Rhodium complexes alkyl

Although extremely rare, a recent report documents the intramolecular S -type displacements of (TPP)Rh-alkyl complexes (TPP = tetraphenylporphyrin) by alcohols and phenols to form THFs (Scheme 19). The intermediate rhodium alkyl complexes are themselves prepared by anti-Markovnikov hydrorhodation, and although the (TPP)Rh-H... 

The results do not prove that in the reaction conditions used the alkyl formation is not reversible, but only that, if it is reversible, the carbon monoxide insertion on both diastereomeric rhodium-alkyls must be much faster than the rhodium-alkyls decomposition. Restricting this analysis of the asymmetric induction phenomena to the rhodium-alkyl complexes formation, two 7r-olefin complexes are possible for each diastereomer of the catalytic rhodium complex (see Scheme 11). The induction can take place in the 7r-olefin complexes formation (I — II(S) or I — II(R)) or in the equilibrium between the diastereomeric 7r-olefin complexes (II(r) and... 

II(S)) and/or to a different reaction rate of the two diastereomeric 7r-olefin complexes to the corresponding diastereomeric alkyl-rhodium complexes (VI(s) and VI(R)). For diastereomeric cis- or trans-[a-methylbenzyl]-[vinyl olefin] -dichloroplatinum( II) complexes, the diastereomeric equilibrium is very rapidly achieved in the presence of an excess of olefin even at room temperature (40). Therefore, it seems probable that asymmetric induction in 7r-olefin complexes formation (I — II) cannot play a relevant role in determining the optical purity of the reaction products. On the other hand, both the free energy difference between the two 7r-olefin complexes (AG°II(S) — AG°n(R) = AG°) and the difference between the two free energies of activation for the isomerization of 7r-com-plexes II(S) and II(R) to the corresponding alkyl-rhodium complexes VI(s) and VI(R) (AG II(R) — AG n(S) = AAG ) can control the overall difference in activation energy for the formation of the diastereomeric rhodium-alkyl complexes and hence the sign and extent of asymmetric induction. 

Ethanol421 is carbonylated to propionic acid, and isopropanol422 is carbonylated to n- and iso-butyric acids. In the latter case it is not known whether the isomerization occurs in the alcohol, the iodide or the rhodium-alkyl complex. 

Direct alkyne insertion into a Rh—Si bond has been observed for the intermediate rhodium silyl complex (dtbpm) Rh[Si(OEt)3] (PMe3) [dtbpm = di(ferf-butyl)phosphino methane] in the hydrosilylation of 2-butyne with triethoxysilane catalyzed by the rhodium alkyl complex (dtbpm)RhMe(PMc3). The crystal structure of (dtbpm)Rh[Si(OEt)3j (PMes) shows that the coordination around the Rh metal is planar with a Rh—Si bond length [2.325(2) A] similar to that found for the complex (Me3P)3RhH(C6F5) Si(OEt)3 (Table ll) . The proposed mechanism for the hydrosilylation reaction of 2-butyne with HSi(OEt)3 yielding mainly the E-isomer of MeCH=C(Me)Si(OEt)3 is outlined in Scheme 36. 

The photosystem with RhCl(CO)(PMe3)2 can also be used for the photodehydrogenation of cyclododecane to cyclododecene, and in general the photosystem can be used for the conversion of alkanes into alkenes/ These observations can be explained on the basis of the conversion shown in Scheme 3.5, where the intermediate rhodium alkyl complex can either undergo insertion of carbon monoxide into the rhodium-carbon bond, or it can undergo )S-hydrogen transfer to yield the alkene. The photoreaction with RhCl(CO)(PMe3)2 can also be used for the insertion of terminal alkynes into aliphatic C-H bonds. 

A number of six-co-ordinate rhodium alkyl complexes are known. From the reaction between (Me2S)3RhCl3 and MeMgl the red-brown tetra-methyl complex (Me2S)3Me4Rh2l2 is isolated the structure, 7.14a, is proposed [48i]. 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

The preparations of a number of rhodium(I) complexes of isocyanides, some of them new, have been described. The newtetrakis(methyl isocyanide) complex, [Rh(CNCH3)4], was isolated as salts of various anions from reactions of RhClj -3H20 or [(l,S-CgH,2)RhCl]2 and this isocyanide ligand (11), and several [Rh(CNR)4]+ alkyl and aryl isocyanide complexes (R= Bu, Pr, /)-C6H4C1, /.-CSH4CH3, and P-C6H4OCH3) have... 

In the case of terminal C=C (1,2 addition units), i.e. when R=R =H and R" (or R111) = polymer chain, two types of hydride migration are possible, namely (i) The Markownikoff s addition which would lead to the formation of B type repeating units and (ii) The anti Markownikoff s addition which would result in the formation of the observed repeating units C. In the case of Markownikoff s type addition the hydride transfer occurs to Ca and results in the formation of branched alkyl-rhodium intermediate complex shown by Structure 2. Whereas when anti Markownikoff s addition occurs, the resulting intermediate alkyl-rhodium complex has linear alkyl ligand as shown by Structure 3. 

Several rhodium(I) complexes have also been employed as ATRP catalysts, including Wilkinson s catalyst, (177),391 421 422 ancj complex (178).423 However, polymerizations with both compounds are not as well-controlled as the examples discussed above. In conjunction with an alkyl iodide initiator, the rhenium(V) complex (179) has been used to polymerize styrene in a living manner (Mw/Mn< 1.2).389 At 100 °C this catalyst is significantly faster than (160), and remains active even at 30 °C. A rhenium(I) catalyst has also been reported (180) which polymerizes MM A and styrene at 50 °C in 1,2-dichloroethane.424... 

The combination of [Rh(Cl(NBD)]2 and ligands 89, 90, 91, or 92 with diphenylsilane asymmetrically reduces aryl alkyl ketones, including acetophenones, in excellent yields and in 81 to 90% ee (Eq. 346).574 The best results are with ferrocene 91 and acetophenone in toluene.575 Other phosphine-substituted ferrocenes do not give comparable results. Rhodium(I) complexes of TADDOL-derived... 

Figure 2 shows the generally accepted dissociative mechanism for rhodium hydroformylation as proposed by Wilkinson [2], a modification of Heck and Breslow s reaction mechanism for the cobalt-catalyzed reaction [3]. With this mechanism, the selectivity for the linear or branched product is determined in the alkene-insertion step, provided that this is irreversible. Therefore, the alkene complex can lead either to linear or to branched Rh-alkyl complexes, which, in the subsequent catalytic steps, generate linear and branched aldehydes, respectively. 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

Rhodium(III) complexes [e.g. (i-Pr,P)2Rh(H)Cl2] in the presence of quaternary ammonium salts are excellent catalysts for the hydrogenolysis of chloroarenes under mild conditions [5] other labile substituents are unaffected. Hydrodehalogenation of haloaryl ketones over a palladium catalyst to give acylbenzenes is also aided by the addition of Aliquat [6]. In the absence of the phase-transfer catalyst, or when the hydrogenation is conducted in ethanol, the major product is the corresponding alkyl-benzene, which is also produced by hydrodehalogenation of the halobenzyl alcohols. 

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

The reversibility of the hydride migration in unmodified rhodium catalysts has been studied intensively by Lazzaroni et al. [56]. Reversible hydride migration will result in aldehydes containing deuterium at the a-carbon, whereas irreversible hydride migration will result in exclusive deuteration of the aldehyde and P-carbon. Mutual reversible alkene coordination and hydride migration ivill result in the formation of deuterated alkenes. Reversible formation of the branched rhodium alkyl will place deuterium at Cl and reversible formation of the linear alkyl complex will provide deuterium at C2 (Scheme 6.4). 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

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