Hydromagnesiation of Alkynes

Conlrarily, Grignard reagent-mediated hydromagnesiation of acetylenes in the presence of 

Aromatic acetylenes participated in the reaction without any complication to show as high stereoselectivity as do dialkylacetylenes. I henyltmelhyDaceiylene shows a rcgioselcctivitv of synthetically acceptable level (ca 9 1) 110] which is further enhanced to an excellent degree (96 4) hy the substitution of a silyl group teq. 3.44) l().54. However, diphenylaectylene no longer afforded the expected ra-produet cleanly as shown in eq. (.3.45) 8.  

I-Silyl-1-alkyncs show as high regioseleelivity ( 95 5) as the stereoselectivity ( 96 4) exemplified in eq. (3.46) 1571. Table 3.6 surveys the reactions of these suhstrales. Apparently, the silyl group plays an important role in determining 

Tabic 3.6. Hydrotnagncsiaiion of (irimelhylsilyl)alkynes wilh Grignard rcagcnis 

Other transition metal-mediated hydromagnesi-ations involve a nickel-catalyzed reaction [9,I(X). However, except for the reaction of eq. (3.56) ]9], further synthetic application of this nickel-catalyzed Grignard exchange reaction has not been examined in more detail. 

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Hydromagnesiation of 1-propynylic alcohols. The hydromagnesiation of alkynes (10,130) has been extended to propynylic alcohols (equation I), which provides a new route to terpenoids.1... 

ON THE MECHANISM OF Cp2TiCl2-CATALYZED HYDROMAGNESIATION OF ALKYNES WITH GRIGNARI)... 

On the Mechanism of CpsTiCli-catalyzed Hydromagnesiation of Alkynes with Grignard Reagents... 

Hydromagnesiation of Alkenes and Alkynes Eiimie Salo and Hirokazu Vrahe... 

Grignard Reagents New Developments Table 3.6. Hydromagnesiation of (irimethylsilyl)alkynes with Grignard reagents... 

Dicyclopentadienyltitanium dichloride is possibly superior to titanium tetrachloride as a catalyst. Nickel(ll) 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]. 

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. 

Hydrotitanation of 1-silyl- and I-stannyl-1-alkynes leads to P-silyl (or p-stannyOalkenyltitanates, thus showing opposite regioselectivity to other related processes (hydroboration, hydroalumination, hydromagnesiation, hydrotitanation, hydrozincation, hydrozirconation).Well-defined alkenes are obtained on further reaction of the alkenylti-tanates, for example, with allylic carbonates an 8 2 displacement occurs to afford 1,4-di-... 

Generation of Optically Active a-Trimethylsilyl-butenolides. Sato and coworkers published a highly efficient method to prepare enantiomericahy pure a,/8-unsaturated butenolides from enantioenriched 4-TMS-3-but)m-2-ol through a Ti-catalyzed hydromagnesiation reaction of the alkyne, car-boxylation using CO2, and a subsequent lactonization (eq 3). This approach was later employed elegantly by Negishi in the synthesis of epolactaene. ... 

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