Allyllithium complex

As far as investigated77, most reactions of the allyllithium-sparteine complexes with electrophiles proceed antarafacially, either as SE2 or anti-SE2 reactions. As a working hypothesis it is assumed that the bulky ligand obliterates the Lewis acid properties of the lithium cation. 

Another example of a [4S+1C] cycloaddition process is found in the reaction of alkenylcarbene complexes and lithium enolates derived from alkynyl methyl ketones. In Sect. 2.6.4.9 it was described how, in general, lithium enolates react with alkenylcarbene complexes to produce [3C+2S] cycloadducts. However, when the reaction is performed using lithium enolates derived from alkynyl methyl ketones and the temperature is raised to 65 °C, a new formal [4s+lcj cy-clopentenone derivative is formed [79] (Scheme 38). The mechanism proposed for this transformation supposes the formation of the [3C+2S] cycloadducts as depicted in Scheme 32 (see Sect. 2.6.4.9). This intermediate evolves through a retro-aldol-type reaction followed by an intramolecular Michael addition of the allyllithium to the ynone moiety to give the final cyclopentenone derivatives after hydrolysis. The role of the pentacarbonyltungsten fragment seems to be crucial for the outcome of this reaction, as experiments carried out with isolated intermediates in the absence of tungsten complexes do not afford the [4S+1C] cycloadducts (Scheme 38). 

Calculations of alkali metal allyl derivatives involving all alkali metals (Li-Cs) indicate a preferred geometry with the metal symmetrically bound in a predominantly electrostatic manner to all three carbon atoms.143 Solution studies of allyllithium in ether indicate the compounds to be highly aggregated in THF complex dynamic behavior is observed. 

On substitution of allyllithium with methyl groups, the structures are distorted tt complexes becoming more jj -like. The previously described allyllithiums are contact ion pairs (CIP) whose dissociation is too low to permit study of the free carbanion. However, this is not the case for a more delocalized system such as 1,3-diphenylallyl whose lithium salts can exist as solvent separated ion pairs (SSIP) in ethereal solutions for which the organic moiety could be treated essentially as a free carbanion55 Boche and coworkers studied the effect of substitution at C(2) in their 1,3-diphenylallyl lithiums on the rotational barriers... 

Preparation of allyltitaniums of the type (allyl)Ti(OiPr) 3 from the corresponding allyl-lithium or -magnesium compounds and ClTi(OiPr)3 by transmetallation and their subsequent synthetic utilization have attracted considerable interest because of the advantageous reactivity of the allyltitaniums as compared to other allylmetal complexes in terms of chemo-, regio-, and diastereoselectivity [3], The preparation of certain allyllithium or -magnesium reagents, however, is not necessarily easy, which would seem to limit the utility of this method. 

Hetero-substituted r)3-allyltitanocenes have also been studied. Functionalized q3-allyltita-nium complexes (X = SiMe3, OPh, SPh) have been prepared by transmetallation of Cp2TiCl with the corresponding allyllithiums, and were found to react regiospecifically with propionaldehyde to give functionalized homoallylic alcohols 11 (Scheme 13.14) [25]. 

The high nucleophilicity of heterosubstituted allyllithium compounds makes them attractive reagents in synthetic organic chemistry. Structural studies of these compounds give a fundamental understanding about the control of the regioselectivity. Often, these studies are difficult due to the tendency of the compounds to form complex fluxional aggregates in solution. Piffl and coworkers have studied the dependency of the oxidation state of sulfur on the structure and electronic properties of the heterosubstituted... 

The lithium-titanium exchange in the allyllithium-sparteine complexes 317 by tetraisopropoxytitanium (TIPT) or chloro-triisopropoxytitanium, resulting in titanates 318, proceeds with strict stereoinversion (equation 84). We assume that—contrary to the lithium-TMEDA complexes—the lithium-(—)-sparteine complexes are weaker Lewis acids and are no longer capable of binding the TIPT in the transition state of the exchange reaction. 

The asymmetric lithiation/substitution of Af-Boc-Af-(3-chloropropyl)-2-alkenylamines 395 by w-BuLi/(—)-sparteine (11) provides (5 )-Af-Boc-2-(alken-l-yl)pyrrolidines 397 via the allyllithium-sparteine complexes 396 (equation 106) . Similarly, the piperidine corresponding to 397 was obtained from the Af-(4-chlorobutyl)amine. Intramolecular epoxide openings gave rise to enantioenriched pyrrolidinols. Beak and coworkers conclude from further experiments that an asymmetric deprotonation takes place, but it is followed by a rapid epimerization a kinetic resolution in favour of the observed stereoisomer concludes the cyclization step. 

W-Substituted 2,4-alkadien-l-ols such as 474 add the alkyllithium/(—)-sparteine complex preferentially to the 2-position to form via the alkoxide the corresponding allyllithium intermediates 475 . Protic workup leads to a mixture of ii/Z-alkenols 476 and 477 on catalytic hydrogenation the -branched alcohols 478 are isolated (equation 130). 

Reductive lithiation of allyl phenyl sulfides.1 This reagent is particularly useful for preparation of allyllithium reagents at temperatures at which the anions are stable. Moreover, regioselectivity in reactions can be achieved by conversion to allyltitanium(IV) complexes by metal exchange with Ti(0-/-Pr)4. Thus the un-symmetrical anion formed from the allyl sulfide 1 with LDMAN reacts with cro-tonaldehyde to give a mixture of 1,2- and 1,4-adducts. The 1,2-adduct 2 can be obtained in high yield as two diastereomers (9 1) by use of the allyltitanium complex (equation I). 

An alternative bonding situation without it interaction has been found by Weiss and Koster (9) for the allyllithium-tetramethylethylenediamine (TMEDA) complex. A low-resolution X-ray structure shows an endless polymer with TMEDA-solvated lithium atoms attached to both terminal allyl carbons. 

The formation of diastereoisomerically pure complexes of 90 with (-)-sparteine is also controlled by crystallisation. Treatment of the indene 89 with BuLi and (-)-sparteine in ether gives, on warming, a yellow precipitate which reacts with carbonyl electrophiles to provide the products 91 typically with good regioselectivity and >95% ee.52 An X-ray crystal structure proved the stereochemistry of the intermediate complex to be that shown as 90b, and hence proved the stereochemical course of the substitution (see section 6.1). The complex is readily decomposed by THF, in the presence of which it rearranges to a racemic V allyllithium. 

Allylalkali metal compounds exhibit dynamic isomer-izations and bond rotations in solntion. In THF solntion, allylalkali metal complexes undergo C bond rotations with activation energies normally in the range of 45-75kJmoL. 1 - AUcylallylalkali metal compounds usually exist as a dynamic mixture of (E) and (Z) isomers (15), with a thermodynamic preference for the (Z) configuration. The rates of isomerization of frani -neopentylallyl metal complexes in THF solution decreases in the sequence Li > Na > K. Numerous studies of the structures and dynamic properties of allyllithium compounds have been reported. " ... 

Methyllithium (and likewise BuLi and allyllithium) also adds to the carbene ligand of (CO)5W[C(OMe)Ph]. However, the reaction of the resulting anionic adduct with Si02/pentane at —40°C yields pentacarbonyl(n -olefin)-W complexes, probably via the intermediary formation of the methyl(phenyl)carbene complex and following rearrangement via 1,2-hydrogen shift ... 

Recently, an improved one-pot method, that is, the metathesis reaction of anhydrous LnCb with three equivalents of allylMgCl, instead of three equivalents of allyllithium in a mixture of THF-l,4-dioxane was developed for the synthesis of neutral triallyl lanthanide complexes. 

In contrast, the approach of bulky alkyl halides from the same side of the benzyl side chain would be difficult due to steric repulsion. Then, DFT calculations of complexes coordinated with i -PrBr were performed and the results implied that complex derived from the TT-allyllithium intermediate 101a was thermodynamically more stable than complex derived from 102a °°. However, the subsequent alkylation step might be disturbed by steric hindrance of the i-Pr group, implying that electrophilic trapping with bulky aUcyl halides such as i-Pr could be controlled in the final step of the alkylation to yield the 3,6-fraw5 isomer as the main product. 

Metalation of limoaeae. Crawford el a . report that limonene (1) undergoes selective metalation at C,o on treatment with the 1 1 complex of n-butyllithium and TMEDA to afford the 2-substituted allyllithium species represented by (2). The metalation is carried out by allowing a mixture of 2 eq. of limonene and 1 eq. of the complex... 

The first example of an Ti -allyllithium was reported for the polymeric 1,3-diphenylallyllithium-diethyl ether complex (93). In this polymer, the lithium atoms lie almost symmetrically above and below the allyl group. The C(l)—C(2>—C(3) angle is quite large, i.e. 13T, similar to that in structures (91) and (92), but the C(2)—Li distance in (93) is shorter than either the C(l)— Li or C(3)— Li distances. TTie structural features of (93) correspond to those found in many allyl transition metal complexes which display Ti -bonding of the transition metal to the allyl group. ... 

Allyltitanium complexes (22) readily add to carbonyl compounds with high regio- and stereo-selection. They are prepared by reaction of a chlorotitanium complex (21) with an allyl-magnesium or -lithium derivative (equation 13). Some of these unsaturated Ti complexes, like (23)-(25) in Scheme 2, obtained from allylmagnesium halides or allyllithium by reaction with titanium tetraisopropoxide or titanium tetramides, are known as titanium ate complexes . The structure of these ate complexes, at least from a formal point of view, can be written with a pentacoordinate Ti atom. Some ate complexes have synthetic interest, as is the case of (allyl)Ti(OPr )4MgBr which shows sharply enhanced selectivity towards aldehydes in comparison with the simple (allyl)Ti(OPr )3. ... 

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