Amongst the many ways of constructing the amide bond, there has been a growing interest in the use of metal-catalysed methods for preparing this important functional group. In this
tutorial review
, ...highlights of the recent literature have been presented covering the key areas where metal catalysts have been used in amide bond formation. Acids and esters have been used in coupling reactions with amines, but aldehydes and alcohols have also been used in oxidative couplings. The use of nitriles and oximes as starting materials for amide formation are also emerging areas of interest. The use of carbon monoxide in the transition metal catalysed coupling of amines has led to a powerful methodology for amide bond formation and this is complemented by the addition of an aryl or alkenyl group to an amide typically using palladium or copper catalysts.
Amides can be prepared from a wide range of precursors with the aid of metal catalysts.
Over the last ten years there has been a huge increase in development and applications of organocatalysis in which the catalyst acts as a nucleophile. Amidines and guanidines are often only thought ...of as strong organic bases however, a number of small molecules containing basic functional groups have been shown to act as efficient nucleophilic catalysts. This
tutorial review
highlights the use of amidine, guanidine, and related isothiourea catalysts in organic synthesis, as well as the evidence for the nucleophilic nature of these catalysts. The most common application of these catalysts to date has been in acyl transfer reactions, although the application of these catalysts towards other reactions is an increasing area of interest. In this respect, amidine and guanidine derived catalysts have been shown to be effective in catalysing aldol reactions, MoritaBaylisHillman reactions, conjugate additions, carbonylations, methylations, silylations, and brominations.
This tutorial review highlights the development and applications of amidine, isothiourea, and guanidine based nucleophilic catalysts for a range of reactions.
The Give and Take of Alcohol Activation Watson, Andrew J. A.; Williams, Jonathan M. J.
Science (American Association for the Advancement of Science),
08/2010, Letnik:
329, Številka:
5992
Journal Article
Recenzirano
Catalysts make alcohols more reactive by taking away hydrogen to create carbonyl compounds and then returning the hydrogen to the final products.
Alcohols are relatively common starting materials for ...chemical reactions, even though they are quite unreactive. For example, reactions that would substitute another functional group (a nucleophile) for OH often fail because the hydroxide group (HO
−
) is difficult to displace—it is a poor leaving group. Alcohols are usually activated by turning the hydroxide into a better leaving group, either by protonating the alcohol or by converting it into a sulfonate or halide. However, both of these activation methods have some disadvantages (
1
). The acidic environment required for protonating the alcohol also protonates and deactivates the incoming nucleophile, especially amines. Conversion of the alcohol into a sulfonate or halide can lead to toxicity problems; many alkyl halides and alkyl sulfonates are mutagenic.
Herein we report the iron-catalyzed β-C(sp3)-methylation of primary alcohols using methanol as a C1 building block. This borrowing hydrogen approach employs a well-defined bench-stable ...(cyclopentadienone)iron(0) carbonyl complex as precatalyst (5 mol %) and enables a diverse selection of substituted 2-arylethanols to undergo β-C(sp3)-methylation in good isolated yields (24 examples, 65% average yield).
The reactivity of alcohols can be enhanced by the temporary removal of hydrogen using a transition metal catalyst to generate an intermediate aldehyde or ketone. The so-formed carbonyl compound has a ...greater reactivity towards nucleophilic addition accommodating the in situ formation of imines or alkenes. The return of hydrogen from the catalyst leads to the formation of new C-N and C-C bonds, often with water as the only reaction by-product.
The direct coupling of unactivated carboxylic acids with amines can be performed in toluene 110 °C in the absence of catalyst. The use of simple zirconium catalysts at 5 mol% loading gave amide ...formation in as little as 4 h.
A general iron-catalyzed methylation has been developed using methanol as a C1 building block. This borrowing hydrogen approach employs a Knölker-type (cyclopentadienone)iron carbonyl complex as ...catalyst (2 mol %) and exhibits a broad reaction scope. A variety of ketones, indoles, oxindoles, amines, and sulfonamides undergo mono- or dimethylation in excellent isolated yields (>60 examples, 79% average yield).
We report a cheap and simple method for the acetylation of a variety of amines using catalytic acetic acid and either ethyl acetate or butyl acetate as the acyl source. Catalyst loadings as low as 10 ...mol% afforded acetamide products in excellent yields at temperatures ranging from 80-120 °C. The methodology can also be successfully applied for the synthesis of a broad range of other amides, including the formation of formamides at 20 °C.
Protoplanetary Disks and Their Evolution Williams, Jonathan P; Cieza, Lucas A
Annual review of astronomy and astrophysics,
01/2011, Letnik:
49, Številka:
1
Journal Article
Recenzirano
Flattened, rotating disks of cool dust and gas extending for tens to hundreds of astronomical units are found around almost all low-mass stars shortly after their birth. These disks generally persist ...for several million years, during which time some material accretes onto the star, some is lost through outflows and photoevaporation, and some condenses into centimeter- and larger-sized bodies or planetesimals. Through observations mainly at IR through millimeter wavelengths, we can determine how common disks are at different ages; measure basic properties including mass, size, structure, and composition; and follow their varied evolutionary pathways. In this way, we see the first steps toward exoplanet formation and learn about the origins of the Solar System. This review addresses observations of the outer parts, beyond 1 AU, of protoplanetary disks with a focus on recent IR and (sub)millimeter results and an eye to the promise of new facilities in the immediate future.
Metal‐free catalysis: A method for the transamidation of primary amides with primary or secondary amines provides access to secondary and tertiary amides, by utilizing catalytic quantities of ...hydroxylamine hydrochloride to activate the chemically robust primary amide group (see scheme). A mechanism of primary amide activation through a hydrogen‐bonding complex is proposed.
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