Allylsilanes can provide synthetic strategies which are not achieved with other allylic metal reagents such as the Grignard, lithium and transition metal reagents. In this review article, the characteristics in the reactions of allylsilanes, especially not only enhanced reactivities towards various electrophiles due to σ-π conjugation but also regio-, stereo-, and chemoselectivities, are surveyed. Moreover, a variety of allylsilanes that have been synthesized and utilized as versatile synthetic equivalents are summarized.
Optically active allylsilanes (up to 95 %ee) which contain an asymmetric carbon atom directly bonded to the silicon atom have been prepared by asymmetric cross-coupling of α- (trimethylsilyl) - benzyl or 1- (trialkylsilyl) ethyl Grignard reagent with alkenyl bromides in the presence of a chiral ferrocenylphoshine-palladium complex ((R) - (S) -PPFA-Pd) as a catalyst. Palladium-catalyzed asymmetric hydrosilylation of 1, 3-dienes also gave optically active allylsilanes. Reaction of the allylsilanes with various electrophiles (t-BuCl/TiCl4, MeCOCl/AlCl3, trioxane/TiCl4, CF3COOD, MCPBA, and palladium (II)) was found to proceed with anti stereochemistry, the electrophiles attacking the double bond selectively anti with respect to the leaving trimethylsilyl group. Reaction of the optically active allylsilanes with aldehydes in the presence of titanium chloride gave optically active erythro homoallylic alcohols with over 90 % enantio- and diastereoselectivities.
In addition to their high oxygenophilicity, organoaluminum compounds are endowed with an ambiphilic character. These properties can be successfully utilized in developing new synthetic reactions with unique selectivities, which include (1) organoaluminum-promoted Beckmann rearrangement of oxime sulfonates, (2) new synthesis of polyamino macrocycles via reductive cleavage of aminals and amidines by diisobutylaluminum hydride, (3) diastereoselective cleavage of chiral acetals by organoaluminum compounds leading to optically active secondary alcohols, allylic alcohols, and β-substituted carbonyl compounds, respectively, and (4) biogenetic-type terpene synthesis using highly oxygenophilic aluminum reagents. These selected examples illustrating the characteristics of organoaluminum chemistry should help in understanding the distinct advantages of organoaluminum reagents in selective organic synthesis.
A variety of organic halides including aryl and alkenyl halides are catalytically converted into α-keto amides in the presence of secondary amines and palladium catalyst. The scope and mechanism of the reaction are described in detail.
Direct introduction of CN- or NO2- into nucleophilic olefins has been the subject of considerable interest in the area of organic synthesis because of their unique synthetic potential. Reviewed herein are intriguing as well as useful examples of such reactions realized only by judicious choice of suitable organoselenium compounds. Direct cyanation of enamines, ketene silyl acetals, silyl enol ethers, and alkenes has been achieved by the use of phenyl selenocyanate, quantitatively prepared by the novel silicon-selenium exchange reaction. The products derived from alkenes were converted stereospecifically into conjugated cyanoalkenes. Direct nitration of alkenes and alkenylsilanes (vinylsilanes) via nitroselenenylation provided nitroselenides again stereospecifically, which were converted into conjugated nitroalkenes and nitroalkenylsilanes, respectively. These reactions have advantages over cyanosulfenylation (B. M. Trost, 1982) and nitromercuration (E. J. Corey, 1978) of alkenes in many respects.
Pyrrolizidine alkaloids have attracted much interest as synthetic targets, and various efficient synthetic routes leading to the pyrrolizidine skeleton have been developed. This review deals with the recent advances in the syntheses of pyrrolizidine alkaloids, especially in the syntheses of pyrrolizidine bases (necines) and macrocyclic bislactonic pyrrolizidine alkaloids.