Unsaturaled compounds catalysts, rhodium complexes

Hydrosilylotion of carbonyl compounds. This rhodium complex is an effective catalyst for hydrosilylation of aldehydes, ketones, a,/3-unsaturated aldehydes and ketones, and a-diketones. Hydrosilylation followed by hydrolysis is equivalent to reduction of either the carbonyl group or the a, 3-un-saturation. 

Besides rhodium catalysts, palladium complex also can catalyze the addition of aryltrialkoxysilanes to a,(3-unsaturated carbonyl compounds (ketones, aldehydes) and nitroalkenes (Scheme 60).146 The addition of equimolar amounts of SbCl3 and tetrabutylammonium fluoride (TBAF) was necessary for this reaction to proceed smoothly. The arylpalladium complex, generated by the transmetallation from a putative hypercoordinate silicon compound, was considered to be the catalytically active species. 

In the 18 years since its discovery, Wilkinson s catalyst has been used to hydrogenate all kinds of unsaturated compounds and great efforts have been made to develop other phosphorus ligand-containing catalysts of both rhodium and other metals. This in turn has led to catalytically active complexes which do not contain phosphorus. In Table 1 the hydrogenation of various unsaturated compounds catalysed by [RhCl(PPh3)3] is summarized. In some cases the addition of deuterium and tritium to the substrate was investigated.84,95,101,102... 

Activity of the Wilkinson catalyst [RhCUPPhsls] in hydrosilylation of carbonyl compounds was already reported by Ojima and co-workers in 1972 (201). The catalyst is highly active in the hydrosilylation of a variety of diaryl, aryl alkyl, and dialkyl ketones, aryl and alkyl aldehydes, a,jS-unsaturated ketones, and esters. Stereoselectivity of hydrosilylation of cyclic ketones depends onbuUd-ness and electronic nature of silanes used. The hydrosilylation of a,/9-unsaturated carbonyl compounds in the presence of [RhCKPPhsls] and other rhodium complexes shows a specific dependence of l,2-/l,4-addition selectivity on the nature of the silane used so that monohydrosilanes undergo 1,4-addition whereas di- and trihydrosilanes give selectively 1,2-addition product (Scheme 29). 

In Section 4, it is described that chlorotris(triphenylphosphine)rhodium(I) (7) is quite an effective catalyst for the hydrosilylation of carbonyl compounds. For this reason, extensive studies on asymmetric hydrosilylation of prochiral ketones to date have been based on employing rhodium(I) complexes with chiral phosphine ligands. The catalysts all prepared in situ are rhodium(I) complexes of the type, (BMPP>2Rh(S)a (8) [40] and (DIOP)Rh(S)Cl (6) [41], and a cationic rhodium(III) complex, [(BMPP)2lUiH2(S)2] Q04 (5) [42], where S represents a solvent molecule. An interesting polymer-supported rhodium complex (V) [41], and several chiral ferrocenylphosphines [43], recently developed as chiral ligands, have also been employed for asymmetric hydrosilylation of ketones. Included in this section also are selective asymmetric hydrosilylation of a,0-unsaturated carbonyl compounds and of certain keto esters. 

Hydrothiolation of unsaturated compounds is an atom economical process, especially the formation of C—S bonds, which are present in many biologically active molecules [146]. Moreover, vinyl sulfides are very convenient synthetic intermediates in organic reactions. Despite the existence of several active catalysts, the stereo- and regioselective controk remain a challenge [147]. Castarle-nas and coworkers reported the selective hydrothiolation of alkynes catalyzed by NHC-based rhodium complexes 62 and 63 (Scheme 10.6) imder mild reaction conditions [147]. A regioselective switch from linear to branched vinyl sulfides was observed when mononuclear compound 62 was used. The authors proposed... 

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... 

Cationic rhodium(I)/chiral biaryl bisphosphine complexes are suitable for the enantioselective [2-I-2-1-2] cycloaddition reactions between electron-rich alkynes and electron-deficient unsaturated compounds (alkynes, nitriles, and isocyanates). Alkynylcarbonyl compounds, alkynylphosphonates, and alkynylphosphine oxides are the best-suited electron-deficient unsaturated compounds for these catalysts. These complexes are also able to catalyze the highly enantioselective synthesis of axially chiral anilides and bezamides. 

When the transition-metal-catalyzed hydrothiolation of unsaturated compounds is performed in the presence of carbon monoxide, carbonylation reactions may proceed with the introduction of sulfanyl groups. In fact, a series of carbonylative thiolation reactions of alkynes and allenes are reported. These reactions provide useful tools to synthetically important organosulfur compounds [104, 105]. For example, the rhodium-catalyzed reaction of alkynes with thiols and CO provides the corresponding thioformylation products regioselectively [106, 107]. Switching the catalyst from rhodium complex to platinum complex leads to a sharp reversal of regioselectivity of CO introduction [108, 109] (Scheme 32). 

Thus, the comprehensive investigations above demonstrate that a variety of multicyclic compounds with quaternary stereogenic centres can be obtained from this class of dien-ynes, depending on the length of the tethering chain as well as on the substituents of the unsaturated functions. The efficiency of cationic Tol-BEM-AP/Rhodium complexes as catalysts for many reactions of this class has been assessed. 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

---