Boron reagent

Asymmetric Reduction of Unsymmetrical Ketones Using Chiral Boron Reagents Review Synthesis 1992, 605. 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

Boron, Reagent 1 X Pressure (kbar) d.r. Yield (%) Case... 

The diastereoselectivity of the addition of metalated croty] compounds 1 to the imine moiety has been constantly improved by changing the metal from magnesium over lithium, zinc, aluminum to boron and tin113 (see Table 7). With the latter a high selectivity in favor of the. vj M-product 3 was achieved. The unh-diastereomer 4 results exclusively from addition of boron reagents to A-arylimines 21. 

Although there are several reports in the literature on boron-mediated amide formations, the boron reagents had to be used in stoichiometric amounts.1-4-5-6-7-8-9 Recently, Yamamoto et al. presented the first truly catalytic method allowing for a direct amide formation from free carboxylic acids and amines as the reaction partners.10-1112 Best results were obtained by using phenylboronic acids bearing electron withdrawing substituents in the meta- and/or para-positions such as 3,4,5-trifluorophenylboronic acid or 3,5-bis(trifluoromethyl)boronic acid as the catalysts. 

Keywords chiral boron reagents, Diels-Alder reaction, stereoselective... 

For a list of methods of preparing alkenes using boron reagents, with references, see Ref. 

Chiral diamino carbene complexes of rhodium have been merely used in asymmetric hydrosilylations of prochiral ketones but also in asymmetric addition of aryl boron reagents to enones. 

The optimal reaction conditions were applied with 59d in the addition of various aryl boronic acids and potassium trifluoroborates to several cyclic and acyclic enones (Fig. 8). Arylboronic acids added to cyclic enones in high yields (89-97%) and with good to excellent selectivities (85-98% ee). Under these conditions, the potassium trifluoroborate reagents reacted at faster rates, but with slightly lower selectivities (83-96% ee). The reactions of acyclic enones with aryl boron reagents gave also excellent yields (83-96%). 

Scheme 7.5 NHC-Pd and NHC-Ni catalysed arylation of carbonyls compounds with boron reagents...

Another extensively developed group of allylic boron reagents for enantioselective synthesis is derived from tartrates.42... 

The synthesis in Scheme 13.49 features use of an enantioselective allylic boronate reagent derived from diisopropyl tartrate to establish the C(4) and C(5) stereochemistry. The ring is closed by an olefin metathesis reaction. The C(2) methyl group was introduced by alkylation of the lactone enolate. The alkylation is not stereoselective, but base-catalyzed epimerization favors the desired stereoisomer by 4 1. 

Petasis reported an efficient addition of vinyl boronic acid to iminium salts.92 While no reaction was observed when acetonitrile was used as solvent, the reaction went smoothly in water to give allyl amines (Eq. 11.54). The reaction of the boron reagent with iminium ions generated from glyoxylic acid and amines affords novel a-amino acids (Eq. 11.55). Carboalumination of alkynes in the presence of catalytic Cp2ZrCl2 and H2O affords vinylalane intermediates, which serve as nucleophiles in the subsequent addition to enantiomerically enriched... 

In boron enolate-mediated aldol reactions, stoichiometric amounts of boron reagents are necessary. However, it has been reported that only a catalytic amount of a boron source is sufficient for boron enolate-mediated aldol reactions in water (Scheme 65).302 It should be noted that even water-sensitive boron enolates can be successfully employed in water. 

Treating boron reagent 45a with an oxazoline compound gives the azaeno-late 52. Subsequent aldol reaction of 52 with aldehyde yields mainly threo-product (anti-53) with good selectivities (Scheme 3-18).38... 

Over the past two decades, chiral allyl- and crotyl-boron reagents have proved to be extremely valuable in the context of acyclic stereoselection. The development of superior allyl-boron reagents, which can give enantio- and diastereoselectivities approaching 100%, has become both challenging and desirable.68... 

Garcia et al.75 have introduced another boron reagent 114 that can also be used in asymmetric allylation reactions. 

Synthesis of the macrolide 6-deoxyerythronolide B 28 is one of the successful demonstrations of double asymmetric induction applied to the construction of complicated natural products.5 Retro synthetic analysis (Scheme 7-7) shows that 28 can be obtained from thio-seco acid 29, which consists of seven propionate building blocks. This is a typical aldol product in which a boron reagent... 

Hydroboration of allylic amines.1 Hydroboration of primary and secondary allylic amines presents problems because amino groups interact with boron reagents. Hydroboration proceeds normally when the amino group is protected by trimethylsilyl groups, and deprotection can be effected by protonolysis in CH,OH. 

Although both boronates and alanates react with allylic bromides, aldehydes and C02 to afford allenic products in satisfactory yield, the alanates are more efficient in additions to ketones (Table 9.14). Boronate reagents do not require a B-C alkyl migration for their preparation. Thus the starting acetylene possesses the structural elements of the product. Additionally, the issue of dummy ligands is irrelevant. 

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