Arylrhodium species

Miura reported a 1,4-shift of rhodium in the reaction of norbornene with arylboronic acids (Scheme 3) where rhodium moves from an alkyl sp2-carbon to an aryl sp2-carbon, and the arylrhodium species further reacts with another molecule of norbornene.30 Polyalkylated phenyls are obtained in good yields. Cesium fluoride facilitates the transmetallation, generating the phenylrhodium complex which then coordinates to the -face of norbornene. 

Cyclobutanones react with arylboronic acids to afford butyrophenones (Equation (60)). Addition of arylrhodium species to the carbonyl group followed by /3-carbon elimination is proposed. 

Investigation using P NMR revealed that a direct transmetallation of the phenyl group from boron to rhodium of the (oxa-n -aUyl)rhodium complex is involved in the reaction of B-Ph-9BBN. The catalytic cycle consists of two steps 1) insertion of the en-one into the aryl-rhodium bond and 2) transmetallation to the (oxa-7r-aUyl)rhodium complex forming an arylrhodium species and the boron enolate (Scheme 3.19). Unfor-... 

A rhodium(l)-catalyzed system in THF is also effective in the Mizoroki-Heck-type reaction of arylsilanediols with acrylates (Scheme 4).53 Interestingly, the use of aqueous THF switches the reaction to 1,4-addition forming /3-arylated esters. The proposed catalytic cycles for these reactions involve 1,4-addition of an arylrhodium species to an acrylate. The change of the reaction pathway is probably because, in aqueous THF, the resultant Rh enolate 6 undergoes protonolysis rather than /3-elimination. Similar Rh-catalyzed 1,4-additions to a,/3-unsaturated carbonyl compounds have been achieved with arylsilicones,54 arylchlorosilanes,55 and aryltrialkoxysilanes.56,57 The use of a cationic Rh-binap complex leads to highly enantioselective 1,4-additions of alkenyl- and arylsilanes.58 583... 

The mechanism of the rhodium catalyzed arylation of imines presumably occurs via an initial transmetallation to produce the arylrhodium species 59 (Figure 1.10). Aryl transfer followed by a second rhodium boron transmetallation (or protonation by water) completes the catalytic cycle. 

In continuous operation consecutive phosphorus-containing products are formed which also influence the activity of the rhodium center and thus contribute to the catalyst deactivation. One of the main degradation products from TPPTS is the sodium salt of m-formylbenzenesulfonic acid, which indicates the insertion of the rhodium atom into the P—C bond. The arylrhodium species in which rhodium has replaced one phosphorus atom presumably exists as a phosphido-bridged dimer which is inactive. This compound may subsequently be converted to a series of consecutive products, e.g., alkyl-diarylphosphines, which act as catalyst poisons. 

Arylrhodium species undergo facile intermolecular 1,2-addition across carbon-carbon triple bonds, with the resulting alkenyl rhodium intermediate providing an entry point for further cascade reactions [121]. As an example, Hayashi applied this concept to the asymmetric synthesis of a number of cycloalkanols 157 starting from ynals 155 and arylboronic acids (Scheme 8.43) [122]. TransmetaUation of the... 

Nevertheless, it is important to underline that in the transmetaUation step from boron to rhodium, it is believed that the Rh-OH complex can coordinate the highly oxophilic arylboronic acid to afford intermediate A (Scheme 5.5). The latter can then deliver the aryl fragment to rhodium in an intramolecular manner to furnish the arylrhodium species. This process occurs under neutrtil conditions but can be accelerated by the addition of a stoichiometric quantity of base. The quaternization of the arylboronic acid will facilitate the rupture of the B-Ar bond (Scheme 5.5) [9b, 14]. 

Aryhhodium species generated upon the cleavage of an Ar-CN bond by a silylrhodium intermediate, derived from the reaction of rhodium(l) with disUanes (see below), can be trapped by aryl halides in an intramolecular manner to give dibenzofurans and carbazoles (Scheme 12) [53]. The arylrhodium species can also... 

Related to the P-carbon elimination of late transition metal alcoholates, P Carbon elimination from rhodium iminyl complexes has also been reported (Scheme 1.44) [61]. Aryl nitriles were eliminated to form arylrhodium species. 

Asymmetric synthesis of 3,4-dihydrocoumarin 41 was achieved by rhodium(I) catalyzed intramolecular reaction of phenol-substituted cydobutanone 42 (Scheme 3.21) [33]. When the reaction of 3,3-disubstituted cyclobutanones was performed in the presence of electron-deficient alkenes, the arylrhodium species generated via 1,4-rhodium shift underwent addition to the alkenes, providing 5-alkylated dihydrocoumarins (Scheme 3.22). 

Work on mechanistic details is in progress, but the present transformation may result from a catalytic cycle that involves the transmetalation between the hydroxo-rhodium(I) 4 and arylboronic acid to give the arylrhodium(I) species 5, and the insertion of enone to the Ar-Rh bond. The hydrolysis of the rhodium(I) enolate with water reproduces the hydroxorhodium species, as shown in Figure 1. The arylrhodium(I) complexes are unstable such as to preclude isolation in pure form, but they have been reasonably speculated to be the key intermediates carrying out various coupling reactions with organic halides and the addition to alkenes and alkynes. 

The transmetallation with organoboron reagent precedes the carbometallative addition of the arylrhodium intermediates 394 to the unsaturated substrate, and the intermediate alkenylrhodium species 395 then undergo hydrolysis to close the catalytic cycle with regeneration of the catalytically active species 393 and formation of the observed enamide products. An intermolecular carborhodation event was proposed to occur in a three-component coupling of arylboronic acids with disubstituted alkynes in the presence of methyl acrylate (Scheme 10.135) [113]. 

The proposed catalytic cycle is described in Scheme 4.23. Transmetallation of aryl-boronate esters to hydroxorhodium species, giving an arylrhodium intermediate, is followed by insertion of the norbomene derivative. Then, the alkylrhodium intermediate, which has no hydrogens for p-hydrogen elimination, undergoes insertion of... 

In 2002, Kurg and Hartwig observed directly aldehyde insertion into an arylrhodi-um species [48]. Arylrhodium complexes are mostly unstable to isolate, but a rhodium analogue of Vaska s complex 74 was easier to handle. The reaction of 74 with 2-naphthaldehyde in benzene-dg at room temperature gave alkoxyrhodium intermediate 75, which was observed directly by and NMR spectroscopy (Scheme 4.36). On treatment of 74 with 2-naphthaldehyde in a mixture of THF-dg and D2O, hy-... 

Monosubstituted cyclobutanones reacted with arylboronic acids in the presence of a Rh(I)-P(f-Bu)3 catalyst to afford butyrophenone derivatives by the addition of an arylrhodium(I) species to the carbonyl group, followed by ring opening of the resulting rhodium(I) cyclobutanolate by P-carbon elimination... 

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