Palladium pincer complex catalyzed stannyl and silyl transfer to propargylic substrates: synthetic scope and mechanism

Pincer complex catalyzed substitution of various propargylic substrates could be achieved using tin- and silicon-based dimetallic reagents to obtain propargyl- and allenylstannanes and silanes. These reactions involving chloride, mesylate, and epoxide substrates could be carried out under mild conditions, and therefore many functionalities (such as COOEt, OR, OH, NR, and NAc) are tolerated. It was shown that pincer catalysts with electron-supplying ligands, such as NCN, SCS, and SeCSe complexes, display the highest catalytic activity. The catalytic substitution of secondary propargyl chlorides and primary propargyl chlorides with electron-withdrawing substituents proceeds with high regioselectivity providing the allenyl product. Opening of the propargyl epoxides takes place with an excellent stereo- and regioselectivity to give stereodefined allenylstannanes. Silylstannanes as dimetallic reagents undergo an exclusive silyl transfer to the propargylic substrate affording allenylsilanes with high regioselectivity. According to our mechanistic studies, the key intermediate of the reaction is an organostannane (or silane)-coordinated pincer complex, which is formed from the dimetallic reagent and the corresponding pincer complex catalyst. DFT modeling studies have shown that the trimethylstannyl functionality is transferred to the propargylic substrate in a single reaction step with high allenyl selectivity. Inspection of the TS structures reveals that the trimethylstannyl group transfer is initiated by the attack of the palladium-tin sigma-bond electrons on the propargylic substrate. This is a novel mechanism in palladium chemistry, which is based on the unique topology of the pincer complex catalysts.     

Palladium pincer complex catalyzed substitution of vinyl cyclopropanes, vinyl aziridines, and allyl acetates with tetrahydroxydiboron. An efficient route to functionalized allylboronic acids and potassium trifluoro(allyl)borates

Palladium-catalyzed boronation of vinyl cyclopropane, vinyl aziridine, and allyl acetate substrates could be accomplished using tetrahydroxydiboron reagent in the presence of SeCSe pincer complex catalyst 1a. These reactions result in allyl boronic acids, which were converted to synthetically useful trifluoro(allyl)borates or allyl boronates. The catalytic transformations proceed under mild and neutral conditions, and therefore many functionalities Br, COOEt, ArSO2(NH), OAc, and SiRMe2 are tolerated. The selectivity of the presented processes is very high, affording the linear products incorporating a trans double bond.     

Ruthenium-catalyzed propargylic substitution reaction of propargylic alcohols with thiols: a general synthetic route to propargylic sulfides

A novel cationic methanethiolate-bridged diruthenium complex [Cp*RuCl(mu2-SMe)2RuCp*(OH2)]OTf (1e) has been disclosed to promote the catalytic propargylic substitution reaction of propargylic alcohols bearing not only terminal alkyne group but also internal alkyne group with thiols. It is noteworthy that neutral thiolate-bridged diruthenium complexes (1a-1c), which were known to promote the propargylic substitution reactions of propargylic alcohols bearing a terminal alkyne group with various heteroatom- and carbon-centered nucleophiles, did not work at all. The catalytic reaction described here provides a general and environmentally friendly preparative method for propargylic sulfides, which are quite useful intermediates in organic synthesis, directly from the corresponding propargylic alcohols and thiols.     

Palladium mediated direct coupling of silylated arylalkynes with propargylic chlorides: an efficient access to functionalized conjugated allenynes

Pd/Cu-catalyzed one-pot reaction of 1-trimethylsilyl-2-arylalkynes with propargylic chlorides in the presence of TBAF is described. The present new procedure is applicable to a wide range of silylated arylalkynes with both electron-withdrawing and electron-donating substituents. Functionalized allenynes can thus be obtained in good yields without prior deprotection of the alkynes.     

An efficient route to highly functionalized benzene derivatives by rhodium-catalyzed dimerization of diynes

An intermolecular efficient dimerization of diynes via Rh-catalyzed cycloaddition is presented. This protocol allows for the expedient synthesis of structurally diverse benzene derivatives, which retain an alkyne unit for post-functionalization to realize molecules with potential medicinal value and extended molecular complexity. The transformation is carried out in toluene and employs Rh(COD)₂OTf₃ as the metal source. Diversely decorated diynes tethered by nitrogen atom and ester were compatible with these conditions and furnished substituted benzenes with excellent selectivity.     

An efficient route to unsymmetrical bis(azolium) salts: CCC-NHC pincer ligand complex precursors

The coupling of N-heterocyclic azoles (imidazoles, benzimidazoles, triazoles) to bromobenzenes (1,2-; 1,3-; or 1,4) in a step-wise, sequential manner was accomplished by manipulation of reaction time and stoichiometry, which provided straight-forward access to unsymmetrical bis(azolium) salts in only three isolation steps from commercially-available starting materials. Eight mono(azole) substituted bromobenzenes, four mono(azolium)bromobenzene salts, twelve unsymmetrical bis(azole)benzenes, and fourteen unsymmetrical bis(azolium) salts, which are precursors for pincer ligand complexes, are reported.     

The propargylic route as efficient entry to monofluoro and gem-difluoro compounds: mechanistic considerations

The dehydroxy-fluorination of propargylic alcohols occurs with a complete regiocontrol and a good to complete stereocontrol, in contrast to the reactions performed on allylic alcohols. The gem-difluorination of propargylic ketones occurs smoothly in contrast to enones which have a very low reactivity towards DAST or Deoxo-fluor™. It is proposed that the large differences in the stabilization energies of the key carbonium ion intermediates (either propargylic or allylic) could explain these strong differences in reactivity during nucleophilic fluorination. The calculations of isodesmic reactions are in full agreement with this proposal.     

A general and efficient FeCl3-catalyzed nucleophilic substitution of propargylic alcohols

A general and efficient FeCl3-catalyzed substitution reaction of propargylic alcohols with carbon- and heteroatom-centered nucleophiles such as allyl trimethylsilane, alcohols, aromatic compounds, thiols, and amides, leading to the construction of C-C, C-O, C-S and C-N bonds, has been developed.     

An efficient diamine.copper complex-catalyzed coupling of arylboronic acids with imidazoles

[formula: see text] A novel diamine.copper complex-catalyzed intermolecular coupling of arylboronic acids (1) with imidazoles (3) is described. In the presence of a catalytic amount of [Cu(OH).TMEDA]2Cl2 (2), arylboronic acids (1) react smoothly with imidazoles (3) in dichloromethane at room temperature to give a variety of N-arylimidazoles (4) in good to excellent yields.     

Short and efficient route to the fully functionalized polar core of scyphostatin

[reaction: see text]. Diastereoselective oxidative dearomatization of benzopyran 5 to the corresponding p-quinol 9b allows a fast, efficient, and versatile entry to scyphostatin's polar epoxycyclohexenone moiety as demonstrated by the preparation of its palmitoyl analogue 16 (R = (CH2)14CH3).     

An efficient and expeditious route to stannylallenes

(Triphenylstannyl)allenes (2) are formed in excellent yield by reacting 3-chloro-1-triphenylstannyl-1-propynes (1) with several alkylcopper(I) species RCu. An 1-alkynyl group could be introduced by using instead of RCu the zinc reagent and Pd[PPh₃]₄ as catalyst.     

Highly efficient synthesis of functionalized indolizines and indolizinones by copper-catalyzed cycloisomerizations of propargylic pyridines

The copper-catalyzed cycloisomerizations of 2-pyridyl-substituted propargylic acetates and its derivatives are described, which offer an efficient route to C-1 oxygenated indolizines with a wide range of substituents under mild reaction conditions. The presented method could be readily applied to the synthesis of indolizinones through a cyclization/1,2-migration of tertiary propargylic alcohols.     

An efficient route from coumarins to highly functionalized N-phenyl-2-quinolinones via Buchwald-Hartwig amination

Multiple substituted N-phenyl-2-quinolinones were obtained in a convenient four-step route from a variety of commercially available coumarins, utilizing customised Buchwald-Hartwig amination protocols for the key reaction. Whereas simple aminolysis of coumarins is limited to non-electron-deficient, sterically unhindered derivatives of aniline under harsh conditions, the present method allows for conversions with multiple substituted aromatic amines, as demonstrated by the example of chlorinated aminosalicylates.     

An efficient route to chiral alpha- and beta-hydroxyalkanephosphonates

Enzymatic kinetic resolution of alpha- and beta-hydroxyphosphonates in combination with ruthenium-catalyzed alcohol isomerization led to a successful dynamic kinetic resolution. A variety of racemic hydroxyphosphonates were efficiently transformed to the corresponding enantiomerically pure acetates (ee up to 99% and yield up to 87%).     

An efficient Pd-catalyzed route to silyl esters

[formula: see text] A one-step, highly selective catalytic route to silyl esters is reported. Commercially available silanes with Si-H functionality were converted to silyl or siloxy esters in the presence of Pd(OAc)2 under mild reaction conditions. This protocol was found to be equally applicable for the modification of multiple silicon centers in one framework and lead to the corresponding polysilyl esters in high yields. A comparison of catalytic efficiency of Pd(OAc)2 versus Pd on carbon was also undertaken.     

An efficient route to aminoanthraquinones and derivatives via a diels-alder reaction.

Cycloadditions of (E)-1-N-carbobenzoxyamino-1,3-butadiene to naphtoquinones followed by aromatization of the adducts and deprotection of the amino group afford regioselective syntheses of −5 and −8 substituted aminoanthraquinones.     

An efficient and highly stereocontrolled route to bulgecinine hydrochloride

(-)-Bulgecinine is a nonproteinogenic amino acid component present in bulgecins A, B, and C, antibiotic glycopeptides derived from Pseudomonas acidophila and Pseudomonas mesoacidophila. In combination with beta-lactam antibiotics, bulgecins exihibit a unique synergistic antibacterial activity against various Gram-negative microorganisms. Utilizing d-serine as a chiral template and employing a highly regio- and stereoselective intramolecular amidomercuration-oxidation protocol in the key pyrrolidine ring forming step, an efficient total synthetic route to enantiopure bulgecinine is reported herein.     

An efficient route to homologated pyranosidic conjugated enals

Mild acidic treatment of pyranosidic α-alkoxy vinyl ethers affords homologated conjugated enals ready for hetero Diels-Alder reactions and for C-glycoside synthesis.     

An efficient catalytic ylide route to vinyl epoxides

The ylide epoxidation catalyzed by telluronium salts provides a simple and extremely efficient route to vinyl epoxides in excellent yields. Both aromatic and aliphatic aldehydes work well. The best results were obtained by using 2 mol% of telluronium salts with Cs₂CO₃ in t-BuOH.