Rhodium catalysis cyclopropanation

Allyl methyl ether (ethyl diazoacetate, rhodium catalysis) and allyl terf-butyl ether (dimethyl diazomalonate, copper catalysis) yield cyclopropanes exclusively. With y-substituted allyl methyl ethers, C-0 insertion is generally strongly favored over cyclopropanation, even with tetraacetatodirhodium as catalyst.In view of these findings, the cyclopropanation of ( )- ,4-dibenzyloxybut-2-ene in moderate yield, only, to give (la,2a,3j5)-31 is notable. 

Lewis acid catalysis of the vinylcyclopropane rearrangement has been reported for vinylcy-clopropane 3, a key intermediate in the synthesis of the plant hormone antheridogen-An. Most recently, a report has appeared describing a highly stereoselective, diethylaluminum chloride promoted rearrangement of vinylcyclopropanes 5 to cyclopentenes 6. The cyclopen tenes appear to be formed directly, in some cases as a consequence of the rhodium-promoted cyclopropanation of enol ethers (see Section 2.4.3.1.3.). ... 

Cyclopropyl ketones are readily isomerized to dihydrofuran derivatives thermally or under catalytic conditions.For example, cyclopropyl ketones 2 and 4 yield dihydrofurans 3 and 5, respectively, thermally or under rhodium catalysis. Such rearrangements occur normally under acid catalysis whereas thermolysis favors the vinylcyclopropane to cyclopentene rearrangement, except for highly functionalized (R = SOjPh) cyclopropanes. 

Rhodium Catalysts. In the early stage of developing rhodium catalysis for intramolecular cyclopropanation, Rh2(OAc>4 was widely used in the construction of bridged or fused polycyclic molecules. For instance, it was used as a catalyst to obtain a tricyclic ketone used as an intermediate of eucalyptol in good... 

One way to form a seven-membered ring would be to add a five-atom component to a two-atom component. This has been achieved using a vinyl cyclopropane 11.141 as the five atom component, and rhodium catalysis... 

Products of a so-called vinylogous Wolff rearrangement (see Sect. 9) rather than products of intramolecular cyclopropanation are generally obtained from P,y-unsaturated diazoketones I93), the formation of tricyclo[2,1.0.02 5]pentan-3-ones from 2-diazo-l-(cyclopropene-3-yl)-l-ethanones being a notable exception (see Table 10 and reference 12)). The use of Cu(OTf), does not change this situation for diazoketone 185 in the presence of an alcoholl93). With Cu(OTf)2 in nitromethane, on the other hand, A3-hydrinden-2-one 186 is formed 160). As 186 also results from the BF3 Et20-catalyzed reaction in similar yield, proton catalysis in the Cu(OTf)2-catalyzed reaction cannot be excluded, but electrophilic attack of the metal carbene on the double bond (Scheme 26) is also possible. That Rh2(OAc)4 is less efficient for the production of 186, would support the latter explanation, as the rhodium carbenes rank as less electrophilic than copper carbenes. 

Another common method for forming cyclopropanes is to react a-diazoketones or esters with olefins under the influence of copper or, better yet, rhodium or ruthenium catalysis. Again a metal carbenoid intermediate is produced which reacts with tire olefin. 

The most frequently used metallic catalysts for acyldiazo- and (alkoxycarbonyl)dia-zomethanes are complexes or salts of rhodium, palladium and copper. Alkenylboronic esters A-silylated allylamines and acetylenes are successfully cyclopropanat-ed with diazocarbonyl compounds under catalysis of one of those metal derivatives. Newly developed metallic catalysts for diazoacetic esters include polymer-bound, quantitatively recoverable Rh(II) carboxylate salts ", Cu(II) supported on NATION ion exchange poly-mer ruthenacarborane clusters, Rh2(NHCOCH3)4 which produces cyclopropanes with substantially enhanced trans (anti) selectivity as shown below and (rj -CsHs)... 

This collection begins with a series of three procedures illustrating important new methods for preparation of enantiomerically pure substances via asymmetric catalysis. The preparation of 3-[(1S)-1,2-DIHYDROXYETHYL]-1,5-DIHYDRO-3H-2.4-BENZODIOXEPINE describes, in detail, the use of dihydroquinidine 9-0-(9 -phenanthryl) ether as a chiral ligand in the asymmetric dihydroxylation reaction which is broadly applicable for the preparation of chiral dlols from monosubstituted olefins. The product, an acetal of (S)-glyceralcfehyde, is itself a potentially valuable synthetic intermediate. The assembly of a chiral rhodium catalyst from methyl 2-pyrrolidone 5(R)-carboxylate and its use in the intramolecular asymmetric cyclopropanation of an allyl diazoacetate is illustrated in the preparation of (1R.5S)-()-6,6-DIMETHYL-3-OXABICYCLO[3.1. OJHEXAN-2-ONE. Another important general method for asymmetric synthesis involves the desymmetrization of bifunctional meso compounds as is described for the enantioselective enzymatic hydrolysis of cis-3,5-diacetoxycyclopentene to (1R,4S)-(+)-4-HYDROXY-2-CYCLOPENTENYL ACETATE. This intermediate is especially valuable as a precursor of both antipodes (4R) (+)- and (4S)-(-)-tert-BUTYLDIMETHYLSILOXY-2-CYCLOPENTEN-1-ONE, important intermediates in the synthesis of enantiomerically pure prostanoid derivatives and other classes of natural substances, whose preparation is detailed in accompanying procedures. 

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

Hubert in 1976 reported that rhodium acetate efficiently catalyzes diazo insertion into an alkene, to give the cyclopropane. In 1979, Southgate and Ponsford reported that rhodium acetate also catalyzes diazo insertion into a C—H bond. Prompted by these studies, Wenkert then demonstrated that cyclization of (58) to (59) proceeded much more efficiently with the rhodium carboxylates than it had with copper salt catalysis (equation 23). ... 

The results in the diazomethane reactions involving zinc(II) chloride catalysis have been explained by invoking a carbenoid intermediate. The properties of such a species will, of course, be sensitive to the nature of the metal and this might explain the different regioselectivity observed when diphenyldiazomethane is decomposed with rhodium and palladium salts in the presence of 5-methylenebicyclo[2.2.1]hept-2-ene (9). With rhodium(II) acetate as catalyst the exocyclic double bond is attacked exclusively, whereas palladium(II) chloride catalysis directs cyclopropanation to the endocyclic double bond. ... 

In the formative years of diazo chemistry it was recognized that copper catalysis both reduced the decomposition temperature of a diazo compound and allowed much more efficient intra- and intermolecular cyclopropanation reactions. With the advent of rhodium- and palladium-based catalysts, the purely thermal method has lost even more ground. 

Enantioselective carbenoid cyclopropanation of achiral alkenes can be achieved with a chiral diazocarbonyl compound and/or chiral catalyst. In general, very low levels of asymmetric induction are obtained, when a combination of an achiral copper or rhodium catalyst and a chiral diazoacetic ester (e.g. menthyl or bornyl ester ) or a chiral diazoacetamide ° (see Section 1.2.1.2.4.2.6.3.3., Table 14, entry 3) is applied. A notable exception is provided by the cyclopropanation of styrene with [(3/ )-4,4-dimethyl-2-oxotetrahydro-3-furyl] ( )-2-diazo-4-phenylbut-3-enoate to give 5 with several rhodium(II) carboxylate catalysts, asymmetric induction gave de values of 69-97%. ° Ester residues derived from a-hydroxy esters other than ( —)-(7 )-pantolactone are not as equally well suited as chiral auxiliaries for example, catalysis by the corresponding rhodium(II) (S )-lactate provides (lS, 2S )-5 with a de value of 67%. 

In contrast to these results, the intramolecular cyclopropanation of (allyldiisopropylsilyl)di-azoacetate 5 occurs only photochemically to give the l-silabicyclo[2.1.0]pentane 10 in good yield. Nitrogen extrusion from 5 occurs neither thermally (180°C) nor under catalysis by rhodium(II) bis(perfluorobutanoate) dimer. 

Ishitani and Achiwa [16] have recently prepared an axially disymmetric rhodium (11) biphenylcarboxylate catalyst, Rh2(S-BDME)4 of Fig. 2, and found that although the transxis diastereoselectivity in its catalysis of the styrene-EDA reaction was poor, the enantiocontrol was better than that observed with the pro-linate catalyst. The biphenyl based catalyst yielded an 87% ee for the cyclopro-panation of 2-naphthylethene and tert-butyl diazoacetate, though again the diastereoselectivity was very low. Use of an additional chiral auxiliary in the diazoester as in the d-menthyl derivative in Eq. (10) furnished a mixture of cyclopropanes, the cis-isomer of which was found to have an ee of 99%. 

The addition of a diazocarbonyl compound to an alkene with metal catalysis is an effective method for the formation of cyclopropanes, as discussed above. However, direct addition to aldehydes, ketones or imines is normally poor. Epoxide or aziridine formation can be promoted by trapping the carbene with a sulfide to give an intermediate sulfur ylide, which then adds to the aldehyde or imine. For example, addition of tetrahydrothiophene to the rhodium carbenoid generated from phenyldiazomethane gave the ylide 131, which adds to benzaldehyde to give the trans epoxide 132 in high yield (4.104). On formation of the epoxide, the sulfide is released and hence the sulfide (and the rhodium complex) can be used in substoichiometric amounts. 

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