Diene rhodium complexes

Fig. 2. Drawing showing the bisphosphine- rhodium complex viewed along the plane of the five-membered ring (diene omitted), and the Z-a-benzamido cinnamic acid substrate in a linear and flat conformation.

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Scheme 14.18 Proposed 1,4-hydrogenation mechanism for the hydrogenation of dienes by cationic rhodium complexes [P2Rh(diene)]+A (54).

A cationic rhodium complex-catalyzed codimerization of 1,3-dienes with alkynes gives the corresponding cyclohexadienes in good yields with high regioselectively, as exemplified in the reaction of 2-methyl-l,3-butadiene with phenylacetylene (Eq. 12) [31]. 

This catalyst system was the first to utilize both terminal alkynes and olefins in the intramolecular reaction. Although a mechanistic rationale for the observed stereoselectivity was not offered, the formation of the single stereoisomer 26 may be rationalized through the diastereotopic binding of the rhodium complex to the diene moiety (Scheme 12.3). This facial selective binding of the initial ene-diene would then lead to the formation the metallacycle III, which ultimately isomerizes and reductively eliminates to afford the product [14]. 

These examples of rhodium-catalyzed [4 + 2] reactions provide the first chemo- and stereoselective carbocycHzations of tethered diene-aUenes, through the facial differentiation of the rhodium-complex. This presumably results in the kinetic formation of a preferred metaUacycle intermediate, which subsequently undergoes alkene insertion... 

NaOH-HiOz) or to carboxylic acids (with w-chloroperbenzoic acid).-190 Double bonds can be hydroborated in the presence of triple bonds if the reagent is 9-BBN.191 On the other hand, dimesitylborane selectively hydroborates triple bonds in the presence of double bonds.192 Furthermore, it is often possible to hydroborate selectively one particular double bond of a nonconjugated diene.191 When the reagent is catecholborane, hydroboration is catalyzed by rhodium complexes, such as Wilkinson s catalyst.194 Enantioselective hydro-boration-oxidation has been achieved by the use of optically active rhodium complexes.195... 

Aryltrimethylstaimanes arylate aldehydes in the presence of a cationic rhodium complex, [Rh(cycloocta-l,5-diene)(MeCN)2]+, yielding secondary alcohols.114... 

Interestingly, the propensity of the boron atom to engage in secondary interactions was also examined by Jacobsen. The interaction of the rhodium complex 60 with a model substrate, namely 5-hexen-l-amine, was monitored by 1H NMR spectroscopy.62 The stronger upheld shifts of the alkene resonances compared to those observed upon coordination of the same substrate to the related boron-free salt [Rh(cod)(DIOP)][ClC>4] (cod = cycloocta-1,5-diene) were attributed to a cooperative behavior of the boron and metal centers of 60 that concomitantly interact with the nitrogen atom and alkene moiety, respectively (Figure 20). 

A chiral rhodium(I)-diene complex catalyses the addition of dimethylzinc to N-tosylarylimines, ArCH=NTs, with ees up to 98%.40... 

Asymmetric hydrogenation of alkenes is efficiently catalysed by rhodium complexes with chiral diphosphite and diphosphoramidite ligands derived from BINOL or diphenylprolinol. Choice of a proper achiral backbone is crucial.341 Highly enantioselective hydrogenation of A-protected indoles was successfully achieved by use of the rhodium catalyst generated in situ from [Rh(nbd)2]SbF6 (nbd = norborna-2,5-diene)... 

Cod dienes, in rhodium complexes, 7, 203 COdH, see Carbon monoxide dehydrogenase Coenzyme B12... 

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