Allylboron compound

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

A second powerful route to functionalized allylboron compounds involves the reaction of an a-haloalkylboronatc and a vinyl organometallic reagent3 4-28-29, 50c-92 04. This method is especially useful for the preparation of allylboron compounds not accessible via the allylorganometal-lic route. Notable examples that fall into this category are ( )-4,4,5,5-tctramethyl-2-[4-(tetrahy-dro-2//-pyran-2-yloxy)-2-butenyl]-l,3,2-dioxaborolane (yield 41 %, 93% E) and (E)- or (Z)-2-(l,l-dimethyl-2-butenyl)-4,4,5,5-tetramethyl-1.3,2-dioxaborolane (yield 77-84%. 98% E or 93% Z). 

Relatively few studies of the reactions of allylboron compounds and ketones have appeared. Ketones are less reactive than aldehydes, and as a result these reactions tend to be much slower and often less diastereoselectivc. The reaction of (Z)-4,4,5,5-tetramethyl-2-[3-(tctrahy-dro-2/A-pyran-2-yloxy)-2-propenyl]-1,.3,2-dioxaborolane and ethyl 2-oxopropanoate, for example, was conducted under 6 kbar pressure at 45 C for 80 hours to give a 9 1 mixture of syn-and antz-diastereomers of 1 in 85% yield49. 

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. 

These reagents are not isolated but are used directly in reactions with aldehydes, after generation of ate complexes via the addition of an alkyllithium reagent or pyridine11. 2-(2-Propenyl)-1,3,2-dioxaborolane is also metalated upon treatment with lithium tetramethylpiperidide, but mixtures of a- and y-substitution products are obtained upon treatment of this anion with alkylating agents20. Consequently, this route to a-substituted allylboron compounds appears to be rather limited in scope. 

Allylboron compounds, synthesis, 9, 170-171 5-Allylboron compounds, cross-coupling reactions, 9, 212 Allylboronic esters, synthesis, 9, 194—195 Allyl boryl ethers, isomerization, 10, 88 Allylchlorodimethylsilanes, for carbonyl allylations,... 

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 haloboration-coupling sequence provided simple access to the stereochemically pure 3,3-disubstituted allylboron compounds 116 via one carbon homologation of the alkenylboron intermediates (Equation (20)).196 Bromoboration of allene afforded 117 which was converted into a tartrate for asymmetric allylboration (Equation (21)).197... 

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 reaction tolerated various functional groups, thus allowing the in situ preparation of allylboron compounds possessing a carbonyl group (Equation (32)).236 The tandem diboration-intramolecular allylboration provided a diastereoselective access to the cycloalkanes bearing 1,3-diols. 

The diboration of allenes afforded another series of allylboron compounds that each have a boryl group at the vinyl carbon. The addition had a strong tendency to occur at the internal double bond of terminal allenes such as 1,2-heptadiene (Equation (33)).237 However, steric hindrance in both allenes and phosphine ligands forced the addition toward the terminal double bond of dimethylallene (Equation (34)).237 On the other hand, addition to the terminal double bond occurred selectively for both monosubstituted and 1,1-disubstituted allenes when Pd(dba)2 was used in the presence of a co-catalyst (RI) such as I2, Arl, and iodoalkenes (Equation (35)).238 The role of co-catalyst was attributed to in situ formation of I-Bpin intermediate, which undergoes oxidative addition and insertion leading to 2-boryl-7r-allylpalladium intermediate. 

Allylboration, Allenylboration, and Propargylboration 9.05.3.3.1 Synthesis and reactions of allylboron compounds... 

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. 

Less is known about the reactions of allylboron compounds however, an ate complex between B-allyl-9-BBN and NaOMe afforded allylarenes in high yields within 0.5-1 h (Equation (209)).898... 

Less is known about the reaction of allylboron compounds however, allylation will occur smoothly in the presence of a base and a palladium catalyst (Scheme 40). The reaction of tri(crotyl)borane with iodobenzene in THF in the presence... 

Allylboron compounds can sometimes be obtained via hydroboration of allenes or 1,3-dienes with dialkylboranes (e.g. see Section 3.10.4.2). Allylboron compounds are particularly readily hydrolyzed, and hydrolysis is accompanied by transposition of the double bond. For example, tricrotylborane reacts with water at room temperature to give 2 mol of 1-butene and 1 mol of crotyldihydroxyborane (Scheme 13). 

Vol. 3, D. H. Solomon, ed.. Step-growth polymerization (1972) 372 pp. A172. C. E. Schildknecht, Allyl Compounds and Their Polymers (Including Polyolefins). Wiley, New York, 1973. There are chapters on allylsilicon compounds, 22 pp. (106) and allylboron compounds, 6 pp. (41). 

Boranes other than those based on a-pinene are particularly useful in allylic transfer to imines to make single enantiomers of unsaturated amines. One good combination is an allylboron compound 80 complexed with an /V-sulfonyl amino alcohol such as 78, derived from nor-ephedrine (see chapter 23) with an /V-silyl imine such as 81. The unsaturated amines 82 are formed in good... 

Allylboron compounds have been the most widely studied of the type 1 allyl organometallics. Of these, the dialkylcrotylboranes isomerize most readily and often require handling at temperatures below -78 °C for isomerization to be suppressed. Thus, it is probably more appropriate to view such compounds ie.g. crotyl-9-BBN) as type III crotyl organometallics. The boratropic isomerization of dialkylcrotylboranes, however, is sensitive to steric factors. For example, the ( )- and... 

Enantiomerically enriched bishomoallylic alcohols could be readily accessed by reactions of 2-vinyloxiranes with chiral allylating agents based on a-pinene in the presence of a catalytic amount of Sc(OTf)3 [116]. The p,v-unsaturated aldehydes were generated in situ via Lewis acid induced rearrangement (1,2-hydride shift) and subsequently trapping by chiral allylboron compounds. The products were obtained in moderate to high yields with excellent enantioselectivities (Scheme 12.46). 

Sawamura also observed that ligands with large bite angle, such as Xantphos, would activate Cu(I)-OR for tr-bond metathesis with B2pin2 to form a Cu—B species 17 usefijl as a formal boryl nucleophile (Scheme 22). Then, the formal Sn2 attack of the Cu—B species on an allylic carbonate would allow y-selective formation of an allylboron compound along with a copper carbonate that would undergo decarboxylation to regenerate the Cu-OR. 

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