Katalysatorfreie enantiospezifische Olefinierung mithilfe von in situ generierten Organocer‐Spezies

Abstract

Wirb eschreiben die In-situ-Synthese von Triorganocer-Reagentien und deren Anwendung in katalysatorfreien Zweifel-Olefinierungen. Diese einzigartigen Cer-Spezies wurden durch neuartige Austauschreaktionen von Organohalogeniden mit n-Bu 3 Ce-Reagentien erzeugt. Durch die geeignete Elektronegativitätv on Cer konnten sowohl die Empfindlichkeit von Organolithium-Verbindungen fürfunktionelle Gruppen als auch die geringere Reaktivitätv on Organomagnesium-Verbindungen ausgeglichen werden. Austauschreaktionen wurden an Aryl-und Alkenylbromiden durchgeführt, die mit chiralen Borpinakolestern enantiospezifische Transformationen eingingen. Abschließend wurden diese neuen Organocer-Spezies eingesetzt, um selektive 1,2-Additionen an enolisierbaren und sterischg ehinderten Ketonen durchzuführen.

The Halogen–Samarium Exchange Reaction: Synthetic Applications and Kinetics

“…Theresult with the iodoester derivative 4i is particularly instructive,since such substituents were not tolerated in halogen-lanthanum exchange reactions,showing that organosamariums are better compatible with more sensitive functional groups than the corresponding organolanthanum derivatives. [11] This observation is as trong argument in favor of ad ependence of the functional group compatibility on the ionic character of the carbon-metal bond. Finally,2-iodothiophene (4j), and 3-iodopyridine (4k), and 3-iodoindole (4l)r eacted with nBu 2 SmCl·4 LiCl (1, 0.60 equiv), furnishing diheteroarylsamarium derivatives 5jl,w hich were further transformed into alcohols 6j-l in 83-85 %y ield (Scheme 3).…”

“…[7] Besides ate-metal reagents such as manganates [8] and cuprates [9] undergo an exchange, but the atom economy [10] of these reactions is moderate.Recently,w eh ave reported an ew halogen/lanthanum exchange reaction and have found that this exchange is at least 10 6 faster than the halogen/magnesium exchange.A lso, the I/La or Br/La exchange displays better functional group tolerance than the halogen/lithium exchange. [11] This led us to postulate that the rate of the halogen/metal exchange depends on the ionic character of the carbon-metal bond of the exchange reagent, and that the functional group compat-ibility also depends on this ionic character and therefore on the electronegativity [12] of the metal (Scheme 1).Forp roving this relation, we turned our attention to samarium which has an electronegativity of 1.17 compared to 1.10 for lanthanum. We therefore predicted that the halogen/ samarium exchange should be slower than the halogen/ lithium, the halogen/lanthanum, and the halogen/cerium exchanges,a nd that the functional group tolerance of the resulting arylsamarium(III) reagent should be better than that of the corresponding lanthanum(III) reagents.Moreover, organosamarium reagents already proved their utility as nucleophiles,b ut only few preparation methods of these organometallics have been reported.…”

“…Recently,w eh ave reported an ew halogen/lanthanum exchange reaction and have found that this exchange is at least 10 6 faster than the halogen/magnesium exchange.A lso, the I/La or Br/La exchange displays better functional group tolerance than the halogen/lithium exchange. [11] This led us to postulate that the rate of the halogen/metal exchange depends on the ionic character of the carbon-metal bond of the exchange reagent, and that the functional group compat-ibility also depends on this ionic character and therefore on the electronegativity [12] of the metal (Scheme 1).…”

“…Next, we turned our attention to the Br/Sm exchange involving the use of aryl bromides,w hich are less expensive than aryl iodides.Whereas nBu 2 SmCl·4 LiCl (1)was asuitable exchange reagent for I/Sm exchange reactions,n oB r/Sm exchange with 4-bromoanisole (7a)t ook place under those conditions even after an extended reaction time ( Table 1, entry 1). In contrast, the mixed species nBu 2 SmMe·5 LiCl (2, 0.70 equiv), inspired by our previous work on lanthanum, [11] reacted at À30 8 8C, leading to full conversion of 4-bromoanisole (7a)i nto the corresponding diaryl(methyl)samarium reagent 8 within 1h.S ubsequent reaction with ketone 9 provided the tertiary alcohol 10 a in 69 %y ield ( Table 1, entry 2). Interestingly,w en oticed that nBu 3 Sm·5 LiCl (3) exchanged all three of its butyl residues,w hereas the related nBu 3 La·5 LiCl was able to exchange only one butyl residue.…”

“…[16] Interestingly, whereas the Br/La exchange on 7a was completed after less than 5min at À50 8 8C, [11] the Br/Sm exchange required 15 min at À30 8 8C, indicating as omewhat slower rate than the corresponding Br/La exchange. [11] Theexchange reagent nBu 3 Sm·5 LiCl (3)was then used to explore the scope of the Br/Sm exchange.F irst, 1-bromo-2fluorobenzene (7b)u nderwent an exchange within 5min at À30 8 8C, furnishing triarylsamarium derivative 11 b,a nd subsequent trapping with aketone provided the tertiary alcohol 10 b in 85 %y ield. Interestingly, [11a] the same reaction with nBuLi led only to degradation of aryl bromide 7b.O ther fluorinated aryl bromides 7c,d furnished the corresponding Scheme 2.…”