Organorhodiums

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

An investigation of different organic solvents, buffer, surfactants, and organorhodium compounds established that the catalytic reduction of tetralin using [ Rh(l,5-hexadiene)Cl 2] proceeds with high efficiency at high substrate-to-catalyst ratios. The reaction occurs at r. t. and 1 atm. pressure in a biphasic mixture of hexane and an aqueous buffer containing a low concentration of a surfactant which stabilizes the catalysts.314... 

Nucleophilic additions of organorhodium species to C=0 and CN multiple bonds constitute important classes of catalytic organic reactions and have experienced significant progress in the last decade. 

Acetylenic esters react with arylboron reagents in the presence of rhodium diphosphine catalyst to give cyclic ketones.409 Equation (61) shows an example which may involve ortfe-metallation and ketone formation. A catalytic, enantioselective reaction was also achieved (Equation (62)). These processes presumably involve unprecedented addition of organorhodium species to the ester carbonyl group. 

Recently, addition of organorhodium species to nitriles has been reported.420 4203 4201 Intermolecular reaction of benzonitrile with phenylborate (accompanied with r//w-aryiation) (Equation (65)), arylative cyclization of acetylenic nitriles (Equation (66)), and cyclization of 2-cyanophenylboronic acid with alkynes or strained alkenes (Equation (67)) are proposed to proceed via this process. 

Although addition of organoiridium compounds to carbonyls and CN multiple bonds has been little investigated in comparison with those of organorhodium compounds, several characteristic reactions have been disclosed recently. 

Once 26 or 27 has been formed, the rhodium-aUcoxide complex is protonated by a nucdeophile molecule, generating the cationic rhodium complex 29 and an alkoxide or phenoxide nucdeophile. This proton transfer step is supported by kinetics experiments and has two effects [14]. Firstly, the organorhodium species is made more electrophilic as a result of the positive charge, and secondly, the nucleophile is rendered more nu-cdeophihc by becoming deprotonated. 

Evans and Nelson examined the stereospecificity of the rhodium-catalyzed allylic alkylation, with the expectation that it would provide additional insight into the mechanism for this particular reaction [16]. They reasoned that the enantiomerically enriched allylic alcohol derivative i would furnish the enantioenriched product iv, provided the initial enyl intermediate ii does not isomerize to the achiral rr-organorhodium intermediate iii prior to alkylation (k2>ki Scheme 10.3). Alternatively, the product of re-... 

Treatment of the enantiomerically enriched acyclic allylic carbonate (S)-l (97% ee) under the standard reaction conditions furnished the allylic alkylation product (S)-14 (95% ee) in 86% yield, with net retention of absolute configuration (Eq. 3). This result implies that the displacement occurs via a classical double inversion process, albeit through a configurationally stable distorted u-allyl or enyl ff+n) organorhodium intermediate. This is supported by the fact that the achiral ff-spedes iii would undoubtedly have afforded the racemate of 14 (Scheme 10.3). Additionally, the enyl (a+n) organo-metallic intermediate provides a model for the regio- and enantiospedfidty observed in the reaction. 

Overall, this book clearly illustrates what we can do in organic synthesis using rhodium catalysis and I have no doubt that it will serve as an excellent reference text for both graduate students and synthetic chemists at all levels in academia and industry. Moreover, I anticipate that this book will stimulate additional research in the area of organorhodium chemistry, and serve to inspire those involved in the development and application of new synthetic methodology. 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

Nakai H, Mizuno M, Nishioka T, Koga N, Shiomi K, Miyano Y, Irie M, Breedlove BK, Kinoshita I, Hayashi Y, Ozawa Y, Yonezawa T, Toriumi K, Isobe K (2006) Direct observation of photochromic dynamics in the crystalline state of an organorhodium dithionite complex. Angew Chem Int Ed 45 6473-6476... 

Well-controlled polymerization of substituted acetylenes was also reported. A tetracoordinate organorhodium complex induces the stereospecific living polymerization of phenylacetylene.600 The polymerization proceeds via a 2-1 -insertion mechanism to provide stereoregular poly(phenylacetylene) with m-transoidal backbone structure. Rh complexes were also used in the same process in supercritical C02601 and in the polymerization of terminal alkyl- and arylacetylenes.602 Single-component transition-metal catalysts based on Ni acetylides603 and Pd acet-ylides604 were used in the polymerization of p-diethynylbenzene. 

CO Chemisorption on Clean Rh(lll) (23). The vibrational spectra of carbon monoxide chemisorbed on Rh(lll) at 300 K as a function of exposure are shown in Figure 2. At very low exposures (less than 0.1 L 1 L = 1 Langmuir 10-6 torr sec) only one peak at 1990 cm- - is observed in the C = 0 stretching region and no ordered LEED pattern is found. By comparison with the infrared spectra of relevant organorhodium compounds (50,... 

Fish RH (2010) A bioorganometallic chemistry overview from Cytochrome P450 enzyme metabolism of organotin compounds to organorhodium-hydroxytamoxifen complexes with potential anti-cancer properties a 37 year perspective at the interface of organometallic chemistry and biology. Aust J Chem 63 1505-1513... 

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