Allyl halides, Grignard syntheses

The method is an extension of the well-known Grignard synthesis in ethers to the use of nonsolvating media, and is a development of procedures previously reported.2-6 A version of it has been employed with straight-chain primary alkyl chlorides, bromides, and iodides from C2 to Cu,5-7 and in solvents (or an excess of the halide) which permit reaction temperatures above 120°, with simple aryl halides such as chlorobenzene and 1-chloro-naphthalene. Branched-chain primary, secondary, and tertiary alkyl halides, allyl, vinyl, and benzyl halides either fail to react or give extensive side reactions. Better results are reported to be obtained in such cases with the use of catalytic quantities of a mixture of an alkoxide and an ether such as diethyl ether or tetrahydrofuran in a hydrocarbon medium, but the products are not, of course, completely unsolvated.4... 

Coupling reactions between allylic halides and alkynylcoppers have been rather heavily investigated as routes to stereochemically defined polyenyl natural products. The CuCl-catalyzed reaction of acetylenic Grignard reagents with chlorine substituted allylic chlorides has been employed in the synthesis of stereochemically pure 1,3,5-undecatrienes... 

Okude, Y., Hirano, S., Hiyama, T., Nozaki, H. Grignard-type carbonyl addition of allyl halides by means of chromous salt. A chemospecific synthesis of homoallyl alcohols. J. Am. Chem. Soc. 1977, 99, 3179-3181. 

Okude Y, Hirano S, Hiyama T, Nozaki H (1977) Grignard-Ty pt Carbonyl Addition of Allyl Halides by Means of Chromous Salt. A Chemospecific Synthesis of Homoallyl Alcohols. J Am Chem Soc 99 3179... 

Mori, K. Simple synthesis of sex pheromones of codling moth and red bollworm moth by the coupling of Grignard reagents with allylic halides. Tetrahedron 30, 3807—3810(1974). 

Pentadienyls may be considered as vinylogues of allyl complexes and accordingly parallels exist, both in their synthesis and reactivity. These are most widely prepared by transmetallation from lithium or Grignard reagents, nucleophilic attack at pentadienyl halides or via hydride abstraction from rj4-pentadiene complexes (Figure 7.13). 

Scattered reports have appeared in the literature of magnesium activation by stirring the solid under an inert atmosphere. The method is exemplified by the synthesis of the aromatic Grignard reagent shown in Figure 1. which frustrated attempts at its preparation by conventional techniques [4]. The development of this method has proved very effective in the synthesis of several allyl- and benzylmagnesium halides [5]. 

The mechanisms of reactions of benzylmagnesium halides can be either homolytic or concerted. As with allylic Grignard reagents, also reversible addition reactions with benzylic Grignard reagents and carbonyl compounds have been reported. In general, yields of normal alcohols are low, and such alcohols are obtained in much better yields by use of dibenzylcadmium [66] or benzyllithium [67]. However, in 1980, it was demonstrated that, by far, the best and quickest results for the synthesis of these normal addition reaction products are obtained by applying benzylic halides in the one-step Li-Barbier reaction under ultrasonic irradiation [68,69]. 

If neither the halides nor their metal derivatives have a stable structure, how can we expect to control their reactions Things are not quite as bad as that. The bromides and chlorides are stable at low temperatures in the absence of Lewis acid catalysts or radical generators. The Grignard reagents often react cleanly to give the less substituted double bond isomer, presumably by the type of process shown in 12. We shall be seeing methods to control transition metal allyls and allyl silanes which make these intermediates valuable reagents in synthesis, and you should finish this chapter in a confident mood without fear of fluxional allyl compounds. 

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