Synthesis of novel rhodium‐xylyl linked N‐heterocyclic carbene complexes as hydrosilylation catalysts

Reaction of ortho‐xylylbis(N‐2,4,6‐trimethylbenzylimidazolinium); xylylbis(N‐butylimidazolinium) and para‐xylylbis(N‐2,4,6‐trimethylbenzylimidazolinium); xylylbis(N‐butylimidazolinium) salts with KOBu^t and [RhCl(COD)]₂ yields ortho‐ and para‐xylylbis{(N‐alkylimidazolidin‐2‐ylidene)chloro(η⁴‐1,5‐cyclooctadiene) rho dium(I)} complexes (2a–d). All compounds synthesized were characterized by elemental analysis and NMR spectroscopy, and the molecular structures of the 2a and 2d were determined by X‐ray crystallography. Triethylsilane reacts with acetophenone derivatives in the presence of catalytic amount of the new rhodium(I)–carbene complexes (2a–d), to give the corresponding silylethers in good yields (83–99%). Copyright © 2008 John Wiley & Sons, Ltd.     

[RhIII(Cp*)]‐Catalyzed ortho‐Selective Direct C(sp2)H Bond Amidation/Amination of Benzoic Acids by N‐Chlorocarbamates and N‐Chloromorpholines. A Versatile Synthesis of Functionalized Anthranilic Acids

A Rh^III‐catalyzed direct ortho‐C?H amidation/amination of benzoic acids with N‐chlorocarbamates/N‐chloromorpholines was achieved, giving anthranilic acids in up to 85 % yields with excellent ortho‐selectivity and functional‐group tolerance. Successful benzoic acid aminations were achieved with carbamates bearing various amide groups including NHCO₂Me, NHCbz, and NHTroc (Cbz=carbobenzyloxy; Troc=trichloroethylchloroformate), as well as secondary amines, such as morpholines, piperizines, and piperidines, furnishing highly functionalized anthranilic acids. A stoichiometric reaction of a cyclometallated rhodium(III) complex of benzo[h]quinoline with a silver salt of N‐chlorocarbamate afforded an amido–rhodium(III) complex, which was isolated and structurally characterized by X‐ray crystallography. This finding confirmed that the C?N bond formation results from the cross‐coupling of N‐chlorocarbamate with the aryl–rhodium(III) complex. Yet, the mechanistic details regarding the C?N bond formation remain unclear; pathways involving 1,2‐aryl migration and rhodium(V)– nitrene are plausible.     

A Convenient Synthesis of N‐Aryl Benzamides by Rhodium‐Catalyzed ortho‐Amidation and Decarboxylation of Benzoic Acids

The rhodium‐catalyzed amidation of substituted benzoic acids with isocyanates by directed C?H functionalization followed by decarboxylation to afford the corresponding N‐aryl benzamides is demonstrated, in which the carboxylate serves as a unique, removable directing group. Notably, less common meta‐substituted N‐aryl benzamides are generated readily from more accessible para‐ or ortho‐substituted groups by employing this strategy.     

Rhodium(II)‐Catalyzed Intramolecular Cycloisomerizations of Methylenecyclopropanes with N‐Sulfonyl 1,2,3‐Triazoles

A novel rhodium(II)‐catalyzed tandem cycloisomerization of methylenecyclopropanes (MCPs) with N‐sulfonyl 1,2,3‐triazoles is disclosed. The reaction produces a series of highly functionalized polycyclic N heterocycles via a rhodium imino carbene intermediate. A distinct feature of this divergent synthesis is that different types of substrates control the reaction pathways. Moreover, several interesting transformations of these products to construct diazabicyclo[3.2.1]octane derivatives are also reported.     

Photoactivated N‐Acyliminoiodinanes Applied to Amination: an ortho‐Methoxymethyl Group Stabilizes Reactive Precursors

N‐Acyliminoiodinanes were characterized for the first time by X‐ray structural analysis. The ortho‐methoxymethyl group and the carbonyl oxygen coordinate to the iodine atom of the iminoiodinane. Activation of the N‐acyliminoiodinane was achieved by photoirradiation at 370 nm, thereby enabling reaction with various silyl enol ethers to give α‐aminoketone derivatives in good to high yield. N‐sulfonyliminoiodinanes bearing ortho substituents were used in photoinduced amination.     

Rhodium‐Catalyzed Regioselective C7‐Functionalization of N‐Pivaloylindoles

An efficient rhodium‐catalyzed method for direct C? H functionalization at the C7 position of a wide range of indoles has been developed. Good to excellent yields of alkenylation products were observed with acrylates, styrenes, and vinyl phenyl sulfones, whereas the saturated alkylation products were obtained in good yield with α,β‐unsaturated ketones. Both the N‐pivaloyl directing group and the rhodium catalyst proved to be crucial for high regioselectivity and conversion.     

Rearrangement products of 3‐methane­sulfonyl‐N‐methyl‐N‐nitro­aniline

Two isomeric products (C₈H₁₀N₂O₄S) of the rearrangement of 3‐methane­sulfonyl‐N‐methyl‐N‐nitro­aniline have been investigated, viz. 3‐methane­sulfonyl‐N‐methyl‐2‐nitro­aniline, which was the main product of the rearrangement, and 5‐methane­sulfonyl‐N‐methyl‐2‐nitro­aniline. In both mol­ecules, the aromatic rings are appreciably deformed towards ortho‐quinonoidal geometry by electronic and steric interactions. The crystal structure is stabilized, in both cases, by weak C—H⋯O hydrogen bonds.     

Catalytic Reaction of Aryldiazoacetates with Indole and Its Derivatives:Profound Effect of N‐1 Substitutent on the Reaction Pathways

The reaction of indole and its derivatives with aryldiazoacetates has been studied in the presence of copper and rhodium catalysts. The electronic property of N-1 substitutent showed significant effect on the reaction pathways. The electron-donating group favored the formation of the β-alkylation products, while the electron-withdrawing group favored the formation of the cyclopropane products. A reaction mechanism was proposed based on the experimental data and previous research results. The structure of aryl group in diazo compounds also affected the yield of the β-alkylation products or the cyclopropane products.     

Cooperative Catalysis for the Highly Diastereo‐ and Enantioselective [4+3]‐Cycloannulation of ortho‐Quinone Methides and Carbonyl Ylides

We describe herein a highly diastereo‐ and enantioselective [4+3]‐cycloannulation of ortho‐quinone methides and carbonyl ylides to furnish functionalized oxa‐bridged dibenzooxacines with excellent yields and stereoselectivity in a single synthetic step. The combination of rhodium and chiral phosphoric acid catalysis working in concert to generate both transient intermediates in situ provides direct access to complex bicyclic products with two quaternary and one tertiary stereogenic centers. The products may be further functionalized into valuable and enantiomerically highly enriched building blocks.     

Rhodium‐Catalyzed PIII‐Directed ortho‐C−H Borylation of Arylphosphines

Transition‐metal‐mediated metalation of an aromatic C?H bond that is adjacent to a tertiary phosphine group in arylphosphines via a four‐membered chelate ring was first discovered in 1968. Herein, we overcome a long‐standing problem with the ortho‐C?H activation of arylphosphines in a catalytic fashion. In particular, we developed a rhodium‐catalyzed ortho‐selective C?H borylation of various commercially available arylphosphines with B₂pin₂ through P^III‐chelation‐assisted C?H activation. This discovery is suggestive of a generic platform that could enable the late‐stage modification of readily accessible arylphosphines.     

Dearomatization of Electron‐Deficient Phenols to ortho‐Quinones: Bidentate Nitrogen‐Ligated Iodine(V) Reagents

Despite their broad utility, the synthesis of ortho‐quinones remains a significant challenge, in particular, access to electron‐deficient derivatives remains an unsolved problem. Reported here is the first general method for the synthesis of electron‐deficient ortho‐quinones by direct oxidation of phenols. The reaction is enabled by a novel bidentate nitrogen‐ligated iodine(V) reagent, a previously unexplored class of compounds which we have termed Bi(N)‐HVIs. The reaction is extremely general and proceeds with excellent regioselectivity for the ortho over para isomer. Functionalization of the ortho‐quinone products was examined, resulting in a facile one‐pot synthesis of catechols, as well as the incorporation of a variety of heteroatom nucleophiles. This method represents the first synthetic application of Bi(N)‐HVIs and demonstrates their potential as a platform for the further development of highly reactive, but also highly tunable, I(V) reagents.     

Cu‐Catalyzed Cross‐Dehydrogenative ortho‐Aminomethylation of Phenols

A highly selective Cu^II‐catalyzed cross‐dehydrogenative ortho‐aminomethylation of phenols with aniline derivatives is described. The corresponding C(sp²)?C(sp³) coupling products were obtained in moderate to excellent yields under mild reaction conditions and with a broad substrate scope. A radical mechanism is proposed.     

Asymmetric Synthesis of Axially Chiral 2‐Aminobiaryls by Rhodium‐Catalyzed Benzannulation of 1‐Arylalkynes with 2‐(Cyanomethyl)phenylboronates

Asymmetric benzannulation of 1‐arylalkynes, where the aryl group is an ortho‐substituted aromatic group, with 2‐(cyanomethyl)phenylboronate was catalyzed by a rhodium complex coordinated with a chiral diene ligand to give high yields of axially chiral 2‐aminobiaryls with greater than 90 % ee.     

CH Oxygenation and N‐Trifluoroacylation of Arylamines Under Metal‐Free Conditions: A Convenient Approach to 2‐Aminophenols and N‐Trifluoroacyl‐ortho‐aminophenols

Direct ortho‐hydroxylation through C?H oxygenation and N‐trifluoroacylation of anilines was achieved in a single step under metal‐free conditions by using a combination of TFA and oxone. The method allowed the formation of functionalised amino phenolic compounds such as ortho‐hydroxy‐N‐trifluoroacetanilides in good yields with broad substrate scope.     

A [4+1] Cyclative Capture Approach to 3H‐Indole‐N‐oxides at Room Temperature by Rhodium(III)‐Catalyzed CH Activation

The rhodium(III)‐catalyzed [3+2] C? H cyclization of aniline derivatives and internal alkynes represents a useful contribution to straightforward synthesis of indoles. However, there is no report on the more challenging synthesis of pharmaceutically important N‐hydroxyindoles and 3H‐indole‐N‐oxides. Reported herein is the first rhodium(III)‐catalyzed [4+1] C? H oxidative cyclization of nitrones with diazo compounds to access 3H‐indole‐N‐oxides. More significantly, this reaction proceeds at room temperature and has been extended to the synthesis of N‐hydroxyindoles and N‐hydroxyindolines.     

Redox‐Neutral Rhodium‐Catalyzed CH Functionalization of Arylamine N‐Oxides with Diazo Compounds: Primary C(sp3)H/C(sp2)H Activation and Oxygen‐Atom Transfer

An unprecedented rhodium(III)‐catalyzed regioselective redox‐neutral annulation reaction of 1‐naphthylamine N‐oxides with diazo compounds was developed to afford various biologically important 1H‐benzo[g]indolines. This coupling reaction proceeds under mild reaction conditions and does not require external oxidants. The only by‐products are dinitrogen and water. More significantly, this reaction represents the first example of dual functiaonalization of unactivated a primary C(sp³)? H bond and C(sp²)? H bond with diazocarbonyl compounds. DFT calculations revealed that an intermediate iminium is most likely involved in the catalytic cycle. Moreover, a rhodium(III)‐catalyzed coupling of readily available tertiary aniline N‐oxides with α‐diazomalonates was also developed under external oxidant‐free conditions to access various aminomandelic acid derivatives by an O‐atom‐transfer reaction.     

Enantioselective Rhodium‐Catalyzed Allylic Alkylation of β,γ‐Unsaturated α‐Amino Nitriles: Synthetic Homoenolate Equivalents

An enantioselective rhodium‐catalyzed allylic alkylation of β,γ‐unsaturated α‐amino nitriles is described. This protocol provides a novel approach for the construction of β‐stereogenic carbonyl derivatives via the catalytic asymmetric alkylation of a homoenolate equivalent. The particularly challenging nature of this transformation is highlighted by the fact that three modes of selectivity must be manipulated, namely regio‐ and enantioselectivity, in addition to geometrical control. The γ‐stereogenic cyanoenamine products can be readily hydrolyzed in situ to afford the β‐substituted carboxylic acids, which in turn provide expedient access to a number of related carbonyl derivatives. Additionally, control experiments indicate that the chiral rhodium‐allyl intermediate facilitates the selective formation of the E‐cyanoenamine products, which is critical since the Z‐isomer affords significantly lower enantiocontrol.     

Photocatalyzed ortho‐Alkylation of Pyridine N‐Oxides through Alkene Cleavage

A photocatalyzed reaction of pyridine N‐oxides with alkenes gives ortho‐alkylated pyridines with cleavage of the carbon–carbon double bond. Benzyl and secondary alkyl groups are incorporated at the ortho position of pyridines in one pot.     

Palladium‐Catalyzed Decarboxylative ortho‐C(sp2)−H Aroylation of N‐Sulfoximine Benzamides at Room Temperature

A palladium‐catalyzed method for the decarboxylative ortho C?H acylation of N‐sulfoximine benzamides is developed at room temperature. The catalytic method enables easy access to various functionalized 2‐aroylaromatic carboxylic acid derivatives in good isolated yields. Based on our mechanistic studies, a Pd(II)/Pd(IV) catalytic cycle that involves aroyl radical intermediate is proposed for the reaction.     

Chiral N,N′‐Dioxide–Scandium(III) Complex‐Catalyzed Asymmetric Friedel–Crafts Alkylation Reaction of ortho‐Hydroxybenzyl Alcohols with C3‐Substituted N‐Protected Indoles

The first Lewis acid catalyzed asymmetric Friedel–Crafts alkylation reaction of ortho‐hydroxybenzyl alcohols with C3‐substituted indoles is described. A chiral N,N′‐dioxide Sc(OTf)₃ complex served not only to promote formation of ortho‐quinone methides (o‐QMs) in situ but also induced the asymmetry of the reaction. This methodology enables a novel activation of ortho‐hydroxybenzyl alcohols, thus affording the desired chiral diarylindol‐2‐ylmethanes in up to 99 % yield and 99 % ee. A range of functional groups were also tolerated under the mild reaction conditions. Moreover, this strategy gives concise access to enantioenriched indole‐fused benzoxocines.