Rhodium Doyle catalysts

We selected a series of rhodium(II) carboxylates, rhodium(II) carboxamidate [5d] (Doyle catalysts 42h, 42i, 42j), and the bridged rhodium(II) carboxylate (Lahuerta catalyst) 42g, as representatives of the various rhodium(II) catalysts generally utilized. Most of the carboxylate and Doyle catalysts were commercially available and were purified by silica gel chromatography prior to use. The Lahuerta catalyst was prepared according to the literature procedure [23]. 

Doyle s rhodium(n) carboxamidate complexes are undisputedly the best catalysts for enantioselective cyclizations of acceptor-substituted carbenoids derived from diazo esters and diazoacetamides, displaying outstanding regio- and stereocontrol.4 These carboxamidate catalysts consist of four classes of complexes pyrrolidinones... 

The development of the first alkyne silylformylation reaction was reported in 1989 by Matsuda [27]. Alkynes were treated with Me2PhSiH and Et3N with 1 mol% Rh4(CO)i2 under CO pressure to produce yS-silyl-a,/ -unsaturated aldehydes (Scheme 5.20). A second report from Ojima detailed the development of rhodium-cobalt mixed metal clusters as effective catalysts for alkyne silylformylation [28]. Shortly thereafter, Doyle reported that rhodium(II) perfluorobutyrate was a highly efficient and selective catalyst for alkyne silylformylation under remarkably mild reaction conditions (0°C, 1 atm CO) [29]. In all these reports, terminal alkynes react regiospedfically with attachment of the silane to the unsubstituted end of the alkyne. The reaction is often (but not always) stereospecific, producing the cis-product preferentially. 

Che has reported that both achiral and chiral rhodium catalysts function competently for intramolecular aziridination reactions of alkyl- and arylsulfonamides (Scheme 17.29) [59, 97]. Cyclized products 87 are isolated in 90% yield using 2 mol% catalyst, PhI(OAc)2, and AI2O3. Notably, reactions of this type can be performed with catalyst loadings as low as 0.02 mol% and display turnover numbers in excess of 1300. In addition, a number of chiral dimeric rhodium systems have been examined for this process, with some encouraging results. To date, the best data are obtained using Doyle s Rh2(MEOX)4 complex. At 10 mol% catalyst and with a slight excess of Phl=0, the iso-... 

Asymmetric C-H insertion using chiral rhodium catalysts has proven rather elusive (Scheme 17.30). Dimeric complexes derived from functionalized amino acids 90 and 91 efficiently promote oxidative cychzation of suifamate 88, but the resulting asymmetric induction is modest at best ( 50% ee with 90). Reactions conducted using Doyle s asymmetric carboxamide systems 92 and 93 give disappointing product yields ( 5-10%) and negligible enantiomeric excesses. In general, the electron-rich carboxamide rhodium dimers are poor catalysts for C-H amination. Low turnover numbers with these systems are ascribed to catalyst oxidation under the reaction conditions. 

Rhodium(II) carboxylate dimers and their carboxamide counterparts have been demonstrated to be exceptionally useful catalysts for carbene transfer processes involving diazocarbonyl substrates [1]. Doyle s seminal work identified Rh2(OAc)4 as the catalyst of choice for a variety of cyclopropanation, C-H insertion, and ylide rearrangement transformations using diazoketones or diazoesters [2]. Important contributions by Taber [3], Padwa [4], and Davies [5] further established the superior catalytic activity of dirho-dium catalysts and the excellent selectivity of rhodium-[Pg.417]

Hodgson et al. (138) chose to investigate a system that had previously been shown to undergo an effective intramolecular addition of a tethered olehn (Scheme 4.72). In his first attempt, using Doyle s Rh2[(5/ )-MEPY]4, the yield of cycloadduct 270 obtained was comparable to that with rhodium acetate, but no asymmetric induction was observed. Changing to the Davies catalysts in dichloromethane resulted in a... 

There are many aspects of these Rh-mediated cyclizations that are yet to be explored. What factors, for instance, govern the ratio of 25 to 26 (Scheme 1)1 Would an Rh catalyst that was more readily polarizable and so more sensitive to electronic effects give a higher proportion of 25 The enantioselective lactone cyclizations of Doyle [15] are particularly intriguing. Attempts toward enantioselective carbocyclization using a chiral rhodium catalyst have to date [16] not... 

An elegant and efficient way for preparation of sulfonium ylides under mild conditions is the so-called Doyle-Kirmse reaction [25,26], which involves transition metal catalyzed decomposition of diazo compounds (usually a-diazocarbonyls) in the presence of sulfides. For the catalytic generation of matallocarbenes from diazo compounds, copper catalysts have traditionally been employed. More recently, rhodium and ruthenium compoimds were reported to be efficient catalysts, especially for the generation of sulfoniiun ylides [27-29]. 

The literature on catalytic cyclization of a-diazoketones has a rather recent history, with the majority of papers originating from the 1980s. The copper catalysts originally used (e. g., CUSO4) suffer from an unspecific product spectrum [5] and have largely been replaced by rhodium catalysts, mainly through the work of Doyle and colleagues [3]. 

Chiral catalysts with structures related to rhodium(II) acetate should principally afford optically pure enantiomeric > -lactones from diazoacetates of type 21. As a matter of fact, Doyle et al. have obtained alkoxy-substituted y-lactones 22 in 85-90% ee (eq. (10)) upon using a Rh2X4-catalyst derived from chiral 2-pyrroli-dinones [18], Related results suggest that the catalyst has a rigid stereochemistry throughout the catalytic cycle [19], which conclusion had already been drawn for enantioselective cyclopropanation [20] (cf. Section 3.1.7). 

Recent and interesting examples of Stevens reactions were reported by Doyle who reacted dithianes with ethyl diazoacetate in the presence of a rhodium catalyst. Seven-membered ring bissulfides were... 

Doyle has shown that the rhodium-catalyzed reaction of allylic sulfides and amines with ethyl diazoacetate produced smoothly the products of 3,2-rearrangement. In contrast with the copper-catalyzed reaction, allylic amines can be used and the yields are good to high (Scheme 44) virtually no cyclopro-panation is observed. These observations demonstrate the superiority of rhodium catalysts compared with either copper ones or the use of light. 

Quite recently, Doyle and coworkers found that dirhodium(II) complexes such as rhodium(II) acetate and Rh2(4S-MEAZ)4 (14) also act as highly active Lewis addcatalysts (1 mol%) for the reaction of trimethylsilylketene and ethyl glyoxalate, affording the P-lactonel5 (Table4.6) [42]. However, theuseofthechiral Rh-complex, Rh2(4S-M EAZ)4, alone afforded almost no asymmetric induction (5% ee for (S)-isomer) (entry 2). The use of quinine (10 mol%) as a cobase catalyst to activate the ketene simultaneously provided exceptional enantiocontrol (99% ee) and enhanced reactivity (entry 5). 

Doyle s catalysts have also been applied to asymmetric intermolecular cyclo-propanation, mainly in the styrene-diazoester reaction. Diazoesters include EDA, dicyclohexylmethyl diazoacetate, and d-menthyl diazoacetate. In general the effectiveness of Doyle s rhodium (II) carboxamidates in enantiocontrol is lower... 

The most significant breakthrough in this area was Doyle s introduction of chiral rhodium (II) carboxamidates (Fig. 4). These catalysts show an exceptional ability to direct highly enantioselective intramolecular cyclopropanation of al-lylic and homoallylic diazoesters, Eq. (19), and diazoamides, Eq. (20). 

In a useful extension of this methodology for enantioselection in intramolecular cyclopropanation, Doyle s group have used chiral rhodium (II) carbox-amidates to effect enantiomer differentiation in reactions of racemic secondary allylic diazoacetates [47]. The catalyst-enantiomer matching approach has also been applied very successfully to intramolecular C-H insertion reactions vide infra). The (R)- and (S)-enantiomers, (10) and (11), respectively, of cyclohex-2-en-1 -yl diazoacetate are displayed in Scheme 7. On exposure to Rh2(4i -MEOX)4 the (R)-enantiomer (10) undergoes cyclopropanation to form tricyclic ketone... 

The most commonly used chiral catalysts are the amino acid based rhodium (II) carboxylates of Hashimoto and Ikegami, and McKervey, and the chiral rhodium (II) carboxamidates of Doyle. The amino acid based catalysts exhibit their highest levels of stereocontrol with non-terminal diazo ketones of structure RCOCN2R1 where Rj H, while the rhodium (II) carboxamidates display high enantiocontrol with diazoacetates. 

Doyle s chiral rhodium (II) carboxamidates have proved to be exceptionally successful for asymmetric C-H insertion reactions of diazoacetates and some diazoacetamides leading to lactones and lactams, respectively. With 2-alkoxyethyl diazoacetates and the Rh2(5S- and 5R-MEPY)4 catalysts, for example, highly enantioselective intramolecular C-H insertion reactions occur, the 5S-catalyst, Eq. (40), and 5R-catalyst furnishing the S- and R-lactone, respectively [58]. A polymer-bound version of Rh2(5S-MEPY)4 has also been applied to the cycliza-tion in Eq. (40) to yield the lactone with 69% ee (R=Me) the catalyst could be recovered by filtration and reused several times, but with decreasing enantiose-lection [59]. 

Two syntheses of the bioactive small molecule (+)-imperanene (197), isolated from Imperata cylindrica, demonstrate that intra- and intermolecular carbenoid C-H insertion can be used as two different means to the same end. The Doyle group reported an intramolecular approach toward this natural product, with diazoester 198 as the cyclization precursor (Scheme 49, top) [140], In the key event, Rh2(4S -MPPIM)4-catalyzed carbenoid insertion led to lactone 199 in 68% yield and 93% ee. Other rhodium catalysts were found to give inferior yields and enan-tioselectivities. Elaboration of 199 to (-i-)-imperanene provided the natural product in 12 steps and approximately 16% overall yield. 

The rhodium-catalyzed cyclization of cycloalkyldiazoacetates 1135 gives mixtures of cis- and //ms-fused lactones when 3.61 is used to generate the chiral catalyst. Recently, Doyle and coworkers found that cis-fused lactones 11.36 could be obtained with a high selectivity when the catalyst was generated from the methyl l-acetylimidazolidin-2-one-4-carboxylate 11.37 [1672],... 

Since then the field of enantioselective catalysis with rhodium(II) complexes containing mepy and mepy-like ligands has been extended appreciably, in particular by the group of M. P. Doyle. Thus, the recent references in this field can be found by searching for the name of the main author M. P. Doyle. The catalysts [Rh2(55-mepy)4] and [Rh2(5R-mepy)4] have been commercialized (REGIS Chemical Company, Morton Grove, IL 60053, USA). 

Experimental evidence for the thermal isomerization from a substituted diazirine to the corresponding diazoalkane was also reported by Doyle et al. (1989 a). They used diazirines as stable diazoalkane precursors which, with the help of the catalyst rhodium(ii) perfluorobutyrate, undergo carbenoid-type reactions. An example is reaction (5-22). Less competition is observed from side reactions that were dominant... 

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