Methyllithium dimer

Earlier, Brown 1J 4> determined the activation energy for the dissociation of tetra-meric to dimeric methyllithium to be EA = 47 + 4 kJ/mol. The two values are in good agreement. 

Generating 6 from 6,6-dibromobicydo[3.1.0]hexane (35) with methyllithium in the presence of styrene, Moore and Moser [55] observed the first [2 + 2]-cycloaddi-tions of 6. Bottini et al. extended the variety of compounds able to trap 6 to 1,3-cyclo-hexadiene [54, 60], furan, 2-methyl furan, 1,3-cyclopentadiene and methyl-substituted 1,3-butadienes [54], In all these reactions, the dimer 38 of 6 is a byproduct or, as in the case of the less reactive trapping agents, even the main product. Hence it is advisable to use a reaction partner of 6, if it is a liquid, as the solvent. 

The cycloadditions of 1-substituted 1,2-cyclohexadienes and among them their dimerization are of interest because of the position selectivity. Does the reaction occur at the substituted or the unsubstituted ethylene subunit For that question to be answered, 1-methyl- (74), 1-phenyl- (75), 1-cyclopropyl- (76), l-(3-phenylpropyl)-(77) and l-trimethylsilyl-l,2-cyclohexadiene (79) were generated from the corresponding 1-substituted 6,6-dibromobicyclo[3.1.0]hexanes with methyllithium. Several of these dibromides are thermolabile, which particularly applies to the phenyl (93) [76] and the cydopropyl derivative [70], In those cases, it is advisable or necessary to prepare the dibromide in situ, that is, the dibromocarbene is liberated from tetrabro-momethane with methyllithium at -60 °C in the presence of the respective cyclopen-tene. Without workup, from the thus formed 6,6-dibromobicyclo[3.1.0]hexane, the 1,2-cyclohexadiene is then generated by addition of methyllithium at -30°C. 

In the solid state NMR study, uncomplexed phenyllithium, assumed to be a tetramer, as well as the TMEDA complexed dimer and the PMDTA complexed monomer were investigated. Both Li and Li isotopes were used in the preparations. The C spectra of the complexes are presented in Figure 12. It is evident that the substitution of Li with Li has profound effects on the Unewidths, especially of the ipso-carbon at ca 180 ppm in the aggregated uncomplexed system (Figure 12a and 12b, respectively). This is in accordance with the previously mentioned study of methyllithium. However, even the other positions are affected by the dipolar couplings to the four quadrupolar lithium cations, but to a lesser extent due to the larger C-Li distances. 

Trialkylated borazines R3B3N3H3 are deprotonated by organolithium reagents, but other reaction pathways also occur.For example, the reaction of Me3B3N3H3 with one equivalent of methyllithium produces the solvated monolithium derivative [(Me3B3N3H2)Li(OEt2)]2 (9.9), which is dimeric in the solid state. The formation of di- or trilithiated derivatives. 

In another series of experiments involving tricyclic dibromocyclopropanes Warner and co-workers have studied the behavior of a propellane derivative towards methyllithium either in the presence or absence of trapping agents. Whereas in the former case a dimer is produced, with diphenylisobenzofuran (DPIBF) two adducts are obtained in a 2 1 ratio in 24% yield [85]. 

Perchlorosilylalkanes will cyclize on catalyzed pyrolysis (Scheme 106) (64CB1111) or in the presence of methyllithium (Scheme 107) (75MI12001). f-Butyllithium will add to vinylchlorodimethylsilane, and the intermediate then cyclizes through what is believed to be a silene intermediate by a head-to-tail dimerization, as is typical of the less hindered silenes (Scheme 108) (77JA2013). 

Various bimolecular assemblies that have been proposed for the transition state are shown in Scheme 13 (14, 19a, 20g). Bicyclic transition state A involves transfer of bridging alkyl group (R) to the terminally located aldehyde, while transition structure B involves reaction between terminal R and bridging aldehyde. The reaction may proceed via mono-cyclic, boat-like six-membered transition state C. Transition structures of types B and C were originally proposed for the reactions of orga-noaluminum compounds and carbonyl substrates (26, 27). Ab initio calculations suggest that methyllithium dimer reacts with formaldehyde through a bicyclic transition state related to A (28). The dinuclear Zn... 

Ab initio calculation of reaction of methyllithium dimer and formaldehyde ... 

More recently the methyl analogue of Re2Clg was prepared from monomeric rhenium pentachloride, as well as from the rhenium benzoate dimer, Re2(02CC6H5)4Cl2, which has the quadruple bond already established (80). Addition of excess methyllithium to ReCls in diethyl ether at —78°C followed by low-temperature isolation of the resultant red crystals produced Li2Re2(CH3)8 2(C2H5)20 in a 16% yield. 

Organometallic chromium dimers have been prepared from CrG2, CrCl3(THF)3, and CrBr2(THF)2 in different reactions. Allyl Grignard reagents form Cr2(allyl)4 (254), while methyllithium in ether produces... 

Cyclic allenes have been obtained in high yields, as illustrated by the synthesis of 1,2-cyclononadiene from the dibromocarbene adduct of the readily available cyclooctene (equation 51).138 The smallest stable cyclic allene known to date is (14) it was prepared from the dibromocyclopropane (13) in high yield.139 A small amount of the tricyclic compound (15) was also obtained (equation 52). The cyclic allene (14) did not undergo dimerization even on prolonged standing at ambient temperatures. In contrast, the unsubstituted analog was detected only at -60 °C by H NMR. It should also be noted that cyclohexa-1,2-diene was generated by the reaction of methyllithium on dibromobicyclo[3.1.0]hexane and trapped as the Diels-Alder adduct.160... 

Transition structures for the lithium-bromine exchange reaction of 1,1-dibromo-alkenes with methyllithium have been located by both the B3LYP and the MP2 levels of theory with the 6-31+G basis set.67 The reaction with methyllithium dimer gave similar results with lower activation energies. These calculations predict both the kinetic and the thermodynamic stereoselectivity correctly. It has been found that predominantly the sterically more constrained bromine atom of 1,1-dibromoalkenes reacted with alkyllithium (dimer) in the kinetic condition. 

Dimethyl-1-cyclohexyl-1-trimethylsiloxy-ethene (191) gives by means of methyllithium/ethylbromide in dimethoxyethane (DME) 1-cyclohexyl-2,2-dimethyl-1-butanone (297)133 Dimerizations occur when substances such as 1-trimethylsiloxy-styrene (192) or 1-trimethylsiloxy-l-cyclopentene (195) are treated with silver oxide/ DMSO to afford l,5-diphenyl-2,5-butadione (198) and 2,2 -dicyclopentanonyl (199)l34 respectively. Under the catalytic influence of Cu2+ ions, 192 plus ben-zenesulfonyl chloride yield phenyl-(l-phenyl-l-ethanone-2-yl)sulfone (200)l3 ... 

Allenes are activated by a diphenylphosphine oxide substituent towards nucleophilic substitution at the j3-carbon atom. Lithium dimethyl-cuprate adds quickly to the 1,2-bond to give, on hydrolysis, the olefin in 16-84% yield, according to the nature of the substituents (76). Optimum conditions were not reported. The intermediate a-copper compound resulting from the addition can be dimerized or reacted with methyl iodide [Eq. (106)]. Similar reactions involving methyllithium are complicated. 

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