Erythro selective aldol condensation using titanium enolates

Condensation of beta-diester titanium enolates with carbonyl substrates: a combined DFT and experimental investigation

The condensation of dialkyl beta-diesters with various aldehydes promoted by TiCl4 has been studied by DFT approaches and experimental methods, including NMR, IR and UV/Vis spectroscopy. Various possible reaction pathways have been investigated and their energy profiles evaluated to find out a plausible mechanism of the reaction. Theoretical results and experimental evidence point to a three-step mechanism: 1) Ti-induced formation of the enolate ion; 2) aldol reaction between the enolate ion and the aldehyde, both coordinated to titanium; and 3) intramolecular elimination that leads to a titanyl complex. The presented mechanistic hypothesis allows one to better understand the pivotal role of titanium(IV) in the reaction.

γ-Silylated α,β-unsaturated amides — Preparation by [1,5]-sigmatropic rearrangement and use as masked dienolate equivalents in carbonyl condensations

The reaction of lithium dienolates derived from N,N-dialkylsenecioamides (1a–1c) with triorganosilyl electrophiles occurs initially at the oxygen atom predominantly, and is followed by an O → C silicon migration to afford the γ-silylated senecioamides (4a–4h). The γ-silylated senecioamide Z-4a undergoes fluoride-ion-mediated condensations with aromatic aldehydes to give kinetic α-(6) and thermodynamic γ-(5) condensation product patterns comparable to lithium dienolates. The TiCl4-mediated reactions with aldehydes gives α-products (6) in a highly syn-selective manner. Possible transition-state models for the syn-selective condensations are discussed and a chair-like transition state featuring bidentate coordination to titanium (11) is proposed.

Organic reaction pathways in the nonaqueous synthesis of metal oxide nanoparticles

Nonaqueous-solution routes to metal oxide nanoparticles are a valuable alternative to the known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review we give a detailed overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape, and, in some cases, also determines the assembly behavior. We have a closer look at the following reaction systems in this order: 1) metal halides in alcohols, 2) metal alkoxides, acetates, and acetylacetonates in alcohols, 3) metal alkoxides in ketones, and 4) metal acetylacetonates in benzylamine. All these systems offer some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. In the second part we present general mechanistic principles for aqueous and nonaqueous sol-gel processes, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents. Depending on the system several mechanisms have been postulated: 1) alkyl halide elimination, 2) elimination of organic ethers, 3) ester elimination, 4) C--C bond formation between benzylic alcohols and alkoxides, 5) ketimine and aldol-like condensation reactions, 6) oxidation of metal nanoparticles, and 7) thermal decomposition methods.

Carbon−Carbon Bond-Forming Reaction of Cyclic Sulfonium Ylides Stabilized by a Carbonyl Group

Rhodium(II)-catalyzed decomposition of diazoketones 1 and 5 bearing a cyclic dithioacetal, in the presence of aldehyde and ClTi(Oi-Pr)(3), afforded both or one of the C=C-bonded products, i.e., ring-enlarged enone 2 and ring-transformed thiophenone 3, that were formed between aldehyde and intermediate bicyclosulfonium ylide. The stereochemistry of the exocyclic C=C bond in the products was exclusively Z. The sulfonium atom that transiently composed the ylide was incorporated into products, but no oxirane was formed.

Carbon−Carbon Bond Formation Reaction of Ethereal Oxonium Ylides via Metal−Enolate Intermediates

First, the carbon-carbon (C-C) bond-forming reaction of aldehydes with bicyclo[m.n.0]-1-oxonium ylides was studied as the ylide was transiently formed in the Rh(II)-catalyzed reaction of a nonenolizable diazoketone, namely, 2-(3-diazo-1,1-dimethyl-2-oxopropyl)-2-methyldioxolane (1). The reaction of 1 with benzaldehyde in the presence of ClTi(Oi-Pr)3 gave the three-carbon, ring-enlarged, and C-C-bonded product 2a (53%). Second, enolizable diazoketone 5 bearing no methyl substituents at the alpha-position was studied under similar catalytic conditions, and the ring-enlarged and C-C-bonded products 19a and 20a were also formed (87%) when titanium compound ClTi(Oi-Pr)3 or Ti(Oi-Pr)4 was used. Similar reactions of diazoketones 27, 29, and 31 bearing a cyclic acetal ring and a longer tethering chain than 5 gave C-C-bonded products 28 (74%), 30 (8%), and 32 and 33 (overall 48%), respectively, albeit 28 and 30 possessed a spiro bisacetal structure. Thus, the hitherto unclarified C-C bond formation of ethereal oxonium ylides with carbonyl electrophiles was realized with the use of an appropriate Lewis acid, for example, ClTi(Oi-Pr)3.

Rhodium-catalyzed asymmetric 1,4-addition of aryltitanium reagents generating chiral titanium enolates: isolation as silyl enol ethers

The addition of aryltitanium triisopropoxide (ArTi(OPr-i )3) to alpha,beta-unsaturated ketones proceeded with high enantioselectivity (94-99.8% ee) in the presence of 3 mol % of [Rh(OH)((S )-binap)]2 in THF at 20 degrees C to give high yields of the titanium enolates as 1,4-addition products. The titanium enolates were converted into silyl enol ethers by treatment with chlorotrimethylsilane and lithium isopropoxide.

Highly Diastereoselective Aldol Reactions with Camphor-Based Acetate Enolate Equivalents

New lithium enolates of alpha-hydroxy ketones, derived from camphor, are evaluated for asymmetric aldol reactions in the presence of lithium chloride. The diastereoselectivity of the reactions between the lithium enolate of 3 and a variety of achiral aldehydes is strongly influenced by the lithium chloride salt. In these instances, the achieved levels of asymmetric induction, typically 95:5 dr, are in the range of those attained in aldol reactions involving the lithium enolate of the methyl ketone 4, which is sterically more demanding. The resulting aldol adducts are easily transformed into beta-hydroxy carboxylic acids, ketones, and aldehydes with concomitant recovery of the camphor, the chiral controller of the process, which can be reused.