Utimoto reagent

K. Takai, K. Sakogawa, Y. Kataoka, K. Oshima, and K. Utimoto 180 PREPARATION AND REACTIONS OF ALKENYLCHROMIUM REAGENTS 2-HEXYL-5-PHENYL-1-PENTEN-3-OL... 

One of the classical solutions to overcome the low reactivity is to render the carbomagnesiation intramolecular. For example, Utimoto and coworkers reported that the reaction of ( )-6-bromo-3-methyl-l-trimethylsilyl-l-hexene (85) with magnesium produces the corresponding Grignard reagent 86, which intramolecularly adds to the vinylsilane moiety from the less hindered side affording a single stereoisomer of cycUzed product 87 (Scheme 60) °°. 

S. Sugihara, M. Utimoto, Synlett, 1998, 313 from S. Matsubara, e-Encyclopedia of Reagents for Organic Synthesis, John Wiley and Sons, Ltd., 2003... 

Scheme 5. General alkylidenation of esters to enol ethers 9 and synthesis of disubstituted ( )-alkenes 10 from aldehydes and chromium(II) reagents according to Takai and Utimoto.

Allylic acetals can be treated with CrCh to obtain the allylic chromium reagent, see K. Takai, K. Nitta and K. Utimoto, Tetrahedron Lett., 1988, 29, 5263. 

Takai, K., Tagashira, M., Kuroda, T., Oshima, K., Utimoto, K., Nozaki, H. Reactions of alkenylchromium reagents prepared from alkenyl trifluoromethanesulfonates (triflates) with chromium(ll) chloride under nickel catalysis. J. Am. Chem. Soc. 1986, 108, 6048-6050. 

Takai, K., Kataoka, Y., Okazoe, T., Utimoto, K. Stereoselective synthesis of (E)-alkenylsilanes from aldehydes with a reagent prepared by chromium(ll) reduction of trimethyl(dibromomethyl)silane. Tetrahedron Lett. 1987, 28, 1443-1446. 

Takai, K., Shinomiya, N., Kaihara, H., Yoshida, N., Moriwake, T., Utimoto, K. Transformation of aldehydes into (E)-l-alkenylboronic esters with a geminal dichromium reagent derived from a dichloromethylboronic ester and CrC. Syn/ett 1995, 963-964. 

Takai K, Tagashira M, Kuroda T, Oshima K, Utimoto K, Nozaki H (1986) Reactions of Alkenylchromium Reagents Prepared from Alkenyl Tiifluoromethanesulfonates (Triflates) with Chromium(n) Chloride under Nickel Catalysis. J Am Chem Soc 108 6048... 

Methylenation of an ester group with this system did not proceed efficiently. As shown in Scheme 5.8, Takai and Utimoto used as an example the methylenation of phenyl benzoate. In this case, the addition of TMEDA was required [15]. The yield of the corresponding vinyl ether was not satisfactory. The reagent system may not have been mild enough for the acid-sensitive product. To a limited extent, it could be made to give acid-sensitive vinyl ether derivatives more efficiently by the addition of excess TMEDA to prevent the decomposition of the product (Scheme 5.9) [16]. 

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