Allyl carbonates transformation reactions

A first example of a combination of a Rh-catalyzed allylic substitution and a Pau-son-Khand annulation reaction has also been developed by the same group [222]. Thus, [RhCl(CO)dppp]2 is able to catalyze both transformations at different reaction temperatures. Treatment of the allylic carbonate 6-106 with the alkyne derivative 6-107 led to a diastereomeric mixture of 6-108 and 6-109 in 63-84% yield, with 6-108 as the main product (Scheme 6/2.23). 

Monoanions derived from nitroalkanes are more prone to alkylate on oxygen rather than on carbon in reactions with alkyl halides, as discussed in Section 5.1. Methods to circumvent O-alkylation of nitro compounds are presented in Sections 5.1 and 5.4, in which alkylation of the a.a-dianions of primary nitro compounds and radial reactions are described. Palladium-catalyzed alkylation of nitro compounds offers another useful method for C-alkylation of nitro compounds. Tsuj i and Trost have developed the carbon-carbon bond forming reactions using 7t-allyl Pd complexes. Various nucleophiles such as the anions derived from diethyl malonate or ethyl acetoacetate are employed for this transformation, as shown in Scheme 5.7. This process is now one of the most important tools for synthesis of complex compounds.6811-1 Nitro compounds can participate in palladium-catalyzed alkylation, both as alkylating agents (see Section 7.1.2) and nucleophiles. This section summarizes the C-alkylation of nitro compounds using transition metals. 

The reactions of allylmetal reagents with carbonyl compounds and imines have been extensively investigated during the last two decades [1], These carbon—carbon bondforming reactions possess an important potential for controlling the stereochemistry in acyclic systems. Allylmetal reagents react with aldehydes and ketones to afford homo-allylic alcohols (Scheme 13.1), which are valuable synthetic intermediates. In particular, the reaction offers a complementary approach to the stereocontrolled aldol process, since the newly formed alkenes may be readily transformed into aldehydes and the operation repeated. 

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]. 

Tab. 10.6 summarizes the application of this transformation to a variety of racemic secondary allylic carbonates using the lithium anion of 4-methoxy-N-(p-toluidine)-benzene sulfonamide. The excellent regioselectivity obtained for this type of substitution provided an important advance in the synthesis of N-(arylsulfonyl)anihnes using the metal-catalyzed allyhc amination reaction. The allyhc alcohol derivatives examined... 

The regio- and diastereoselective rhodium-catalyzed sequential process, involving allylic alkylation of a stabilized carbon or heteroatom nucleophile 51, followed by a PK reaction, utilizing a single catalyst was also described (Scheme 11.14). Alkylation of an allylic carbonate 53 was accomplished in a regioselective manner at 30 °C using a j-acidic rhodium(I) catalyst under 1 atm CO. The resulting product 54 was then subjected in situ to an elevated reaction temperature to facilitate the PK transformation. 

Needless to say, /i-keto esters are important compounds in organic synthesis. Their usefulness has been considerably expanded, based on Pd-catalysed reactions of allyl / -keto carboxylates 399. Cleavage of the allylic carbon-oxygen bond and subsequent facile decarboxylation by the treatment of allyl / -keto carboxylates with Pd(0) catalysts generate the 7i-allylpalladium enolates 400, 401. These intermediates undergo, depending on the reaction conditions, various transformations which are not possible by conventional methods. Thus new synthetic uses of / -keto esters and malonates based on Pd enolates have been expanded. These reactions proceed under... 

As discussed above, the cross-coupling reaction of organosilicon compounds proceeds stereospecifically, depending on the reaction conditions. Thus, the transformation C—Si C —C is demonstrated to be accompanied by chirality transfer. Now, the question arises of how to prepare organosilicon compounds whose chiral allylic carbon is substituted by a silyl group. The most accessible solution is asymmetric hydrosilylation of olefins [35]. We studied asymmetric hydrosilylation of 1-substituted 1,3-butadienes using... 

Our first approach to 1 is based on a retrosynthetic analysis depicted in Fig (8). The crucial step to construct the cw-fused bicyclic ring skeleton of 1 is the intramolecular allylic amination of a cw-allylic carbonate 25. The paUadium-catalyzed allylation takes place with retention of the configuration [76] and requires the c/s-isomer 25 for the ring closure. Compound 25 may be derived from keto acid 24 through a sequence of reactions including esterification, O-methoxycarbonylation, removal of the Boc and benzylidene groups, dehydrative cyclization, reductive alkylation and ureido formation. The last five transformations are to be conducted in a successive manner, i.e., without isolation of the intermediates. The 4-carboxybutyl chain of 1 may be installed by the reaction of O-trimethylsilyl (TMS) cyanohydrin 23 with a di-Grignard... 

In these reactions the metal cleaves the allylic carbon-hydrogen bond to eliminate HCl. Since HCl is formed a base may be added to improve the 5neld of the 7t-allylpalladium complex and allow less substituted olefins to be transformed into 7r-allylpalladium complexes. Reaction of olefins with PdCl2 in acetic acid in the presence of sodium acetate , or with PdCl2(PhCN)2 in the presence of sodium carbonate gives the corresponding t-allyl complexes in good yield ... 

Several other types of domino reactions have been employed in the synthesis of natural products. Diastereoselective conversion of allylic carbonate 173 into enone 174 was one key transformation in a total synthesis of (+)-3-isorauniticine 175 (Scheme 27).Treatment of allylic sulfonamide 173 with a palladium catalyst re-gioselectively forms a 7r-allylpalladium intermediate by carbonate displacement. Carbopalladation of the pendant alkene, carbonylation, a second intramolecular alkene insertion, and /3-hydride elimination delivers a 67 22 11 mixture of stereoisomers of which enone 174 is the major product (isolated in 45-53% yield). Carbopalladation products can also undergo anion capture reactions. For instance, during the synthesis... 

The majority of recent efforts to develop catalytic allylic substitution reactions have focused on enantioselective transformations. Most enantioselective reactions form a stereocenter at one of the allylic carbons, and this new stereocenter is formed within the coordination sphere of the metal. Less common are enantioselective substitutions that form products containing a new stereocenter at the nucleophile. These reactions have been considered challenging because the stereochemistry is set at an atom that is outside of the direct coordination sphere of the metal. 

Copper(II)-catalyzed Boryl Addition to Allylic Carbonates. The conversion of allylic carbonates to chiral a-substituted allylboronates was also investigated by Hoveyda, who was able to accon ilish this transformation with a Cu(II)-NHC complex. This reaction proceeds in a vinylogous fashion to Sawamura s, but under these conditions, (E)- and (Z)-allylic carbonates undergo substitution to produce opposite enantiomers of product with similar yields and selectivity. This methodology is also tolerant of substitution at the a- or -position and is effective for di-or trisubstituted alkyl (linear or branched) or aryl alkenes delivering a quaternary a-chiral allylboronate product with up to 98% enantioselectivity (eq 49). 

Intramolecular trapping of Pd(II)-jj -aUyl intermediates by nucleophiles such as phenols is possible, a transformation exemplified by reaction of Pd(0)/(5, 5)-Trost ligand with phenol allyl carbonates 29, which brings about the construction of chiral quaternary centers and the formation of chiral chromans 30 (eq 20). ... 

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