Stoichiometric base

NaOMe solutions, no correlation was found between reaction rates and either or stoichiometric base concentration but where the rates were successfully correlated by a linear free energy equation similar to those given above. 

A practical and efficient set of conditions were developed using a stoichiometric base catalyst, l,4-diazabicyclo[2,2,2]octane (DABCO)... 

All these samples are problematic in one way or another. Some are not obtained reproducibly and others are difficult to characterize or prepare in quantity. It is extremely important to synthesize high-spin polymers systematically and stoichiometrically, based on the strict molecular design described in the last section. 

Enantioselective deprotonations of meso substrates such as ketones or epoxides are firmly entrenched as a method in asymmetric synthesis, although the bulk of this work involves stoichiometric amounts of the chiral reagent. Nevertheless, a handful of reports have appeared detailing a catalytic approach to enantioselective deprotonation. The issue that ultimately determines whether an asymmetric deprotonation may be rendered catalytic is a balance of the stoichiometric base s ability... 

A number of workers have made progress on this front. Asami and coworkers have anchored the stoichiometric base on the solid phase to realize a catalytic desymmetrization using lithiated diamine 135. Andersson has shown that slow addition of LDA results in an improvement in enantioselectivity when using his bicyclic base 136, while Ahlberg has illustrated that a stoichiometric base such as lithiated 1,2-dimethylimidazole results in an efficient catalytic system using diamine 137. Alexakis has published a smdy involving a number of chiral ethane-and propane-diamines in the catalytic deprotonation of cyclohexene oxide. Enan-tioselectivities observed are moderate, with diamine 138 providing the desired product in 59% ee and 80% yield. ... 

The choice of a suitable initiator represents an important step in creating a well-defined polymerization system in terms of initiation efficiency and confrol over propagation. The entire system can only be designed on a stoichiometric base when a quantitative and fast initiation occurs. This is of enormous importance, because the composition of the entire polymerization mixture needs to be varied within small increments in order to control the microstructure. The catalyst needs to be carefully selected from both chemical and practical points of view. Schrock [5,10,12,109,110] and Grubbs systems [6], both highly active in the ROMP of strained functionalized olefins, can offen be used. Since fhe preparafion and in particu-... 

As shown in Schemes 13.30 and 13.32, LDA is commonly used as the stoichiometric base, and in the presence of DBU. Recent systematic screening of a variety of lithium amide bases confirmed the superior performance of LDA [64]. It was, however, also found that in the presence of DBU, w-BuLi can be used with similar efficiency [64]. Ahlberg et al. have found it is beneficial to replace the commonly used LDA by 2-(lithiomethyl)-l-methylimidazole (64, Scheme 13.33) [66], Under these conditions, 20 mol% of O Brien s base 60 (Schemes 13.28 and 13.33) afford 93% ee in the isomerization of cyclohexene oxide [66]. Similarly, 2-lithio-l-... 

Following this step there is continued dissolution, which removes whatever hyperfine particles may have resulted during sample preparation. After removing these, further dissolution breaks down the outer surface of the residual layer at the same rate that alkalis are replaced by hydrogen at the interface between fresh mineral surfaces and the residual layer. This releases all constituents to the solution. Release is now stoichiometric, based on solution chemistry and surface morphological results. Thus, the reaction is surface-controlled (Velbel, 1985). 

Brook rearrangements may be carried out with either catalytic or stoichiometric base. With catalytic base, the reaction can be considered an equilibrium between 41 and 42. The strength of the Si-0 bond (about 500-520 kJ mol-1) compared with the Si-C bond (about 310-350 kJ mol-1) means that, provided the anion 33 forms reasonably rapidly (some degree of stabilisation is required), Brook rearrangement (alkoxide formation) is favoured over retro-Brook. Organolithiums 33 may be present as intermediates in the catalytic Brook rearrangement, but their reactivity cannot be exploited under these conditions. 

An alternative disconnection of the alkoxide requires the addition of a silyllithium reagent to an enone. Addition of stoichiometric base to the alcohol 51 produces an alkoxide 52, but no evidence of Brook rearrangement to generate 53 was found on protonation of the product. However, alkoxide 52 must exist in equilibrium with some of the organolithium 53, since alkylation with a soft electrophile (Mel) produced 54.41 The equilibrium concentration of the organolithium 53 is lessened in this case by the impossibility of O-Li coordination. 

In presence of fluorides the exchange reactions are reversible. This is not the case in presence of stoichiometric bases. The stoichiometry with sodium hydroxide is such that 3 moles of base are consumed for each mole of chlorinated product (equation 78) ... 

The generation of copious amounts of inorganic salts can similarly be largely circumvented by replacing stoichiometric mineral acids, such as H2S04, and Lewis acids and stoichiometric bases, such as NaOH, KOH, with recyclable solid acids and bases, preferably in catalytic amounts (see later). 

At the early stage of Heathcock s biomimetic total syntheses of discorhabdins [108], a 5-ejco Heck cyclization was employed for the synthesis of 3,6,7-functionalized indole. As highlighted in Scheme 42, when precursor 237 was exposed to catalytic palladium acetate, tri-o-tolylphosphine, and stoichiometric base, indole 238 was smoothly produced in 89% yield. Subsequently, the total syntheses of discorhabdin C (239) and discorhabdin E (240) were accomplished using indole 238 as the common intermediate. 

Polymer-supported equivalents of the widely used organic base 4-(dimethylamino)pyridine (DMAP) were soon developed, but many of their reported applications are as a stoichiometric base. Resin 37 (Scheme 10.10), containing a polyethylene imine) matrix was the first supported system to be prepared. Those materials were more efficient catalysts than DMAP itself, under the same conditions, for the hydrolysis of p-riilropheriyl esters in aqueous solution [175, 176],... 

The animation of aryl chlorides is desirable due to their prevalence and lower costs as compared to aryl bromides and iodides. Aryl chlorides are less prone to oxidative addition to palladium(O) and therefore require more activating ligands. An early example of ArCl amination was reported by M. Beller and co-workers.52 They found a palladacycle derived from Pd(OAc)2 and (o-tol)3P, for example, trawj-di(p-acetato)-bis[o-(di-o-tolylphosphino)benzyl]dipalladium(II). This catalyst allows for the amination of electron deficient aryl chlorides (13) with piperidine in the presence of catalytic LiBr and KO/-Bu as the stoichiometric base. The high temperature required (135 °C) also gives rise to a minor amount of the meta aminated product 15, likely due to aryne formation. It is unclear from this example whether a true oxidative addition to the aryl chloride is occurring, or whether the para product 14 also results from aryne formation, as a 1 1 mixture of products 14 and 15 also results without palladacycle present. 

Chelating ferrocene phosphine L18 was reported by Hartwig to efficiently catalyze the amination of most aryl chlorides with any type of primary aliphatic amine, imine, or hydrazine at 80-100 °C with NaOf-Bu in DME. Base sensitive aryl chlorides, or those containing acidic protons, may be aminated using LiHMDS as the stoichiometric base. Impressively, catalyst loadings as low as 0.005 mol% can be used. 

Hartwig was able to show that LiHMDS could serve as both an ammonia equivalent and as the stoichiometric base for the amination reaction catalyzed by Pd(f-Bu)3P.94 The reaction conditions tolerate many aryl bromides and chlorides, but due to the size of LiHMDS, or/Ao-substituted haloarenes are not acceptable. As before, acidic workup reveals the aniline by cleavage of the N-Si bond. 

A complex reaction is one in which the reaction rates depend on the concentrations of the reacting substances and on the concentrations of the final or intermediate products of the reactions. For complex reactions the overall stoichiometry is frequently not known, so the rate cannot be related to the stoichiometry. However, the stoichiometric-based rate expression of Eq. (4.1.3) is found to be generally applicable to all reactions, although the order of the reaction v- with respect to the species i is not necessarily its stoichiometric coefficient and need not be integer or positive. 

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