New strategic reactions for organic synthesis: catalytic asymmetric C-H activation alpha to nitrogen as a surrogate for the mannich reaction

The asymmetric C-H activation reactions of methyl aryldiazoacetates are readily induced by the rhodium prolinate catalyst Rh(2)(S-DOSP)(4) (1) or the bridged prolinate catalysts Rh(2)(S-biDOSP)(2) (2a) and Rh(2)(S-biTISP)(2) (2b). The C-H activation of N-Boc-protected cyclic amines demonstrates that the donor/acceptor-substituted carbenoids display remarkable chemoselectivity, which allows for highly regioselective, diastereoselective, and enantioselective reactions to be achieved. Furthermore, the reactions can display high levels of double stereodifferentiation and kinetic resolution. The C-H activation is caused by a rhodium carbenoid induced C-H insertion. The potential of this chemistry is demonstrated by a very direct synthesis of threo-methylphenidate.     

Isotope effects and the nature of selectivity in rhodium-catalyzed cyclopropanations

The mechanism of the dirhodium tetracarboxylate catalyzed cyclopropanation of alkenes with both unsubstituted diazoacetates and vinyl- and phenyldiazoacetates was studied by a combination of (13)C kinetic isotope effects and density functional theory calculations. The cyclopropanation of styrene with methyl phenyldiazoacetate catalyzed by Rh(2)(octanoate)(4) exhibits a substantial (13)C isotope effect (1.024) at the terminal olefinic carbon and a smaller isotope effect (1.003-1.004) at the internal olefinic carbon. This is consistent with a highly asynchronous cyclopropanation process. Very similar isotope effects were observed in a bisrhodium tetrakis[(S)-N-(dodecylbenzenesulfonyl)prolinate] (Rh(2)(S-DOSP)(4) catalyzed reaction, suggesting that the chiral catalyst engages in a very similar cyclopropanation transition-state geometry. Cyclopropanation with ethyl diazoacetate was concluded to involve an earlier transition state, based on a smaller terminal olefinic isotope effect (1.012-1.015). Density functional theory calculations (B3LYP) predict a reaction pathway involving complexation of the diazoesters to rhodium, loss of N(2) to afford a rhodium carbenoid, and an asynchronous but concerted cyclopropanation transition state. The isotope effects predicted for reaction of a phenyl-substituted rhodium carbenoid with styrene match within the error of the experimental values, supporting the accuracy of the theoretical calculations and the rhodium carbenoid mechanism. The accuracy of the calculations is additionally supported by excellent predictions of reaction barriers, stereoselectivity, and reactivity trends. The nature of alkene selectivity and diastereoselectivity effects in these reactions is discussed, and a new model for enantioselectivity in Rh(2)(S-DOSP)(4)-catalyzed cyclopropanations is presented.     

A new route to hindered tertiary amines

The Rh(2)(OAc)(4)-stabilized carbenoid derived from dimethyl diazomalonate has been found to insert into the N-H bond of sterically hindered secondary aliphatic amines to afford hindered tertiary aliphatic amines in quite satisfactory yields. For example dimethyl 2-(dicyclohexylamino)propanedioate was formed in 85% yield from dicyclohexylamine, and the severely hindered dimethyl 2-(2,2,6,6-tetramethyl-1-piperidinyl)propanedioate was formed in 38% yield from 2,2,6,6-tetramethylpiperidine. The Rh(2)(OAc)(4) - dimethyl diazomalonate reaction was found to work also for arylalkylamines and diarylamines. In these cases, small amounts of products resulting from formal insertion of the carbenoid into an aromatic C-H bond were detected. Substitution at ortho positions caused the yield of C-H insertion products to increase. Other diazo compounds, viz. ethyl diazoacetoacetate, 2-diazocyclohexane-1,3-dione, and 2-diazo-5,5-dimethylcyclohexane-1,3-dione, performed satisfactorily in Rh(2)(OAc)(4)-catalyzed reactions with arylalkylamines and diarylamines, but led to complicated reaction mixtures with dialkylamines.     

Highly diastereoselective and enantioselective C-H functionalization of 1,2-dihydronaphthalenes: a combined C-H activation/Cope rearrangement followed by a retro-Cope rearrangement

The Rh2(S-DOSP)4-catalyzed reaction of vinyldiazoacetates with dihydronaphthalenes results in a highly enantioselective (91-99.6% ee) and diastereoselective (>98% de) C-H functionalization. The apparent intermolecular C-H insertion was demonstrated to be a combined C-H activation/Cope rearrangement followed by a retro-Cope rearrangement.     

Dirhodium(II) tetra(N-(dodecylbenzenesulfonyl)prolinate) catalyzed enantioselective cyclopropenation of alkynes

Dirhodium tetrakis((S)-N-(dodecylbenzenesulfonyl)prolinate) (Rh(2)(S-DOSP)(4)) is an effective chiral catalyst for the enantioselective cyclopropenation of alkynes by methyl aryldiazoacetates. [reaction: see text]     

Rh2(S-biTISP)2-catalyzed asymmetric functionalization of indoles and pyrroles with vinylcarbenoids

Asymmetric functionalization of N-heterocycles by vinylcarbenoids in the presence of catalytic amounts of Rh(2)(S-biTISP)(2) has been successfully developed. This bridged dirhodium catalyst not only selectively enforces the reaction to occur at the vinylogous position of the carbenoid but also affords high levels of asymmetric induction.     

Combined C-H activation/cope rearrangement as a strategic reaction in organic synthesis: total synthesis of (-)-colombiasin a and (-)-elisapterosin B

The total synthesis of (-)-colombiasin A (2) and (-)-elisapterosin B (3) has been achieved. The key step is a C-H functionalization process, the combined C-H activation/Cope rearrangement, between methyl (E)-2-diazo-3-pentenoate and 1-methyl-1,2-dihydronaphthalenes. When the reaction is catalyzed by dirhodium tetrakis((R)-(N-dodecylbenzenesulfonyl)prolinate), Rh(2)(R-DOSP)(4), an enantiomer differentiation step occurs where one enantiomer of the dihydronaphthalene undergoes the combined C-H activation/Cope rearrangement while the other undergoes cyclopropanation. This sequence controls the three key stereocenters in the natural products such that the remainder of the synthesis is feasible using standard chemistry.     

Exploring stereogenic phosphorus: synthetic strategies for diphosphines containing bulky,highly symmetric substituents

Diphosphine ligands bearing highly symmetric, bulky substituents at a stereogenic P atom were prepared, exploiting established protocols, which include the use of chiral synthons such as 3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholidine-2-borane (3a) and phenylmethylchlorophosphine borane (10) and the enantioselective deprotonation of dimethylarylphosphine boranes. However, only (Bu(t)())(Me)PCH(2)CH(2)P(Bu(t)Me (8a) could be prepared from 3a. The diphosphines (S,S)-1,2-bis(mesitylmethylphosphino)ethane, ((S,S)-8b) and (S,S)-1,2-bis(9-anthrylmethylphosphino)ethane ((S,S)-8c), which contain 2,6-disubstituted aryl P-substituents, were prepared by Evans' sparteine-assisted enantioselective deprotonation of P(Ar)(Me)(2)(BH(3)) (Ar = mesityl or 9-anthryl), but the enantioselectivity did not exceed 37% ee. The asymmetrically substituted, methylene-bridged diphosphine (2R,4R)-(Ph)(CH(3))PCH(2)P(Mes)(CH(3)) ((2R,4R)-12) (Mes = mesityl) was prepared by the newly developed stereospecific reaction of the enantiomerically pure chlorophosphine borane PCl(Ph)(Me)(BH(3)) (10) with the racemic, monolithiated dimethylmesitylphosphine borane P(Mes)(Me)(CH(2)Li)(BH(3)). Diastereomerically pure (2R,4R)-12 was obtained with 86% ee. The rhodium(I) derivatives [Rh(COD)(P-P)]BF(4) containing the diphosphine ligands 8a, 8b, and 12, as well as the previously reported (S,S)-1,2-bis(1-naphthylphenylphosphino)ethane ((S,S)-8d), were prepared and tested in the enantioselective catalytic hydrogenation of acetamidocinnamates. The best catalytic result (98.6% ee) was obtained with [Rh(COD)(8d)](+) as catalyst and methyl Z-alpha-acetamidocinnamate as substrate. Some of the catalytic results are discussed in terms of the preferred conformations of the substituents at phosphorus, as calculated by molecular modeling.     

Mechanism and origin of enantioselectivity in the Rh2(OAc)(DPTI)3-catalyzed cyclopropenation of alkynes

The mechanism of cyclopropenations of alkynes with ethyl diazoacetate catalyzed by Rh2(OAc)4 and Rh2(OAc)(DPTI)3 (1) is studied by a combination of kinetic isotope effects and theoretical calculations. With each catalyst, a significant normal 13C KIE was observed for the terminal acetylenic carbon, while a very small 13C KIE was observed at the internal acetylenic carbon. These isotope effects are predicted well from canonical variational transition structures for cyclopropenations with intact tetrabridged rhodium carbenoids. A viable mechanism based on the recently proposed importance of a [2 + 2] cycloaddition on a tribridged rhodium carbenoid could not be identified. An explanation for the enantioselectivity with DPTI ligands is described.     

Enantioselective synthesis of cyclopropylphosphonates containing quaternary stereocenters using a D2-symmetric chiral catalyst Rh2(S-biTISP)2

[reaction: see text] Rh(2)(S-biTISP)(2)-catalyzed reactions of dimethyl aryldiazomethylphosphonates generate donor/acceptor-substituted rhodium carbenoid intermediates capable of cyclopropanation of styrenes in high yields (85-96%), diastereoselectivity (> or =98% de), and enantioselectivity (76-92% ee).     

Practical synthesis of Dirhodium(II) tetrakis[N-phthaloyl-(S)-tert-leucinate

An efficient and reliable procedure for the preparation of dirhodium(II) tetrakis[N-phthaloyl-(S)-tert-leucinate], Rh2(S-PTTL)(4), a universally effective catalyst for a range of enantioselective carbene transformations, is described. The N-phthaloylation of (S)-tert-leucine by the method of Bose with essentially no racemization is a key to this process.     

Enantioselective/anion-selective incorporation of tris(ethylenediamine) complexes into 2D coordination spaces between tripalladium(II) supramolecular layers with D-penicillaminate

The formation of a cocrystallized coordination compound, [Pd(3)(D-pen)(3)](2)·[M(en)(3)](ClO(4))(3) (D-H(2)pen = D-penicillamine; M = Co(III) or Rh(III)), from [Pd(3)(D-pen)(3)] and [M(en)(3)](ClO(4))(3) is reported. In this compound, only the Δ-configurational [M(en)(3)](3+) cations were incorporated when its racemic (Δ/Λ) isomer was employed. Besides this enantioselective incorporation of complex cations, this compound was found to show the selective incorporation of ClO(4)(-) as the anion species.     

Electrosynthesis of Rh2(dpf)4(R) where dpf = N,N'-diphenylformamidinate anion and R = CH3, C2H5, C3H7, C4H9 or C5H11

The electrosynthesis of Rh(2)(dpf)(4)(R) where dpf is the N,N'-diphenylformamidinate anion and R = CH(3), C(2)H(5), C(3)H(7), C(4)H(9) or C(5)H(11) was carried out in THF containing 0.2 M tetra-n-butylammonium perchlorate (TBAP) and one of several alkyl iodides represented as RI. The initial step in the reaction involved a one-electron reduction of the Rh(2)(4+) unit in Rh(2)(dpf)(4) to its Rh(2)(3+) form followed by a homogeneous reaction involving electrogenerated [Rh(2)(dpf)(4)](-) and the alkyl iodide in solution to give Rh(2)(dpf)(4)(R). The homogeneously generated Rh(2)(5+) product was then immediately reduced by a second electron at the potential where [Rh(2)(dpf)(4)(R)](-) is generated, giving [Rh(2)(dpf)(4)(R)](-) which contains a Rh(2)(4+) center as a final product of an electrochemical ECE mechanism. The electrosynthesized [Rh(2)(dpf)(4)(CH(3))](-) derivative could be reoxidized to Rh(2)(dpf)(4)(CH(3)) on the reverse potential sweep and both forms of the CH(3) bonded derivative were in situ characterized by cyclic voltammetry combined with UV-visible and/or ESR spectroscopy. The reversible Rh(2)(4+/3+) process of Rh(2)(dpf)(4) is located at E(1/2) = -1.11 V in THF, 0.2 M TBAP while the electrogenerated Rh(2)(dpf)(4)(R) products are substantially easier to reduce, with E(p) values for the Rh(2)(5+/4+) couples ranging from -0.50 to -0.54 V vs. SCE depending upon the specific R group.     

Dirhodium tetracarboxylate derived from adamantylglycine as a chiral catalyst for carbenoid reactions

[reaction: see text] The dirhodium tetracarboxylate, Rh2(S-PTAD)4, derived from adamantylglycine, is a very effective chiral catalyst for carbenoid reactions. High asymmetric induction was obtained in Rh2(S-PTAD)4-catalyzed intramolecular C-H insertion (94% ee), intermolecular cyclopropanation (99% ee), and intermolecular C-H insertion (92% ee).     

Rhodium chemzymes: Michaelis-Menten kinetics in dirhodium(II) carboxylate-catalyzed carbenoid reactions

Rhodium carboxylate-mediated reactions of diazoketones involving cyclopropanation, C-H insertion, and aromatic C-C double bond addition/electrocyclic ring opening obey saturation (Michaelis-Menten) kinetics. Axial ligands for rhodium, including aromatic hydrocarbons and Lewis bases such as nitriles, ethers, and ketones, inhibit these reactions by a mixed kinetic inhibition mechanism, meaning that they can bind both to the free catalyst and to the catalyst-substrate complex. Substrate inhibition can also be exhibited by diazocompounds bearing these groupings in addition to the diazo group. The analysis of inhibition shows that the active catalyst uses only one of its two coordination sites at a time for catalysis. Some ketones exhibit the interesting property that they selectively bind to the catalyst-substrate complex. The similarity of the kinetic constants from different types of reactions with similar diazoketones, regardless of the linking unit or the environment of the reacting alkene, suggests that the rate-determining step is the generation of the rhodium carbenoid. A very useful rhodium carboxylate catalyst for asymmetric synthesis, Rh(2)(DOSP)(4), shows slightly slower kinetic parameters than the achiral catalysts, implying that enantioselectivity of this catalyst is based on slowing reactions from one of the enantiotopic faces of the reactant, rather than any type of ligand-accelerated catalysis. A series of rhodium catalysts derived from acids with pK(a)s spanning 4 orders of magnitude give very similar kinetic constants.     

Direct Catalytic Addition of Alkylnitriles to Aldehydes by Transition‐Metal/NHC Complexes

Direct catalytic addition of alkylnitriles to aldehydes allows for an atom‐economical access to β‐hydroxynitriles under proton transfer conditions. Direct use of alkylnitriles as pronucleophiles has been hampered due to their low acidity resulting in an inability to generate α‐cyano carbanions in a catalytic manner. A transition metal/N‐heterocyclic carbene (NHC) complex prepared from [{Rh(OMe)(cod)}₂] and an imidazolium‐based carbene was identified as an effective catalyst to promote the reaction with as little as 1.25 mol % of catalyst loading. The corresponding Rh complex, derived from chiral triazolium salt, rendered the reaction enantioselective, albeit with moderate enantioselectivity.     

Asymmetric intermolecular C-H activation,using immobilized dirhodium tetrakis((S)-N-(dodecylbenzenesulfonyl)- prolinate) as a recoverable catalyst

[reaction: see text] Heterogenization of dirhodium tetrakis((S)-N-dodecylbenzenesulfonyl)prolinate) (Rh(2)(S-DOSP)(4)) can be readily achieved on a pyridine functionalized highly cross-linked polystyrene resin. The immobilized complex is readily recycled and exhibits excellent catalytic activity for asymmetric intermolecular C-H activation by means of rhodium carbenoid induced C-H insertion.     

An efficient enantioselective synthesis of (S)-(-)-acromelobic acid

[reaction: see text] A highly efficient enantioselective synthesis of (S)-(-)-acromelobic acid (1) was achieved via asymmetric hydrogenation of dehydroamino acid derivative (3) using (R,R)-[Rh(DIPAMP)(COD)]BF(4) catalyst followed by removal of protective groups in >98% ee and good over all yield. The key intermediate (3) was prepared from the commercially available citrazinic acid (4) in six steps.     

A general method for preparation of metal carbenes via solution- and polymer-based approaches

A new general, synthetically simple, and safe method for the preparation of metal carbene complexes, which is based on diphenyl sulfonium salts as carbenoid precursors, has been developed, and its scope and applications were studied. In general, deprotonation of a sulfonium salt with a base results in a sulfur ylide, which, in turn, reacts with an appropriate metal precursor to give the corresponding metal carbene complex. Thus, starting from benzyldiphenylsulfonium salt, the complexes (PCX)Rh=CHPh (X = P, N) were prepared in quantitative yield. Syntheses of Grubbs' catalyst, (PCy(3))(2)Cl(2)Ru=CHPh, and of Werner's carbene, [Os(=CHPh)HCl(CO)(P(i)Pr(3))(2)], were achieved by this method. Novel trans-bisphosphine Rh and Ir carbenes, ((i)Pr(3)P)(2)(Cl)M=CHPh, which could not be prepared by other known methods, were synthesized by the sulfur ylide approach. The method is not limited to metal benzylidenes, as demonstrated by the preparation of the Ru vinyl-alkylidene, (PCy(3))(2)Cl(2)Ru=CH-CH=CH(2), methoxycarbonyl-alkylidene, (PCy(3))(2)Cl(2)Ru=CH(CO(2)Me), and alkylidene (PCy(3))(2)Cl(2)Ru=CH(CH(3)), (PCy(3))(2)Cl(2)Ru=CH(2) compounds. The problem of recycling of starting materials as well as the issue of facile purification of the product metal carbene complex were addressed by the synthesis of a polymer-supported diarylsulfide, the carrier of the carbenoid unit in the process. Based on the sulfur ylide route, a methodology for the synthesis of metallocarbenes anchored to a polymer via the carbene ligand, using a commercial Merrifield resin, was developed.     

The F/Ph rearrangement reaction of [(Ph3P)3RhF], the fluoride congener of Wilkinson's catalyst

The fluoride congener of Wilkinson's catalyst, [(Ph(3)P)(3)RhF] (1), has been synthesized and fully characterized. Unlike Wilkinson's catalyst, 1 easily activates the inert C-Cl bond of ArCl (Ar = Ph, p-tolyl) under mild conditions (3 h at 80 degrees C) to produce trans-[(Ph(3)P)(2)Rh(Ph(2)PF)(Cl)] (2) and ArPh as a result of C-Cl, Rh-F, and P-C bond cleavage and C-C, Rh-Cl, and P-F bond formation. In benzene (2-3 h at 80 degrees C), 1 decomposes to a 1:1 mixture of trans-[(Ph(3)P)(2)Rh(Ph(2)PF)(F)] (3) and the cyclometalated complex [(Ph(3)P)(2)Rh(Ph(2)PC(6)H(4))] (4). Both the chloroarene activation and the thermal decomposition reactions have been shown to occur via the facile and reversible F/Ph rearrangement reaction of 1 to cis-[(Ph(3)P)(2)Rh(Ph)(Ph(2)PF)] (5), which has been isolated and fully characterized. Kinetic studies of the F/Ph rearrangement, an intramolecular process not influenced by extra phosphine, have led to the determination of E(a) = 22.7 +/- 1.2 kcal mol(-)(1), DeltaH(++) = 22.0 +/- 1.2 kcal mol(-)(1), and DeltaS(++) = -10.0 +/- 3.7 eu. Theoretical studies of F/Ph exchange with the [(PH(3))(2)(PH(2)Ph)RhF] model system pointed to two possible mechanisms: (i) Ph transfer to Rh followed by F transfer to P (formally oxidative addition followed by reductive elimination, pathway 1) and (ii) F transfer to produce a metallophosphorane with subsequent Ph transfer to Rh (pathway 2). Although pathway 1 cannot be ruled out completely, the metallophosphorane mechanism finds more support from both our own and previously reported observations. Possible involvement of metallophosphorane intermediates in various P-F, P-O, and P-C bond-forming reactions at a metal center is discussed.