Alkylation through organometallics

Hydrocarbons with suitably acidic (allylic, benzylic, propargylic, acetylenic) hydrogens can be transformed to organometallic derivatives, which then can react with alkylating agents to yield alkylated products. 

Alkali metal alkyls, particularly n-butyl lithium, are the most frequently used reagents to form metallated intermediates.246 247 In certain cases (di- and triphenyl-methane, acetylene and 1-alkynes, cyclopentadiene) alkali metals can be directly applied. Grignard reagents are used to form magnesium acetylides and cyclopenta-dienyl complexes.248 Organolithium compounds with a bulky alkoxide, most notably M-BuLi-ferf-BuOK in THF/hexane mixture, known as the Lochmann-Schlosser reagent or LICKOR superbase, are more active and versatile reagents.249-252 

Taking advantage of this favorable equilibrium, 2-methyl-2-heptene was transformed to (Z)-3-methyl-3-octene with good selectivity 256 

Alkylation of isoprene at the methyl group was achieved through the reaction of the corresponding metal derivative (fert-BuOK-lithium tetramethylpiperidine, THF) with 1-bromoheptane (60% yield).257 Regioselective monoalkylation of isoprope-nylacetylene can be performed in high yields through the dilithium derivative 258 

Alkylation of aromatics at benzylic positions can also be performed. Me3SiCH2K is a highly selective reagent for this process 253 

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Derivatives of 9-BBN. Like other boron hydrides, a variety of proton sources (ROH, RCO2H, RSO3H, HX (X = Cl, Br, OH, SH, 02P(0H)2, NHR)) as well as boron halides can be effectively employed to prepare useful derivatives from 9-BBN-H. The synthetic value of B-MeO-9-BBN lies principally in the preparation of B-alkyl derivatives through organometallic reagents as described above. Asa byproduct in other processes, it is also easily converted to 9-BBN-H with BMS (eq 17). ... 

The first hint of organometallic rare earth (RE) chemistry to appear in the literature was the attempted synthesis of trimethyl cerium which appeared as a footnote in a paper in 1902 concerned with the reactivity of metallic cerium and lanthanum toward hydrogen and ethylene. Following subsequent attempts reported in 1935 to prepare RE alkyls through the exposure of a lanthamun mirror to methyl radicals, triethyl scandiiun and yttriiun were claimed in 1938. However, this was before the advent of the seminal... 

As described in Section 2.3.2, vinylaziridines are versatile intermediates for the stereoselective synthesis of (E)-alkene dipeptide isosteres. One of the simplest methods for the synthesis of alkene isosteres such as 242 and 243 via aziridine derivatives of type 240 and 241 (Scheme 2.59) involves the use of chiral anti- and syn-amino alcohols 238 and 239, synthesizable in turn from various chiral amino aldehydes 237. However, when a chiral N-protected amino aldehyde derived from a natural ot-amino acid is treated with an organometallic reagent such as vinylmag-nesium bromide, a mixture of anti- and syn-amino alcohols 238 and 239 is always obtained. Highly stereoselective syntheses of either anti- or syn-amino alcohols 238 or 239, and hence 2,3-trans- or 2,3-as-3-alkyl-2-vinylaziridines 240 or 241, from readily available amino aldehydes 237 had thus hitherto been difficult. Ibuka and coworkers overcame this difficulty by developing an extremely useful epimerization of vinylaziridines. Palladium(0)-catalyzed reactions of 2,3-trons-2-vinylaziri-dines 240 afforded the thermodynamically more stable 2,3-cis isomers 241 predominantly over 240 (241 240 >94 6) through 7i-allylpalladium intermediates, in accordance with ab initio calculations [29]. This epimerization allowed a highly stereoselective synthesis of (E) -alkene dipeptide isosteres 243 with the desired L,L-... 

Other bonds that merit attention are those connecting C(7) through C(ll). These could be formed by one of the many methods for the synthesis of ketones. Bond disconnections at carbonyl centers can involve the 0=C-C(a) (acylation, organometallic addition), the C(a)-C((3) bond (enolate alkylation, aldol addition), or C((3)-C(7) bond (conjugate addition to enone). 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

These reactions are typically sketched through an organometallic complex which for Ti is TiX4 with X a ligand species. We assume that two of the X ligands are tightly bound such as with Cl, and that the alkyl R and the olefin = can be bound on the other two sites, as shown in Figure 1 1-7. 

Much effort has been devoted to activate hydrocarbons, particularly saturated compounds, through the formation of organometallic compounds and transformation of the latter to substituted derivatives. A number of transition-metal complexes have been found to insert into carbon-hydrogen bonds leading to stable alkyl metal hydrides ... 

Imidothioesters are easily obtained through 5-alkylation of thioamides, and we have noted (see Section 2.8.3) a general preparation of such compounds from isothiocyanates via addition of an organometallic and 5-alkylation. 

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