Rhodium allyl

Fig. 13.16. Preparation of a Rh/Si02 catalyst by the reaction of rhodium allyl with surface Si-OH groups.

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 interpretation of the formation of the Ci3-lactone requires a sequence of mechanistical pathways which are unknown so far in rhodium-catalysis. Two proposals for the mechanism were given in Equation 12. The mechanism of path B is similar to that shown for palladium catalysis. A rhodium Cg-carboxylate complex is formed which under further incorporation of butadiene could yield the lactone. In the mechanism of path A three molecules of butadiene react with the starting rhodium compound forming a C- 2 Chain, which is bound to the rhodium by two n -ally1 systems and one olefinic double bond. Carbon dioxide inserts into one of the rhodium allyl bonds thus forming a C- 3-carboxyl ate complex, which yields the new C-13-lactone. 

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. ... 

The Paloc group was developed as an amino acid protective group that is introduced with the p-nitrophenyl carbonate (H2O, dioxane, 68-89% yield). It is exceptionally stable to TFA and to rhodium-catalyzed allyl isomerization, but it is conveniently cleaved with Pd(Ph3P)4 (methylaniline, THF, 20°, 10 h, 74-89% yield). ... 

Acetylenic epoxides are reduced readily to the olehnic epoxide, provided the resulting epoxide is not allylic (27). In the latter case, one might surmise that hydrogenolysis could best be avoided by use of rhodium in a neutral nonpolar solvent (81) or a Lindlar catalyst (13). Reduction of l,2-epoxydec-4-yne over Lindlar catalyst gave (Z)-l,2-epoxydec-4-ene in 95% yield (69). Hydrogenation ceased spontaneously. 

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. ... 

When the substrate is an allylic alcohol or amine, the addition is generally anti," though the stereoselectivity can be changed to syn by the use of catecholborane and the rhodium complexes mentioned above. Because the mechanism is different, use of this procedure can result in a change in regioselectivity as well, [e.g., styrene PhCH=CH2 gave PhCH(OH)CH3]. ... 

Asymmetric cyclizahon was also successful in the rhodium-catalyzed hydrosilyla-tion of silyl ethers 57 derived from allyl alcohols. High enanhoselectivity (up to 97%... 

A fourth focus of catalytic chemistry in our laboratory has been iridium-catalyzed asymmetric allylic substitution. Dr. Toshimichi Ohmura had been studying additions to rhodium and iridium allyl and benzyl complexes in hopes of developing... 

Allyl groups attached directly to amine or amide nitrogen can be removed by isomerization and hydrolysis.228 These reactions are analogous to those used to cleave allylic ethers (see p. 266). Catalysts that have been found to be effective include Wilkinson s catalyst,229 other rhodium catalysts,230 and iron pentacarbonyl.45 Treatment of /V-allyl amines with Pd(PPh3)4 and (V,(V -dimethylbarbi Lurie acid also cleaves the allyl group.231... 

Besides the formation of carbenes from diazo compounds and the hydroformyla-tion, rhodium (as described previously for palladium) has also been used as catalyst in domino processes involving cycloadditions. Thus, Evans and coworkers developed a new Rh(I)-catalyzed [4+2+2] cycloaddition for the synthesis of eight-membered rings as 6/2-105 using a lithium salt of N-tosylpropargylamines as 6/2-104, allyl carbonates and 1,3-butadiene (Scheme 6/2.22) [221]. The first step is an al-... 

Scheme 6/2.23. Rhodium-catalyzed domino allylation/Pause-Khand process...

With catalysts such as nickel and rhodium for which it has been shown that 1-2 hydrogenolysis is seriously competitive with 1-3 hydrogenolysis, there is no need to assume that ir-olefin/allyl hydrogenolysis occurs (but neither can it with certainty be excluded). This conclusion is likely to be true for other catalysts such as cobalt and iron which also favor complete hydrocarbon fragmentation to methane. 

---