Allylboronic acid

One of the most general preparative routes to allyl- and 2-butenylboranes involves the reaction of an allylic organometallic species and an electrophilic borylating reagent. Various esters of allylboronic acid have been prepared in this way2,4-5. 

Recent optimization studies reveal that the yield of 2-(2-propenyl)-1,3,2-dioxaborolane-4,5-di-carboxylate esters (i.e., the tartrate ester modified allylboronates) is improved by using triiso-propyl borate as the borylating agent1. The improved yields are directly related to the increased efficiency of the preparation of the intermediate allylboronic acid. 

The allyl boronate esters (R,R)- and (S,S)-1 are prepared by reaction of allylboronic acid, CH2=CHCH2B(OH)2 with l- and D-diisopropyl tartrate.1... 

Dimethyl allylboronates (e.g., 17,18) are sensitive to hydrolysis, and the allylboronic acids are unstable in the presence of O2. The pinacoi and tartrate esters, however, are quite stable and many have been purified by distillation. We routinely monitor the isomeric purity of the crotyl reagents by capillary GC analysis (ref 3e). 

The enantioselective addition of ally organometallics to carbonyls has become one of the workhorses of organic synthesis. Dennis Hall of the University of Alberta reports (J. Am. Chem. Soc. 125 10160, 2003) the scandium triflate catalysis chiral allylboronic acids become more effective tools. The best of these, the Hoffmann camphor derivative 2, adds to aldehydes under Sc(OTf), catalysis with excellent enantiomeric excess. The reaction works equally well for methallyl, and for the E and Z crotyl boronic acids. The crotyl derivatives react with the expected high diastereocontrol. A limitation to the boronate additions is that branched chain aldehydes give low yields. 

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... 

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. 

Houk and Li also performed calculations on the reaction between allylboronic acid and acetaldehyde6 (Scheme 3.VI). Transition state A, in which the methyl group of acetaldehyde occupies an equatorial position is more stable than transition state B by 5.5kcal/mol. Thus, the theoretical studies support the transition-state models in Scheme 3.IV proposed by Hoffmann and others. 

Triallylborane reacts with aldehydes to form esters of diallylboronic, allylboronic or boric acids depending upon the stoichiometry of the reactants (Chart 9)92). Similarly, ketones afford esters of either diallylboronic or allylboronic acids. The third allyl moiety fails to react with ketones as a result of severe steric crowding around the boron atom as shown in Chart 9 2). 

Three compounds, diallyl (1), (Af-allyWV-phenylamino)-diallylborane (2), (8 11B 44 ppm) and an allylboronic acid derivative (3) (8 nB 32 ppm) were identified by NMR as the products of the reaction of A113B with PhN02 (Scheme 2). 

Handling, Storage, and Precautions allylboronates are typically handled as a solution in toluene (0.5-1.0 M) and transferred by syringe under an inert atmosphere. The reagent, stored neat or as a solution in toluene over 4A molecular sieves under an argon atmosphere in a refrigerator (—20 °C), is stable for several months. In the presence of water, (1) rapidly hydrolyzes to DIPT and the achiral allylboronic acid. 

By a similar method, the (Z)-crotylborate is synthesized from cA-2-butene in 70-75% yield with a 98% isomeric purity. The tartrate esters of allylboronic acids are an excellent reagent for asymmetric allylboration of carbonyl compounds. Allyl(diisopinocampheyl)borane [51] and the allylic boron derivatives of ester and amide, such as camphordiol [52], pinanediol [53], 1,2-diphenyl-1,2-ethylenediamine [54], have also been successfully used for asymmetric allylboration of carbonyls. 

Scheme 20 Reactivity of 29 in Suzuki-Miyaura coupling of allylboronic acid pinacol ester derivatives...

The S,S-pincer5, cocatalyzed by TsOH, converts allylic alcohols to allylboronic acids for direct allylation of aldehydes. ... 

The cyclic allylboronic acids 81 provided the trisubstituted (Z)-olefins 82 when treated with alkaline hydrogen peroxide (Equation 2). If, instead of boracycles 81, the cyclic dienylic boronic acids 83 are used in the previous reaction, the stereodefined, trisubstituted dienylic carbinols 84 are formed (Equation 3) <2002AGE152>. 

The cyclic allylboronic acids are excellent substrates also for diastereoselective dihydroxylation. Scheme 7 demonstrates the utility of this reaction in preparing functionalized xylitol derivative 85 with >20 1 diastereoselec-tivity <2002AGE152>. 

The aoss-coupling reaction of bis(pinacolato)diboron with allyl acetates provides the pinacol esters of allylboronic acids in high yield (Scheme The reaction is effectively catalyzed by Pd(dba)2 in DMSO at 50 °C. The reaction more or less accompanies the formation of biallyl, the amount of which markedly depends on the kind of leaving group of the al-lyhc substrates. E)- and (Z)-Cinnamyl acetate and a-phenylallyl acetate aU provide trans-cinnamylboronic acid ester selectively and in almost the same yield, which suggests that the reaction proceeds via ir-allylpalladium as a common intermediate. 

The desired diene-containing compounds such as 48 can also be synthesized using the Suzuki-Miyaura cross-coupling reaction. For example, cross-coupling of the aromatic diiodide 49 with allylboronic acid pinacol ester 50 readily furnished 48 [23]. After exposure of this diene 48 to an RCM reaction, followed by DDQ oxidation, the naphthalene 51 was produced in good yield (Scheme 17.8). 

SCHEME 17.8 Synthesis of substituted naphthalenes from 1,2-diiodobenzenes—application of allylboronic acid pinacol ester. (Adapted with permission from Ref. [10]. Copyright (2009) American Chemical Society). 

Conjugate addition of allylboronic acid pinacol ester to nons)mimet-ric dialkylidene ketones with an in situ generated phosphonite nickel catalyst. 

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