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]. 

In intramolecular cyclopropanation, Doyle s catalysts (159) show outstanding capabilities for enantiocontrol in the cyclization of allyl and homoallyl diazoesters to bicyclic y-and <5-lactones, respectively (equations 137 and 138)198 205. The data also reveal that intramolecular cyclopropanation of Z-alkenes is generally more enantioselective than that of E-alkenes in bicyclic y-lactone formation198. Both Rh(II)-MEPY enantiomers are available and, through their use, enantiomeric products are accessible. In a few selected cases, the Pfaltz catalyst 156 also results in high-level enandoselectivity in intramolecular cyclopropanation (equation 139)194. On the other hand, the Aratani catalyst is less effective than the Doyle catalyst (159) or Pfaltz catalyst (156) in asymmetric intramolecular cyclo-propanations201. In addition, the bis-oxazoline-derived copper catalyst 157b shows lower enantioselectivity in the intramolecular cyclopropanation of allyl diazomalonate (equation 140)206. 

Doyle and coworkers found that the Rh2(4S-MACIM)4 catalyst (9.94) provides higher ds-selectivity than related Doyle catalysts (9.25-9.27) in the reaction of substrate (9.95), where the insertion takes place highly selectively into one of four C-H bonds, affording essentially a single product (9.96). °... 

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

The role of Lewis acids in the formation of oxazoles from diazocarbonyl compounds and nitriles has primarily been studied independently by two groups. Doyle et al. first reported the use of aluminium(III) chloride as a catalyst for the decomposition of diazoketones.<78TL2247> In a more detailed study, a range of Lewis acids was screened for catalytic activity, using diazoacetophenone la and acetonitrile as the test reaction.<80JOC3657> Of the catalysts employed, boron trifluoride etherate was found to be the catalyst of choice, due to the low yield of the 1-halogenated side-product 17 (X = Cl or F) compared to 2-methyI-5-phenyloxazole 18. Unfortunately, it was found that in the case of boron trifluoride etherate, the nitrile had to be used in a ten-fold excess, however the use of antimony(V) fluoride allowed the use of the nitrile in only a three fold excess (Table 1). 

Doyle MP (2004) Metal Carbene Reactions from Dirhodiimi(II) Catalysts. 13 203-222 Drudis-Sol6 G, Ujaque G, Maseras F, Lledos A (2005) Enantioselectivityin the Dihydroxyla-tion of Alkenes by Osmiimi Complexes. 12 79-107... 

In 2005, Doyle et al. reported an original sequence of two successive intramolecular cyclopropanations involving a bis(diazoacetates), using a sterically encumbered oxaimidazolidine carboxylate dirhodium(II) catalyst, Rh2[(45, 5)-BSPIM]4. An excellent result, depicted in Scheme 6.16, was obtained resulting from a double diastereoselection. 

It has been pointed out earlier that the anti/syn ratio of ethyl bicyclo[4.1,0]heptane-7-carboxylate, which arises from cyclohexene and ethyl diazoacetate, in the presence of Cul P(OMe)3 depends on the concentration of the catalyst57). Doyle reported, however, that for most combinations of alkene and catalyst (see Tables 2 and 7) neither concentration of the catalyst (G.5-4.0 mol- %) nor the rate of addition of the diazo ester nor the molar ratio of olefin to diazo ester affected the stereoselectivity. Thus, cyclopropanation of cyclohexene in the presence of copper catalysts seems to be a particular case, and it has been stated that the most appreciable variations of the anti/syn ratio occur in the presence of air, when allylic oxidation of cyclohexene becomes a competing process S9). As the yields for cyclohexene cyclopropanation with copper catalysts [except Cu(OTf)2] are low (Table 2), such variations in stereoselectivity are not very significant in terms of absolute yields anyway. 

Rh-carboxamide complexes (86), (103), and (104) introduced by Doyle el al. are by far the best cata ysts or t e eye ization of 2- and 3-alkenyl diazoesters and diazoamides, although the catalyst... 

Drent E, van Broekhoven JAM, Doyle MJ, Wong PK (1995) In Fink G, Muhlhaupt R, Britzinger HH(eds) Ziegler catalysts. Springer, Berlin Heidelberg New York, pp 481-496... 

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... 

Arylation of activated double bonds with diazonium salts in the presence of copper catalysts is known as the Meerwin reaction. The reaction is postulated to either proceed through an organocopper intermediate or through a chlorine atom transfer from chiral CuCl complex to the a-acyl radical intermediate. Brunner and Doyle carried out the addition of mesityldiazonium tetrafluoroborate with methyl acrylate using catalytic amounts of a Cu(I)-bisoxazoline ligand complex and were able to obtain 19.5% ee for the product (data not shown) [79]. Since the mechanism of the Meerwin reaction is unclear, it is difficult to rationalize the low ee s obtained and to plan for further modifications. 

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... 

Doyle et al. have demonstrated the catalyst-dependent diastereoselectivity in Rh(ii) complex-catalyzed reaction of cinnamyl methyl ether 36 and ethyl diazoacetate 35 (Scheme 6). " The change of the diastereoselectivity of the products 38a and 38b with different Rh(ii) catalyst provides strong evidence that Rh(ii) catalyst is associated with the ylide in the rearrangement process. The moderately high level of asymmetric induction (4-69% ee) is also observed with allyl iodide (Equation (4)). In this case, the chiral metal complex must be in the product-forming step, because free iodo ylide is achiral. 

Desymmetrization strategy in enantioselective oxonium ylide formation/[l,2]-shift reaction has been reported by Doyle and co-workers.With dirhodium(ii) tetrakis[methyl l-(3-phenylpropanoyl)-2-oxoimidazolidine-4(3 )-carboxylate] [Rh2(43 -MPPIM)4] as the catalyst, up to 88% ee is obtained (Equation (7)). 

Allylic oxidation offers another route to ketones. Michael P. Doyle of the University of Maryland has found (/ Am. Chem. Soc. 2004,126, 13622) that Rh caprolactam is a very active (0.1 mol %) catalyst for this conversion. 

Doyle and co-workers have employed Rh2(pfb)4 as a highly selective catalyst for the room temperature synthesis of silyl ethers from alcohols and triethylsilane.159 The selectivity of the catalyst is demonstrated by reactions of olefinic alcohols, in which hydrosilylation is not competitive with silane alcoholysis when equimolar amounts of silane and alcohol are employed. High yields (>85%) of triethylsilyl ethers are obtained from reactions of alcohols such as benzyl alcohol, 1-octanol, 3-buten-l-ol, cholesterol, and phenol. Tertiary alcohols are not active in this system. 

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