Organolithium compounds alkenes

A more promising procedure for the formation of alkenes from tosylhydrazones is represented by the Shapiro reaction It differs from the Bamford-Stevens reaction by the use of an organolithium compound (e.g. methyl lithium) as a strongly basic reagent ... 

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

Compound 388 is an acylating agent for electron-deficient alkenes, in a Michael addition process. It is formed by treating molybdenum hexacarbonyl with an organolithium compound, followed by quenching the intermediate 387 with boron trifluoride (equation 104). The structure of 388 (R = Ph) can be elucidated by NMR spectroscopy. Other examples of enantioselective and diastereoselective Michael-type additions involving lithium-containing intermediates in the presence of chiral additives can be found elsewhere in the literature . 

Support-bound silanes are useful linkers and intermediates, and can be prepared by several routes. The most versatile approaches include the reaction of resin-bound organolithium compounds with chlorosilanes [36,37] and the hydrosilylation of resin-bound alkenes (Figure 4.6). Further transformations of resin-bound silanes are discussed in Section 3.11.2. 

The Shapiro Reaction, a variation on the Bamford-Stevens Reaction, is the base-induced reaction of tosylhydrazones to afford alkenes. This reaction is carried out with two equivalents of an organolithium compound. 

Polystyrene-bound silanes are usually prepared by reaction of organolithium compounds with resin-bound silyl chlorides [12, 13]. The C-Si bonds of aryl-, heteroaryl-, vinyl-, and allylsilanes are stable towards alcoholates or weak reducing agents, but can be cleaved under mild conditions by treatment with acids or fluoride to yield a hydrocarbon and a silyl ester or silyl fluoride. Several linkers of this type have been tested and have proven useful for the preparation of unfunctionalized arenes and alkenes upon cleavage from insoluble supports. 

To the best of our knowledge, X-ray structural data of complexes with simple dihapto interactions between a lithium atom and the n system of an alkene or alkyne ligand are unknown, but there is some spectroscopic evidence for weak it interactions in solutions of 3-alkenyllithium compounds from 7Li-and H-NMR data (4). Interactions of this sort are presumably important in addition (polymerization) reactions between organolithium compounds and alkenes or alkynes. 

Fig. 10.36. Preparation of an alkene by adding a Grignard reagent or an organolithium compound to a ketone and subsequent workup with an acid that is strong enough to induce an El elimination.

In the majority of cases, organolithium compounds and Grignard reagents contain polarized but covalent carbon—metal bonds. Lithioalkanes, -alkenes, and -aromatics, on the one hand, and alkyl, alkenyl, and aryl magnesium halides, on the other hand, are therefore formulated with a hyphen between the metal and the neighboring C atom. Only lithiated alkynes and alkynyl Grignard reagents are considered to be ionic—that is, species with carbon, metal bonds similar to those in LiCN or Mg(CN)2. 

As discussed later, some examples for the formation of three- and four-membered ring products have been reported from substituted alkenes that generate more stabilized organolithium compounds after cyclization. 

Taylor and Wei have also developed a versatile method for the synthesis of cyclopentanes employing readily available organolithium compounds as difunctional, conjunctive reagents. This strategy represents an anionic [3 + 2] approach to substituted cyclopentanes. The reactions of lithiated alkenes 149 with activated alkenes 150 afford cyclopentane derivatives 151 in reasonable yield and, in some cases, with excehent stereocontrol. The alkenes 150 must be added over extended times to minimize polymerization processes. The... 

As shown in Scheme 11, Hoppe and coworkers have accomplished the first enantios-elective intramolecular carbolithiation reaction of alkenes by fusion of the concepts of the intramolecular carbolithiation reaction and the asymmetric deprotonation. The efficiency of this method has been demonstrated by the extension to other substrates by these and other authors78. However, this topic has been excellently reviewed by Hoppe and Christoph in Chapter 17 of Vol. 1 of The Chemistry of Organolithium Compounds and therefore it will not be treated in this chapter. 

C. Mixed-Metal Organolithium Compounds with Unsaturated (Alkene, Arene) Uganda... 

Butyllithium is an excellent reagent for the dehydrohalogenation of 1-chloroalkenes or 1,1-dichloroal-kanes to terminal alkynes the organolithium compound must be added dropwise to avoid alkylation of the intermediate carbenoids, leading to alkenes. Three equivalents of alkyllithium are necessary to convert the 1,1-dichloroalkane to the alkyne (two equivalents from 1-chloroalkenes Scheme 45). 

The base catalyzed decomposition of arylsulfonylhydrazones of aldehydes and ketones to provide alkenes is called the Bamford-Stevens reaction. When an organolithium compound is used as the base, the reaction is termed the Shapiro reaction. The most synthetically useful protocol involves treatment of the substrate with at least two equivalents of an organolithium compound (usually MeLi or BuLi) in ether, hexane, or tetramethylenediamine. The in s/ft formed alkenyllithium is then protonated to give the alkene. The above procedure provides good yields of alkenes without side reactions and where there is a choice, the less highly substituted alkene is predominantly formed. Under these reaction conditions tosylhydrazones of a,(3-unsaturated ketones give rise to conjugated dienes. It is also possible to trap the alkenyllithium with electrophiles other than a proton. 

Table 1. Arylcyclopropanes from Alkenes and Carbenes, Generated from Arylhalomethane or Aryldihalomethane with Organolithium Compounds... 

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