Transition-metal-catalyzed aromatic C–H functionalization assisted by the phosphorus-containing directing groups

Transition-metal-catalyzed C–H bond functionalization has attracted tremendous attention as an increasingly important protocol for the construction of C–C and C–X bonds.In particular,C–H bond functionalization assisted by the phosphorus-containing directing groups has made significant progress in the past few years.A series of valuable phosphorous compounds,especially phosphine ligands have been provided conveniently.This review comprehensively describes recent advances in transition-metal-catalyzed C–H bond functionalization assisted by the phosphorus-containing directing groups,highlights its potential applications in organic synthesis,and gives some insights into future advances.

Rh-catalyzed Transient Directing Group Promoted C-H Amidation of Benzaldehydes Utilizing Dioxazolones

Transition-metal catalyzed C--H functionalization of benzaldehydes is of great interest in organic synthesis. Herein, we developed a transient directing group assisted amidation of benzaldehydes catalyzed by rhodium catalyst. With the employment of 10 mol% of 4-trifluoromethyl aniline, the in situ generated imine groups as the directing group efficiently enable this transformation. By using this protocol, a wide range of benzaldehydes were efficiently converted into the corresponding N-（2-formylphenyl）benzamides utilizing dioxazolones as the nitrogen source.

Palladium-catalyzed ortho-C–H silylation of biaryl aldehydes using a transient directing group

A strategy for Pd-catalyzed ortho-C–H silylation of biaryl aldehydes enabled by a transient auxiliary is presented. This protocol provides a broad range of ortho-silylated biaryl aldehydes in good yields. The silyl moiety can be further functionalized under mild conditions, rendering the silylated products useful building blocks. Notably, this protocol also offers an opportunity to establish a platform for expeditious synthesis of structurally diverse axially chiral biaryl aldehydes via sequential atroposelective interannular C–H functionalization/intraanular C–H silylation.

Representations of Free Groups on Rational Line Q

A.M.W. Glass and S.H.McCleary have given the 2 transitive representation of the countable free l group F η(1<η≤ω 0 ).In this paper we shall give the highly ordered transitive representation of countable free groups on the rational line Q, which generalizes their results. As applications,we obtain the highly ordered transitive representation for the direct product of countable free groups,and the inverse limit of countable free groups must be an action on the set Q.

Symmetry Groups and New Exact Solutions of(2+1)-Dimensional Dispersive Long-Wave Equations

Using the modified CK＇s direct method, we derive a symmetry group theorem of （2＋1）-dimensional dispersive long-wave equations. Based upon the theorem, Lie point symmetry groups and new exact solutions of （2＋1）- dimensional dispersive long-wave equations are obtained.

Existence of Almost Resolvable Directed 7-Cycle Systems

Let ARDkCS（v） denote an almost resolvable directed k-cycle system of order v. It is clear that a necessary condition for the existence of an ARDkCS（v） is v=1（mod k）. For k：3,4,5 and 6, the existence of an ARDkCS （v） had been completely solved. This paper shows that there exists an ARD7CS（v） if and only if v≡1 （rood 7） and v≥8.

Remote ether groups-directed regioselective and chemoselective cycloaddition of azides and alkynes

Remote ether groups could be used as directing groups to prepare fully substituted 5-ether-1,2,3-triazoles with exclusive 1,5-regioselectivities and excellent chemoselectivities. Ether group could coordinate with iridium catalyst by lone-pair electron at a distance(up to four σ bonds) away from alkyne to control the regioselectivity by weak coordination effect. The cycloaddition reaction chemoselectively occurred at the propargyl ether moiety of diyne to give unique fully substituted 4-alkynyl-triazole.

Weakly coordinating group directed rhodium-catalyzed unconventional site-selective C-H olefination of indolizines at the 8-position

A rhodium-catalyzed directing group promoted selective C-H olefination reaction of indolizines at the 8-position is re ported.Di-olefination at 2,8-positions also achieved with silver hexafluoroantimonate as an additive under similar reaction conditions.Weakly coordinating groups,such as ketone,alde hyde,amide and ester,were used as directing groups.The ester group can be removed under acid conditions and therefore is used as a traceless directing group.

Nano palladium catalyzed C(sp^(3))-H bonds arylation by a transient directing strategy

Reported herein is the first example of heterogeneous palladium catalyzed C(sp^(3))-H bonds arylation by a transient-ligand-directed strategy.Using supported palladium(metallic state) na nopariticles as catalyst,a wide range of aryl iodides undergo the coupling with various o-methylbenzaldehyde derivatives to assemble a library of highly selective and functionalized o-benzylbenzaldehydes.The stability of the catalyst was easily recovered four runs without significant loss of activity.The XPS analysis of the catalyst before and after reaction indicated that the reaction might be carried out by a catalytic cycle starting with Pd~0.

Highly Stereoselective Synthesis of 2-Azido-2-Deoxyglycosides via Gold-Catalyzed S_(N)2 Glycosylation

Highly stereoselective synthesis of 2-azido-2-deoxyglucosides and 2-azido-2-deoxygalactosides is achieved via a gold-catalyzed S_(N)2 glycosylation.The glycosyl donors feature a designed 1-naphthoate leaving group containing an amide group.Upon gold activation of the leaving group,the amide group is optimally positioned to direct an S_(N)2 attack by an acceptor via H-bonding interaction.Both 2-azido-2-deoxyglucosyl/galactosyl donor anomers can undergo stereoinversion at the anomeric position,affording the opposite anomeric glycoside products with excellent levels of stereoselectivity or stereospecificity and in mostly excellent yields.This S_(N)2 glycosylation accommodates a broad range of acceptors.The utility of this chemistry is demonstrated in the synthesis of a trisaccharide featuring three 1,2-cis-2-azido-2-deoxyglycosidic linkages.

Some recent advances in transition-metal-catalyzed ortho SP^2 C–H functionalization using Ru, Rh, and Pd

In the past decade,transition-metal-catalyzed C–H functionalization by weak coordination has emerged as a practical and powerful tool to access many valuable chemicals.Two classes of weakly coordinating directing groups,commonly occurring functional groups,and easily removable auxiliaries,have been found to be efficient and practical for C–H activation reactions.This mini-review contains examples of recent research advances on transition-metal-catalyzed SP2 C–H functionalization via weak coordination,using Ru,Rh,and Pd.A number of weakly coordinating functional groups(e.g.,ketone,ester,carbamate,tertiary amide,ether,thioether,alcohol,and some others)are covered.As the field of transition-metal-catalyzed C–H functionalization continues to develop and more synthetically useful chemo-,regio-,and enantioselective reactions catalyzed by transition metal via weak coordination are discovered,this promising and attractive strategy will play a more important role in modern organic synthesis.

Palladium-catalyzed multi-acetoxylation of 1,3-disubstituted 1H-pyrazole-5-carboxylates via direct C(sp^2)-H or C(sp^3)-H bond activation

A palladium-catalyzed multi-acetoxylation of 1,3-disubstituted 1H-pyrazole-5-carboxylate derivatives containing multiple potential reactive sites is described. Therein, the sequence of this process has been appropriately investigated. The protocol mainly provides the di- and tri-acetoxylated products for 1,3- diarylpyrazoles. Besides, it is found that the acetoxylation of C（sp3）-H bond is prior to that of C（sp2）-H bond under structurally competitive conditions.

Rhodium catalyzed regioselective arene homologation of aryl urea via double C–H bond activation and migratory insertion of alkyne

A convenient rhodium catalyzed oxidative arene homologation of aniline derivatives with symmetrical or unsymmetrical alkynes using Cu（OAc）2 as oxidant is described. Urea group is shown to be effective as a directing group for initial ortho C–H activation. Two migratory insertion events of alkyne into Rh–C bond occur successively, both with complete regioselectivity. This method is particularly useful for synthesis of polyarenes with different substituents, which has not been reported with conventional protocol. A mechanism has been proposed to explain the observed data.

Rh(Ⅲ)-Catalyzed Diverse C-H Functionalization of Iminopyridinium Ylides

Divergent synthesis of useful skeletons has been realized via rhodium(Ⅲ)-catalyzed C-H activation of iminopyridinium ylides and coupling with various unsaturated coupling reagents.Isocoumarins and isoquinolones were obtained via cleavage of the C-N or N-N bond in the ylidic di recti ng group,while fluorinated alkenes were delivered with the di recti ng group in tact.The reactions occurred with wide substrate scopes and good efficiency under redox-neutral and air-tolerant conditions.Representative products exhibit solid-state fluoresce nt property and bioactivity of in hibiti on toward huma n cancer cells.

Rhodium-catalyzed formal[4+3]annulation reaction of N-methoxybenzamides with gem–difluorocyclopropenes:A combination of experimental and theoretical studies

A rhodium-catalyzed[4+3]cycloaddition reaction between N-methoxybenzamides and gem–difluorocyclopropenes is described.The reaction offers a mild and efficient approach towards the synthesis of fluorinated 2 H-azepin-2-ones with broad substrate scope.A consecutive HOAc-assisted C–N bond formation and fluorine elimination are involved as key steps for success as illustrated by detailed DFT studies.

Palladium-Catalyzed Methionine-Facilitated β and γ C(sp^(3))–H Arylation of N-Terminal Aliphatic Amino Acids of Peptides

A new method for post-assembly modification of peptides via the Pd-catalyzed endogenous group-directed sp^(3) C—H arylation with aryl iodides is developed.The sulfide side chain of methionine(Met)residue can facilitate the arylation of bothβandγmethyl C—H bonds of N-terminal aliphatic amino acids of peptides via formation of 5 or 6-membered palladacycle intermediates.The reactions can proceed in moderate to good yield and with excellent diastereoselectivity.

Arene C-H Iodination Using Aryl Iodides

Metathesis reactions represent powerful synthetic tools that have been used in a number of fields from the synthesis of natural product to functional material preparation.However,the C-H metathesis reaction is extremely rare.Herein,we report the first Pd(Ⅱ)-catalyzed C-H iodination of arenes using 2-nitrophenyl iodides as the mild iodinating reagents via a formal metathesis reaction.Unusual C-I bond formation occurred with aryl iodides in preference to competing C-C coupling in this reaction.Assisted by aliphatic carboxyl directing groups,a range of hydrocinnamic acids and related arenes could be selectively iodinated at either meta-or orthopositions of the phenyl ring.Remote diastereoselective C-H activation was also promising.This method might unfold a novel approach to iodinate challenging substrates.

Regiodivergent Access to 2- or 3-Substituted Indanones: Catalyst-Controlled Carboacylation via C–C Bond Activation

Indanones are ubiquitous in biologically active compounds.Intramolecular hydroacylation of aldehydes and alkenes is an efficient and atomeconomic route to indane rings.However,these reactions are limited to the transfer of a hydride to the alkene.The transfer of aryl groups enabling the formation of C–C bonds during the cyclization would be a new method for the synthesis of substituted indanones.This report describes the regiodivergent carboacylation of alkenes with ketones to furnish both 2-and 3-substituted indanones in a regiocontrolled manner.

Palladium-catalyzed 2-pyridylmethyl-directed β-C(sp^3)—H activation and cyclization of aliphatic amides with gem-dibromoolefins:A rapid access to γ-lactams

The direct Pd-catalyzed β-C（sp3）--H activation and cyclization of aliphatic amides bearing a removable 2-pyridylmethyl directing group with gem-dibromoolefins is described for the first time to construct a variety of γ-lactams, The resulting products with Z- and E-configurations can be easily separated and purified after the reaction, demonstrating the effectiveness and applicability of the method herein developed.