Allylboronate stereoselective addition

FIGURE 3 General structure of allylboronates and their stereoselective addition to aldehydes. 

Intramolecular additions generally follow the same trends of stereoselectivity as observed in the bimolecular reactions. Eor example, allylic boronates ( )- and (Z)-118 provide the respective trans- and cis-fused products of intramolecular aUylation. As shown with allylboronate ( )-118, a Yb(OTf)3-catalyzed hydrolysis of the acetal triggers the intramolecular aUylboration and leads to isolation of the trans-fused product 119 in agreement with the usual cyclic transition structure (Eq. 96). 

Complexes of unsymmetrically substituted conjugated dienes are chiral. Racemic planar chiral complexes are separated into their enantiomers 84 and 85 by chiral HPLC on commercially available /f-cyclodextrin columns and used for enantioseletive synthesis [25]. Kinetic resolution was observed during the reaction of the meso-type complex 86 with the optically pure allylboronate 87 [26], The (2R) isomer reacted much faster with 87 to give the diastereomer 88 with 98% ee. The complex 88 was converted to 89 by the reaction of meldrum acid. Stereoselective Michael addition of vinylmagnesium bromide to 89 from the opposite side of the coordinated Fe afforded 90, which was converted to 91 by acetylation of the 8-OH group and displacement with EtjAl. Finally, asymmetric synthesis of the partial structure 92 of ikarugamycin was achieved [27],... 

The addition to 1,3-dienes afforded a new class of allylboron compounds. The platinum(0)-phosphine catalysts stereoselectively yielded or-1,4-addition products 131233 234 and 133216 235 for 2,3-disubstituted butadiene, 1,3-cyclohex-adiene, and 1,3-pentadiene by Txr-coordination of a diene to a platinum catalyst (Equations (30) and (31)). In contrast, phosphine-free Pt(dba)2 resulted in the selective formation of a 1,2-addition product 134216 for 1,3-pentadiene (Equation (31)). The corresponding chiral allyl boronates were synthesized by diboration of dienes with 123 or 124.234 235... 

Kabalka et al. have investigated the Pd-catalyzed eross-eoupling reaction of MBH acetates and bis(pinacolato)diboron to produee 3-substituted-2-alkoxy-carbonyl allylboronates 371, whieh ean be further transformed into stable allyl trifluoroborate salts 372 by addition of exeess aqueous KHF2. The allylboronate 371 and allyltrifluoroborate derivatives 372 react with aldehydes to afford functionalized homoallyhc alcohols 373 and 374, respectively, stereoselectively in the presence of Lewis acid (Scheme 3.166). ... 

The addition of allylboronates to aldehydes was first discovered in 1974 [429]. This reaction has since found tremendous use in the stereoselective synthesis of acetate and propionate units embodied in numerous natural products (Equation 76, Figure 1.39) [430]. The tartrate-based chiral allylboronates, for example, have become one of the... 

In the last example, cis-carbocupration of alkynoic ester 14 provides a weakly nucleophilic and configurationally imstable 1-alkoxycarbonyl alkenylcopper intermediate (15) the presence of HMPA as additive was crucial to avoid erosion of the E/Z stereoselectivity in the alkylation step with electrophile 16 [39, 40]. Under these conditions, 3,3-disubstituted allylboronates 17 were prepared in over 20 1 selectivity, and these reagents were subsequently employed in the stereoselective preparation of quaternary carbon centers (Section 6.4.2.2). 

The use of optically pure a-chloroalkyl boronic esters as electrophiles, e.g. the dicyclohexyl boronate 18 obtained from Matteson s elegant methodology [41], lends access to a-alkyl substituted allylboronates (e.g., 19) with very high diastereomeric purity by way of a net inversion of configuration (Equation 9) [42]. Likewise, alkenylmetal fragments react with chiral dichloromethylboronate 20 to afford optically pure a-chloroallylboronates such as 21 (Equation 10) [43]. Subsequent addition of these a-substituted reagents to aldehydes is highly stereoselective. Furthermore, the chloride... 

Yang and Cheng observed an unusual variant of allene diborylation where an aryl or alkenyl halide can act as a co-catalyst with a phosphine-free palladium catalyst [51]. As shown with the formation of allylboronate 29, the addition is completely regiose-lective and highly stereoselective (Equation 17). Examples with aryloxy-substituted allenes were also successfully demonstrated. Notably, Pd(dba)2 does not promote the diboration of allenes without the co-catalyst, and a new mechanism involving oxidative addition of an I-B bond to the palladium center was proposed to rationalize these observations. 

Although most reactions of allylboronates involve aldehydes, other electrophilic partners are also possible. As exemplified in Equation 40, ketones can also react with allylboronates, yielding tertiary homoallylic alcohols [109]. Additions with simple ketones are much slower than similar reactions with aldehydes, and the stereoselectivity in these additions is quite variable, depending on the difference in size between the two substituents on the ketone. This was also noted in a recent study of binaph-thol-derived allylboronates, where high levels of enantioselection (>96% ee) were obtained in the reaction of 3,3 -(CF3)2-BINOL allylboronate 82 with several aromatic ketones (Equation 41) [110]. 

In the recent development of a one-pot borylation of allylic acetates (e.g., 37) followed by in situ addition to sidfonyl-imines, Szabo and co-workers observed that the palladium catalyst is required for the addition of the transient allylboronate 38 to the N-sulfonyl imine (Equation 46) [59]. The authors proposed the intermediacy of a bis-allylpalladium species to explain this result. These additions are highly regio- and stereoselective. 

Diboration of 1,3-Dienes, Allenes, Enones, and Other Unsaturated Molecules. As described previously in the original article, the Pt[0] catalyzed addition of 1,3-dienes with B2pin2 affords various allylboronic esters (eq 4). Reactions using phosphine-based catalysts such as Pt(PPh3)4 stereoselectively produce els -1,4-addition products with Z-configuration for aliphatic and cyclic dienes. The reaction mechanism suggests... 

Roush has crafted a class of allyl/vinyl diboron reagents that participate in sequential aldehyde addition 1,5-anti-diols reactions and enable the expeditious synthesis of 1,5-diols. The hydroboration reaction of allene 34 furnishes 35, treatment of which with aldehydes results in the formation of 36 (Scheme 5.6) [64]. These allylboronate adducts subsequently undergo addition to a second set of aldehydes, generating 1,5-anti-diols 37 with high diastereoselectivity (S 20 1) and optical purity (S 89% ee). Use of the bulkier boronate 38 gives 1,5-syn-diols 39 with equally high stereoselectivity (dr >14 1, ee>91%) [65]. 

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