Boronates allylboronates

Allylboronates prepared from simple diols display appreciable reactivity, but eyelie boronate derivatives prepared from 1,2- or 1,3-diols display considerably less. The commonly employed pinacol esters are among the least reactive members of this class. 2-Allyl-3-methyl-l,3,2-oxaza-... 

Allylboranes and Allylboronates With Stereogenic Centers to Boron... 

An extremely attractive feature of the route outlined at the beginning of this section for the transformation of boronates 3 or 4 to a-substituted allylboron compounds 5 is that reagents with very high enantiomeric purity (> 90% ee) may be prepared when precursors such as 3 and 4, and therefore also ate complex 1, contain a suitable diol chiral auxiliary17. The following syntheses of (S)-68, lib9, and 1310 illustrate this feature. 

Olefin metathesis of vinylboronates [102] and allylboronates [103, 104] has been investigated over the past few years because organoboranes are versatile intermediates for organic synthesis. Cross metathesis of vinylboronate 108 and 2-butene 109, for example, yields the boronate 110, which can be converted to the corresponding vinyl bromide 111 with high Z selectivity. Vinyl iodides can be obtained analogously. It should be noted that vinyl bromides and vinyl... 

Enyne metathesis starting either from acetylenic boronates and homoallylic alcohols [104a,c] or from propargyl alcohols and allylboronates [104b] has recently been described. The resulting boronated dienes can be converted to allenes or cycloaddition products. The cross metathesis of vinylcyclopropyl-boronates directed toward the total synthesis of natural products has very recently been investigated by Pietruszka et al. [104d]. 

Roush reported another tartrate boronate, (E )-y-[(menthofuryI)-dimethyl silyl]-allylboronate 130, for untz -a-hydroxyallyation of aldehydes. Reagent 130 can be obtained from commercially available menthofuran, which was selected... 

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... 

If the presence of sensitive functional groups poses problems of chemose-lectivity in the use of hard allylic metal reagents, allylboronate derivatives also can be accessed by a milder transmetalation of allylic tin species with boron halides.This approach has been used by Corey in the synthesis of chiral bis(sulfonamido)boron reagents such as the medially 1 reagent 15 (Eq. 19) (see section Chiral Boronate Derivatives ). ... 

Allylic boronates are more stable to atmospheric oxidation and are thns mnch easier to handle than the corresponding allylic boranes. The stability of the boronate reagents arises from the partial donation of the lone pairs of electrons on the oxygen atoms into the empty p-orbital of boron. This mesomelic effect is responsible for the npfield shift of the boron atom in NMR compared to that of allylic boranes (compare allylboronate 31 and allylborane 32). ... 

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). 

Many of the recent advances in synthetic applications of allylic boron reagents have focused on the use of these reagents as key components of tandem reactions and one-pot sequential processes, including multicomponent reactions. The following examples briefly illustrate the range of possibilities. Most cases involve masked allylboronates as substrates, and the tandem process is usually terminated by the allylboration step. 

Intermolecular Allylboration. A tandem aza[4+2] cycloaddition/allyl-boration three-component reaction has been designed based on the prece-dented carbocyclic [4- -2] cycloaddition/allylboration and a snbsequent one-pot variant. Thns, the thermal reaction between hydrazonobutadienes 138, A-substitnted maleimides, and aldehydes provides polysnbstituted a-hydroxy-alkylpiperidines 141 via the cyclic allylboronate intermediate 139 and the proposed chairlike transition stmctnre 140 (Eq. 103). Monoactivated dienophiles like acrylates fail to react with heterodienes 138 bnt the scope of aldehydes is very broad both ahphatic and aromatic aldehydes are snitable, inclnding electron-rich ones. An inverse electron-demand variant to access the corresponding dihy-dropyran derivatives via the intermediacy of enantiomerically enriched pyranyl allylic boronate 76 has been snbsequently developed (see Eq. 64). ° ... 

As described above in Eq. 43, simple allylboronates can be transformed into more elaborated ones using olefin cross-metathesis. " Treatment of pinacol allylboronate 31 with a variety of olefin partners in the presence of Grubbs second-generation catalyst 142 smoothly leads to formation of 3-substituted allylboronates 143 as cross-metathesis products (Eq. 104). Unfortunately, these new allylic boronates are formed as mixtures of geometrical isomers with modest E/Z selectivity. They are not isolated but rather are treated directly with benzaldehyde to give the corresponding homoallylic alcohol products in good yields (Table A). 

Allylboronates of type 103 react with equivalent amounts of aldoximes 102 (equation 73) giving allylhydroxylamines 104 in good yields. Similar reactions of aldoximes and glyoxylate oxime ethers with allyl bromide and indium also provide hydroxylamines. Additions of substituted allyl boronates to oximes produce mixtures of stereoisomers with ratio highly dependent on the steric size of substituents in both molecules. Addition of allyltri-n-butyltin to aldoxime ether 105 (equation 74) was found to proceed with a considerable diastereoselectivity. 

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. 

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... 

The rearrangement process depicted in Figure B5.1 involves interaction of the vacant p orbital on boron with the alkene. The presence of n-donor substituents on boron such as -OR or -NR2 reduces the electron deficiency on boron and suppresses the rearrangement. Thus allylboron derivatives with two oxygen substituents, for example, are stable at room temperature and their ( )- and (Z)-isomers can be prepared isomerically pure (see below). 

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... 

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. 

Alkoxy-l-boronobutadiene reacted with maleimides, acrylates, and acrylamides to provide cyclic allyl boronates which undergo allylboration with aldehydes (Equation (148)).409 Analogous syntheses of cyclic allylboron compounds via [2+4]-cycloaddition is discussed in Section 9.05.2.2.1. 

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. 

R,R)-dimethyl tartrate allylboronate and acetaldehyde showed that transition state A is more stable than B by 1.75 kcal/mol (Scheme 3.1s). The major force for the energy difference is an attractive Coulomb interaction between the ester oxygen and the boron-complexed aldehyde carbonyl group The distance between the two interacting charges is shorter in A (3.28 A) than in B (4.11 A). The authors concluded that the repulsive nln interaction proposed initially might play a lesser role than speculated previously. 

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). 

These 1,3-budienyl-l-boronic esters are of particular interest since their Diels-Alder cycloaddition leads to allylboronates with a fixed Z configuration, their double bond which is part of the cyclohexenyl moiety. One possible preparation of these compounds is the... 

A theoretical study of the effects of structure and substituents on reactivity in allylboration has recently been completed [116]. Electron delocalization from the oxygen of an attacking aldehyde to the boron p-type atomic orbital is crucial in the allylboration reaction. The ab initio molecular orbital study indicates that the complex between the allylic borane and the aldehyde is weak. The relative elec-trophilicity of the boron atom in allylboron reagents is estimated by projecting out the unoccupied reactive orbital having the maximum amplitude onto the boron p-type atomic orbital. Two factors which are considered to be of importance in the reaction include the electron accepting level of the reactive unoccupied orbital and the efficiency of localization of the orbital in the unoccupied MO space. 

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