoride, hydroxide or allyl anion in the gas phase have been performed to determine the mechanism for both elimination and substitution reactions.6 The elimination reactions have... 

If the donor is a sulfur atom, its lone pair is practically nonbonding (the electronegativities of sulfur and carbon are similar), above 2 and scheme c is no longer valid. If the sulfur is modeled by a carbanion, the sulfur-substituted diene and the dienophile are represented by the pentadienyl and the allyl anions, respectively. Their HOMOs will both be nonbonding. Hence FO theory predicts that placing the sulfur on the dienophile and the attractor on the diene may be slightly more favorable than the opposite substitution pattern. 

In the catalytic presence of tetrabutylammonium fluoride, a trimethylsilyl group is cleaved from AKtrimethylsilyl)rnethylbenzylimine to form the resonance-stabilized 2-aza-allyl anion which undergoes a Michael addition reaction with, for example, methyl acrylate, giving y-aminoesters.333 These types of aminoesters serve as a starting material for the elaboration of diversely substituted pyrrolidones.334... 

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