Rhodium II mandelate

The intramolecular reaction between diazo ketones and benzenes is an effective way to generate a range of bicyclic systems.7 The earlier copper-based catalysts have largely been superseded by rho-dium(ll) salts. Unlike the case in the intermolecular reactions, rhodium(ll) acetate is the catalyst that has been most commonly used. Studies by McKervey,133 136 however, indicated that rhodium(II) mandelate, which would be expected to generate a slightly more electrophilic carbenoid than rhodium(ll) acetate, often gave improved yields. 

Rhodium(II) mandelate-catalyzed decomposition of diazoester 257 involves intermolecular addition of the intermediate carbenoid to the triple bond to afford the carbenoid intermediate 258. The latter again reacts... 

A similar strategy starts with an intramolecular alkyne insertion of an a-diazo ketone. Thus, rhodium(II)-catalyzed reaction of 2-(6,8-nonadien-1-ynyl)-or-diazoacetophenones 112 at 0°C in dichloromethanc affords cyclopen%]azulenones 115 in 50-58% yield879. Due to the enhanced solubility, rhodium(II) mandelate has proved to be superior to the acetate. 

The [3+4] annulation approach to the hydroazulenes is achieved with high asymmetric induction (greater than 90% de) by using (/ )-pantolactone as a chiral auxiliary (Table 7). The nature of the catalyst has a considerable effect on the level of asymmetric induction. A sterically crowded catalyst, such as rhodium pivalate, results in much lower diastereoselectivity than rhodium(II) acetate or rho-dium(II) hexanoate. Consequently, even though the enantiomers of rhodium(II) mandelate exhibit double stereodifferentiation with the (/ )-pantolactone auxiliary (entries 5,6), both catalysts are bulky and result iinferior asynunetric induction compared to that obtained with an uncrowded achiral catalyst (entries 1-3). 

More highly substituted aromatics have also been studied in the course of natural product synthesis. For example, rhodium(II) mandelate-catalyzed cyclization of diazoketone 41 produces the ring expanded product 42, which on hydrogenations furnishes the tricyclic lactone 43. ... 

In 1990, Brunner [5], McKervey [6], and Ikegami [7] and their respective coworkers independently introduced chiral rhodium(II) carboxylates for asymmetric diazocarbonyl transformations. At that time the only chiral rhodium(II) carboxylates known were those derived from (R) and (S)-mandelic acid which had been prepared by Cotton and co-workers [8] for structural and chiroptical studies. Enantiopure carboxylates (1) on a dirhodium core (substituents varied from H, Me, and Ph to OH, NHAc, and CFj) were assessed by Brunner [5] for enantioselective cyclopropanation of alkenes with ethyl diazoacetate. McKervey... 

The choice of transition-metal catalysts can play an important role in determining reaction pathways as shown in the model studies toward the synthesis of harringtonolide. The epimeric C-H insertion products 80 are obtained in 75% and 40-50% yield, respectively, with Rh2(tpa)4 and rhodium mandelate. In contrast, bis(A -t-butylsalicyl-aldiminato) copper(II) generates the very labile cycloheptatriene 82 (50% yield), which is converted to the more stable isomer 83 upon treatment with l,8-diazabicyclo[5.4.0]undec-7-ene (DBU). ... 

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