Cycloadditions RhCl

The bicyclo[6.1.0]nonatriene 46 is transformed to the 8,8-dihydroindene on treatment with a catalytic amount of [Rh(CO)2Cl]2 [24], The stereoselectivity depends on the reaction temperature. [5 + 2]cycloaddition of vinylcyclo-propanes 47 with alkynes is catalyzed by RhCl(PPh3)3 in the presence of silver triflate to give seven-membered rings 48 [25]. (Scheme 17 and 18)... 

In the presence of nickel(0), tethered diene-VCPs react to produce eight- and five-membered ring products (Scheme 2). Palladium(O) and cobalt(m) were also tried but produced only decomposition products. However, in the presence of Wilkinson s catalyst (RhCl(PPh3)3), tethered diene-VCP 1 was cleanly converted to triene 4 in 91% yield. Although the desired cycloaddition reaction was not obtained, the cleavage of the cyclopropane ring was encouraging.22... 

A cycloaddition process between the Rh=C bond of the allenylidene derivative 38 and the C=C bond of the terminal alkyne has been evoked in the formation of the zwitterionic 71-aUyl-allenyl complexes 81 (Scheme 28), the initially formed metaUacyclobutenes 80 evolving into 81 by formation of carbene intermediate [RhCl(P/-Pr3)2(=CHCR=C=C=CPh2)] (R = Ph, p-MeC6H4, SiMe3) and subsequent migration of one of the phosphine ligands from the metal to the carbene carbon atom [205]. 

A combination of the alkyne cyclotrimerization catalyzed by RhCl(PPh3)3 with a 1,3-dipolar cycloaddition provides rapid access to 3-dihydroindenylmethyl-3,7-diaza[3.3.0]-octane 61 in excellent yield thus five new bonds, four stereocenters, and three rings are created in a one-pot operation (Scheme 7.18) [41]. 

The first examples of metal-mediated [5-1-2] cycloadditions between VCPs and tethered alkynes were reported in 1995 (Tab. 13.1) [21]. Initial success was obtained by treating alkyne-VCP 54 with Wilkinsoris catalyst ([RhCl(PPh3)3]) in refluxing toluene for 48 h to produce cycloadduct 55 in 84% yield. Further investigations led to the development... 

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. 

RhCl(CO)2]2 was discovered early in our work as a new catalyst for the [5+2] cycloaddition of VCPs and alkynes. This catalyst proved to be impressively effective for many of the previously problematic cases and, more generally, allowed the reactions to proceed under mild conditions. Initial interest in [RhCl(CO)2]2 was prompted, in part, by the expectation that it would be less stericaUy encumbered than Wilkinson s catalyst. 

The reaction of VCP 79 illustrates the performance of the rhodium(I) dimer (Tab. 13.4). For reference, attempts to effect [5+2] cycloadditions with this substrate (79) and [RhCl(PPh3)3]/silver triflate resulted only in the formation of complex product mixtures. In remarkable contrast, when this same substrate was treated with 5 mol% [RhCl(CO)2]2 for 20 min in toluene at 110°C, the [5+2] cycloadduct 80 was obtained in 80% yield. Despite these significant advantages, tethered alkene-VCPs are not successfully converted with this catalyst. 

The stmctural complexity and biological activity of the cyathane family of diterpenes has stimulated considerable interest from synthetic chemists, as reflected in the number and diversity of approaches reported thus far [42]. Our own strategy for cyathane synthesis is based on a rhodium-catalyzed [5+2] cycloaddition. The precursor for this reaction was fashioned ultimately from commercially available and inexpensive (S)-(-)-limonene. Treatment of the ketone 139 with 5 mol% [RhCl(CO)2]2 in 1,2-dichloro-ethane gave cycloadduct 140 (Scheme 13.14) in 90% yield and in analytically pure form after simple filtration through a plug of neutral alumina [43]. 

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. 

Alkynes are poor dienophiles in the Diels- Alder reaction decomposition occurs by an attempted thermal intramolecular Diels-Alder reaction of dienynes at 160 °C. In contrast, the Ni-catalysed [4+2] cycloaddition of the dienyne 50 proceeded smoothly at room temperature using tri(hexafluoro)isopropyl phosphite to give 51, which was converted to the yohimbine skeleton 52 [15]. The same reaction is catalysed by RhCl(Ph3P)3 in trifluoroethanol [16]. Intramolecular Diels-Alder reactions of the 6,8-dieneyne 53 and the 1,3,8-triene 55, efficiently catalysed by [Rh(dppe)(CH2CH2)2]SbF6 at room temperature, gave 54 and 56 [17],... 

The Rh-catalyzed cycloisomerization of 1,6-enynes that occurs via rhodium vinylidene-mediated intermediates was first described in 1988 by Grigg et al. (471). Cyclization of 1,6-enynes (I, Scheme 57) with [RhCl(cod)]2/P(p-FC6H4)3, where cod=l,5-lyclooctadiene as the catalyst generates a metal-vinylidene that undergoes [2 + 2] cycloaddition and... 

Cycloaddition. In the presence of RhCls 3H2O and i-Pr2NEt alkynes are trimerized to provide substituted benzenes, a mixture of a diyne and an alkyne forms a 1 1-adduct. ... 

The first example of the asymmetric [5+2] cycloaddition of alkene-VCP 517 was reported using a Chiraphos-Rh catalyst, which gave c/s-fused bicyclo[5.3.0]decene 518 in 80% yield and 63% ee (Scheme 2-75, eq. 1). The reaction of alkyne-VCP 519 catalyzed by [ RhCl(C2H4)2 2] complex in the presence of NaBAr 4 gave 520 with 99% ee (Scheme 2-75, eq. 2). ... 

In an intramolecular Mizoroki-Heck-type alkenylation of alkenes, Wilkinson s catalyst [RhCl(PPh3)3] (84) proved superior than other rhodium compounds (Scheme 10.29) [56]. Notably, the selectivities of the cyclization of various 2-bromo-1,6-dienes were found to be improved with Wilkinson s catalyst when compared with those observed with palladium complexes. Thereby, the corresponding l,2-bis(methylene)cyclopentanes, which themselves are valuable substrates for further cycloaddition reactions, such as 1,3-diene 86, could be isolated in high yields. 

Roulland and coworkers utilized a [2-t2-t2] cycloaddition and an RCM-aromatization strategy in an innovative approach to the central core of the landomycinone natnral prodncts [46]. As shown in Scheme 17.24, the triyne 124 was successfully cycUzed with the catalyst RhCl(PPhj)j to afford compound 125, after which vinyUithium addition resulted in the hemiketal diene 126. Immediate... 

The use of heteroatom-linked diynes enables the efficient synthesis of substituted heterofluorenes. For example, substituted carbazoles were synthesized by the RhCl(PPh3)j-catalyzed [2+2+2] cycloaddition of tosylamide-hnked 1,6-diynes with terminal alkynes (Scheme 21.10) [13]. 

Hanzawa, Saito, and their coworkers developed a rhodium(I)-cata-lyzed intramolecular hetero-[4 -I- 2] cycloaddition of a-alkynyl-vinyl oximes in 2007 [12], By using [RhCl(cod)]2 and AgSbFe as the catalyst system in hexafluoroisopropanol (HFIP), bicyclic pyridine derivatives were formed in good yields (Scheme 2.8). 

Another [2+2+2] cycloaddition reaction, using alkenyl isocyanates and alkynes, affords the vinyl amides 195 and 196 in good yields. The catalysts system used is [RhCl(C2H4)]2... 

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