Lithium carbanion

Dimethylpropanoyl)-l, 2,3,4-tetrahydroisoquinolincs 16 form dipole-stabilized lithium carbanions on deprotonation, but their addition to aldehydes or methyl ketones proceeds nevertheless with low simple diastereoselectivity22 23. However, a high preference for the formation of the w-diastereomer is observed after transmetalation with magnesium bromide22"24. 

The lithium carbanion derived from 7i-tricarbonyl(2-ethylpyridine)chromium adds to noneno-lizable aldehydes with high induced and simple diastereoselectivity10,11. 

The addition of anhydrous cadmium iodide or chloride on allylic lithium carbanions causes changes in the regioselectivity, but information on the stereochemical results is scarce31-42-43. 

With titanated 2-alkenyl carbamates, the opposite regioselectivity can also be observed. Lithiated l-(4-methylphenylsulfonyl)-2-alkenyl diisopropylcarbamates, after metal exchange with chlorotriisopropoxytitanium, add to aldehydes y-selectivelylls. The less reactive titanat-ing reagent tetraisopropoxytitanium does not apparently react with these stabilized lithium carbanions, because in its presence a-selectivity is retained (Section 1.3.3.3.1.3.2.). 

The problem can be solved by the transformation of the lithium carbanions into the more reactive trichlorotitanium intermediates via the stannanes. Finally, the (- )-sparteine complex of (5)-( )-l-methyl-2-butenyl diisopropylcarbamate105 (Section 1.3.3.3.1.2.) is apparently transmetalated by tetraisopropoxytitanium with inversion of configuration, leading to homoaldol products with moderate diastereomeric excess103. 

Phenylpropionic acid (10) was obtained in low yield and low enantiomeric purity. Although not necessarily detrimental, here configurational instability of the intermediate lithium carbanion pairs 9/epi-9 is the major reason for the insufficient result. This issue will be examined more closely in Section III.A. 

Similarly, the reaction of 1-chloropentyhnagnesium chloride (56), derived from 1-chloro-l-iodopentane, with a-sulfonyl lithium carbanions (52) affords 1,2-di- or 1,1,2-trisubstituted olefins (57) in moderate to good yields (equation 14). This reaction represents an elegant preparation of olefins from sulfones. 

TABLE 1. Reaction of a-sulfonyl lithium carbanions (52) with chloromethylmagnesium chloride (53)... 

Thus, magnesium carbenoid 38, generated from 1-chloroalkyl phenyl sulfoxide (37) in THF at —65°C with 2.8 eq of i-PrMgCl, reacts with a-sulfonyl lithium carbanion to lead to 1,2-di- and 1,1,2-trisubstituted olefins (60). Yields are better in such conditions as compared to the reaction described in equation 14. 

The proposed mechanism of this reaction is composed by an initial S v2-type nucleophilic substitution reaction of 113 with the nucleophilic a-sulfonyl lithium carbanion to give the alkylmagnesium species (114) having a sulfonyl group at the / -position. Then, a -elimination reaction of magnesium sulfinate from the intermediate (114) occurs... 

TABLE 4. Synthesis of alkylidenecyclopropanes (118) from 1-chlorocyclopropyl phenyl sulfoxide (116) and a-sulfonyl lithium carbanions through magnesium cyclopropy-Udene (117)... 

The electrophilic reaction of magnesium alkylidene carbenoids with other nucleophiles than the original Grignard reagent can also be carried out. For example, treatment of magnesium alkylidene carbenoid 157, derived from 147, with a-sulfonyl lithium carbanion afforded allenes 159 in moderated yields (equation 39/. ... 

The proposed mechanism is as follows First, the a-sulfonyl lithium carbanion attacks the electrophilic carbenoid carbon atom to give the vinyhnagnesium intermediate (158). As the sulfonyl moiety is a good leaving group, a /-elimination takes place to afford the allenes (159). 

Thus, a-sulfinyl lithium carbanion of 1-chloroethyl p-tolyl sulfoxide was reacted with 1,4-cyclohexanedione mono ethylene ketal (195) to afford the adduct (196) in quantitative yield. The adduct was treated with ferf-butylmagnesium chloride (magnesium alkoxide was initially formed) followed by isopropylmagnesium chloride to result in the formation of magnesium /3-oxido carbenoid 197. The /3-oxido carbenoid rearrangement then takes place to give one-carbon expanded magnesium enolate 198. Finally, an electrophile was... 

The chemistry mentioned above could also be applied to aldehydes. As an example, p-anisaldehyde and 1-chlorobutyl p-tolyl sulfoxide were used as shown in Table 16. Thus, treatment of a-sulfinyl lithium carbanion of 1-chlorobutyl p-tolyl sulfoxide with... 

Application of the method described above to unsymmetrical cyclic ketones such as 2-substituted cyclohexanones gave 2,7-disubstituted and 2,2,7-trisubstituted cycloheptanones (Scheme 8). Treatment of a-sulfinyl lithium carbanion of 1-chloroethyl p-tolyl sulfoxide with 2-substituted cyclohexanones (210a and 210b) afforded adducts as a mixture of two diastereomers. The main adducts were first treated with f-BuMgCl followed by i-PrMgCl (4 equiv) at 0°C to room temperature to give the magnesium /3-oxido carbenoid 211. The... 

Stork, Jacobson, and Levitz (51) have recently reported that the reaction of the lithium carbanion 234 with benzaldehyde followed by reduction with sodium borohydride gave the phenylcarbinol 235. The sequence of events in the transformation of 234 to 23S was shown to be as depicted below. Convincing spectral evidence was obtained for 236, 238, and 239. Thus, the hemi-orthoamide tetrahedral intermediate 237 which was generated in situ gave the ami nobenzoate 238, the expected product from stereoelectronic control. 

With this end in view, phenyldimcthylsilyl tri-n-butylstannane was added under the influence of zero-valent palladium compound with high regioselectivity and in excellent yield to the acetylene 386 to give the metallated olefin 387 (Scheme 56). The vinyl lithium carbanion 388 generated therefrom, was then converted by reaction with cerium(lll) chloride into an equilibrium mixture (1 1) of the cerium salts 389 and 390 respectively. However, the 1,2-addition of 389 to the caibonyl of 391, which in principle would have eventually led to ( )-pretazettine, did not occur due to steric reasons — instead, only deprotonation of 391 was observed. On the other hand, 390 did function as a suitable nucleophile to provide the olefinic product 392. Exposure of 392 to copper(II) triflate induced its transformation via the nine membered enol (Scheme 55) to the requisite C-silyl hydroindole 393. On treatment with tetrafluoroboric acid diethyl ether complex in dichloromethane, compound 393 suffered... 

The reaction was extended to 1,3-dienyltins, giving the corresponding lithiated reagents, both in terminal633-635 or internal636 position, which can be substituted by alkoxy groups637. Recently, a 1,3,5-trienyl lithium carbanion was prepared in a similar way from 22 and used for a further synthesis (equation 48)638. 

This volume, which complements the earlier one, contains 9 chapters written by experts from 7 countries. These include a chapter on the dynamic behavior of organolithium compounds, written by one of the pioneers in the field, and a specific chapter on the structure and dynamics of chiral lithium amides in particular. The use of such amides in asymmetric synthesis is covered in another chapter, and other synthetic aspects are covered in chapters on acyllithium derivatives, on the carbolithiation reaction and on organolithi-ums as synthetic intermediates for tandem reactions. Other topics include the chemistry of ketone dilithio compounds, the chemistry of lithium enolates and homoenolates, and polycyclic and fullerene lithium carbanions. 

Different results are obtained when lithium carbanions react with octahedral pentacarbonylcarbene complexes, (CO) ML (M = Cr, Mo, W), strongly depending on the type of carbene ligand as well as on the type of lithium carbanion used ... 

By reaction of cationic carbonyl complexes with lithium carbanions, neutral acyl complexes are prepared. Whereas treatment of [> -CpFe(CO)3]BF4 with (a) PhLi gives the expected > -CpFe(CO)2 [C(0)Ph] in 80% yield, with (b) MeLi only traces of > -CpFe(CO)2 [C(0)Me] can be detected . This complex and other phosphane-substituted acyl compounds of the type f -CpM(CO)L[C(0)Me] [M = Fe, Ru L = CO, PPh3, P(hex)j], as well as >/ -CpMo(CO)2P(hex)3[C(0)Me] (prepared by different routes), are protonated with and alkylated with [R3 0]BF4 reversibly, yielding cationic hydroxy- and alkoxy(methyl)carbene complexes, respectively . The formation of the ( + )- and ( —)-acetyl complex / -CpFe(C0)(PPh3)[C(0)Me] from the ( + )-and ( —)-conformers of optically active > -CpFe(C0XPPh3)[C(0)0-menthyl] and MeLi occurs with inversion of configuration at the asymmetric iron atom . 

Excluding the a-P-, a-Si-substituted carbanions which are listed in Table 2, there exist relatively few simple a-P-substituted carbanions whose structures are known. References to the crystal structures of some tri (alkyl or aryl) substituted phosphines are listed in Table 4. Few if any of these compounds have been utilized as synthetic reagents. Only two synthetically useful phosphorus-stabilized carbanions of Group la or Ila metal cations have been examined by X-ray diffraction analysis. Hie lithium carbanion of 2-benzyl-2-oxo-l,3,2-diazaphosphorinane (198) crystallizes as a monomeric bis-THF solvate (199) with a tricoordinate lithium atom. The magnesium salt of diethoxyphosphinyl acetone (200) is cluirac-terized as an intramolecularly chelated trimer similar in structure to [Mg(acac)]3. The Cu salt of this 3-keto phosphorus-stabilized anion exists only as a monomer. 

---