Enantioselectivity hydrometallation

The bicychc structure 7 is desymmetrized by reductive C-O bond splitting in the presence of the Ni complex of (5)-BINAP and diisobutylalumium hydride (DIBAL-H). This couple ensures enantioselective hydrometalation-elimination from one of the enantiotopic carbon atoms in the bridge to the C-O bond, affording dihydronaphthalenol (f )-8 in 88% yield and 98% e.e. In the following steps, protection of 8 as the sUyl ether 9 was completed before addition of bromine to the double bond in the presence of triethylamine. This intermediate was treated with DBU 1,8-diazabicyclo[5,4,0]undec-7-ene, strong, crowded base) without isolation, and on regioselective elimination of hydrogen bromide, vinylbromide 10 was isolated in 83% yield. 

Brown s discovery of the hydroboration reaction in the 1950s opened new avenues for the selective functionalization of olefins. The recognition that acyclic olefins can have well-defined conformational biases allowed the development of diastereoselective, substrate-controlled processes and the advent of the principles of acyclic stereocontrol. Given the central role the hydroboration of olefins has played, it is hardly surprising that the earliest examples in this field involve such transformations. For the organic chemist, diastereoselective and enantioselective hydrometalation reactions have rapidly become an indispensable tool equally useful in simple olefin functionalizations and in the stereoselective construction of highly complex molecules. 

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

Spectacular enantioselection has been observed in hydrogenation (cf. Section 2.2) [3] and hydrometallation of unsaturated compounds (cf. Section 2.6) [4], olefin epoxidation (cf Section 2.4.3) [5] and dihydroxylation (cf Section 3.3.2) [6], hydrovinylation (cf Section 3.3.3) [7], hydroformylation (cf Section 2.1.1) [4a, 8], carbene reactions [9] (cf Section 3.1.10), olefin isomerization (cf Section 3.2.14) [10], olefin oligomerization (cf Section 2.3.1.1) [11], organometallic addition to aldehydes [12], allylic alkylation [13], Grignard coupling reactions [14], aldol-type reactions [15], Diels-Alder reactions [12a, 16], and ene reactions [17], among others. This chapter presents several selected examples of practical significance. 

Metal catalyzed hydrometalations, specifically hydroboration and hydrosi-lylation, are particularly attractive due to the potential for enantiocontrol by the use of chiral ligands coordinated to the metal center. While the first metal catalyzed hydroalumination was described more than 40 years ago, the first synthetically useful enantioselective hydroalumination was described in 1995 and the scope and synthetic utility of this reaction are only just emerging. 

Liang T, Nguyen KD, Zhang W, Krische MI (2015) Enantioselective ruthenium-catalyzed carbonyl allylation via alkyne-alcohol C-C bond forming transfer hydrogenation aUene hydrometallation vs. oxidative coupling. J Am Chem Soc 137 3161-3164... 

In the realm of olefin hydrometalation, the hydroboration reaction has largely dominated the scene. This is in no small part due to the early work by Brown in the development of this reaction class to a level of sophistication that permitted its evolution as a reliable and common tool in the service of complex molecule synthesis. There are, however, closely related catalytic asymmetric hydrometalation reactions of olefins that provide alternatives to enantioselective hydroboration namely, the hydroalumination [14] and hydrosilylation reactions [16]. 

---