Raji Chem World: December 2011

DOI: 10.1002/chem.201102984

DOI: 10.1002/chem.201103171

Graphical abstract: Synthesis of chromans via Pd-catalyzed alkene carboetherification reactions

A new method for the construction of 2-substituted and 2,2-disubstituted chromans via Pd-catalyzed carboetherification reactions between aryl/alkenyl halides and 2-(but-3-en-1-yl)phenols is described. These reactions effect formation of a C–O bond and a C–C bond to afford the chroman products in good yield, and also provide stereoselective access to tricyclic chroman derivatives.

Chem. Commun., 2012, 48, 609-611

Chien-Hong Cheng,

DOI: 10.1002/chem.201102996

Masahiro Miura,

DOI: 10.1002/anie.201106825

Aryne arylation: A new method of direct arylation is reported for aniline substrates. The reaction uses benzyne to synthesize a variety of aminobiaryls under mild conditions (see scheme), requiring no stoichiometric metalation or transition-metal catalysis. An ene mechanism is implicated, and conveys excellent functional group tolerance relative to metal-mediated processes.

DOI: 10.1002/anie.201106150

An efficient and clean method to construct C [BOND]

C bonds has been developed via the acylation reaction of acetanilides with benzylic alcohols using tert-butyl hydroperoxide (TBHP) as oxidant catalyzed by palladium acetate in the presence of triflic acid (TfOH). The acylation reactions exhibit excellent reactivities, and up to 95% yield of the corresponding aryl ketone could be obtained under the optimal conditions.

DOI: 10.1002/adsc.201100417

Lutz Ackermann,

DOI: 10.1002/adsc.201100487

Selective synthesis of functionalized naphthalenes and indenones by palladium-catalyzed cyclization of o-haloacetophenones and terminal alkynes in the presence of a secondary amine is reported. Under a nitrogen atmosphere, the palladium-catalyzed reaction of o-haloacetophenones with terminal alkynes in the presence of wet secondary aminesgenerated 1-(dialkylamino)-3-aryl/alkylnaphthalenes. When the reaction was conducted in air, 1-indenone-3-carbaldehydes were obtained. The synthesized β-arylnaphthalenes 4a–f emit light in a range from 386 to 452 nm with quantum yields from 0.10 to 0.48 in cyclohexane. The absorption and emission efficiency of 4d were finely tuned by the acidity of the environment, and this might be useful in the use of 4d as a fluorescent pH sensor.

The direct C–H alkynylation of 1,3,4-oxadiazoles with 1,1-dibromo-1-alkenes has been accomplished by using a combination of CuBr/LiOtBu in PEG-400 (a green solvent) at 80 °C. The products were formed in high yields (73–86 %) in 2 h. No additional ligand or volatile solvent was required and the conversion was ecofriendly.

DOI: 10.1002/ejoc.201101542

Graphical abstract: Direct amide formation from unactivated carboxylic acids and amines

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.

Chem. Commun., 2012, 48, 666-668

Calculating oxidation states of different metals should be pretty familiar. Here’s what you do. Take a typical compound – FeCl3, for instance. Treat every bond between the metal and a different atom as if it were an ionic bond. That means the more electronegative elements (like chlorine, say, or oxygen) bear negative charges, and the less electronegative element (such as the metal) bears the positive charge.

If the compound is neutral, the sum of the oxidation states also has to be neutral. (If the compound has a charge, you adjust the oxidation states accordingly so that their sum equals the charge).

Now here’s a fun exercise. Try applying the same rules to carbon.

It’s going to feel a little bit weird. Why? Because there are two key differences.

First, carbon is often more electronegative (2.5) than some of the atoms it’s bound to (such as H, 2.2). So what do you do in this case?
Secondly, unlike metal-metal bonds, carbon-carbon bonds are ubiquitous. So how do you deal with them?

Two answers.

In a C-H bond, the H is treated as if it has an oxidation state of +1. This means that every C-H bond will decrease the oxidation state of carbon by 1.
Any two bonds between the same atom do not affect the oxidation state (recall that the oxidation state of Cl in Cl-Cl (and that of H in H-H) is zero. So a carbon attached to 4 carbons has an oxidation state of zero.

So unlike metals, which are almost always in a positive oxidation state, the oxidation state of carbon can vary widely, from -4 (in CH4) to +4 (such as in CO2). Here are some examples.

(Don’t forget that this is called a “formalism” for a reason. The charge on the carbon isn’t really+4 or –4. But the oxidation state formalism helps us keep track of where the electrons are going, which will come in handy very soon).