Imine reaction with allylboronates

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

The reactions of ailylboranes, -silanes and -stannanes with carbonyl compounds and imines always take place with double-bond migration, and the structural stability of the reagent is a common problem that is encountered ( 2.7). The reactions of ailylboranes and -boronates occur without catalysis, while those of allylsilanes and -stannanes usually require the presence of a Lewis add [253], The mechanism of the reactions of allylboron derivatives is concerted, and the addition occurs via a six-membered cyclic transition state. A slightly distorted chair transition state model in which the oxygen of the carbonyl group is coordinated to the boron atom is usually invoked (Figure 6.42). Various steric and polar interactions dictate whether the Si or Re face of the prochiral aldehyde is attacked (models C j... 

Hoffmann et al have reported the addition of allyl(dimethoxy)borane to linear and branched a-aryl-aldimines (entries 16-19, Table 2). The absence of a-deprotonation may be explained by a delicate balance between the basicity and the reactivity of the allylboronate. Allyl(dimethoxy)borane should thus be considered the reagent of choice in reactions with enolizable aldimines. Reactions are conveniently carried out at 25 °C in CH2CI2 and work-up is performed using triethanolamine to break up amine-boronate complexes. Allyl(dimethoxy)borane also adds to the cyclic imine, A -piperideine, in 90% yield. The reported yields for the addition of allyllithium and allylmagnesium chloride to A -piperideine are low. 

Reactions of allylboronates with imines [119,120] and oximes [119,121-123] have also been documented. These reactions are attractive because they lead to homoallylic amines as products. They are much slower than similar reactions with aldehydes and the additions are often less selective. Prediction of their stereochemical outcome is complicated by the possibilities that they may proceed via boat-like transition states, and the imine or oxime substrate might undergo E-Z isomerization under the harsh conditions of the additions. Wuts and co-workers, however, observed that (E)-3-tri-alkylsilyl-substituted reagents like 85 are particularly effective for additions to N-ben-zyl imine derivatives (Equation 44) [124]. A few examples of enantioselective additions have been reported [125,126], one of the more successful being the addition of the 2-carboxyester allylboronate 86 to imine 87, yielding exo-methylene y-lactam 88 as a pure enantiomer (Equation 45) [127]. 

Recently, Kobayashi and co-workers reported an interesting variant for the synthesis of homoallylic amines, dubbed an ammonia fixation reaction [128]. With this method, allylboronates are reacted with aldehydes in a solution of ethanolic ammonia. Despite the highly basic conditions, optically pure protected a-hydroxy aldehydes such as 89 can be employed without any observed racemization, and this approach was apphed to the synthesis of aminosugar derivatives via aminoalcohol product 90 (Equation 47). Unfortunately, the addition of a chiral camphor-based allylboronate to benzaldehyde led only to a low enantioselectivity (34% ee). Reactions with the ( )- and (2)-pinacol crotylboronates lead to the same diastereoselectivity seen in the corresponding reactions with aldehydes, affording the respective anti and syn products from a reaction mechanism that most likely involves the intermediacy of primary imines. 

Z- or -Configured allylboronates 46b bearing a trisubstituted olefin moiety could also be employed in the developed reaction sequence affording access to optically active fS,y-disubstituted derivatives 29 (Scheme 4.12). Importantly, the reactions with both aldehydes 38 and imines 47 proved fully diastereospecific. While reactions with Z-46b led to diastereoselective formation of trans-29, the allylboration of -46b gave CIS-configured 29 exclusively. Excellent enantioselectivities (>95% ee) obtained in the synthesis of lactams 29b (X = NMe) are worth noticing. 

Enantiomerically pure homoallylic amines are very important chiral building blocks for the synthesis of natural products. However, enantioselective methods for homoallylamine are quite undeveloped. In 1995, Itsuno and co-workers reported the first example of enantioselective allylation of an imine (Scheme 7) [13]. The reaction of N-trimethylsilylbenzaldimine 19 with a chiral allylboron reagent 20 in ether at -78 °C afforded the corresponding homoallylamine 22 in 73% ee. 

Cross-coupling reactions 5-alkenylboron boron compounds, 9, 208 with alkenylpalladium(II) complexes, 8, 280 5-alkylboron boron, 9, 206 in alkyne C-H activations, 10, 157 5-alkynylboron compounds, 9, 212 5-allylboron compounds, 9, 212 allystannanes, 3, 840 for aryl and alkenyl ethers via copper catalysts, 10, 650 via palladium catalysts, 10, 654 5-arylboron boron compounds, 9, 208 with bis(alkoxide)titanium alkyne complexes, 4, 276 carbonyls and imines, 11, 66 in catalytic C-F activation, 1, 737, 1, 748 for C-C bond formation Cadiot-Chodkiewicz reaction, 11, 19 Hiyama reaction, 11, 23 Kumada-Tamao-Corriu reaction, 11, 20 via Migita-Kosugi-Stille reaction, 11, 12 Negishi coupling, 11, 27 overview, 11, 1-37 via Suzuki-Miyaura reaction, 11, 2 terminal alkyne reactions, 11, 15 for C-H activation, 10, 116-117 for C-N bonds via amination, 10, 706 diborons, 9, 167... 

Several reviews on allylboron compoimds and odier allylmetal reagents and their additions to carbonyl compoimds and imines have appeared [1-6], including two excellent chapters in a book from Wiley-VCH [1,2]. Accordingly, diis chapter does not aim to provide a comprehensive accoimt on die chemistry of allylboronates, but presents instead an overview of die main preparative methods for allylboronates, with particular focus on advances reported in die past five years, and on several recent synthetic applications such as the Lewis add-catalyzed additions to carbonyl compounds and tandem allylation reactions. 

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