Vinylic boron compounds

Yamamoto Yatagai Maruyama Sonoda Murahashi J. Am. Chem. Soc. 1977, 99. 5652, Bull. Chem. Soc. Jpn. 1977, 50. 3427. For other methods of dimerizing vinylic boron compounds, see Rao Kumar Devaprabhakara J. Organomet. Chem. 1979, 179, C7 Campbell Brown J. Org. Chem. 1980, 45, 549. 

Reaction of the anion 3 with trialkyl boron compounds gives adducts which eliminate the toluenesulfonate group in a 1,2-alkyl shift (eq 3). The resultant vinyl boron compounds are capable of undergoing Suzuki-type cross-coupling reactions. Using this method, Ichikawa et al. were able to convert 3 into functionalized aryl adducts which were then cyclized in a 5-endo fashion to give monofluorinated adducts. ... 

As described in the previous section, it was discovered that vinylic boron compounds readily react with vinylic halides to give coupling products. Consequently, we next attempted to examine the reaction of 1-alkenylboranes with haloarenes which have also sp hybridized carbon-halogen bonds, and found that the reaction takes place smoothly. Representative results are exhibited in Table 4. 

This reaction has the advantage that only one product (11) (head-to-head coupling product) is formed. Additional results are shown in Table 5. Aromatic bromides and iodides easily react with vinylic boron compounds, but organic chlorides do not participated, except the reactive allylic and benzylic derivatives. Heteroaromatic halides can be used as coupling partners. Or// o-substituents on benzene ring do not give difficulty. Thus, the cross-coupling reaction is used for the synthesis of benzo-fused heteroaromatic compounds (eq 5) (6). 

Mardniec, B., Pietraszuk, C., Kownacki, 1. and Zaidlewicz, M. (2005) Vinyl- and arylsilicon, germanium, and boron compounds, in Comprehensive Organic Functional Group Transformations II, Vol. 1 (eds A.R. Katrytzky and R.J.K. Taylor), Elsevier, Amsterdam, pp. 941-1024. 

Enyne 112a having a boron moiety on the alkyne gives vinyl boron 113a [Eq. (6.83)], whose carbon-boron bond can be converted to a carbon-carbon bond. Enyne metathesis followed by Diels-Alder reaction of the resulting diene with DMAD gives tricyclic compound 114 [Eq. (6.84)]. ... 

The ability to provide highly functionalized reagents, such as unsaturated silanes and vinyl boronates, starting from terminal olefins is one of the most attractive attributes of CM, particularly when traditional methods for the preparation of such compounds are not synthetically straightforward. Therefore, significant research has been undertaken to determine the broad-spectrum chemoselectivity of olefin metathesis catalysts. In many cases, the use of a CM protocol in reagent synthesis is completely orthogonal to alternative methods of preparation. 

The (E) and (Z) forms of the vinylboron compounds 278 and 279 can be prepared by hydroboration of alkynes and haloalkynes, and their reactions with the (E)- or (Z)-vinyl iodides or bromides 280 and 281 proceed without isomerization the four possible isomers 282-285 can be prepared in high purity. However, for the efficient preparation of the (Z,Z)-dienes 287, the diisopropyl ester of (Z)-alkenylboronic acid 286 should be used. Other boron compounds give poor yields... 

Hydroboration of allenes 65 with pinacolborane in the presence of Pt(DBA)2 and a trialkylphosphine provides either the allyl boronate 66 or the vinyl boronate 67 regioselectively, depending on the stereoelectronic factors of the phosphine employed (Equation 2) <1999CL1069>. Allyl and vinyl boronates are synthetically important because of their ability to undergo nucleophilic addition to carbonyl compounds as well as transition metal-catalyzed cross-coupling. 

Ruthenium-catalyzed olefin cross-metathesis (ring-closing metathesis, RGM) between terminal alkenes and vinyl-boronic acid or esters has recently been developed for the synthesis of ( )-l-alkenylboron compounds from alkenes.459,460 The efficiency of protocol was proved in the synthesis of a key intermediate of epothilone 490 292 461 (Equation (84)). The vinyl boronate was given almost exclusively the trans-adduct. 

Unusual and otherwise difficult to obtain boronates can be made by addition of a xanthate to allyl or vinyl boronates. The reaction proceeds in the same way as for ordinary olefins and compounds containing a boronate... 

The carbon-boron heterocycle, 3-phenyl-3-benzoborepin, exhibits oxidative stability upon exposure to air, an unusual feature for a trivalent boron compound. In Table XVI are recorded the chemical shift data for the vinyl protons for the benzometallepins of B, Sn, and Si. The PMR spectrum of 3-phenyl-3-benzoborepin exhibits vinyl proton resonances at lower fields than would be expected for an olefinic boron compound (compared to trivinylboron or 4,5-dihydroborepin see Table XV), and also at lower field than the benzostannepin derivative (217). The shift to lower field of 0.4 to 0.8 ppm may be consistent with the presence of a ring current, which would require the participation of the Bp orbital in the 7r-electron system. Support for increased electron density at boron might be provided from B NMR measurements, but such data have not yet been reported. Complexation of boron, which converts the... 

As much recent work has concerned dimesitylboryl species, it is worth remarking that the related compounds (22)-(24) also readily yield carbanions. Unlike mesityl compounds, (22) cannot be deproton-ated at the 4 -methyl group, while compounds (23) have some special properties due to the great increase in steric hindrance around boron. Compounds (24) promise to be of utility due to their very ready solvolysis with water or alcohols in the presence of caudytic quantities of mineral acid. n>ey are even selectively hydrolyzed in preference to vinyl as well as alkyl groups (equation 23). ... 

There are many other ways of making boronic acids and you are referred to Suzuki s review37 for a full discussion. A Suzuki coupling would occur if one of these boron compounds 236 was combined with an aryl or vinyl halide or triflate in the presence of Pd(0) and an oxy-anion. 

The dotted arrows on the transmetallation step 243 show only what joins to what and are not intended as a serious mechanism. Indeed a better mechanism might involve addition of RO to the boron atom before transmetallation. This process can be used to couple aryl to aryl, vinyl to vinyl, and aryl to vinyl (either way round ). As boron compounds are much less toxic than tin compounds, the Suzuki coupling is often preferred industrially. Because each partner in the coupling reaction is marked in a different way - one with a boron atom and one with a halide - we can be sure that we shall get cross coupling reactions only. 

The a,(3-unsaturated ester (8), aldehyde (9), and nitro (10) compounds participate as electrophiles in a number of useful conjugate addition reactions. Copper-catalyzed addition of Grignard reagents provides access to aryl butanoic acid derivatives substituted with an oxetane (equation 1 in scheme 13.7). The ester, aldehyde, and nitro electrophiles also undergo mild Rh-catalyzed additions of aryl and vinyl boronic acids (equation 2 through equation 4 in scheme 13.7). Interestingly, the unsaturated aldehyde participates readily in an amine conjugate addition to afford oxetane substituted 3-amino-acetaldehyde derivatives. 

Figure 5.1. Chiral boron compounds for asymmetric allyl addition to achiral primary, secondary, and tertiary alkyl, vinyl, and aryl aldehydes, and their typical enantioselectivities (a-e at -78°, g-j at -100°). (a) [17] (b) [18] (c) [19] (d) [19] (e) [20] (f-h) [21-24] (i-j) [25].

In Section 12.7.B, the Stille coupling reaction reacted a vinyl tin compound with a vinyl triflate, in the presence of palladium(O). Tetravalent tin complexes add to aldehydes and ketones, in the presence of a Lewis acid. Allyltin complexes are, by far, the most widely used of these compounds.297 a typical example is taken from the work of Keck, in which a chiral aldehyde (455) was treated with allyltributyltin, in the presence of various Lewis acids. S As shown in Table 12.19, a mixture of syn (456) and anti (457) products was obtained. The ratio of 456/457 was dependent on the structure of the R group in 455, the solvent and the Lewis acid.The anti product (457) was obtained by using the tert-butyldimethylsilyloxy derivative (sec. 7.3.A.i) of 455 with 2 equivalents of boron trifluoride in dichloromethane. The syn product is obtained preferentially when the benzyloxy derivative of 455 is used with titanium tetrachloride in dichloromethane.298... 

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