Allylboration reactions compounds

Increasing interest is expressed in diastereoselective addition of organometallic reagents to the ON bond of chiral imines or their derivatives, as well as chiral catalyst-facilitated enantioselective addition of nucleophiles to pro-chiral imines.98 The imines frequently selected for investigation include N-masked imines such as oxime ethers, sulfenimines, and /V-trimcthylsilylimines (150-153). A variety of chiral modifiers, including chiral boron compounds, chiral diols, chiral hydroxy acids, A-sull onyl amino acids, and /V-sulfonyl amido alcohols 141-149, have been evaluated for their efficiency in enantioselective allylboration reactions.680... 

P.y-Unsaturated (allylic) boron derivatives (not alkyl-, aryl- and vinylboranes) readily react with organic compounds containing multiple bonds to form novel carbon-carbon bonds (Scheme 1). In the last 30 years, allylboration reactions have been widely used in organic synthesis.1... 

Hoffmann has continued his pioneering work in asymmetric allylboration and synthesized several heterocyclic compounds (26). Soderquist has developed a new reagent, chiral 5-allyl-10-trimethylsilyl-9-borabicyclo[3.3.2]-decane for allylboration reactions (27). 

The reaction of trimethylsilylacetylene with triallylborane in CDCI3 proceeds stepwise and involves 1,1- and 1,2-allylboration <1968IZV1923, 1999JOM(580)234>. Vinylborane 116 readily cyclizes into compounds 117 and 118 (Scheme 49). 

The thermal reaction of dipropynylsilanes with AII3B also involves 1,1- and 1,2-allylboration of the triple bond (or vice versa) (Scheme 50). l-Sila-3-bora-2-ethylidene-4-cyclopentene derivatives 119 are formed quantitatively at room temperature. They undergo an intramolecular 1,2-allylboration on heating for 3h to give the corresponding bicyclic compounds 120 quantitatively <2002JOM(649)232>. 

The reaction of the tetrayne pentasilane with triallylboranes furnishes a mixture of silole 121 and compound 122 in a 1 5 ratio (Scheme 51). An excess of AII3B did not induce further allylboration with the remaining C C bonds even upon prolonged heating <2002JOM(649)232>. 

A neat hydroformylation - allylboration - hydroformylation reaction sequence reported by Hoffmann el al. afforded the pyrano-fused oxepanes 135 and 136 (48% total yield ca 1 1 ratio) from the precursor 134 the compound 134 required a number of steps for synthesis from the alcohol 130, the ethynyl ether 131, and the boronate 133 (by addition of 132) in moderate overall yield. Dess-Martin oxidation of 135 and 136 then afforded the final lactones 137 and 138 [01NJC102],... 

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. 

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

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 key step to the tricyclic compound 17 seems to be the stereoselective aza Diels-Alder reaction (20) of the intermediate enamine dienophile 18 with azadiene adduct 19 to produce the imine complex 21. Allylboration of C=N bond in the latter proceeds also stereoselectively (21, arrow) furnishing the diborylated compound 22. 

The N=0 bond is not included in Scheme 1 because the allylboration of nitro compounds has not been studied yet. We decided to investigate this reaction. 

Following this route, the authors achieved the synthesis of 10 analogues of compound 58, in a high global yield, where the crucial step of this total synthesis was the efficient catalytic enantio-, regio-, EIZ-, and diastereoselective three-component inverse electron demand hetero-[4+2] cycloaddition/allylboration sequence. This key process provides a rare example of an enantioselective hetero-Diels-Alder reaction involving acyclic 2-substituted enol ethers. Additionally, these compounds were evaluated for antimicrobial activity, and two of them showed more activity than the original thiomarinol H. 

The stability of allylboronates and the high level of diastereoselectivity in their additions to carbonyl compounds and imines represent very attractive attributes in organic synthesis. The examples in this chapter clearly show that recent advances in the preparation of allylboronates will help in furthering their applications. The development of efficient catalytic enantioselective allylboration methods and the invention of more powerful and more elaborate tandem reaction processes constitute two emerging areas for further development. 

It is known that hydroboration reaction of cyano groups gives iminoborane species which dimerize to form B-N four-membered ring (cyclodiborazane) in the case of appropriate borane used [16]. When this reaction is used for the bifunctional monomers, formation of polymeric materials consisting of cyclodiborazane units can be expected. Concerning the preparation of poly(cycIodiborazane)s, we have recently reported two new methods, i.e., allylboration polymerization of dicyano compounds with triallylborane [17], and polycondensation between bis(silylimine)s and chlorodialkylboranes [18]. 

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