Multi Grignard reaction

You must make sure that you have this reaction at your hngertips, since it will appear several times thronghont the rest of yonr organic chemistry conrse. Let s practice a bit. We will begin with a few problems that are just one-step syntheses (Grignard) to make snre yon got it. Then, the last two problems in this problem set will be multi-step syntheses (using a Grignard reaction and other reactions we have seen). 

A multi-step (Grignard reaction, elimination, oxidative cleavage) procedure for chain degradation of carboxylic acids (esters) (see 1st edition). 

In 1990, Hahn and Tompkins reported an interesting multi-carbon homologation of alkyl halides by the reaction of magnesium carbenoid with a Grignard reagent (Scheme 2) . 

Of the various organometallic reactions employed in carbon-carbon bond formation, the metal acetylide, Grignard, and Reformatsky reactions have been the most prevalent in the synthesis of vitamin A (7) and carotenoids [1]. However, there has been considerable interest in the applications of transition metals and, for the preparation of symmetrical carotenoids, low valent titanium. This article is divided according to the metals used in carbon-carbon bond formation, with further subdivisions based on reaction type and/or putative reactive intermediates. For multi-step syntheses, only the steps that employ metals in carbon-carbon bond formation are discussed and, whenever possible, the reaction conditions and yields are indicated on the reaction schemes. 

Silacyclobutane monomers with aryl and alkylaryl substituents were prepared in multi-gram quantities from reaction of commercially available 1,1 -dichloro-1 -silacyclobutane and 1 -chloro-1 -methyl-1 -silacyclobutane precursors with suitable Grignard reagents. Allyl-substituted AB and AB2 type monomers were prepared from treatment of intermediate diorganochlorosilanes and organodichlorosilanes, respectively, with allylmagnesium bromide. Selected structures of the monomers used in this work are represented in Figure 2. 

The TaClj-catalysed reaction between 1-alkenes and n-alkyl Grignard reagents has resulted in the formation of two types of o-alkylmagnesiums. The proposed multi-step mechanistic pathway for this reaction involves the formation of labile / ,/ -dialkyl substituted tantalacyclopentanes as key intermediates. 

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