The Enantioselectivity and the Stereochemical Course of Copper‐Catalyzed Intramolecular CH Insertions of Phenyliodonium Ylides

The Cu‐catalyzed intramolecular CH insertion of phenyliodonium ylide 1b was investigated at 0° in the presence of several chiral ligands. Enantioselectivities varied in the range 38–72%, and were higher than those resulting from reaction of the diazo compound 1c at 65°. The intramolecular insertion of the enantiomerically pure methyl diazoacetate (R)‐ 20 and of the corresponding phenyliodonium ylide (R)‐ 21 proceeded to (R)‐ 23 with retention of configuration with [Cu(hfa)₂] (hfa=hexafluoroacetylacetone=1,1,1,5,5,5‐hexafluoropentane‐2,4‐dione) and [Rh₂(OAc)₄]. These results are consistent with a carbenoid mechanism for the Cu‐catalyzed insertion with phenyliodonium ylides. However, the insertion of the perfluorosulfonated phenyliodonium ylide (R)‐ 29 afforded with [Cu(hfa)₂] as well as with [Rh₂(OAc)₄] the cyclopentanone derivative 30 as a cis/trans mixture with only 56–67% enantiomeric excess.     

Synthesis of oxygen-containing heterocyclic compounds based on the intramolecular O-H insertion and Wolff rearrangement of α-diazocarbonyl compounds

The addition products of Ti(IV)-enolate derived from β-keto α-diazo carbonyl compound to ketones or α,β-unsaturated compounds were subjected to Rh₂(OAc)₄-catalyzed and photo-induced diazo decomposition. The Rh₂(OAc)₄-catalyzed reaction afforded intramolecular O-H insertion products, while the photo-induced reaction gave Wolff rearrangement/intramolecular nucleophilic addition products. The transformations represent new approaches to tetrahydrofuran and γ-butyrolactone derivatives.     

Diazo Compounds and Phenyliodonium Ylides in Inter- and Intramolecular Cyclopropanations Catalyzed by Dirhodium(II). Synthesis and Chiral Resolution by GC versus HPLC

Summary. The dirhodium(II)-catalyzed intermolecular cyclopropanation of a set of olefins with either diazo free phenyliodonium ylides or diazo compounds afforded cyclopropanes derived from Meldrum’s acid, dimethyl malonate, (silanoxyvinyl)diazoacetates, 3,3,3-trifluoro-2-diazopropionate, ethyl diazo(triethyl)- and (dimethylphenyl)silylacetate with moderate to high yield in either racemic or enantio-enriched forms. The intramolecular cyclopropanation of triethylsilyl-substituted allyl diazoacetates in the presence of the chiral rhodium(II) catalyst [Rh₂(s-nttl)₄] in toluene afforded the corresponding cyclopropanes with up to 37% ee. An efficient chiral separation method based on enantioselective GC and HPLC was developed. The method provides information about the chemical yields of the cyclopropane products, enantioselectivity, substrate specifity, and catalytic activity of the chiral catalysts used in the inter- and intramolecular cyclopropanation reactions and avoids time-consuming work-up procedures.     

Rhodium(II)‐Catalyzed Inter‐ and Intramolecular Cyclopropanations with Diazo Compounds and Phenyliodonium Ylides: Synthesis and Chiral Analysis

Different classes of cyclopropanes derived from Meldrum's acid (=2,2‐dimethyl‐1,3‐dioxane‐4,6‐dione; 4 ), dimethyl malonate ( 5 ), 2‐diazo‐3‐(silyloxy)but‐3‐enoate 16 , 2‐diazo‐3,3,3‐trifluoropropanoate 18 , diazo(triethylsilyl)acetate 24a , and diazo(dimethylphenylsilyl)acetate 24b were prepared via dirhodium(II)‐catalyzed intermolecular cyclopropanation of a set of olefins 3 (Schemes 1 and 4–6). The reactions proceeded with either diazo‐free phenyliodonium ylides or diazo compounds affording the desired cyclopropane derivatives in either racemic or enantiomer‐enriched forms. The intramolecular cyclopropanation of allyl diazo(triethylsilyl)acetates 28, 30 , and 33 were carried out in the presence of the chiral dirhodium(II) catalyst [Rh₂{(S)‐nttl)₄}] ( 9 ) in toluene to afford the corresponding cyclopropane derivatives 29, 31 and 34 with up to 37% ee (Scheme 7). An efficient enantioselective chiral separation method based on enantioselective GC and HPLC was developed. The method provides information about the chemical yields of the cyclopropane derivatives, enantioselectivity, substrate specifity, and catalytic activity of the chiral catalysts used in the inter‐ and intramolecular cyclopropanation reactions and avoids time‐consuming workup procedures.     

Sulfur ylides. 10. Modified method for the synthesis of pyrrolizine- and indolizinediones

A modified method for the synthesis of derivatives of pyrrolizinedione and indolizinedione was proposed. The method is based on the intramolecular cyclization of keto-stabilized sulfur ylides generated in situ from diazo ketones and Me₂S in the presence of Rh₂(OAc)₄.     

Rhodium(II)-Catalyzed CH Insertions with {[(4-Nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane

The [Rh₂(OAc)₄]-catalyzed decomposition of {[(4-nitrophenyl)sulfonyl]imino}phenyl-λ³-iodane (NsN?IPh) resulted in formal insertions into CH bonds, activated by phenyl or vinyl groups, or by O-substituents. Scope and limitations of the reaction were investigated. Yields of up to 84% were achieved in the most favorable cases. Yields were enhanced by electron-releasing substituents and decreased by steric hindrance. Aziridination competed with allylic insertion with olefinic substrates. The insertion reaction proceeded with retention of configuration. With chiral Rh^II catalysts, a modest asymmetric induction was observed. A mechanism involving direct insertion by a Rh-complexed nitrene into the CH bond is proposed.     

Tetrakis[(4S)-4-phenyloxazolidin-2-one]dirhodium(II) and Its Catalytic Applications for Metal Carbene Transformations

The synthesis and X-ray structure of the binuclear complex tetrakis[(4S)-4-phenyloxazolidin-2-one]-dirhodium(II) ([Rh₂{(4S)-phox}₄]) are reported. Structure-selectivity comparisons are made for typical metal carbene transformations, such as inter- and intramolecular cyclopropane formation, intermolecular cyclopropene formation and intramolecular C–H insertions of diazoacetates and diazoacetamides. The enantioselectivity achieved in the [Rh₂{(4S)-phox}₄]-catalyzed reactions is intermediate between that of [Rh₂{(5S)-mepy}₄] and [Rh₂{(4R)-bnox}₄], which were described previously (mepy = methyl 5-oxopyrrolidine-2-carboxylate; bnox = 4-benzyloxazolidin-2-one). In contrast to other catalyzed intermolecular cyclopropane formations, those using [Rh₂{(4S)-phox}₄] result preferentially in formation of the cis-cyclopropane.     

Reactions of Diazo Compounds with Imines. Preliminary communication

The [Rh₂(OAc)₄]-catalyzed addition of methyl diazoacetate to N-benzylideneaniline ( 1a ) afforded the imine cis- 2 in 35% yield. Under catalysis by chiral Rh^II catalysts, however, only racemic 1a was produced, and the yield was low. In the presence of dimethyl maleate, aziridine formation was suppressed, and an intermediate ylide 6 was trapped as cycloadduct 7 . No aziridines were obtained, however, from 1b, 1c , and 3 . The iminium salt 8 reacted with (trimethylsilyl)diazomethane in the absence of [Rh₂(OAc)₄] via dipolar cycloaddition followed by extrusion of N₂ to 10 .     

Rh(II)-catalyzed formation and rearrangement of trifluoroacetyl-containing sulfur ylides

A facile, convenient, efficient, and high yielding Rh(II)-catalyzed formation and rearrangement of trifluoroacetyl-containing sulfur ylides are reported. It is a Rh₂(OAc)₄-catalyzed [2,3]-sigmatropic rearrangement of sulfur ylide intermediates generated from α-diazo compounds.     

重氮化合物与醇的插入反应合成α-烷氧基-β-二羰基化合物

研究了在过渡金属配合物催化下α-重氮-β-二羰基化合物与醇的插入反应, 考察了重氮化合物的结构、醇的结构、催化剂的性质、反应溶剂和反应温度对这一反应的影响. 发现当重氮化合物与甲醇的物质的量比为1∶10, 1 mmol% Rh₂(OAc)₄为催化剂和回流的苯的条件下, 反应能够以高的化学产率生成α-甲氧基-β-二羰基化合物. 手性醇衍生的重氮乙酰乙酸酯反应的产物中两种非对应异构体的比例为3∶2～1∶1.     

Rhodium(II)-Catalyzed Decomposition of β,γ-Unsaturated Diazo Compounds

The Rh^II-catalyzed decomposition of β,γ-unsaturated diazo ketones 1 in the presence of MeOH leads via vinylogous Wolff rearrangement to γ,δ-unsaturated esters 6 (Schemes 1 and 2). A modest asymmetric induction is achieved when the reaction is carried out with chiral tetrakis(pyrrolidinecarboxylato)- or tetrakis(oxazolidinonato)dirhodium(II) complexes. Vinyl and phenyl diazoacetates 11 and 20 , respectively, or 1-diazo-3-phenyl-propan-2-one ( 25 ), when subjected to the same reaction conditions, react by OH insertion with MeOH (Schemes 3–5). In the absence of MeOH, phenyl diazoacetates 20 and 25 undergo intramolecular CH insertion to 22 and 26 , respectively. Intramolecular CH insertion occurs with N-aryldiazoamides 23 even in the presence of MeOH (Scheme 5).     

Rhodium-catalyzed cyclopropanations of 2-aryl-2H-chromenes with dialkyl malonate esters. A comparison of α-diazo derivatives and phenyliodonium ylides

Rhodium-catalyzed reactions of 2-aryl-substituted 2H-chromenes with α-diazo esters prepared from dimethyl and tert-butyl methyl malonates were investigated, and the results were compared with reactions carried out with phenyliodonium ylides prepared from the same esters. The phenyliodonium ylide prepared from dimethyl malonate was found to give superior yields of cyclopropane products compared to the corresponding α-diazo equivalent. However, this result was reversed with tert-butyl methyl malonate when Rh₂(S-TBSP)₄ was used to decompose the diazo compound. All reactions gave 1,1-cyclopropane diesters as single diastereomers.     

Enantioselectivity and cis/trans-Selectivity in Dirhodium(II)-Catalyzed addition of diazoacetates to olefins

The Rh¹¹-catalyzed carbenoid addition of diazoacetates to olefins was investigated with [Rh₂{(4S)-phox}₄] ( 1 ;phox = tetrakis[(4S)-tetrahydro-4-phenyloxazol-2-one]), [Rh₂{(2S)-mepy}₄] ( 2 ; mepy = tetrakis[methyl (2S)-tetrahydro-5-oxopyrrole-2-carboxylate]), and [Rh₂(OAc)₄] ( 3 ). While catalysis with 2 and 3 afford preferentially trans-cyclopropanecarboxylates, the cis-isomers are the major products with 1 . In general, the enantioselectivities achieved with 1 and 2 are comparable. Additions catalyzed by 1 are strongly sensitive to steric effects. Highly substituted olefins afford cyclopropanes in only poor yield. The preferential cis-selectivity observed in reactions catalyzed by 1 is attributed to dominant interactions between the ligand of the catalyst and the substituents of both olefin and diazoacetate, which overrule the steric interactions between olefin and diazoacetate in the transition state for carbene transfer.     

Synthesis of oxazole and furan derivatives via Rh2(OAc)4-catalyzed C≡X bond insertion of cyclic 2-diazo-1,3-diketones with nitriles and arylacetylenes

Abstract

A convenient and efficient procedure for the synthesis of 2-substituted-6,7-dihydrobenzo[d]oxazol-4(5H)-ones and 2-aryl-6,7-dihydrobenzofuran-4(5H)-ones through a Rh₂(OAc)₄-catalyzed C≡X (X?=?N, C) insertion of cyclic 2-diazo-1,3-diketones with nitriles and aromatic alkynes has been developed. This reaction uses readily available starting materials and stable cyclic 2-diazo-1,3-diketone compounds, with desired products formed in good to high yields. A tentative mechanism involving a C≡X bond insertion and 1,5-dipolar electrocyclization/ring opening and cyclization sequence for this reaction is proposed.     

The methoxycarbonylcarbene insertion into 1,3-dithiolane and 1,3-oxathiolane rings

Treatment of substituted 1,3-dithiolanes and 1,3-oxathiolanes with methyl diazoacetate in the presence of Rh₂(OAc)₄ effects ring expansion to the corresponding substituted 1,4-dithiane-2-carboxylates and 1,4-oxathiane-3-carboxylates. The sulfur ylides initially generated in these reactions undergo Stevens rearrangement in competition with both [2,3]-C-C-sigmatropic rearrangement and intramolecular fragmentation. In the case of 2-styryl-substituted 1,3-oxathiolane and 1,3-dithiolane, ring expansion on one-, three- and four-carbons subsequently takes place.     

Intramolecular C–H insertion catalyzed by dirhodium(II) complexes using CO2 as the reaction media

Abstract

In this work, the intramolecular C–H insertion of diazoacetamides catalyzed by dirhodium(II) complexes and using CO₂ as solvent is disclosed. The expected lactams were obtained in yields over 97%. The asymmetric intramolecular C–H insertion was also achieved and the β-lactam 14 was obtained in >97% yield and 65% ee using the chiral dirhodium(II) catalyst Rh₂(S-PTTL)₄. Finally, the dirhodium(II) complex Rh₂(OAc)₄ was used in two consecutive cycles in which complete conversion to the lactam was observed.     

Intramolecular asymmetric C–H insertion of N-arylalkyl,N-bis(trimethylsilyl)methyldiazoamides mediated by chiral rhodium(II) catalysts. Synthesis of (R)-β-benzyl-γ-aminobutyric acid

The enantio- and site-selectivity of the intramolecular C–H insertion reactions of acyclic N-arylalkyl, N-bis(trimethylsilyl)methyl α-diazoacetamides, and α-carboalkoxy-α-diazoacetamides 1a–g, catalyzed by chiral Rh(II) carboxamidates and Rh(II) carboxylates were studied. In general, the reaction showed good to excellent chemoselectivity. Regioselectivity for most of the reactions was high, but was also found to be influenced by the structure of the diazo substrate and the chiral Rh(II) catalyst employed. The highest enantioselectivity for the reactions catalyzed by chiral Rh(II) carboxamidates was 69% and Rh₂(4R-MEOX)₄ was found to be the most effective. For the chiral Rh(II) carboxylate catalyzed reactions, the highest ee obtained was 75% and Rh₂(S-PTTL)₄ is the optimal catalyst. The method was applied toward the synthesis of a GABA analogue, (R)-β-benzyl-γ-aminobutyric acid.     

Construction of fluorinated pyrazole derivatives via a one-pot tandem C–H insertion/electrophilic fluorination reaction

A series of 4-fluoro-pyrazole derivatives were synthesized in moderate to good yields via a one-pot tandem procedure involving an Rh₂(OAc)₄-catalyzed C–H insertion and an electrophilic fluorination with N-fluorobenzenesulfonimide (NFSI).     

Radiation reduction of binuclear Rh(II) complexes in aqueous-methanol solutions

Radiation reduction of binuclear [Rh₂(OAc)₂(phen)₂(H₂O)₂](OAc)₂, [Rh₂(OAc)(tpy)₂Cl₂]Cl·2H₂O and [Rh₂Cl₂(HCOO)₂(bpy)₂]·4H₂O complexes in aqueous-methanol solution have been studied. The reduction yields as equal to ca. 6 equiv/100eV and the rate constants of reactions: complex+e _solv ⁻ as equal to 2.9·10¹⁰, 3.2·10¹⁰ and 3.7·10¹⁰ M⁻¹·s⁻¹, respectively, have been determined. On the basis of electronic spectra it has been shown that Rh(II) compounds were reduced giving several Rh(I) complexes being in equilibrium. The mechanism of the processes has been discussed.     

C2-Symmetric 4,4′,5,5′-Tetrahydrobi(oxazoles) and 4,4′,5,5′-Tetrahydro-2,2′-methylenebis[oxazoles] as Chiral Ligands for Enantioselective Catalysis Preliminary Communication

The synthesis of a series of enantiomerically pure, C₂-symmetric 4,4′,5,5′-tetrahydro-2,2′-methylenebis[oxazoles] and 4,4′,5,5′-tetrahydro-2,2′-bi(oxazoles) is reported. Copper complexes with anionic tetrahydromethylenebis[oxazole] ligands are efficient catalysts for the enantioselective cyclopropane formation from olefins and diazo compounds (up to 96% ee in the reaction of styrene with menthyl diazoacetate). Tetrahydrobi(oxazole)iridium(I) complexes were found to catalyze transfer hydrogenations of aryl alkyl ketones with i-PrOH (up to 91% ee). Tetrahydrobi(oxazole)palladium complexes can be used as enantioselective catalysts for allylic nucleophilic substitution (up to 77% ee in the reaction of PhCH?CHCH(OAc)Ph with NaHC(COOMe)₂).