Lithium hydride elimination

Starting with l,l-bis(trimethylsilyl)ethylene (5) in hexane or diethyl ether as the solvent we obtained another dimeric product, a monolithiumorganic compound 8 which was shown not to be formed by lithium hydride elimination from the 1,4-dilithiobutane derivative 6, the only product in THF as the solvent. Obviously the vinyllithium derivative 7, primarily formed in the same manner as vinyllithium from ethylene [4], in contrast to vinyllithium [4] does not add further lithium atoms but adds itself to the starting material 5 yielding 8 ... 

Dimerization as well as lithium hydride elimination can be avoided by replacing a (3 hydrogen atom in 5 by an alkyl or a phenyl group. Thus 9a as well as 9b upon the addition of lithium metal yield stable vicinal dilithiumorganic compounds 10 which as 1,2-dilithioethane derivatives interestingly neither lose lithium hydride nor add to the starting material ... 

Keywords Dilithiovinylsilanes / Lithium Hydride Elimination / Reductive Metalation / Solvent Effect / Vinyllithium Dimerization... 

In all these reactions polylithiumorganic intermediates have to be postulated. We have shown for 1,1-dilithioethanes, as well as cis- and tra 5-dilithioethylene, the ease of lithium hydride elimination through independent synthesis of these reactive intermediates [15, 16] 1,2-dilithioethane can be trapped in a small amount at -120 °C [17],... 

So we propose a general mechanism (Scheme 3) for the reaction of vinylsilanes with lithium metal, which should also allow a general access to vicinal and geminal dilithiovinylsilanes by repetitive addition of lithium metal to the C=C-double bond and subsequent elimination of lithium hydride. In order to explore this synthetic approach the reduction of a series of either a- or P-substituted vinylsilanes with lithium was examined, here the substituent R H) in 15 and 19 is introduced to prevent the last lithium hydride elimination. 

Scheme 3. Dilithiovinylsilanes 15 and 19 from vinylsilanes by repetitive lithium addition and lithium hydride elimination.

Thus, besides two additions of lithium and two consecutive lithium hydride eliminations a [ 1,4]-proton shift has occurred. In order to prove the proposed reaction mechanism all three intermediates, i.e., the vinyllithium compound (E)- A, R = Ph, and the trilithium compounds 25 and 26, were synthesized independently. 

R = SiMea.Interestingly, the vinyllithium compound is not stable under these conditions but adds to the starting material with formation of 39, a dimeric monolithium derivative (Scheme 9). The formation of 39 by lithium hydride elimination from 4 could be excluded experimentally. 

The addition of lithium metal to vinylsilanes is a suitable approach to vinyllithium compounds as the addition of lithium metal is usually followed by lithium hydride elimination, especially when performing the reaction in less polar solvents like toluene. So far, the formation of vicinal or geminal dilithiovinylsilanes by two consecutive lithium addition and lithium hydride elimination sequences is... 

The organolithium addition to pyridines or other nitrogen aromatic heterocycles is a well-established general synthetic method. It is usually used to achieve overall substitution on the ring, via lithium hydride elimination or oxidation of the dihydrointermediate [18]. 

According to recent calculations by Houk and Rondan 11c, 10.2 kcal/mol (42.7 kJ/mol) higher in energy than Ila, is the transition state for the lithium hydride elimination which was found to take place within 8 h at room temperature... 

Due to intramolecular coordination (see Sect. 2.7) 4,4-dilithio-l-butene 33 is more stable and can be prepared by a direct mercury-lithium exchange reaction although lithium hydride elimination yields a conjugated system 97 Rearrangement to a cyclopropylcarbinyl species 34, however, was not observed (see Sect. 2.7). 

Neither -butyllithium nor hexyllithium can be stored indefinitely as they eliminate, if slowly, lithium hydride to form 1-butene or 1-hexene, respectively. The activation parameters ( a 30 kcal/mol, Ig A 12.8) for the lithium hydride elimination from octyllithium in decane have been determined in the temperature range of 120-150 °C. /m-Butyllithium gives isobutene. sec-Butyllithium shows the most pronounced propensity for decomposition to lithium hydride and 1- or 2-butene and isobutene. To secure shelf-lives of a few months, alkyllithiums in (cyclo)aliphatic solvents should be stored in a deep freezer (at -15 to -25 °C). 

Lithium Iodide. Lithium iodide [10377-51 -2/, Lil, is the most difficult lithium halide to prepare and has few appHcations. Aqueous solutions of the salt can be prepared by carehil neutralization of hydroiodic acid with lithium carbonate or lithium hydroxide. Concentration of the aqueous solution leads successively to the trihydrate [7790-22-9] dihydrate [17023-25-5] and monohydrate [17023-24 ] which melt congmendy at 75, 79, and 130°C, respectively. The anhydrous salt can be obtained by carehil removal of water under vacuum, but because of the strong tendency to oxidize and eliminate iodine which occurs on heating the salt ia air, it is often prepared from reactions of lithium metal or lithium hydride with iodine ia organic solvents. The salt is extremely soluble ia water (62.6 wt % at 25°C) (59) and the solutions have extremely low vapor pressures (60). Lithium iodide is used as an electrolyte ia selected lithium battery appHcations, where it is formed in situ from reaction of lithium metal with iodine. It can also be a component of low melting molten salts and as a catalyst ia aldol condensations. 

Another route that has been recommended recently is oxidation of the As-stenyl acetate to the corresponding 7-kctone, conversion to the p-tosylhydrazone, and elimination with lithium hydride. Overall yields are in the range 20-50%.2... 

In the absence of base, vinylmercury reagents and lithium tetrachloropalladate(II) react with alkenes to form ir-allylpalladium complexes arising from addition of the vinylpalladium chloride to the alkene followed by palladium hydride elimination, a reverse readdition and rr-allyl formation (equation 10).31... 

The insertion reaction is stereospecific and syn. Moreover the /S-hydride elimination is also syn. For acyclic alkenes there is free rotation in the organopalladium intermediate so that the more stable /ra .v-alkene is formed. Electron-withdrawing groups in the alkene also increase the rate of the insertion reaction and give higher yields generally, but the reaction is limited to relatively sterically unhindered alkenes. In general, polar solvents such as DMF or acetonitrile are most commonly used. There are several common additives which aid in the reaction. These include lithium or tetraalkylammonium chlorides and bromide, silver salts, or cuprous iodide, but exactly how they function is unknown at present. 

Conjugated alkenes which are part of aromatic rings are occasionally carbolithiated, but unless they are stabilised by further conjugation the products are unstable. For example, organolithiums will add to naphthalene, but the product eliminates lithium hydride to give the... 

If di(/butylainino)silane, a colorless liquid (bp 50 °C/3 mbar) easily prepared in tetrahydrofuran from commercially available dichlorosilane and rbutylamine in a molar ratio of 1 4, is freated at 0 °C with an equimolar amount of nbutyl lithium in npentane, lithiation of one N-H group as well as cyclization combined with elimination of hydrogen occurs (Eq. 2). Reactions of this type are known from the literature [4, 5] to proceed at higher temperature or in the presence of small amounts of an alkali metal or its hydride. We therefore assume that traces of lithium hydride formed in a side reaction might have catalyzed the formation of compound 4a. 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

The previous cycloaddition reaction discussed is believed to proceed through an aldimine anion (19). Such delocalized anions can also be generated by treatment of suitable aldimines with a strong base. Subsequent cyclocondensation with a nitrile produces imidazoles [25-28]. The 2-azaallyl lithium compounds (19) are made by treatment of an azomethine with lithium diiso-propylamide in THF-hexane ( 5 1) (Scheme 4.2.9) [29. To stirred solutions of (19) one adds an equimolar amount of a nitrile in THF at —60°C. Products are obtained after hydrolysis with water (see also Section 2.3). If the original Schiff base is disubstituted on carbon, the product can only be a 3-imidazoline, but anions (19) eliminate lithium hydride to give aromatic products (20) in 37-52% yields (Scheme 4.2.9). It is, however, not possible to make delocalized anions (19) with R = alkyl, and aliphatic nitriles react only veiy reluctantly. Examples of (20) (Ar, R, R, yield listed) include Ph, Ph, Ph, 52% Ph, Ph, m-MeCeUi, 50% Ph, Ph, p-MeCeUi, 52% Ph, Ph, 3-pyridyl, 47% Ph, Ph, nPr, 1% [25]. Closely related is the synthesis of tetrasubstituted imidazoles (22) by regioselective deprotonation of (21) and subsequent reaction with an aryl nitrile. Even belter yields and reactivity are observed when one equivalent of potassium t-butoxide is added to the preformed monolithio anion of (21) (Scheme 4.2.9) [30]. 

Summary Vinylsilanes are known to react with lithium metal either to 1,2-dilithioethanes by reduction or to 1,4-dilithiobutanes by reductive dimerization. The reaction of substituted vinylsilanes with lithium metal is employed in the approach to vicinal and geminal dilithiated vinylsilanes by two consecutive additions of lithium metal and subsequent eliminations of lithium hydride. A mechanistic investigation in the reactivity of a- and (3-substituted vinylsilanes towards lithium metal discloses several new reaction pathways, whereby the choice of solvent plays an important role in apolar solvents like toluene vinyllithium compounds are obtained. Compound 14, R = Ph, which is not stable under the reaction conditions, finally affords the 1,4-dilithium compound 27. Compound 18, R = SiMes, on the other hand either adds to the starting vinylsilane (forming the monolithium compound 39) or shows an unusual dimerization to 47, which is studied in detail. 

When starting from ( )-l-o-bromophenyl-2-trimethylsilylethene 31 [30] the postulated trilithium compound 26 is obtained by addition of lithium metal to 32 in this case diethoxymethane (DEM) [31] is the most suitable solvent. Compound 26, however, eliminates lithium hydride very easily, so only a small amount (7 %) of the trimethyl derivatives (erythro and threo 33, 1 4) is obtained. As the main product 28 (66 % yield) is again observed, additionally 9 % of ( )-l-o-tolyl-2-trimethylsilylethene originating from 32 is found (Scheme 7). 

Finally the intermediate 25 is synthesized by adding lithium metal to E)- A, R = Ph, in the more polar solvent THF, here not only the [l,4]-proton shift is supressed, neither does 14 eliminate lithium hydride under the reaction conditions, probably a solvent separated ion pair has to be anticipated for the lithium in the benzylic position of 25 as the cause for this stability. 

Thermopsine can also be prepared from 13a-hydroxylupanine (LXXXI). Dehydrogenation gives an enamine (LXXXII) whose lactam group may be reduced with lithium aluminum hydride. Elimination of water with phosphorus pentoxide gives a diene (LXXXIII) which on oxidation with ferricyamide gives a good yield of thermopsine 52). 

Additionally, if the initiation reaction is more rapid an the chain propagation, a very narrow molecular weight distribution, MJM = 1 (Poisson distribution), is obtained. Typically living character is shown by the anionic polymerization of butadiene and isoprene with the lithium alkyls [77, 78], but it has been found also in butadiene polymerization with allylneodymium compounds [49] and Ziegler-Natta catalysts containing titanium iodide [77]. On the other hand, the chain growth can be terminated by a chain transfer reaction with the monomer via /0-hydride elimination, as has already been mentioned above for the allylcobalt complex-catalyzed 1,2-polymerization of butadiene. 

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