By hydromagnesiation

Thirty years ago it was reported that reactions of Grignard reagents with 1-alkenes, catalysed by titanium tetrachloride, lead to organomagnesium compounds, formally derived by addition of HMgX to the carbon-carbon double bond [50,51], e.g. 

The reaction has since been extended to a variety of alkenes (including dienes and styrenes) and alkynes [52, 53], and it has been established that the hydrogen atoms transferred are 3-hydrogen atoms from the Grignard reagent. 

Dicyclopentadienyltitanium dichloride is possibly superior to titanium tetrachloride as a catalyst. Nickel(n) compounds are also active, but with these catalysts concurrent addition of the organomagnesium compounds to carbon-carbon multiple bonds (see Section 4.1) causes complications. Examples of hydromagnesiation by Grignard reagents are listed in Table 3.8. As will be seen from entries in Table 3.8, the stereochemistry of addition to alkynes is syn. The regiochemistry is also usually predictable in the relevant examples in Table 3.8, one regioisomer is obtained mainly or exclusively. In other cases, however, mixtures are formed [54]. 

More recently, a related reaction has been reported, in which active forms of magnesium hydride, prepared in situ or pre-prepared, undergo addition to alkenes, catalysed by titanium or zirconium(iv) halides, to give dialkylmagnesium compounds [56, 59, 64]  

These reactions give high yields with 1-alkenes, as shown, but are less satisfactory with alkynes or non-terminal alkenes. Examples of these reactions are also given in Table 3.8, and procedures for hydromagnesation by both Grignard reagents and magnesium hydride follow. 

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The best potential precursors of the pentacyclopropylcyclopentadienyl cation 6+ have recently been prepared. The Grignard reagent 13, which was obtained by hydromagnesiation of easily accessible dicydopropylacetylene (12), gave, upon reaction with butyl formate and... 

Hydromagnesiation butenolides.1 Reaction of optically pure y-trimethylsilyl propargylic alcohols with isobutylmagnesium bromide catalyzed by Cp2TiCl2 (14,... 

The hydromagnesiation of acetylenes, catalysed by titano- and zirconocenes is well known. A regioselective reaction occurs only if one of the substituents on the triple bond is silicon or an aryl group. Studies on its mechanism were reported by Sato.2 7 Cai has applied this reaction for the synthesis of a number of polysubstituted alkenes (Scheme p).28-28 -2 ... 

HydromagnesiatUm vinylsilanes. The hydromagnesiation (10, 130-131) of 1-trimethylsilyl-1 -alkynes with isobutylmagnesium bromide followed by alkylation affords (Z)-l,2-dialkylvinylsilanes in high yield. 

Furanes. Hydromagnesiation of 3-trimethylsilyl-2-propyne-l-ol (1) (11, 163) followed by reaction with an aldehyde or ketone provides E-3-lrimethylsilyl-2-alkcne-l,4-diols (2). These are converted into 3-trimethylsilyl-2,5-dihydrofurancs (3) on dehydration... 

The asymmetric dihydroxylation followed by internal lactonization has been used by Sato and coworkers [101] in their efficient preparation of enantiomeri-cally pure butenolides and furans (Scheme 26). AD of enyne 68 using AD-mix-p, afforded diol 69 in 74% yield and 97% ee. Hydromagnesiation generated the vi-... 

As far as the role of the transition metal catalysts in the hydromagnesiation is concerned, other hydrometallation reactions such as hydroa-lumination 1108] and the recently reported hydro-zincation 1109) are catalyzed by the same class of titanium complexes, so all of these reactions can be grouped together. In fact, these reactions show quite similar applicabilities to particular unsaturated compounds and similar rcgioselcctivi-ties with unsymmetrical substrates (eqs. 3.64 and 3.65). 

Hydromagnesiation of a terminal olefin followed by the nickel-catalyzed vinylation with excess vinyl bromide provides a one-pot method of two-carbon homologation of the parent olefin as shown in eq. (3.22) [37],... 

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