Vinylic organoboranes

Adds to alkynes, giving vinylic organoboranes that can be oxidized with H2O2 to yield aldehydes (Section 8.4). 

Among a variety of dienophiles the simplest one ethylene, and 1-alkenes react sluggishly.3 Consequently, their equivalents are highly desirable and a few such compounds have been introduced.4 Vinylic organoboranes are good candidates for that purpose, provided they would be sufficiently reactive and the carbon-boron bond of the adduct could be transformed into the carbon-hydrogen bond without isomerization of the double bond. Here, our studies in this direction are described.5... 

The hydroboration of acetylenes also involves cis addition, leading to stereodefined vinylic organoboranes ... 

The protonolysis of alkenyldialkylboranes to give -alkenes can be conducted, in most cases, under neutral conditions using methanol. More hindered alkenyldisiamylboranes react less well, unless a small amount of a carboxylic acid is added. A variety of Z-alkenyl pheromones was prepared using this method. The synthesis of trans-alkenes and unsymmetrical ketones was also accomplished using vinylic organoborane chemistry.7... 

Brown, H.C., C. Subrahmanyam, T. Hamaoka, N. Ravindran, D.H. Bowman, S. Misumi, M.K. Unni, V. Somayaji, N.G.J. Bhat. Vinylic Organoboranes. A Convenient Stereospacific Synthesis of (Z)-l-Halo-l-alkenes from l-AUqmes via (E)-TAlkenylborane Derivatives with Halogens, Org. Chem. 54 6068—6075 (1989). 

H. C. Brown and J. B. Campbell, Synthesis and Applications of Vinylic Organoboranes , Aldrichimica Acta, 1981,14, 3. 

H. C. Brown Organic Synthesis Today and Tomorrow (The Rich Chemistry of Vinylic Organoboranes, B. M. Trost and C. R. Hutchinson, Eds.), pp. 121—137. Pergamon (1981). 

Organoboranes are reactive compounds for cross-coupling[277]. The synthesis of humulene (83) by the intramolecular cross-coupling of allylic bromide with alkenylborane is an example[278]. The reaction of vinyiborane with vinyl-oxirane (425) affords the homoallylic alcohol 426 by 1,2-addition as main products and the allylic alcohol 427 by 1,4-addition as a minor product[279]. Two phenyl groups in sodium tetraphenylborate (428) are used for the coupling with allylic acetate[280] or allyl chloride[33,28l]. 

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... 

Scheme 24 Brown mechanism for the conjugate addition of organoboranes to methyl vinyl ketone...

While die above reactions represent only a small fraction of die reactions known for palladium, they form the basis of a powerful methodology for building carbon structures. Several variations have been developed which utilize certain types of reactants and give particular types of products. All diese variations, however, contain a common theme. In each case an electron-deficient reagent (e.g., a vinyl halide or aromatic triflate) reacts with an election-rich reagent (e.g., an alkene, an organoborane, or an organotin) witii the formation of a new carbon-carbon bond. In that sense diese reactions are related to die reactions between carbon nucleophiles and carbon electrophiles discussed previously in diis chapter. They are quite different, however, because diey proceed only in the presence of Pd(0). In fact they proceed only in die coordination sphere of Pd(0). The ability of Pd(0) to catalyze diese reactions is nearly unique We will now examine some of die more common processes. 

The chemistry of unsaturated organoboranes often differs markedly from that of their saturated analogues. Both vinylic and allylic boranes react readily with many substrates toward which trialkylboranes are inert90 . Allylic boranes can be synthesized selectively via direct hydroboration of an appropriate allene or conjugated diene and are of immense synthetic importance 91 93). Mikhailov in his book and review 7,94) has documented the synthetic applicability of allylic boranes with caution of high thermal reactivity with respect to allylic rearrangement. For example, (l-methyl-2-propenyl)dialkylboranes rearrange spontaneously to the 2-butenyl isomer even at -78 °C (Eq. 40). 

Organoborane complexes have shown a high efficiency for polymerizing vinyl monomers at room temperature as evidenced by numerous investigations developed by Chung et al. [94-97]. The oxidation of organoborane leads to the formation of borane peroxide able to initiate the polymerization ... 

Though oxidation with acidic hydrogen peroxide is rarely used, alkyl- and aryl-boronic acids are readily attacked with a migratory order of Bu PhCH2 > Bu > Bu" > Ph > vinyl Me. It tqqiears, therefore, that the reaction might be useful for selective oxidations as well as for oxidations of base lal e organoboranes. ... 

Oxidation with trimethylamine N-oxide is not complicated by side reactions and proceeds in a stepwise fashion such that one primary alkyl group is oxidized at 25 C, the second at 65 C and the third at 120 C. ° The trimethylamine evolved can be estimated and thus the method can be used for the quantitative estimation of organoboranes. The rates of oxidation are not markedly solvent dependent and there is a clear cut ease of oxidation in the order tertiary alkyl > secondary cycloalkyl > secondary alkyl > primary alkyl > branched primary alkyl > vinyl. Some examples of useful differentiation are shown in equations (23) and (24). ... 

Organoboranes (such as aryl and vinyl boronic acids) have also demonstrated to be valuable reagents for the synthesis of organomercurials, particularly in the presence of functional groups that are not compatible with the more reactive organolithium or -magnesium reagents. 

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