Dimethylzinc, transmetalation

As Scheme 16 shows, vinyl(methyl)zinc compounds 17 were obtained from the hydrozirconation of terminal alkynes with Cp2ZrHGl, followed by treatment with dimethylzinc. The initially formed vinyl zirconium complex undergoes rapid transmetallation with ZnMe2 to yield the product.44... 

At this point, the other ligands were re-examined with t-butylethyne, but no improvement could be made. Clearly, the greater steric bulk of the zinc species resulting from t-butylethyne substantially diminished the stereoselection of the catalyst. In a final attempt to decrease the steric demand of the zinc species, dimethylzinc (3 equiv) was employed as the transmetalation reagent, which should have resulted in formation of the slightly less demanding zinc species (Table 2.1.3.1). 

When the ligand system consists of chlorine atoms, monoalkyl- and monoaryl-titanium(IV) compounds are readily made from TiCU by a chlorine atom replacement. The parent compound in this series, MeTiCb, is made conveniently from the reaction of TiCU and dimethylzinc without the use of ethereal solvents, the preparation being carried out in n-pentane or dichloromethane. Other methylating species can be used, such as MeMgBr, MeLi and MeAlCh. When MeLi is used in diethyl ether, an equilibrium takes place between different complexation states of Ti in the resulting MeTiCU with the solvent (equation 8). Transmetallation from organotin compounds like (15 equation 9) has also been utilized to make trichlorotitanium compounds. ... 

The allenylpalladium 27, formed from the carbonate 26, undergoes 5-exo cycliza-tion to give 28 and its 3-exo cyclization, without f-E. elimination, generates the bicyclo[3.1.0]hexane system 29, which has no -hydrogen to be eliminated. Thus transmetallation of 29 with the tin reagent 30, followed by reductive elimination affords the azabicyclo[3.1.0]hexane 31 diastereoselectively [4], The synthesis of (—)-a-thujone was achieved by diastereoselective domino cyclization of the chiral carbonate 32, and trapping with dimethylzinc to give 33 as a key reaction [5],... 

Wipf and coworkers have used the Zr-Zn transmetalation, aldehyde addition methodology for the rapid, stereocontrolled preparation of a//-(E)-poly-enes [82] in the total syntheses of (+)-curacin A [83] and ( )-nisamycin [84]. Synthesis of the eastern side chain of ( )-nisamycin was accomplished in two steps from alkyne 79 (Scheme 17). Hydrozirconation of 79 followed by addition to cyclohexylcarboxaldehyde in the presence of dimethylzinc afforded the allylic alcohol 80. Elimination of the newly formed hydroxyl group was accomplished via the corresponding trifluoroacetate. Addition of alkenylzirconocenes... 

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