The Rhodium l -Catalyzed Alder-Ene Reaction

The bond reorganization process shown in Eq. (1) is formally known as an Alder-ene reaction, where X=H. The classic example involving the thermal reaction of an alkene having allylic hydrogen(s) (an ene) with an electron-deficient unsaturated compound (an enophile) was first reported by Alder in 1943 [1]. 

Modem Rhodium-Catalyzed Organic Reactions. Edited by P. Andrew Evans Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30683-8 

Substrates possessing an allene that participate in the Alder-ene reaction are less common, but a few examples are known. Malacria [11] and Livinghouse [12] have independently used cobalt to effect intramolecular allenic Alder-ene reactions but the scope of these reactions was not investigated. Sato has performed an allenic Alder-ene reaction to form five-membered rings, using stoichiometric amounts of titanium [13], and Trost has shown that 1,3-dienes can be prepared via an intermolecular Alder-ene reaction between allenes and enones using a ruthenium(II) catalyst [14]. 

As evidenced by the plethora of reviews cited in the preceding paragraphs, certain substrates and transition metals that affect the intra- and intermolecular Alder-ene reaction have been extensively studied. However, new ways to attain this synthetically useful reaction are valuable since some of these processes are completely substrate-dependent and involve metal catalysts with low functional group compatibility. This chapter details the role of rhodium(I) catalysts in achieving the formal Alder-ene reaction. 

The first example involving a rhodium catalyst in an ene reaction was reported by Schmitz in 1976. An intramolecular cyclization of a diene occurred to give a pyrrole when exposed to rhodium trichloride in isobutanol (Eq. 2) [15]. Subsequently to this work, Grigg utilized Wilkinson s catalyst to effect a similar cycloisomerization reaction (Eq. 3) [16]. Opplozer and Eurstner showed that a n -allyl-rhodium species could be formed from an allyl carbonate or acetate and intercepted intramolecularly by an alkene to afford 1,4-dienes (Eq. 4). Hydridotetrakis(triphenylphosphine)rhodium(l) proved to be the most efficient catalyst for this particular transformation. A direct comparison was made between this catalyst and palladium bis(dibenzylidene) acetone, in which it was determined that rhodium might offer an additional stereochemical perspective. In the latter case, this type of reaction is typically referred to as a metallo-ene reaction [17]. 

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I 8 The Rhodium(l)-Catalyzed Alder-Ene Reaction Tab. 8.9 Scope of halogen shift reaction... 

The ether-tethered allenyne 248 undergoes a rhodium(l)-catalyzed intramolecular allenic Alder ene reaction to afford the ( )-3,6-dihydropyran 249 as the major product (Equation 111) <2002JA15186>. Likewise, ether tethered enynes can undergo rhodium(i)-catalyzed cycloisomerizations to afford 3,6-dihydropyrans <2005JA10180>. 

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