Lithium alkene complexes

It is clear from these experiments that the presence of ethylene catalyses the fixation of nitrogen in lithium complexes. This assisted complexation was also observed with methyl-substituted ethylene and butadiene. It is a characteristic property of lithium-alkene complexes, as experiments performed with other lithium complexes have so far not yielded such ternary complexes. If one can easily anticipate that the fractional positive charge on the lithium in LiC2H4 and Li(C2H4)2 facilitates the coordination of N2 with, presumably, a a-donation to lithium, and possibly, to a weaker extent, p-donation from the metal, it is difficult to rationalize why LiC2H2 and LiC2H4 behave so differently with respect to nitrogen, for instance. 

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

The structure of catalyst 428 was proposed as a result of the several experiments shown in Sch. 60 and discussed below [89]. Firstly, it was observed that treatment of ALB catalyst 394 (Sch. 51) with methyllithium produced a solution from which the hexacoordinate aluminum species 434 (M = Li) could be crystallized in 43 % yield. The same compound could also be obtained from solutions prepared from 394 and nBuLi, and the sodium enolate of 425. Solid-state X-ray analysis of this compound revealed that it has the same structiu-e as the species 417 (Sch. 56) isolated by Feringa and coworkers during the preparation of ALB with excess BINOL (Sch. 55) [86]. The tris-BINOL(tris-lithium) alimunum complex 434 is not the active catalyst in the Michael addition of phosphonate 425 to cyclohexenone because the use of this material as catalyst gave the Michael adduct 426 in 28 % yield and 57 % ee which is dramatically lower than obtained by use of catalyst 428 (Sch. 59). In addition, the use of catalyst 434 (M = Li) gave the alkene product 429 in 13 % yield, a product that was not seen with catalyst 428. Additional evidence comes from the reaction between 425 and cyclopentenone with catalyst 434 (M = Li) which gives the adduct 427 in 78 % yield and 12 % ee. 

Reversal correlates with the presence of lithium ion and also with the involvement of betaine species. These two risk factors are interrelated because lithium halides rapidly cleave oxaphosphetane 31 or 32 (Scheme 8) at — 70°C resulting in the reversible formation of the betaine lithium halide complexes 40 or 41, respectively (18b). Donor solvents shift the equilibrium toward the oxaphosphetane by coordinating the lithium halides and thereby promote stereospecific decomposition to the alkenes. If the solvent is not an effective lithium coordinating agent, then 40 and 41 decompose slowly, and the risk of... 

The reaction was anticipated to proceed through an initial ligand exchange between the lithium allenyl alkoxide and the diisopropoxytitanium alkene complex, followed by stereoselective carhometallation (Scheme 10.154). 

Lithium alkyls or Grignard reagents generally reduce the alkene complex to Fp. This problem can sometimes be overcome by using LiCuR instead. 

The reaction of nonstabilized ylides with aldehydes can be induced to yield trans 3 kenes with high stereoselectivity by a procedure known as the Schlosser modification of the Wittig reaction." In this procedure, the ylide is generated as a lithium halide complex and allowed to react with an aldehyde at low temperature, presumably forming a mixture of diastereomeric betaine-lithium halide complexes. At the temperatures under which the addition is carried out, fragmentation to an alkene and triphenylphosphine oxide does not occur. This complex is then treated... 

Alkenes. —Reviews on Ziegler-Natta catalysis and the stereoregular and sequence-regular polymerization of butadiene have been published and the stereoselective oligomerizations of isoprene by lithium and palladium catalysts have been compared. Semi-empirical MO calculations suggest that Ziegler-Natta polymerization proceeds via a bis-alkene complex and a metallacyclo-pentane intermediate. ... 

Ca.ta.lysts, A small amount of quinoline promotes the formation of rigid foams (qv) from diols and unsaturated dicarboxyhc acids (100). Acrolein and methacrolein 1,4-addition polymerisation is catalysed by lithium complexes of quinoline (101). Organic bases, including quinoline, promote the dehydrogenation of unbranched alkanes to unbranched alkenes using platinum on sodium mordenite (102). The peracetic acid epoxidation of a wide range of alkenes is catalysed by 8-hydroxyquinoline (103). Hydroformylation catalysts have been improved using 2-quinolone [59-31-4] (104) (see Catalysis). 

Seven procedures descnbe preparation of important synthesis intermediates A two-step procedure gives 2-(HYDROXYMETHYL)ALLYLTRIMETH-YLSILANE, a versatile bifunctional reagent As the acetate, it can be converted to a tnmethylenemethane-palladium complex (in situ) which undergoes [3 -(- 2] annulation reactions with electron-deficient alkenes A preparation of halide-free METHYLLITHIUM is included because the presence of lithium halide in the reagent sometimes complicates the analysis and use of methyllithium Commercial samples invariably contain a full molar equivalent of bromide or iodide AZLLENE IS a fundamental compound in organic chemistry, the preparation... 

The pyrolysis of sodium chlorodinuoroacetate is still a widely used, classical method for generating difluorocarbene, especially with enol and allyl acetates [48, 49, 50, 51] (equation 21) A convenient alternative that avoids the hygroscopic salt uses methyl chlorodifluoroacetate with 2 equivalents of a lithium chlonde-hexa-methylphosphoric triamide complex at 75-80 °C in triglyme [52], Yields are excellent with electron-rich olefins but are less satisfactory with moderately nucleophilic alkenes (4-5% yields for 2-bulenes)... 

These alkene isomers are separately available (4) by treatment of threo-S-trimethylsilyloctan-4-ol, prepared by reduction of the corresponding ketone with DIBAL in pentane at —120°C, with base or acid. The preparation of 5-trimethylsilyloctan-4-one itself illustrates three general procedures the addition of alkyl lithium reagents to vinylsilanes to generate a-lithiosilanes, the preparation of complex /5-hydroxysilanes, as diastereoisomeric mixtures, and the oxidation of such compounds to /8-ketosilanes... 

The cyclopentadienyl group is another interesting ligand for immobilization. Its titanium complexes can be transformed by reduction with butyl lithium into highly active alkene hydrogenation catalysts having a TOF of about 7000 h 1 at 60 °C [85]. Similar metallocene catalysts have also been extensively studied on polymer supports, as shown in the following section. 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

The reaction (a Schlenk dimerization) between the phosphine-borane-substituted alkene nPr2P(BH3) (Me3Si)C=CH2 and elemental lithium in THF yields the complex [(pmdeta)Li Prn2P(BH3) (Me3Si)CCH2]2 123 which in the solid state has a lithium bound to the BH3 hydrogens of the ligand, and no Li-C(carbanion) contact (pmdeta = N,N,N, N",N"-pentamethyldiethylenetriamine).85... 

For acyclic allylic substrates the situation is more complex, since a larger number of reactive conformations, and hence corresponding transition states, compete. Thus, methyl cinnamyl derivatives 163 (X = OAc), upon treatment with lithium dimethylcuprate, mainly gave the Sn2 substitution product 166 (entry 1, Tab. 6.6 and Scheme 6.34) [80]. The preference for the Sn2 product is expected, since deconjugation of the alkene system is electronically unfavorable. 

Better results were obtained for the carbamate of 163 (entry 3) [75, 80). Thus, deprotonation of the carbamate 163 with a lithium base, followed by complexation with copper iodide and treatment with one equivalent of an alkyllithium, provided exclusive y-alkylation. Double bond configuration was only partially maintained, however, giving 164 and 165 in a ratio of 89 11. The formation of both alkene isomers is explained in terms of two competing transition states 167 and 168 (Scheme 6.35). Minimization of allylic strain should to some extent favor transition state 167. Employing the enantiomerically enriched carbamate (R)-163 (82% ee) as the starting material, the proposed syn-attack of the organocopper nucleophile could then be as shown. Thus, after substitution and subsequent hydrogenation, R)-2-phenylpentane (169) was obtained in 64% ee [75]. 

---