Herstellung von festen, substituierten Allylzinkreagentien und deren Reaktionen mit Elektrophilen

Abstract

Die Reaktion von verschiedenen allylischen Chloriden und Bromiden mit Zinkstaub in der Gegenwart von Lithiumchlorid und Magnesiumpivalat (Mg(OCOtBu) 2 )i n THF führt zu allylischen Zinkreagentien, welchen achE ntfernen des Lçsungsmittels als feste Zinkreagentien erhalten werden, die eine exzellente thermische Stabilität aufweisen. Diese allylischen Reagentien kçnnen sowohl in Pd-katalysierten Kreuzkupplungen mit PEPPSI-IPent, als auch in regiound stereoselektiven Additionen an aromatische Ketone und Aldehyde verwendet werden. Acylierungen mit verschiedenen Säurechloriden führen regioselektiv zu den entsprechenden homoallylischen Ketonen, wobei die neue C-C-Bindung immer an der sterischstärker gehinderten Seite gebildet wird.

Regioselective Transition‐Metal‐Free Allyl–Allyl Cross‐Couplings

“…[18] In line with this rationalization we find that hetero dimers of type 8 represent the most stable structures in solution;t his is indicated by the significant exergonicity of the exchange reaction of (LiCl) 2 (sol) 4 with (RZnBr) 2 (sol) 2 (DG sol (THF) = À14.4 kJ mol À1 , DG sol (DMPU) = À27.1 kJ mol À1 )a tt he B2PLYP-D3(FC)/def2-TZVPP level [19] including implicit SMD/B3LYP/6-31G(d) solvation. [20] Indeed, 1 Ha nd 13 CNMR shifts in THF obtained during the LiCl-mediated oxidative addition of zinc to the carbon-bromine bond of prenyl bromide (2b)c orrelate quite well with theoretically calculated chemical shifts of lithium zinc dimer 8-THF at the mPW1K/IGLO-III level of theory [21] (see the Supporting Information). Substituted allylic organometallics often show high diastereoselectivity in reactions with electrophiles due to their rather ordered cyclic or acyclictransition states.…”

“…[12] Recently,wehave described convenient mild preparations of functionalized allylic zinc reagents and demonstrated their utility in synthesis. [13] In contrast to most reactive allylic organometallics,t hese allylic zinc reagents tolerate various functional groups.T herefore,w ee nvisioned that the allylallyl cross-coupling between such functionalized allylic zinc reagents and substituted allylic halides may provide access to ab road range of functionalized 1,5-dienes of type 3.H erein, we report ah ighly regioselective head-to-tail cross-coupling leading to products of type g,a'-3 and tolerating sensitive functional groups such as esters and nitriles.…”