Rhodium complexes allyl

NMR spectroscopy has shown that the allyl ligand in both platinum and rhodium allyl complexes at room temperature in deuterochloroform solution has all terminal hydrogens magnetically equivalent (20). This phenomenon may result from an interchange of the four allyl protons via a short-lived cr-allyl intermediate or transition state. As seen in Fig. 8, for such a rearrangement to take place a rotation around the C(l)-C(2) bond occurs, interchanging protons 1 and 2 concurrent with a rotation around the C(2)-C(3) bond interchanging protons 3 and 4. 

The Nozaki groups showed that by employment of a chiral Rh catalyst based on a bidentate phosphine-phosphite ligand, 1,3-pentadiene can be selectively converted into the branched unsaturated aldehyde that was chiral (Scheme 4.13) [80]. The reaction proceeds via a symmetrical rhodium allyl complex and preferentially produces the iso-product. This is in strong contrast to the reaction with non-substituted 1,3-butadiene. In this regard, there is a possibility of running the reaction in a stereoselective manner (see Section 4.3.3.5) [80]. A similar steering effect has been observed with other 4-substituted 1,3-butadienes, such as vinyl cyclohexene, 3-methyl-1,3-butadiene, or 1-phenyl-l,3-butadiene [80, 81]. 

Several rhodium-allyl complexes were immobilized on to solid supports. For example, the reaction of Rh( j -03115)3 with the surface hydroxyl groups of partially dehydroxylated silica led to the formation of the surface organometallic complex (=SiO)(=SiOX)Rh(77 -C3Hs)2 (where X is H or Si=), with the evolution of propene. ... 

Another difference between the two mechanisms is that the former involves 1,2 and the latter 1,3 shifts. The isomerization of 1-butene by rhodium(I) is an example of a reaction that takes place by the metal hydride mechanism, while an example of the TT-allyl complex mechanism is found in the Fe3(CO)i2 catalyzed isomerization of 3-ethyl-l-pentene. " A palladium acetate or palladium complex catalyst was used to convert alkynones RCOCSCCH2CH2R to 2,4-alkadien-l-ones RCOCH= CHCH = CHCHR. ... 

In the Rh-BINAP-catalyzed allyl amine isomerization step used in Takasago s Menthol process, the catalyst is inhibited by water through the formation of a hydroxyl-bridged rhodium trinuclear complex [ Rh(BINAP) i(/<2-0H)2]C104 [61]. 

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]. 

Figure 7.3 Proposed structures of 7C-allyl rhodium siloxide complexes immobilized on silica.

Scheme 7.2 Reactions of the surface 7t-allyl rhodium siloxide complex with trimethylphosphine.

Scheme 7.3 Reaction of surface Jt-allyl rhodium siloxide complex with carbon monoxide.

Rhodium immobilized complexes were also found to be effective catalysts of the addition of HSiMe(OSiMe3)2 and HSi(OEt)3 to various allyl ethers. The data presented in Table 7.4 confirm a high catalytic activity of catalysts 1, 3 and 5 in the conversion of allyl ethers into the corresponding silyl derivahves, but, unfortunately, only in the case of allyl phenyl ether did the catalytic achvity remained unchanged up to 10 cycles. ICP analysis of the rhodium solid catalysts after hydrosilylation tests revealed a high concentration of rhodium. Therefore, the decrease in catalytic activity of 1 does not depend only on leaching of rhodium from the silica surface. 

I 7 Well-Defined Surface Rhodium Siloxide Complexes and Their Application to Catalysis Table 7.5 Hydrosilylation of 1-hexadecene and allyl ethers by polyhydrosiloxane (7.1) . 

Subsequently to rhodium coordination with the enyne to form X, oxidative addition with the allyl chloride affords a rhodium-7r-allyl complex. Then isomerization... 

Gyclization/hydrosilylation of enynes catalyzed by rhodium carbonyl complexes tolerated a number of functional groups, including acetate esters, benzyl ethers, acetals, tosylamides, and allyl- and benzylamines (Table 3, entries 6-14). The reaction of diallyl-2-propynylamine is noteworthy as this transformation displayed high selectivity for cyclization of the enyne moiety rather than the diene moiety (Table 3, entry 9). Rhodium-catalyzed enyne cyclization/hydrosilylation tolerated substitution at the alkyne carbon (Table 3, entry 5) and, in some cases, at both the allylic and terminal alkenyl carbon atoms (Equation (7)). 

Since conjugated dienes form stable rc-allyl complexes with [Co(CO)4]2, they undergo hydroformylation very slowly to give saturated monoaldehydes in low yields.8 Mixtures of mono- and dialdehydes are usually formed in rhodium-catalyzed hydroformylations.71 72 Saturated monoaldehydes were isolated, however, when 1,3-butadiene and 1,3-pentadiene were hydroformylated in the presence of rhodium dioxide.70... 

Although much effort has been devoted to decarbonylation of cyclopropylcarbonyl metal complexes (vide supra), only (cyclopentadienyl)dicarbonyliron (Fp) derivatives have been successfully decarbonylated either photochemically22,24 or using Wilkinson s rhodium catalyst [(PPh3)2RhCl]2 (equation 35). Further decarbonylation by irradiation led to metallacyclopentane formation, whereas thermal decomposition resulted in the formation of the corresponding Cp(CO)Fe(allyl) complexes. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

The nature of the oxidation products is traceable to the nature of the rhodium-alkene interaction. Terminal alkenes and internal ones (e.g. cycloheptene), which form 77-complexes of rhodium(I), e.g. [RhCl(alkene)2]2, are selectively converted into methyl ketones, whereas alkenes which form 7r-allylic complexes of rhodium(III) (e.g. cyclopen-tene) give alkenyl ethers via oxidative substitution of the alkene by the solvent alcohol.204... 

Although C—H insertion reactions rarely occur in intermolecular reactions with diazoacetates, these are common side reactions with diazomalonates3132 (equation 10) and diazo ketones (with a-allyl vinyl ethers).33 Several mechanistic pathways are available to generate the products of an apparent direct C—H insertion reaction and these include dipolar intermediates, ir-allyl complexes and ring opening of cyclopropanes.1 Oxidative problems due to the presence of oxygen are common with copper catalysts, but these are rarely encountered with rhodium catalysts except in systems where the carbenoid is ineffectively captured.34... 

Substituted 2-(/J-oxoalkyl)tetrahydrofurans have been prepared in acceptable yields by a one-step aldol-type reaction of allyl ketones and 4-methyl-l-phenyl-4-penten-l-one, catalyzed by a rhodium(I) complex-tin(II) chloride mixture (equation 180)652. This type of catalyst mixture has also been used for both linear653 and cyclic654 codimers in a similar fashion. 

Allyl complexes (pseudo-rotations, dynamic NMR studies, 1, 416 with tungsten carbonyls and isocyanides, 5, 688-689 rc-Allyl complexes with Cr, 5, 305 with Cr(II), 5, 300 with Cr(III), 5, 300 and cyclodextrins, 12, 789 in enyne carbometallation, 10, 328 with rhodium, 7, 220-221 (j-Allyl complexes, with iron, 6, 98 5-Allyldiisopinocampheylboranes, in asymmetric allylboration, 9, 198... 

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... 

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