Rhodium-Mediated Cycloaddition

Transition-Metal-Mediated Aromatic Ring Construction, First Edition. Edited by Ken Tanaka. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

1 Intermolecular Reactions Catalyzed by Neutral Rhodium Complexes 

Conjugated oligomers, possessing donor and acceptor substituents, have been a prominent class of compounds in material science. Two dimensionally conjugated benzenes, bearing three pairs of donor-acceptor substituents, were synthesized by 

2 Intermolecular Reactions Catalyzed by Cationic Rhodium Complexes 

R = alkyl, alkenyl, aryl, SiMes E = C02Me, COjEt, C02lBu 

---

The rhodium-mediated reaction of 69 with 2,3-dihydrofuran (a formal dipolar cycloaddition of a cyclic diazo dicarbonyl compound with a vinyl ether) yields 70. Corrqiound 70 can be transformed in a number of steps to 71 a,b <96TL2391>. 

Kotha and Brahmachary353 prepared some constrained a-amino acids using a rhodium mediated [2 + 2 + 2] cycloaddition reaction. The indane type of a-amino acids were synthesized by reacting diynes with monoynes using Wilkinson s catalyst354. Thus, the reaction of diyne 615 with 616 afforded a-amino acid derivative 617 (equation 176). 

Chemical Aspects of Rhodium-Mediated 1,3-Dipolar Cycloaddition... 

Substituted 4,5-dihydroazepines 321 (e.g., R1 = Bn, R2 = R3 = H, R4 = M e 82% yield) may be prepared in high yield by a rhodium-mediated hetero-[5+2]-cycloaddition of the cyclopropyl imines derived from 318 on reaction with the primary amines 319, with dimethyl acetylenedicarboxylate 320 (Equation 48) <2002JA15154>. 

M. C. Pirrung and Y. R. Lee, Hydroxy direction of the rhodium-mediated dipolar cycloaddition of cyclic carbenoids with vinyl ethers, J. Chem. Soc., Chem. Commun., (1995) 673-674. 

In the examples illustrated above (equations 62-64), the heteroatom had an attached proton, which transferred readily. In the absence of such a proton, the initially-formed ylide can undergo other reactions. One of the more interesting of these is 1,3-dipolar cycloaddition, as illustrated by the conversion of (160) to (161) (equation 66). Rhodium-mediated diazo addition to an ether is currently the method of choice for preparing such oxonium ylides. 

A rhodium-mediated carbene addition has been employed as the key step in a synthesis of furans. The precursors were synthesized on TentaGel-NHi resin, which was transformed into an amide (135). Subsequent formation of imides 136 with malonic ethyl ester chloride and reaction with tosyl azide gave solid-phase-bound diazo imides 137. Reaction with Rh2(OAc)4 in the presence of electron-deficient alkynes produced substituted furans 139 via the intermediate isomiinch-none 138 through a sequence of a [2-i-3]-cycloaddition to the alkyne and subsequent cycloreversion. The yields of the reaction varied in the range 50-70% (Scheme 36) [52]. 

Transition metal-mediated cycloaddition and cyclization reactions have played a vital role in the advancement and applications of modem synthetic organic chemistry. Rhodium-catalyzed cycloadditions/cyclizations have attracted significant attention because of their versatility in the transformations of activated and unactivated acetylenes, olefins, allenes, etc. These reactions are particularly valuable because of their ability to increase molecular complexity through a convergent and highly selective combination of acyclic components. In addition, these reactions allow for the preparation of molecules with chemical, biological, and medicinal importance with greater atom economy. Recent developments in rhodium-catalyzed cycloaddition and cyclization reactions are described in this section. 

Very recently, the enantioselective synthesis of a completely orfho-fused [ll]helicene-like molecule has been achieved via the rhodium-mediated intramolecular double [2+2+2] cycloadditions (Scheme 21.30) [34]. 

For an example of a rhodium carbenoid mediated [4S+1C] cycloaddition, see Schnau-belt J, Marks E, Reissig HU (1996) Chem Ber 129 73... 

Vinyl Fischer carbenes can be used as three-carbon components in Ni(0)-mediated and Rh(l)-catalyzed [3 + 2 + 21-reactions with alkynes (Schemes 48 and 49)142 and with allenes (Schemes 50 and 51).143 All three of the proposed mechanisms for the [3 + 2 + 2]-cycloadditions involve an initial carbene transfer from chromium to nickel or rhodium (Schemes 49, 52, and 53). As is seen from the products of the two [3 + 2 + 2]-reactions with 1,1-dimethylallene, although the nickel and rhodium carbenes 147G and 147K appear similar, the initial insertion of the allene occurs with opposite regioselectivity. 

Intermolecular [4+2]-cycloaddition of vinylallenes with alkynes is efficiently mediated by means of an electronically tuned rhodium catalyst (Scheme 16.81) [91]. A five-membered rhodacycle is formed from the vinylallene. Coordination followed by insertion of an alkyne to the rhodacycle generates a seven-membered rhodacycle, from which rhodium(I) is eliminated reductively to produce a cyclohexatriene, leading to the aromatic compound. 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

While the perfluorinated acetates do prefer insertion, they are still capable of forming 1,3-dipoles and have demonstrated interesting effects on the regioselectivity of intramolecular cycloaddition reactions, presumably through Lewis acid-mediated effects on the dipolarophile [83]. Other chemoselectivity effects have been noted in the intramolecular cycloaddition reactions and may or may not be partially induced by conformation and sterics [84]. It was further demonstrated thaL when possible, O-H insertion is the predominant outcome over other types of insertion for rhodium]II)-car-benes, independently of the catalyst. However, cycloaddition reactions have been demonstrated to be hgand-dependent [85]. 

Pirrung [153] has described the synthesis of ( )-pongomol 66 via the rhodium(II)-mediated reaction of the diazacyclohexane dione 67, to afford the fused bicyclic ketone 68 (Scheme 19.8). Moreover, this group [154] also detailed a similar approach in their synthesis of ( )-isoeuparin 69 (Scheme 19.9). Pirrung and Lee [155] expanded their rhodium(II)-mediated dihydrofuran cycloaddition strategy, for the conversion of the... 

---