Isoprene complexes with rhodium

In 1987, Matsuda et al. reported the first Rh-catalyzed intermolecular [4+2] cycloaddition (Scheme 2-65). The cationic rhodium-complex-catalyzed codimerization of isoprene (491) with alkynes 390, affording the corresponding cyclohexadienes 492 in 44-85%, yields and excellent regioselectivity. 

The alkylation of phenols with isoprene and isoprene derivatives can be catalyzed over rhodium complexes of TPPTS [22], In the case where the isoprene derivative is myrcene, methanol is added to the reaction mixture. The apparent benefit is to increase reaction rates with the highly water-soluble myrcene substrate. The alkylation of 2-naphthol with myrcene is shown in Eq. (2). In 2 h at 100 °C in aqueous methanol a 40% conversion of myrcene is observed (160 turnovers). 

Similarly, the reaction of isoprenes with methyl acetylacetate can be accomplished over rhodium complexes of TPPTS [23], The example with myrcene is shown in Eq. (3). In this case rates are also increased upon addition of methanol. For example, in water alone as the solvent 174 turnovers of myrcene are observed in 1 h at 90 °C, whereas 297 turnovers are observed under otherwise identical conditions in water/methanol (75 25). No difference in reaction selectivity is observed in the presence of methanol. In both cases the selectivity to the isomeric products shown in Eq. (3) is 99%. 

Representative products are XLII and XLIV . With butadiene in benzene the rhodium complex XLIIIa reacts fast at RT, whereas the iridium complex XLIIIb requires 2 h at reflux. Other dienes give complex products with XLIIIa. The complex XLIIIb reacts with isoprene (12 h at reflux) but fails to react with cyclic dienes. 

The use of the cobalt triad carbonyls as catalysts continues to provide many papers for this report. Publications cover the silylformylation of 1-Hexyne catalyzed by diodium-cobalt carbonyl clusters the formation of hydroxycarbene cobalt carbonyl derivatives, the use of rhodium cluster carbonyls in the water-gas shift reaction Rh4(CO) 2> and Co3Rh(CO)] 2 catalysts for the hydrosilation of isoprene, cyclohexanone and cyclohexenone catalytic reduction of NO by CO and the carbonylation of unsaturated compounds The chemistry of iridium carbonyl cluster complexes has been extended by making use of capping reactions with HgCl2and Au(PPh3)Q... 

Cobalt, Rhodium, and Iridium. —Cations of structure [Co(diene)(PRs)3]+ have been prepared (diene = butadiene or isoprene) they are fluxional in solution, but low temperature n.m.r. spectra indicate a square pyramidal structure with diene in the basal plane. The benzoquinone complex (87) has been prepared and undergoes successive mono- and di-protonation at oxygen methylation with Mel also occurs at oxygen to yield (88). Cobaltacyclopentadiene complexes Cp(PPh3)Co(CRi=CR CR =CR ) (RSR =Me, Ph, or CO Me) react with... 

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