Boranes, unsaturated

In 1989 Yamamoto et al. reported that the chiral (acyloxy)borane (CAB) complex 3 is effective in catalyzing the Diels-AIder reaction of a number of a,/ -unsaturated aldehydes [5]. The catalyst was prepared from monoacylated tartaric acid and bo-... 

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. 

Trialky I boranes react rapidly with methyl vinyl ketone (and other a,j8-unsaturated ketones) to yield, after hydrolysis, methyl ketones of the indicated structure 4). The reaction with acrolein is analogous to give jS-alkylpropionaldehydes (5). The process is inefficient in that only one of the three alkyl groups of the borane is converted into product, but the rapidity and ease of carrying out the reaction may be adequate... 

The tartrate ester modified allylboronates, the diisopropyl 2-allyl-l,3,2-dioxaborolane-4,5-di-carboxylates, are attractive reagents for organic synthesis owing to their ease of preparation and stability to storage71. In the best cases these reagents are about as enantioselective as the allyl(diisopinocampheyl)boranes (82-88% ee with unhindered aliphatic aldehydes), but with hindered aliphatic, aromatic, a,/l-unsaturated and many a- and /5-alkoxy-substituted aldehydes the enantioselectivity falls to 55-75% ee71a-b... 

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine A-oxide for the oxidation of the intermediate vinyl boranes. 

Addition of unsaturated boranes to methyl vinyl ketones Hydrocarboxylation of triple bonds Addition of acyl halides to triple bonds 1,4-Addition of acetals to dienes... 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

A novel, unprecedented sulfoxide-directed borane reduction of the a,P-unsaturated sulfoxide to the saturated sulfide was discovered. 

The detection of the structural effects of B—C rr-bonding in unsaturated open-chain organoboranes should prove more difficult than in boracyclo-polyenes for several reasons. First, as already mentioned, there is the conformational freedom of the open-chain boranes, which reduces B(2pz)—C(2pz) overlap (cf. 17 vs. 19). Second, there is the linear overlap of 34 versus the cyclic, Htickel aromatic overlap of 35. 

One such reported example is the synthesis of polypropylene-6-polymethyl-methacrylate (PP-6-PMMA) copolymers utilizing metallocene catalysis and the borane chemistry. In the initial step, PP with chain-end olefinic unsaturations was prepared using metallocene catalysts such as Et(Ind)2ZrCl2/MAO. The unsaturation sites were then hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce borane-terminated PP (43) (Fig. 30), which was selectively oxidized and interconverted to a... 

The catalytic cycle for hydroboration is now widely accepted and direct examples of several intermediate species have been isolated and well characterized (Scheme 3).5-7 These now include (j-borane complexes, which have in some instances been found to be catalytic precursors for hydroboration.8-10 Oxidative addition of an H—B bond to a coordinatively unsaturated metal fragment... 

Preparation of phenyl (P-carbomethoxyethyl) phosphine-borane — Reaction of a primary phosphine-borane with an unsaturated ester... 

The utilization of phosphine-boranes as stable forms of the phosphine has been exploited for addition reactions with alkenes. With several types of a,P-unsaturated compounds, Michael addition of primary phosphine-boranes has been accomplished in excellent yield for the preparation of secondary phosphine-boranes (Equation 3.30).443... 

Reaction of a primary phosphine-borane with an unsaturated ester443... 

Bourumeau, K., Gaumont, A.-C., and Denis, J.-M., Hydrophosphinylation of a,P-unsaturated esters by primary phosphine-boranes a useful entry to symmetrical and unsymmetrical phosphine-boranes, Tetrahedron Lett., 38, 1923, 1997. 

Hydroboration, the addition of a boron-hydrogen bond across an unsaturated moiety, was first discovered by H. C. Brown in 1956. Usually, the reaction does not require a catalyst, and the borane reagent, most commonly diborane (B2H6) or a borane adduct (BH3-THF), reacts rapidly at room temperature to afford, after oxidation, the /AMarkovnikov alkene hydration product. However, when the boron of the hydroborating agent is bonded to heteroatoms which lower the electron deficiency, as is the case in catecholborane (1,3,2-benzodioxaborole) 1 (Scheme 1), elevated temperatures are needed for hydroboration to occur.4 5... 

A modified version of the Brown-Negishi reaction using B-alkylcatechol-boranes was reported (Scheme 32). This novel method is based on a simple one-pot procedure involving the hydroboration of various substituted alkenes with catecholborane, followed by treatment with catalytic amount of oxygen/DMPU/water and a radical trap. Efficient radical additions to a,ft-unsaturated ketones and aldehydes have been reported. Primary alkyl radicals are efficiently generated by this procedure and the reaction has been applied to a 300 mmol scale synthesis of the y-side chain of (-)-perturasinic... 

Tetraborane(lO) reacts, as does diborane, readily with unsaturated hydrocarbons. Alkenes give access to B2,B4-alkylene bridged B4H8(RCH-CHR) organo-boranes [37] while alkynes, allenes and l-ene-3-ynes lead to a variety of carba-boranes [38-40]. 

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