Alkoxide, elimination

The pH of the solution is of overwhelming importance in determining the course of these hydrolyses. In basic solution, oxygen elimination is dominant. This is because the unprotonated nitrogen substituent is a very poor leaving group and is also more effective at facilitating the alkoxide elimination by electron donation ... 

Scheme 7 Enantioselective coupling, f>-alkoxide elimination sequence...

Scheme 8 Diene cyclization followed by 3-alkoxide elimination...

In a reaction similar to the (>-alkoxide elimination reactions seen with zir-conocenes, catalytic Rh(OH)(cod)2 and 2 eq. of arylboronic acids gave cyclic products 165 from enynes 166 (Scheme 35) [100]. In this reaction, transmet-allation of Rh - OR with B - Ph gave Rh - Ph species 167, which inserted into the alkyne, cyclized to 168, and finally underwent [>-alkoxidc elimination to provide Rh-OCH3. This reaction is limited to the formation of five-membered rings, but it can also undergo cascade type reactions of enediynes to give multicyclic products [100]. 

The above intramolecular diene cyclizations are likely to proceed through a similar set of reactions as shown in Scheme 6.2 for the intermolecular variants. Thus, as depicted in Scheme 6.6, formation of the zirconacyclopropane at the less hindered terminal alkene (—> ii), generation of the tricyclic intermediate iii, Zr—Mg exchange through the intermediacy of zirconate iy and 3-H abstraction and Mg alkoxide elimination in v may lead to the formation of the observed product. Additional kinetic and mechanistic studies are required before a more detailed hypothesis can be put forward. 

The syn addition of the adducts suggests a mechanism different from that observed in rhodium-catalyzed alcoholysis and aminolysis reactions. Mechanistic investigations from the Tautens laboratory have revealed that the most likely mechanism involves an enantioselective carbopalladation followed by a (3-alkoxide elimination to afford the ring-opened product." ... 

To avoid the formation of ketenes by alkoxide elimination, ester enolates are often prepared at low temperatures. If unreactive alkyl halides are used, the addition of BU4NI to the reaction mixture can be beneficial [134]. Examples of the radical-mediated a-alkylation of support-bound a-haloesters are given in Table 5.4. Further methods for C-alkylating esters on insoluble supports include the Ireland-Claisen rearrangement of O-allyl ketene acetals (Entry 6, Table 13.16). Malonic esters and similar strongly C,H-acidic compounds have been C-alkylated with Merrifield resin [237,238]. 

Alkyllithium reagents attack the unsubstituted carbon of terminal epoxides. In the presence of excess amide base, the intermediate alkoxides eliminate to form alkenes (Scheme 10) <2004JA12250>. 

A / -elimination reaction also occurs with linear and cyclic glycol mono-alkyl ether radicals. In this case the nucleofugal leaving group is the alkoxy function, RO, as shown in Scheme 7 for base catalysis and a cyclic system in which alkoxide elimination leads to ring-opening [63] ... 

The transient zirconocene butene complex, 105, has proved to be useful in a number of organic transformations. For example, butene substitution of zirconocene alkene complexes with alkoxy-substituted olefins results in /3-alkoxide elimination to furnish the zirconocene alkoxy compounds (R = Me, 123 R = Bnz, 124) (Scheme 16).50,51 Addition of propargyl alcohols to the zirconocene butene complex, 105, affords homoallylic alcohols. These reactions are of limited utility owing to the lack of stereoselectivity or formation of multiple products. Positioning the alkoxide functional group further down the hydrocarbyl chain allows synthesis of cyclopropanes, though mixtures of the carbocycle and alkene products are obtained in some cases (Scheme 16).52... 

Detailed studies from these laboratories shed light on the mechanistic intricacies of asymmetric catalytic carbomagnesations, allowing for an understanding of the above trends in regio- and stereoselectivity [6]. Importantly, our mechanistic studies indicate that there is no preference for the formation of either the anti or the syn (EBTHI)Zr-olefin isomers (e.g.,8 anti vs 8 syn) it is only that one metallocene-alkene diastereomer (syn) is more reactive. Our mechanistic studies also indicate that zirconacyclopentane intermediates (i in Scheme 3) do not spontaneously eliminate to the derived zirconocene-alkoxide Zr-Mg ligand exchange is likely a prerequisite for the alkoxide elimination and formation of the terminal alkene. 

The addition of carbon nucleophiles to the carbene carbon atom generates a new C-C bond. Organolithium reagents add to alkoxycarbene complexes to give alkyl metalates which undergo alkoxide elimination upon addition of acid or exposure to silica gel and thus may serve as precursors for non-heteroatom-stabilized diarylcarbene complexes (Scheme 1). [14b]... 

Organozirconium compounds are attractive intermediates in an impressive array of synthetic methodology and have received much attention in recent years [14], Among various possibilities for the generation of organozirconium reagents, low-valent zirconium-induced (3-alkoxide elimination reaction is a particularly attractive protocol. 

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