Rhodium cyclopropenation

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. 

More recent work employing diphosphine ligands has focused on both new substrates for hydroboration and also new hydroborating agents. Specifically, Gevorgyan has successfully employed cyclopropenes 56 as substrates, with pinacolboranes 13 as the borane source.20 Impressive enantioselectivities were obtained with a range of diphosphines, for example, with rhodium complexes of NORPHOS (>99% ee), PHANEPHOS (97% ee), BINAP (94% ee), and Tol-BINAP (96% ee), all with near perfect m-selectivity (see Scheme 8). 

As discussed in Section 3.1.6, cyclopropenes can react with rhodium complexes [38,585,587-589,1061,1063] or other transition metal derivatives to yield vinylcarbene complexes (see Section 3.1.6). This reaction will proceed particularly smoothly with strained cyclopropenes, because these can already isomerize thermally to vinylcarbenes [1064]. Hence the formation of vinylcarbene complexes from alkynes can proceed by initial cyclopropanation, followed by reaction of the resulting cyclopropene with the complex L,M. 

In the example shown in Figure 4.4 either of these mechanisms leads to insertion of the alkyne into the C-Rh double bond of the initially formed acylcarbene rhodium complex. The resulting vinylcarbene complex undergoes intramolecular cyclopropanation of the 1-cyclohexenyl group to yield a highly reactive cyclopropene, which is trapped by diphenylisobenzofuran. 

Over the last few years it has become clear that rhodium(II) acetate is more effective than the copper catalysts in generating cyclopropenes.12 126 As shown in Scheme 28,12S a range of functionality, including terminal alkynes, can be tolerated in the reaction with methyl diazoacetate. Reactions with phenyl-acetylene and ethoxyacetylene were unsuccessful, however, because the alkyne polymerized under the reaction conditions. 

Intramolecular cycloaddition of a diazo ketone to a cyclopropene. Rhodium) II) acetate is markedly superior to copper or copper(II) sulfate as the catalyst for cyclopropanation of l,4-diacetoxy-2-butyne with /-butyl diazoacetatc. The product (1) was converted by known steps into the diazo ketone 2. In the presence of rhodium(II) acetate, 2 undergoes intramolecular cycloaddition to the cyclopropene double bond to give the highly strained tricyclic pentanone derivative 3 in 30% yield. C oppcr catalysts are less efficient for this conversion. 

Unsaturated ethers. The efficient insertion of carboalkoxycarbenes into the O—H bond of alcohols catalyzed by Rh(II) acetate (5, 571-572) extends to reactions with unsaturated alcohols. For this reaction copper(II) triflate is usually comparable to rhodium(II) alkanoates. Insertion predominates over cyclopropanation in the case of ethylenic alcohols. In reactions with acetylenic alcohols, cyclopropenation can predominate over insertion because of steric effects, as in reactions of HC=CC(CH3)2OH where the insertion/addition ratio is 36 56. 

The mechanism of cyclopropenations of alkynes with ethyl diazoacetate, catalysed by (AcO)4Rh2 and (DPTI)3Rh2(OAc), has been 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 detected at the internal acetylenic carbon. These isotope effects are consistent with the canonical variational transition structures for cyclopropenations with intact tetrabridged rhodium carbenoids but not with a 2 + 2-cycloaddition on a tribridged rhodium carbenoid structure.99... 

Dirhodium(II) tetrakis(carboxamides), constructed with chiral 2-pyrroli-done-5-carboxylate esters so that the two nitrogen donor atoms on each rhodium are in a cis arrangement, represent a new class of chiral catalysts with broad applicability to enantioselective metal carbene transformations. Enantiomeric excesses greater than 90% have been achieved in intramolecular cyclopropanation reactions of allyl diazoacetates. In intermolecular cyclopropanation reactions with monosubsti-tuted olefins, the cis-disubstituted cyclopropane is formed with a higher enantiomeric excess than the trans isomer, and for cyclopropenation of 1-alkynes extraordinary selectivity has been achieved. Carbon-hydro-gen insertion reactions of diazoacetate esters that result in substituted y-butyrolactones occur in high yield and with enantiomeric excess as high as 90% with the use of these catalysts. Their design affords stabilization of the intermediate metal carbene and orientation of the carbene substituents for selectivity enhancement. 

Chiral rhodium(II) carboxamides are exceptional catalysts for highly enantio-selective intermolecular cyclopropenation reactions (50). With ethyl diazoacetate and a series of alkynes, use of dirhodium(II) tetrakis[methyl 2-pyrrolidone-5-(R)-carboxylate], Rh2(5R-MEPY)4, in catalytic amounts ( 1.0 mol %) results in the formation of ethyl eyelopropene-3-earboxylates (eq 4) with enantiomeric excesses... 

Alternative rhodium(II) carboxamide catalysts derived from 4-(R)-benzyloxa-zolidinone (47 -BNOXH) and 4-(S)-isopropyloxazolidinone (4S-IPOXH) provided only a fraction of the enantioselection obtained with Rh2(MEPY)4 catalysts. Whereas cyclopropenation of 1-hexyne with ethyl diazoacetate in the presence of Rh2(5R-MEPY)4 resulted in 15 (eq 4, R = n-Bu) with 54% ee, Rh2(47 -BNOX)4 gave the same compound in 5% ee, and Rh2(4S-IPOX)4 provided only 6% ee. 

Analysis of 13C distribution in recovered alkynes using C4 atom as an internal standard led to experimental KIEs as collected in Table 4. For both catalysts significant isotope effect was observed for the terminal acetylenic carbon. Experimental KIEs are consistent with cyclopropenation via intact tetrabridged rhodium carbenoids and do so to support [2+2] cycloaddition. DFT calculations using B3LYP functional were complicated and did not give conclusive results. 

Table 4 Experimental 13C KIEs for cyclopropenations of 1-pentyne or 1-hexyne with ethyl diazoacetate in the presence of rhodium catalyst...

The addition of alkoxycarbonylcarbene derived by catalysed decomposition of methyl diazoacetate to several simple, and in particular terminal, alkynes leads to low yields S7), but the reaction with 1 -trimethylsilylalkynes proceeds reasonably efficiently subsequent removal of the silyl-group either by base or fluoride ion provides a route to l-alkyl-3-cyclopropenecarboxylic acids. In the same way 1,2-bis-trimethylsilyl-ethyne can be converted to cyclopropene-3-carboxylic acid itself58 . The use of rhodium carboxylates instead of copper catalysts also generally leads to reasonable yields of cyclopropenes, even from terminal alkynes 59). 

Addition of a rhodium carbenoid to an alkyne leads to a cyclopropene derivative. In an intramolecular context, the fused cyclopropene moiety is unstable and undergoes ring opening to generate a rhodium vinyl carbenoid entity, which can then undergo cyclopropanation or cyclopropena-tion, carbon hydrogen insertion, and ylide generation. This is illustrated... 

Without question, the most important developments in this field over the past 10 years have been in the area of enantioselective hydroborations. New chiral catalyst systems are typically tested in hydroborations of vinyl arenes, as reactions using HBcat and a cationic rhodium catalyst are well known to give selective formation of the unusual branched isomer. In related studies, enantiopure 2,2-disubstituted cyclopropyl boronates were easily prepared via the catalytic asymmetric hydroboration of 3,3-disubstituted cyclopropenes using a number of chiral neutral rhodium complexes (equation 13). Directing groups, such as esters and alkoxymethyl substituents, were necessary for achieving... 

Polymer-supported benzenesulfonyl azides have been developed as a safe diazotransfer reagent. ° These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, paUadium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Diazoketones and diazoesters with alkenes to give the cyclopropane derivative, usually with a transition-metal catalyst, such as a copper complex. The ruthenium catalyst reaction of diazoesters with an alkyne give a cyclopropene. An X-ray structure of an osmium catalyst intermediate has been determined. Electron-rich alkenes react faster than simple alkenes. ... 

Extensive work by Binger has shown that palladium, nickel and rhodium complexes can effect the cyclooligomerization of alkylated cyclopropenes. The mode of coupling is often dependent upon the particular metal catalyst employed (equation 92). 

Mo2(C5Hs)2(CO)/ or [Mo( -MeC=CMe)(PhS(0)SPh)( /-C5H5)] produce complexes derived from fission of the cyclopropene C(l)-C(3) bond. However, treatment of the last of these carbocycles with the binuclear rhodium complex [Rh2(ju-CO)2( /-C5Me5)2] results in the cleavage of the carbon-carbon double bond and formation of the bridged carbene complex 313. ... 

Benzoyldiazomethane reacts with a terminal alkyne in the presence of a rhodium catalyst to give cyclopropene 406. The resulting cyclopropene was allowed to react with another terminal alkyne in the presence of tetra-carbonyldichlorodirhodium, affording oxepin 407 (92JA5881). 

The rhodium(I) catalyzed hydroformylation of cyclopropenes to give ds-aldehydes has been mentioned 62). 

The second approach involves several examples of intramolecular reactions which employ cyclopropenes with a diazoketone substituent. In these cases, copper and rhodium catalysts effectively promote the formation of bridged bicyclo[1.1.0]butanes. ... 

An elegant extension of this method is the tandem formation of two rings by rhodium catalyzed intramolecular addition of a diazo ketone to an enyne giving cyclopropene 3. ... 

The kinetics of the isomerization in tetrachloroethene at 413-433 K show an E/ of 25 1 kcal mol an enthalpy of activation of 24 + 1 kcal moP and an entropy of activation of — 22.4 eu. Changing the solvent to benzonitrile does not increase the rate and is consistent with a nonpolar transition state such as a vinylcarbene, but the entropy value indicates a more concerted mechanism. Indenes were also produced when a solution of the cyclopropene in cyclohexane was heated with copper stearate at 60 C for 30 minutes or when a solution in dichloromethane was heated with trifluoracetic acid, or when 1,3,3-triphenylcyclopropene was treated with [(C2H4)PtCl2]2- Reactions catalyzed by rhodium(I) heptafluorobutanoate have also been reported. ... 

A solution of l,2-bis(acetoxymethyl)-3-(diazoacetyl)cyclopropene (6, X = OAc) (195 mg, 8 mmol) in EtOH-free CHC13 (10 mL) was added dropwise with stirring to a solution of rhodium(II) acetate dimer... 

The rhodium catalyst also efficiently promotes the cyclopropenation of alkyncs with high diastereoselectivities or with enantioselectivities up to 98 %. Compared with the related cyclo-propanations this process seems to be more dependent on the size and the configuration of the alkyl group of the diazoester114. If the 5-catalyst is employed the configuration at the cyclo-propene is predominantly S, w hile the 7 -catalyst induces the opposite configuration. 

Enantioselective cyclopropenation.2 This chiral rhodium(U) catalyst can effect highly enantioselective cyclopropenation of 1-alkynes with alkyl diazoacctates. The enantioselectivity increases with the steric size of the ester group, and the size and polarity of the alkyne substituent also affects the enantioselectivity. The highest selectivity is observed with d-menthyl diazoacetatcs as a result of double diastercosclection. 

Cyclopropenation. Cyclopropenes can be formed from alkynes by reaction with methyl diazoacetate using a rhodium(ll) carboxylate as catalyst. The reaction is not particularly subject to steric hindrance, but polar groups (CH2COOCH3) inhibit cyclopropenation markedly. Insertion reactions compete with cyclopropenation in the case of acetylenic alcohols. ... 

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