Reactions with allylboronates structure

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). 

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... 

Relatively few studies of the reactions of allylboronates and ketones have appeared. The reaction of (85) and ethyl pyruvate, for example, was conducted under 6 kbar pressure at 45 C for 80 h to give a 9 1 mixture of diastereomers (86a) and (86b). The stereochemistry of this reaction parallels that seen with crotyl-9 BBN (Figure 10) in that the structure of the major isomer is consistent with a transition state in which the —C02Et unit adopts an equatorial position. The same result could occur, however, via a boatlike transition state with an axial —C02Et group. 

It is to be expected that the extent of relative diastereoselection will depend on the difference in size of the R substituent relative to Me. The examples summarized in Table 4 are generally supportive of this thesis, particularly the reactions of (1) with aldehydes (146) and (147) that are much more selective than with the structurally less complex aldehyde (145). The data cited for reactions of pinacol allylboronate... 

Transition state structures (chair and twist-boat) for the reaction of formaldehyde with allylborane (255) and allylboronic acid have been located with ab initio calculations at the... 

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],... 

To explain the stereochemical outcome of the reaction of allylic boron reagents with carbonyl compounds, Houk and Li carried out calculations on the transition structures of the model reaction of formaldehyde and allylboronic acid6 (Scheme 3.V). The bimolecular complex formed initially between allylboronic acid and formaldehyde would rearrange via a six-membered transition state to form an intermediate. Calculations show that chair transition state A is 8.2kcal/ mol more stable than twist-boat transition structure B, clearly confirming that the six-membered chairlike transition-state model is a legitimate scheme to predict the stereochemical outcome of the boron allylation reaction. 

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