Rhodium acyclation catalyst

Having established that pure enantiomer ( S,ZR)-77 was capable of undergoing remarkably regioselective and diastereoselective C-H activation, it followed that highly efficient enantiomeric differentiation of rac-77 could be accomplished.199 Hence, the Rh2(5Y-MEPY)4-catalyzed reaction of rac-77 effectively gave close to a 1 1 mixture of enantioenriched (lY)-78 (91% ee) and ( R)-79 (98% ee) (Equation (68)). Other equally spectacular examples of diastereo- and regiocontrol via chiral rhodium carboxamide catalysts in cyclic and acyclic diazoacetate systems have been reported.152 199 200 203-205... 

Alkenylstannanes are usually reported to be resistant to hydrogenation with typical metal catalysts, though a rhodium(l) catalyst has been used at 100atm to reduce 3-hydroxy-l-alkenylstannanes.220 Diimine has been used to reduce some acyclic and cyclic methylstannylalkene compounds, but lower yields were obtained with tributylstan-nylalkenes (Equation (72)).221,198... 

Recently, a new class of phosphabarrelene/rhodium catalysts has been developed, which for the first time allows for hydroformylation of internal alkenes with very high activity and which proceeds essentially free of alkene isomerization [36-38]. Two examples, results of hydroformylation of an acyclic and a cyclic internal alkene substrate, are depicted in Scheme 2. 

Similar to the intramolecular insertion into an unactivated C—H bond, the intermolecular version of this reaction meets with greatly improved yields when rhodium carbenes are involved. For the insertion of an alkoxycarbonylcarbene fragment into C—H bonds of acyclic alkanes and cycloalkanes, rhodium(II) perfluorocarb-oxylates 286), rhodium(II) pivalate or some other carboxylates 287,288 and rhodium-(III) porphyrins 287 > proved to be well suited (Tables 19 and 20). In the era of copper catalysts, this reaction type ranked as a quite uncommon process 14), mainly because the yields were low, even in the absence of other functional groups in the substrate which would be more susceptible to carbenoid attack. For example, CuS04(CuCl)-catalyzed decomposition of ethyl diazoacetate in a large excess of cyclohexane was reported to give 24% (15%) of C/H insertion, but 40% (61 %) of the two carbene dimers 289). 

In a direct competition between 1,2- and 1,5-insertion into methylene C —H bonds, the relative proportion of products depends on the rhodium carboxylate employed. Rhodium(II) pivalate is the most efficient catalyst so far found for the cyclization of methyl 2-diazo-10-undecenoate. In contrast, rhodiumfll) trifluoroacetate gives a 52 48 ratio of cyclic 5/acyclic 6 products. 

Although the vast majority of this review has been concerned with homogeneous systems, supported catalyst and single-crystal studies of rhodium are important topics that have also been considered by some researchers. This topic extends and dovetails nicely with the discussion of the interactions of acyclic and cyclic polyamine ligands and Schiff... 

Thus changing the ligands on dirhodium(II) can provide a switch which, in some cases, can turn competitive transformations on or ofT146. Other examples include the use of dirhodium(II) carboxamides to promote cyclopropanation and suppress aromatic cycloaddition146. For example, catalytic decomposition of diazoketone 105 with dirhodium(II) caprolactamate [Rh2(cap)4] provides only cyclopropanation product 106. In contrast, dirhodium(II) perfluorobutyrate [Rh2(pfb)4] or dirhodium(II)triphenylacetate [Rh2(tpa)4] gave the aromatic cycloaddition product 107 exclusively (equation 100)l46 148. Although we have already seen that rhodium(II) acetate catalysed decomposition of diazoketone 59, which bears both aromatic and olefinic functionalities, afforded stable norcaradiene 60 (equation 70)105, the rhodium(II) acetate catalysed carbenoid transformation within an acyclic system (108) showed no chemoselectivity (equation 101). However, when dirhodi-um(II) carboxamides were employed as catalysts for this type of transformation, only cyclopropanation product 109 was obtained (equation 101). ... 

Keywords Asymmetric hydrosilylation, optically active alcohols, amines, Chiral Titanocene Catalysts, Acyclic Imines, Cyclic Imines, Chiral Rhodium Catalysts, aromatic ketones... 

The results of the hydroformylation of internal olefins are reported in Table 9. In the case of (Z)- and (E)-2-butene, the same fare of the unsaturated carbon atom is formylated with either a rhodium- or platinum (—)-DIOP-containing catalytic system. With the rhodium catalyst, when an acyclic olefin is used as the substrate, the same fare is always attacked, and it is only the notation but not the geometric requirement that is different for (E)-l-phenyl-1-propene. The only exception is represented by bicyclo[2,2,l]heptene. However, using (—)-CHIRAPHOS instead of (—)-DIOP, also bieyelo[2,2,l]heptene behaves like internal butenes. No regularity is observed for the cobalt or ruthenium (—)-DIOP catalytic systems. With the same system, only in 3 cases out of 15 the face of the prochiral atom preferentially formylated has different geometric requirements. 

An example is the rhodium catalyzed hydroformylation reaction, which is an industrially important homogenous catalytic process [3]. In contrast, it is amazing that such an important transition-metal catalyzed C/C bond-forming process has been employed only rarely in organic synthesis [4]. Part of the reason stems from the difficulty in controlling stereoselectivity. Even though some recently developed chiral rhodium catalysts allow for enantio- and diastereoselective hydroformylation of certain specific classes of alkenes [5, 6], only little is known about the diastereoselective hydroformylation of acyclic olefins [7, 8]. 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

BICYCLO[2.2.nHEPTA-2,5-DIENE)[1,4-BIS(DIPHENYLPHOSPHINO)BUTANE]RHODIUM(l)TETRAFLUOROBORATE Table 4 Reduction of Acyclic Homoallylic Alcohols with Catalyst (1)... 

Palladium and rhodium are good catalysts for partial reduction of acyclic... 

The flexibility of acyclic systems adds another element to the analysis of substituent directive effects. Some of the best examples of stereoselective reductions involve allylic alcohols and the rhodium catalyst B. 

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