Rhodium-nucleophile complex

The halide exchange protocol also allows the use of other nucleophiles such as activated methylenes. The rhodium iodide complex was found to be the most... 

Once 26 or 27 has been formed, the rhodium-aUcoxide complex is protonated by a nucdeophile molecule, generating the cationic rhodium complex 29 and an alkoxide or phenoxide nucdeophile. This proton transfer step is supported by kinetics experiments and has two effects [14]. Firstly, the organorhodium species is made more electrophilic as a result of the positive charge, and secondly, the nucleophile is rendered more nu-cdeophihc by becoming deprotonated. 

Coordinated secondary amines can also be alkylated, but only after deprotonation by a strong base generates a suitable nucleophile. Work on rhodium(III) complexes of ethylenediamine12 has been extended to nickel(II) complexes of various fully saturated macrocycles such as cyclam (Scheme l).13,14 The methylated cyclam complex is kinetically inert, unlike the isomer with all four methyl groups on the same side of the ring, which is obtained on reaction of the preformed tetramethyl cyclam with nickel ions. 

This mechanism does not explain what is happening in the rhodium(m) complex, [Rh(bpy)3]3+, where exchange occurs first at the 6-position. Increasing the charge on the metal ion will increase the tendency towards attack by a nucleophile and also the acidity of the pyridine ligand. Nucleophilic attack will always occur at the 4- and 6-positions of... 

Enynes (77) can react with arylboronic acids in the presence of a catalytic amount of a rhodium(I) complex under mild conditions to give (Z)-l-(l-arylethyl- (g) idene)-2-vinylcyclopentanes (78).100 In analogy, the rhodium-catalysed cyclization of the cyano-substituted alkynes (79) with arylboronic acids has been reported to produce the cyclic ketone (80) as the first example of a nucleophilic addition of an Rh(I) species to a cyano group.101... 

Platinacyclobutane complex 118 undergoes equilibrium heterolytic scission of the exocyclic carbon-carbon bond to form a cationic allyl complex and the organic enolate ion (Equation 35) <1993OM3019>. Similar dissociative ionization was previously reported for rearrangements of iridium and rhodium metallacyclobutane complexes formed by nucleophilic alkylation < 1990JA6420>. This carbon-carbon bond activation is generally associated with reversible central carbon alkylation of Jt-allyl complexes (Section 2.12.9.3.3), but the homolytic equivalent has recently been... 

Vinyl bromides are directly aminocarbonylated by nickel carbonyl and amines. Very similarly, Rh (CO)i6 and BU4NCI as cat yst convert allylphosphates to. -y-unsaturated amides via rr-allylrhodium complexes (equation 43). Although palladium(O) complexes are more reactive than rhodium(I) complexes, palladium(O) complexes undergo side reactions, like reductive elimination in the presence of carbon monoxide, and direct nucleophilic attack by amines. 

Prior to these investigations by HCC the promotional effect of iodide on the oxidative addition of Mel was investigated by others [9, 39, 40]. Foster demonstrated that the rate enhancement of this reaction in anhydrous medium was attributable to increased nucleophilicity of the rhodium catalyst with added iodide. The rationale for this observation was the generation of an anionic rhodium carbonyl complex, [Rh(CO)2l(L)]. Generation of this species was observed only with iodide added to certain neutral Rh species. No rate enhancement occurred with iodide added to the anionic complex, [Rh(CO)2l2] [39]. Similarly, in solvents with a high water concentration, iodide salts exhibited no rate enhancement in the presence of [Rh(CO)2l2] [11]. Maitlis and co-workers, in more recent investigations, reported a promotional effect of iodide in aprotic solvents on the oxidative addition of CH3I on [Rh(CO)2l2] [9a, 9c]. 

Although the two-step oxidative addition mechanism of, for example, Mel to square-planar complexes is known to proceed generally by electrophilic attack of the methyl group at the metal atom, many cases are known where attack of a nucleophile, such as 1 , precedes this step. Another example this year reveals a similar situation, electrophilic attack of Mel at a rhodium(I) complex competing with a second path which depends on solvent addition. Scheme 7 outlines the... 

Summary Mixed functionalized octa( -alkyl/halogenalkylsilsesquioxanes) with different side chains has been prepared by acid-catalyzed cohydrolysis of w-alkyl- and lialogenalkyltrichlorosilanes. The silsesquioxane mixtures were separated in preparative amounts with normal-phase HPLC. Mixed functionalized octa( -alkyl/halogenalkylsil-sesquioxanes) with different side chains are suitable examples of an organically modified silica gel. They can be modified by nucleophilic substitution of the halogen atom with -PPh and rhodium-phosphino complexes are be formed. 

Rate constants for exchange of axial water molecules in the tetra-y -carboxylato-rhodium(ii) complex (2) by nucleophiles L (pyridine, picolinic acid, niacin, or isonicotinic acid) have been determined using T-jump. The rate-determining step... 

The four most common methods for the synthesis of late transition metal enolates are oxidative addition to halocarbonyl compoxmds, ligand metathesis with main group enolates or silyl enol ethers, nucleophilic addition of anionic metal complexes to halocarbonyl electrophiles, and insertion of an a,3-imsaturated carbonyl compoimd into a metal hydride. Examples of these synthetic routes are shown in Equation 3.47-Equation 3.50. Equation 3.47 shows the synthesis of a palladium enolate complex by oxidative addition of ClCHjC(0)CHj to Pd(PPh3), Equation 3.48 shows the synthesis of a palladium enolate complex by the addition of a potassium enolate to an aryl Pd(II) halide complex, and Equation 3.49 shows the synthesis of the C-bound W(II) enolate complex in Figure 3.7 by the addition of Na[( n -C5R5)(CO)jW] to the a-halocarbonyl compound. Finally, Equation 3.50 shows the synthesis of a rhodium enolate complex by insertion of but-l-en-3-one into a rhodium hydride. This last route has also been used to prepare enolates as intermediates in reductive aldol processes. - ... 

The insertions of imines into late transition metal-carbon bonds are even less common. In one case, the insertion of an imine into Ni- and Pd-acyl bonds occurs with 2,1-regiochemistry to form an aminoalkyl product (Equation 9.80). Tlus reaction is likely to occur through a polar transition state formed by attack of a nucleophilic nitrogen at the electrophilic acyl carbon. One set of examples of 1,2-insertions of imines into late metal-carbon bonds have been reported. This example involves insertion of N-aryl aldimines into rhodium-aryl complexes containing a labile pyridine ligand (Equation 9.81). The rates of these reactions were inverse order in added pyridine, suggesting that the reaction occurs by an intramolecular migratory insertion mechanism after replacement of the coordinated pyridine by the imine. 

As more acidic oxygen nucleophiles added preferentially to 7, it was reasoned that carboxylic acids would be good candidates for the ring-opening oxabicyclic alkenes to form the corresponding allylic esters. While certain transition metals, most notably palladium, are known to react with allylic esters to form Jt-allyl intermediates, rhodium-phosphine complexes are less reactive [11]. The product allylic ester formed... 

Probably the most important industrial application of a transition-metal nucleophile is the Monsanto process for carbonylating methanol, using soluble rhodium-carbonyl complexes in the presence of iodide. The catalyst is in fact [Rhl2(CO)2] (Forster 1979, and references therein), and the catalytic cycle is shown in Scheme 21. The substrate for the rhodium catalyst is methyl iodide, which oxidatively adds to yield [Rh(Me)(I)3(CO)2] . 

Diazo compounds, with or without metal catalysis, are well-known sources of carbenes. For synthetic purposes a metal catalyst is used. The diazo compounds employed are usually a- to an electron-withdrawing group, such as an ester or a ketone, for stability. In the early days, copper powder was the catalyst of choice, but now salts of rhodium are favoured. The chemistry that results looks very like the chemistry of free carbenes, involving cyclopropanation of alkenes, cyclopropenation of alkynes, C-H insertion reactions and nucleophilic trapping. As with other reactions in this chapter, free carbenes are not involved. Rhodium-carbene complexes are responsible for the chemistry. This has enormous consequences for the synthetic applications of the carbenes - not only does the metal tame the ferocity of the carbene, but it also allows control of the chemo-, regio- and stereoselectivity of the reaction by the choice of ligands. 

The reaction of ethyl methallylic amine under HAM conditions in common organic solvents may lead via intramolecular hydrocarboxylation to the corresponding lactam 2 as a result of the fast nucleophilic attack of the nitrogen atom at the carbonyl group in the intermediate rhodium-acyl complex (Scheme 5.98) [35a, 82]. This path could be suppressed by running the reaction in supercritical... 

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