Rhodium catalysts conjugate additions

For rhodium-catalyzed conjugate addition using organosilanes, several other conditions have been reported [34]. Cationic rhodium catalysts such as [Rh(COD)2]BF4 and ]Rh(COD)(MeCN)2]BF4 are more active than neutral rhodium catalysts such as ]Rh(OH)(COD)]2. 

Additionally, the efficient synthesis of substituted phenylalanine-type amino acids using a rhodium catalyzed, conjugate addition of arylboronic acids has been described. The reactions are run in water using a low loading (0.5 mol%) of the rhodium catalyst [28]. 

Maddaford and co-workers have reported the diastereoselective synthesis of C-gly-coside 34m by use of rhodium-catalyzed conjugate addition [27] (Scheme 4.13). The reaction is efficiently catalyzed by cationic rhodium catalysts such as [Rh(cod)2]BF4,... 

The Meyer-Schuster-type conversion of propargylic alcohols into conjugated enones was found to proceed efficiently in the presence of NHC-Au catalysts. " This process could be coupled with a subsequent asymmetric rhodium-catalyzed conjugate addition, thus expanding the synthetic scope of the transformation. ... 

A novel rhodium-catalysed conjugate addition of indium reagents to electron-deficient olefins has been reported. " The reaction takes place in THF/MeOH at 110 C using arylindium dichlorides, a rhodium(I)-BINAP complex as catalyst, and a,/ -unsaturated ketones and lactones in good yields. The addition of methanol was found crucial for an efficient transformation, and NMR studies indicating that methanol promotes the catalytic cycle leaving the indium organometallic unaltered. 

Hayashi investigated the asymmetric, rhodium-catalyzed conjugate addition reaction of organoboronic acids to a variety of unsaturated electrophilic acceptors [46, 144-147). The first successful example of such a transformation involved the use of a Rh-BINAP catalyst (187, Equation 34) [144. As shown for cyclohexenone, the rhodium-catalyzed addition of phenylboronic acid takes place to afford 188 in 96% ee and >99% yield. 

The addition of allcenes to alkenes can also be accomplished by bases as well as by the use of catalyst systems consisting of nickel complexes and alkylaluminum compounds (known as Ziegler catalysts), rhodium catalysts, and other transition metal catalysts, including iron. These and similar catalysts also catalyze the 1,4 addition of alkenes to conjugated dienes, for example. 

Similar reactions have been carried out on acetylene. Aldehydes add to alkynes in the presence of a rhodium catalyst to give conjugated ketones. In a cyclic version of the addition of aldehydes, 4-pentenal was converted to cyclopen-tanone with a rhodium-complex catalyst. In the presence of a palladium catalyst, a tosylamide group added to an alkene unit to generate A-tosylpyrrolidine derivatives. ... 

Scheme 65 outlined a proposed mechanism in which rhodium served as catalyst for the conjugate addition, and the C-Bi bond was proposed to be oxidatively added to the Rh complex. 

It is worth noting that conjugate addition can also be effected by the addition of aryl- and alkenylboronic acid reagents to enones in the presence of rhodium catalysts. This new catalyst system has the following advantages ... 

Maddaford reported the diastereoselective synthesis of C-glycosides 29 using conjugate addition catalyzed by cationic rhodium catalysts such as [Rh(COD)2]BF4 (Eq. 1) [24]. Addition of phosphine hgands to the reaction system inhibited the conjugate addition. It is likely that the enone 28 derived from the pyranose is less reactive toward the conjugate addition. 

Oi and Inoue recently described the asymmetric rhodium-catalyzed addition of organosilanes [35]. The addition of aryl- and alkenyltriaUcoxysilanes to u,y9-unsaturated ketones takes place, in the presence of 4 mol% of a cationic rhodium catalyst generated from [Rh(COD)(MeCN)2]BF4 and (S)-B1NAP in dioxane/H20 (10 1) at 90°C, to give the corresponding conjugate addition products (Eq. 3). The enantioselectivity is comparable to that observed with the boronic acids, as the same stereochemical pathway is applicable to these reactions (compare Scheme 3.7). 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

Vinyl boronates 244 undergo radical addition to a,/3-unsaturated ketones 245 in the presence of a rhodium catalyst and (V)-BINAP to yield the 1,4-addition product 246 in high ee and yield <1998TL8479>. Under the reaction conditions, the boronates undergo hydrolysis to provide the corresponding boronic acids, which further undergo conjugate addition with the enone (Scheme 42). 

A wide variety of unsaturated substance can be hydroformylated by cobalt or rhodium catalysts but conjugated alkenes (e.g., butadiene) may give a number of products including hydrogenated monoaldehydes. The mechanism is different, since addition of M H to dienes leads to alicyclic species which may be present as a-bonded intermediate or as h3allyls. 

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