Rhodium complexes isonitriles

Murahashi described a new pyrrole synthesis involving a rhodium complex-catalyzed reaction of isonitriles (e.g., 3) with 13-dicarbonyl compounds 4 to afford the pyrroles 5 <01OL421>. This process is believed to proceed by chemoselective activation of the a-C-H bond of the isonitrile even in the presence of the more acidic dicarbonyl derivative. 

The diboration of alkenes has been more challenging to develop. Table 16.5, which is also adapted from the Miyaura review, summarizes a series of diborations of alkenes. These examples show that complexes of group 10 metals lacking phosphine or isonitrile ligands have been most active for diese additions. Gold and rhodium complexes have also been shown to catalyze the diboration of vinylarenes. 

The structure of rhodium(II) carboxylate-derived carbene complexes has been assessed both by quantum mechanical calculations [19,20] and by the study of rhodium(II) carboxylate isonitrile complexes [20,21]. Recent investigations [20] suggest, that also in these highly electrophilic carbene complexes there is a significant n backbonding from rhodium to carbon. 

The [Rh ri -TMPP)2] [TMPP = tris(2,4,6-trimethoxyphenyl)phosphine] complex reacts reversibly with CO and isonitriles to form stable square-planar diamagnetic carbonyl and isonitrile complexes (13, 14). In the octahedral [Rh ri -TMPP)2] complex, the two TMPP ligands are coordinated to rhodium(II) with the two P donors in mutual cis positions. Four methoxy... 

The photochemical activation of solvents has afforded several complexes of the type Tp M(Ar)X(L) (Table III). Thus, in the presence of P(OMe)3, Tp IrH2 ( -COE) (207) photolytically activates benzene to afford 321, the mechanism of which was explored at some length (Section III-B. 1 and III-B.2). The rhodium analogue of 321 (324) was obtained under comparable conditions, but from the rhodium(I) complex Tp Rh( " -1,3-COD) (171), " which also activates benzene in the presence of t rt-butylacrylate to afford 326. The bis(isonitrile) complex Tp Rh(CNCH2 Bu)2 (509) photolytically activates benzene to afford... 

A problem is that the Pauson-Khand reaction uses two equivalents of cobalt. More efficient versions, many of them catalytic, using other metals have been developed. These include carbonyl complexes of titanium, molybdenum, tungsten (Scheme 7.15), rhodium and ruthenium (Scheme 7.16). Rhodium, iridium and iron (Scheme 7.17) have also been used with two alkynes to give cyclopentadienones, often as complexes 7.59. A version of the Pauson-Khand reaction employing a nickel catalyst and an isonitrile in place of CO has been developed. The product is an imine, which can be hydrolysed to a cyclopentenone. 

Square planar / -complexes of rhodium are known for engaging in catalytic cycles based on oxidative addition chemistry. A recently developed example is reaction (5), involving insertion of an isonitrile into an aryl C —H bond. " Initial dissociation of phosphine is likely, providing 14-electron [RhCl(L)(CNR)]. The [Cp Re(CO)L] fragment was very readily produced... 

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