Protonation of 2H‐Azaphosphirene Complexes: P?N Bond Activation and Ring‐Expansion Reactions

P? N bond activation of 2H‐azaphosphirene complexes 1 and 2 by using triflic acid led to ring expansion in the presence of nitriles. In the absence of nitriles, the reaction surprisingly afforded two haptomeric N‐protonated 1‐aza‐3‐phospha‐butadiene complexes in the case of complex 1 , whereas the N‐protonated 2H‐azaphosphirene complex [H‐ 2 ]⁺ was characterized by NMR spectroscopy.

     

Terminal Phosphanido Rhodium Complexes Mediating Catalytic P?P and P?C Bond Formation

Complexes with terminal phosphanido (M PR₂) functionalities are believed to be crucial intermediates in new catalytic processes involving the formation of P P and P C bonds. We showcase here the isolation and characterization of mononuclear phosphanide rhodium complexes ([RhTp(H)(PR₂)L]) that result from the oxidative addition of secondary phosphanes, a reaction that was also explored computationally. These compounds are active catalysts for the dehydrocoupling of PHPh₂ to Ph₂P PPh₂. The hydrophosphination of dimethyl maleate and the unactivated olefin ethylene is also reported. Reliable evidence for the prominent role of mononuclear phosphanido rhodium species in these reactions is also provided.     

Decomposition of 1,1‐dimethyl‐1‐silacyclobutane on a tungsten filament—evidence of both ring C?C and ring Si?C bond cleavages

The decomposition of 1,1‐dimethyl‐1‐silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum ultraviolet laser single photon ionization time‐of‐flight mass spectrometry. It is found that the decomposition of DMSCB on the W filament to form ethene and 1,1‐dimethylsilene is a catalytic process. In addition, two other decomposition channels exist to produce methyl radicals via the Si? CH₃ bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage of a ring C? C bond and a ring Si? C bond, respectively, to form diradical intermediates, followed by the breaking of the remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C? C bond is dominant over that of propene via an initial cleavage of a ring Si? C bond. When the collision‐free condition is voided, secondary reactions in the gas‐phase produce various methyl‐substituted 1,3‐disilacyclobutane molecules. The dominant of all is found to be 1,1,3,3‐tetramethyl‐1,3‐disilacyclobutane originated from the dimerization of 1,1‐dimethylsilene. Copyright © 2010 John Wiley & Sons, Ltd.     

Copper‐Catalyzed Radical/Radical C?H/P?H Cross‐Coupling: α‐Phosphorylation of Aryl Ketone O‐Acetyloximes

The selective radical/radical cross‐coupling of two different organic radicals is a great challenge due to the inherent activity of radicals. In this paper, a copper‐catalyzed radical/radical C H/P H cross‐coupling has been developed. It provides a radical/radical cross‐coupling in a selective manner. This work offers a simple way toward β‐ketophosphonates by oxidative coupling of aryl ketone o‐acetyloximes with phosphine oxides using CuCl as catalyst and PCy₃ as ligand in dioxane under N₂ atmosphere at 130 °C for 5 h, and yields ranging from 47 % to 86 %. The preliminary mechanistic studies by electron paramagnetic resonance (EPR) showed that, 1) the reduction of ketone o‐acetyloximes generates iminium radicals, which could isomerize to α‐sp³‐carbon radical species; 2) phosphorus radicals were generated from the oxidation of phosphine oxides. Various aryl ketone o‐acetyloximes and phosphine oxides were suitable for this transformation.     

Extended Reaction Scope of Thiamine Diphosphate Dependent Cyclohexane‐1,2‐dione Hydrolase: From C?C Bond Cleavage to C?C Bond Ligation

ThDP‐dependent cyclohexane‐1,2‐dione hydrolase (CDH) catalyzes the C C bond cleavage of cyclohexane‐1,2‐dione to 6‐oxohexanoate, and the asymmetric benzoin condensation between benzaldehyde and pyruvate. One of the two reactivities of CDH was selectively knocked down by mutation experiments. CDH‐H28A is much less able to catalyze the C C bond formation, while the ability for C C bond cleavage is still intact. The double variant CDH‐H28A/N484A shows the opposite behavior and catalyzes the addition of pyruvate to cyclohexane‐1,2‐dione, resulting in the formation of a tertiary alcohol. Several acyloins of tertiary alcohols are formed with 54–94 % enantiomeric excess. In addition to pyruvate, methyl pyruvate and butane‐2,3‐dione are alternative donor substrates for C C bond formation. Thus, the very rare aldehyde–ketone cross‐benzoin reaction has been solved by design of an enzyme variant.     

Synthesis of 1,3‐Diamines Through Rhodium‐Catalyzed C?H Insertion

Schließen und öffnen : N‐Boc‐N‐alkylsulfamide sind geeignete Substrate für die Titelreaktion. Die oxidative Cyclisierung im ersten Schritt ist hoch chemoselektiv sowie stereospezifisch und diastereoselektiv. Mit neuen Verfahren zur Öffnung der dabei erhaltenen Sechsringheterocyclen werden unterschiedlich geschützte 1,3‐Diamine zugänglich (siehe Schema).

     

Synthesis of 1,3‐Diamines Through Rhodium‐Catalyzed C?H Insertion

A grand opening : N‐Boc‐N‐alkylsulfamides are effective substrates for the title transformation. Oxidative cyclization is highly chemoselective as well as being both stereospecific and diastereoselective. With the advent of new protocols that facilitate ring opening of the six‐membered‐ring heterocyclic products, access to differentially protected 1,3‐diamines has been made possible (see scheme).

     

Metal‐Free Reductive Cleavage of C?N and S?N Bonds by Photoactivated Electron Transfer from a Neutral Organic Donor

A photoactivated neutral organic super electron donor cleaves challenging arenesulfonamides derived from dialkylamines at room temperature. It also cleaves a) ArC NR and b) ArN C bonds. This study also highlights the assistance given to these cleavage reactions by the groups attached to N in (a) and to C in (b), by lowering LUMO energies and by stabilizing the products of fragmentation.     

Rhodium(III)‐Catalyzed C?C and C?O Coupling of Quinoline N‐Oxides with Alkynes: Combination of C?H Activation with O‐Atom Transfer

[Cp*Rh^III]‐catalyzed C H activation of arenes assisted by an oxidizing N O or N N directing group has allowed the construction of a number of hetercycles. In contrast, a polar N O bond is well‐known to undergo O‐atom transfer (OAT) to alkynes. Despite the liability of N O bonds in both C H activation and OAT, these two important areas evolved separately. In this report, [Cp*Rh^III] catalysts integrate both areas in an efficient redox‐neutral coupling of quinoline N‐oxides with alkynes to afford α‐(8‐quinolyl)acetophenones. In this process the N O bond acts as both a directing group for C H activation and as an O‐atom donor.     

Nickel‐Catalyzed Chemo‐, Regio‐ and Diastereoselective Bond Formation through Proximal C?C Cleavage of Benzocyclobutenones

The first catalytic intermolecular proximal C1 C2 cleavage of benzocyclobutenones (BCB) without prior carbonyl activation or employing noble metals has been developed. This protocol operates at room temperature and is characterized by an exquisite chemo‐, regio‐ and diastereoselectivity profile, constituting a unique platform for preparing an array of elusive carbocyclic skeletons.     

Palladium(II)‐Catalyzed C?H Activation/C?C Cross‐Coupling Reactions: Versatility and Practicality

Pick your Pd partners : A number of catalytic systems have been developed for palladium‐catalyzed C? H activation/C? C bond formation. Recent studies concerning the palladium(II)‐catalyzed coupling of C? H bonds with organometallic reagents through a Pd^II/Pd⁰ catalytic cycle are discussed (see scheme), and the versatility and practicality of this new mode of catalysis are presented. Unaddressed questions and the potential for development in the field are also addressed.

     

The Morita?Baylis?Hillman Reaction of Chiral Highly Oxygenated Cyclopent‐2‐enones

The Morita? Baylis? Hillman (MBH) reactions of (4S,5R,7R,8R)‐ and (4R,5R,7R,8R)‐4‐hydroxy‐7,8‐dimethoxy‐7,8‐dimethyl‐6,9‐dioxaspiro[4.5]dec‐2‐en‐1‐ones ( 2 and 3 , resp.) with aldehydes using various catalysts were studied. A combination of Bu₃P/phenol in THF was found being optimum conditions giving the corresponding MBH adducts with high diastereoisomeric ratios. After separation, each stereomerically pure isomer of the MBH adducts was subjected to hydrolysis employing 1% aq. CF₃COOH (TFA) in a water bath of an ultrasonic cleaner to afford the corresponding polyhydroxylated cyclopentenones in good yields.     

Nickel‐Catalyzed Enantioselective C?C Bond Formation through C?O Cleavage in Aryl Esters

We report the first enantioselective C C bond formation through C O bond cleavage using aryl ester counterparts. This method is characterized by its wide substrate scope and results in the formation of quaternary stereogenic centers with high yields and asymmetric induction.     

Ruthenium‐Catalyzed C?C Bond Cleavage in Lignin Model Substrates

Ruthenium–triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox‐neutral C C bond cleavage of the β‐O‐4 lignin linkage of 1,3‐dilignol model compounds. A mechanistic pathway involving a dehydrogenation‐initiated retro‐aldol reaction for the C C bond cleavage was proposed in line with experimental data and DFT calculations.