Lithium allyl complexes

Summary Several lithium l,3-diphospha-2-sila-allyl complexes 3a-f and the diphosphino-dichlorosilane 2 have been prepared and characterized. The hydrolysis and substitution reactions of these compounds are described yielding a number of phosphino- and diphosphino-silaphosphenes 5a-d, 4a,b and 6. The compounds have been characterized by NMR and by X-ray analyses in the cases of 2, 3a-c and 4a. 

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. 

This complex is not the actual catalyst for the hydrovinylation, but needs to be activated in the presence of a suitable co-catalyst. The role of this additive is to abstract the chloride ion from the nickel centre to generate a cationic allyl complex that further converts to the catalytically active nickel hydride species. In conventional solvents this is typically achieved using strong Lewis acids such as Et2AlCl. Alternatively, sodium or lithium salts of non-coordinating anions such as tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BARF) can be used to activate hydrovinylation... 

The titanium compounds, derived from the configurationally stable lithium-(—)-sparteine complexes 349a,b prepared from primary allyl carbamates, undergo lithium-titanium exchange with chlorotriisopropoxytitanium to form the allyltitanates... 

In the case of the protonation of allyl anions 54 (Structures 6), no systematic governing of the selectivity is possible at present. Due to the existence of complex mixtures of lithium allyl aggregates, these systems are extremely sensitive to small changes in reaction conditions. 

One of the first significant advances in the chemistry of TT-allylpalladium complexes was the discovery that alkenes could be directly converted into the corresponding allyl complex by substitution into the allylic C—H bond. A variety of recipes have now been reported that can accomplish this transformation. Initially, palladium chloride17-23 or its more soluble forms, sodium or lithium tetrachloropalladate24-27 and bisacetonitrile palladium dichloride,28-30 in alcohol or aqueous acetic acid solvent were employed. The use of palladium trifluoroacetate, followed by counterion exchange with chloride, represents the mildest and most effective means available to accomplish this reaction.31... 

Similarly, lithium enolates of ketones add to allyl acetates via Pd° catalysis by a double inversion process,110 and potassium ketone enolates have been shown to add to preformed rr-allyl complexes with inversion (equations 156 and 157)."1... 

Vinyl substitution occurs with conjugated dienes as well as with alkenes, employing aryl-, vinyl-, methyl-, alkoxycarbonyl- or benzyl-mercury reagents and lithium tetrachloropalladate(II), but the products are usually rr-allylpalladium complexes if the reactions are carried out under mild conditions (equation 8).24,25 The ir-allylic complexes may be decomposed thermally to substituted dienes26 or reacted with nucleophiles to form allylic derivatives of the nucleophile. Secondary amines, for example, react to give tertiary allylic amines in modest yields, along with dienes and reduced dienes (equation 9).25... 

Palladium coordinates to one face of the diene promoting intramolecular attack by the alcohol on the opposite face. The resulting <7-alkyl palladium can form a 71-allyl complex with the palladium on the lower face simply by sliding along to interact with the double bond. Nucleophilic attack of chloride from the lithium salt then proceeds in the usual way on the face opposite palladium. The overall addition to the diene is therefore cis. 

TABLE 10. Dynamics of reorganization of allylic lithium TMEDA complexes, 0.3 M in diethyl ether-4io from 13C NMR line shape analysis... 

Lithium lanthanum jr-allyl complexes, LiLn(All)4 dioxane, where Ln = Ce, Nd, Sm, Gd, Dy have been synthesized and used as catalysts in the polymerization of butadiene. The data show the predominance of 1,4-trans product. The catalytic activity of the lanthanides was nearly the same as evidenced by the percent yield in the range 78-90. 

Pentadienyls may be considered as vinylogues of allyl complexes and accordingly parallels exist, both in their synthesis and reactivity. These are most widely prepared by transmetallation from lithium or Grignard reagents, nucleophilic attack at pentadienyl halides or via hydride abstraction from rj4-pentadiene complexes (Figure 7.13). 

The reaction of ZrCl4 with two equivalents of the lithium allyl compound (tBuMe2SiCH)2CHIi-TMEDA in toluene yielded the reduced complex [(lBuMe2... 

Similar selectivities are encountered with substrates 228. In the case of Irons-22, the product with overall retention, trans-2, can be obtained predominantly by conducting the reaction in the presence of lithium chloride, which blocks the coordination sites on palladium in the intermediate 7t-allyl complex and thus suppresses isomerization of the starting material via syn migration of acetate from the metal to the rc-allyl ligand. A large excess of amine has been found to further enhance the diastereoselectivity of this process. 

In the course of an investigation into the syntheses and polymerization activity of bis(allyl)zirconium precatalysts, Eisen and co-workers found that the treatment of ZrCU with 2 equiv. of the lithium allyl, (tBuMe2SiGH2)2CHLi(TMEDA), in toluene afforded ( BuMezSiCH CH Zr -Cl LilTMEDA) 2 in 48% yield (Scheme 2).6 The authors concluded that reduction of MCI4 to MC13 with concomitant formation of the allyl dimer and LiCl occurs first, before reaction with further 2 equiv. of lithium allyl yields the bis(allyl)zirconium(m) complex 2. 

This volume begins with two procedures in the area of catalytic asymmetric synthesis. The first procedure describes the synthesis of (R)-2-Dl PH ENYLPHOSPHI NO-2 -METHOXY-1,1 -BINAPHTHYL (MOP), a chiral ligand that has proven very useful in palladium-catalyzed hydrosilylation of olefins and palladium-catalyzed reduction of allylic esters by formic acid. The next procedure describes the catalytic asymmetric synthesis of nitroaldols using a chiral LANTHANUM-LITHIUM-BINOL COMPLEX, illustrated by the synthesis of (2S,3S)-2-NITRO-5-PHENYL-1,3-PENTANEDIOL. 

One way in which allyl lithiums can be made by direct deprotonation is when the allyl system has a substituent not at one end but in the middle, and when the substituent is of the kind we used in chapter 7 for directing aromatic lithiation. The simplest examples are amides such as 48 that react with two molecules of BuLi to give a dilithium derivative11 often represented as an allyl complex 50b though it is probably a lithium o-complex 50a. These amide derivatives react with aldehydes and ketones to give lactones,12 e.g. 53 via adducts such as 52. 

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