Intermolecular RhCl

The Rh-Rh distance is 3.12 A, long compared with Rh-Rh single bonds (2.624A in Rh2(MeCN) J([, 2.73 A in Rh4(CO)12) there is a weaker (3.31 A) intermolecular attraction. Dipole moment and IR studies indicate that the structure is retained in solution and is, therefore, a consequence of electronic rather than solid-state packing effects. Furthermore, it is found for some other (but not all) [RhCl(alkene)2]2 and [RhCl(CO)(PR3)]2 systems. SCF MO calculations indicate that bending favours a Rh-Cl bonding interaction which also includes a contribution from Rh—Rh bonding [56b]. 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

Alkoxy-VCP 163 was found to be a very competent reagent in the intermolecular [5+2] cycloaddition (Tab. 13.12). With some minor optimization of the previous reaction conditions, namely the use of 1,2-dichloroethane (DCE) as solvent at a higher concentration (0.5 M) and reaction temperature (80 °C), the reaction was found to be complete in minutes in some cases with 0.5 mol% [RhCl(CO)2]2, while still providing good to excellent yields of cycloheptenone products. Significantly, reactive functionahty, including unprotected alcohols and carboxylic acids, is tolerated in the reaction. The reaction is also readily scaled, with comparable isolated yields over a 100-fold increase in scale. The formation of products in minutes is of consequence, as such reactions allow for the more time-efficient reahzation of synthetic goals. 

Replacement of the 2-naphthyl groups by 2-dimethylaminomethylphenyl groups in H2(npOEP) also led to a rhodium porphyrin being able to extract leucine from water, however, the situation is complicated by dimerization of the rhodium porphyrin due to intermolecular amine-rhodium bonding [286]. A rhodium complex of a trifunctional chiral bis(2-hydroxynaphthyl)porphyrin related to the above-mentioned RhCl(npOEP) system was used to separate diastereomers formed via two-point fixation of amino acids [287],... 

C-Arylglycoside 137 is synthesized by the RhCl(Ph3P)3-catalysed intermolecular reaction of 135 with 136 [57],... 

Intermolecular reactions of dienes, allenes, and methylenecy-clopropanes with alkenes are mediated by RhCl(PPh3)3, although mixtures of products are usually formed (eqs 50-51). ... 

As part of comparative studies, Iyer [47] reported the use of Vaska s complex [IrCl(CO)(PPh3)2l (92) in intermolecular Mizoroki-Heck-type reactions of methyl acrylate (1) and styrene (2). Aryl iodides could be used as electrophiles, while bromobenzene, chlorobenzene and aliphatic halides gave no desired product. The catalytic activity was found to be lower than that observed when using Wilkinson s complex [RhCl(PPh3)3] (84). Thus, a higher reaction temperature of 150 °C was mostly required. In contrast to the corresponding cobalt-catalysed reaction, however, Vaska s complex (92) proved applicable to orf/io-substituted aryl iodides (Scheme 10.33). 

Intermolecular direct arylations of heteroarenes, such as indoles, pyrroles or (benzo)furans, were, thus far, predominantly achieved with palladium catalysts (see Chapter 10). However, rhodium complexes proved also competent for the direct functionaUzations of various valuable heteroarenes with comparable or, in some cases, improved catalytic performance. Thus, rhodium-catalyzed C—H bond functionalizations of various N-heterocycles, were elegantly developed by Bergman, Ellman and coworkers. Here, the use of a catalytic system comprising [RhCl(coe)2]2 and PCys led to direct arylations of unprotected benzimidazoles with aryl iodides... 

The coordinately unsaturated Rh catecholate complex [(triphos)Rh(3,5-DBCat)] (40) has been synthesized by oxidative addition of 3,5-DBQ (31) to the 16-electron fragment [(triphos)RhCl] [triphos = MeC(CH2PPh2)3]. Below 10 C, complex 40 in dichloromethane or acetonitrile picks up dioxygen in a reversible manner to form [(triphos)(Rh (h -02)(h -3,5-DBSQ)] (41), which promotes the oxygenation of 28 to give 31 (97 %), 29 (5 %), and 30 (3 %) (Scheme 22). The characteristic features of the oxygenation of 28 catalyzed by 41 are as follows (i) the process takes place in an intermolecular fashion, (ii) the catechol is oxygenated into intermediates either of the... 

The first report on rhodium-catalyzed [2 + 2 + 2] cycloaddifion of alkynes is the intermolecular cyclotrimerization of dimethyl acetylenedicarboxylate (DMAD) catalyzed by a neutral rhodacyclopentadiene/arsine complex in 1968 [6]. After this initial report, various neutral rhodium(I) complexes were developed for intermolecular [2 + 2 + 2] cycloaddition of internal alkynes (Scheme 4.1) [7-13], Among them, (T) -cyclopentadienyl)rhodium(I) complexes [7-9,13] are the best-investigated catalysts. Neutral rhodium(ni) complexes have also been employed as catalysts [14,15], A RhCls/amine system effectively catalyzes [2 + 2 + 2] cycloaddition of internal alkynes [15]. 

Selective intermolecular [2 - - 2 - - 2] cycloaddition of terminal alkynes is more difficult to achieve than internal alkynes, due to their various reactivities toward transition-metal complexes in addition to the regioselectivity problem. Indeed, a neutral rhodium(I)/phosphine complex such as RhCl(PPh3)3 generally reacts with terminal alkynes to give not cyclotrimers but linear dimers [20]. The neutral rhodium(I) and rhodium(III) complexes could be applied to intermolecular [2 - - 2 - - 2] cycloaddition of terminal alkynes (Scheme 4.5) [13,15a and b]. 

In the pyridine synthesis, the cationic rhodium(I)/biaryl bisphosphine complexes are widely applicable catalysts for both intermolecular and intramolecular [2 + 2 + 2 cycloaddition of alkynes with nitriles under mild conditions. Wilkinson s complex [RhCl(PPh3)3] is effective for intramolecular [2-f 2-1-2] cycloaddition of diyneni-triles under microwave heating. 

Intermolecular Rhodium-Caialyzed [5+2] Cycloadditions In 1998, Wender et al. reported the first examples of intermolecular metal-catalyzed [5+2] cycloadditions of VCPs with alkynes [32]. While several catalysts have been proved to be efficient in promoting intramolecular [5+2] cycloadditions of VCPs and alkynes, the intermolecular [5+2] process has been Umited until recently to the use of [RhCl(CO)2]2 [33]. The initial study revealed that internal, terminal, electron-rich, and electron-poor alkynes all participated in the [5+2] cycloaddition with VCPs, giving... 

The use of [RhCl(CO)2]2 in intermolecular [5+2] cycloadditions often require heating, which in turn promotes competing cyclotrimerization of alkyne starting materials, decomposition of the VCP, or formation of undesired secondary isomerization products. Such transition metal-catalyzed intermolecular cycloadditions pose particular chemo-and regioselectivity challenges as well as entropic penalties not encountered in intramolecular processes, as the latter benefit from tether-derived alignment and proximity of reactive functionalities not possible in the former. In this context, Wender et al. have recently demonstrated that the cationic rhodium(I) complex, [RhCCioHsKcod)]" SbFe, promoted the remarkably efficient intermolecular [5+2] cycloaddition of 1-alkoxy-VCP 37, and 1-alkyi-VCP 42... 

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