Alkenylboron Compounds

An alkyl- or alkenylboron compound, as a suitable organoboron component (a borane, boronic acid or ester) can be prepared through hydroboration of an... 

In addition to alkenylboron compounds, alkenylalane, alkenylzirconium, alkenyltin, alkenyl copper, and alkenylmagneslura reagents are reported to undergo a related alkenyl-alkenyl coupling reaction to give l,3-a1kad1enes. 

New mechanistic studies with [Cp2Ti(CO)2] led to the observation that the tita-nocene bis(borane) complex [Cp2Ti(HBcat)2] (Hbcat = catecholborane) generated in situ is the active catalyst.603 It is highly active in the hydroboration of vinylarenes to afford anti-Markovnikov products exclusively, which is in contrast to that of most Rh(I)-catalyzed vinylarene hydroboration. Catecholborane and pinacolborane hydroborate various terminal alkynes in the presence of Rh(I) or Ir(I) complexes in situ generated from [Rh(COD)Cl2] or [Ir(COD)Cl2] and trialkylphosphines.604 The reaction yields (Z)-l-alkenylboron compounds [Eq. (6.107)] that is, anti addition of the B—H bond occurs, which is opposite to results found in catalyzed or uncatalyzed hydroboration of alkynes ... 

Alkenylboron compounds cyclopropanations, 9, 181 haloetherification, 9, 182 hydrogenation and epoxidation, 9, 182 metal-catalyzed reactions, 9, 183 metallic reagent additions, 9, 182 via radical addition reactions, 9, 183 5-Alkenylboron compounds, cross-coupling reactions, 9, 208 Alkenyl complexes with cobalt, 7, 51 with copper, 2, 160, 2, 174 with Cp Re(CO) (alkene)3 , 5, 915-916 with dicarbonyl(cyclopentadienyl)hydridoirons, 6, 175 with gold, 2, 255... 

A diastereoselective Rh(I)-catalysed conjugate addition reaction of aryl- and alkenyl-boronic acids to unprotected 2-phenyl-4-hydroxycyclopentenone (207) has been investigated. The free OH group on the substrate was found to be responsible for the (g) stereochemistry, which is cis for arylboronic derivatives (208). In the case of the alkenylboronic compounds, the stereochemistry can be tuned to either a cis (with a base as additive) or trans addition (209) (with CsF as additive), without the need for protecting groups.249... 

An iridium(m) hydride catalyzed intramolecular alkyne hydroalkoxylation can be used to construct isochromans 545 bearing a C(3)-spiroacetal moiety (Equation 226) <2005OL5437>. Novel isochromans bearing an array of C(3)-spiroacetal moieties are accessible via the hydroboration of alkynediols with disiamylborane followed by treatment of the resulting alkenylboron compounds with alkaline hydrogen peroxide <1998CL81>. 

Fig. 16.18. Representative mechanism of the Pd-catalyzed C,C coupling of an organoboron compound. The elementary steps, discussed in the text, are (1) complexation, (2) oxidative addition, (3) transmetalation of the alkenylboron compound to afford an alkenylpalladium compound, (4) reductive elimination, and (5) dissociation of the coupled product from the metal. - Note Regarding the arrangement of the ligands around the metal center of the individual intermediates and the details of the transmetalation the present mechanistic analysis is less complete than the mechanistic analysis of other Pd-catalyzed C,C couplings, namely the Stille coupling (Figure 16.27) or Heck reaction (Figure 16.35, part II), which have been investigated in great detail.

Arylalkenes are accessible not only by way of an arylation of alkenylboron compounds (Figure 16.18) but also via an alkenylation of arylboron compounds. Figure 16.19 exemplifies... 

Fig. 13.14. Representative mechanism of the Pd-catalyzed C,C coupling of an organoboron compound. The elementary steps, discussed in the text, are (1) complexation, (2) oxidative addition, (3) transmetallation of the alkenylboron compound to afford an alkenylpalladium compound, (4) reductive elimination, and (5) dissociation of the coupled product from the metal.

Arylalkenes are accessible not only by way of an arylation of alkenylboron compounds (example in Figure 13.14) but also via an alkenylation of arylboron compounds. Figure 13.15 exemplifies this for a Pd-catalyzed reaction of an arylboronic acid with iodoalkenes with widely variable substitution patterns. The addition of KOH increases the reactivity of the arylboronic acid in this coupling and in similar ones. The base converts the boronic acid into a negatively charged boronate ion A. This ion A is transmetallated faster than the neutral boronic acid by the Pd(II) intermediate the boronate ion is a superior nucleophile and replaces the iodide ion in the Pd(II) complex particularly fast. 

Alkenylzirconium compounds 34 obtained by the hydrometallation of alkynes were transformed to 1-alkenylboron compounds via transmetallation with B-halo-9-BBN or B-chlorocatecholborane62,63 (Scheme 5). Mercuration followed by transmetallation to BH3 was advantageous over the lithiation route in the synthesis of indole-3-boronic acid 35.64... 

Scheme 11 Dehydrogenative hydroboration giving 1 -alkenylboron compounds.

Haloboration of terminal alkynes yielded stereodefined (Z)-2-halo-l-alkenylboron compounds e.g., 114, 115 that allowed further functionalization of the resulting C-Br or G-B bond. 2,2-Disubstituted-l-alkenylboronic esters were stereoselectively obtained when the haloboration was followed by cross-coupling with 1-alkynylzinc chlorides (Equation (18)).192 The iodoboration-conjugate addition sequence gave alkenyl iodides that were used as intermediate for the total synthesis of deoxyepothilone derivatives (Equation (19)).193-195... 

The stannylboration of alkynes was followed by cross-coupling of the G-Sn bond in the synthesis of stereodefined 1-alkenylboron compounds 143 (Equation (48)).261 There was a large accelerating effect of boryl substituents on thermal ring closing to the methylene cyclobutenes at 140 °G. 

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. 

Scheme 32 Cross-coupling 1 -alkenylboron compounds. The left parts come from vinylboron compounds and the right parts from halides or triflates (Equation (203)).

Alkenylboron compounds couple with the representative organic halides or triflates (Scheme 28). Hexaalkylbenzene was synthesized by sixfold alkenylation (55) of hexabromobenzene followed by catalytic hydrogenation of the double bonds 11521. The reaction of 1-alkenylborane with 1-bromo-l-alkyne stereose-lectively provided ( )-enyne (56) which was then converted into ( ,Z)-hexa-deca-10,12-dienal, a sex pheromone of the melonworm 11531. Due to the difficulty of purification of a geometrical mixture, the stereoselective synthesis is critical for such dienes or trienes. The PGEi derivatives (57) were synthesized... 

Fluorination of Alkenylboron Compounds and Arylboronic Acids and Derivatives... 

Hydroborates in the presence of an ester or of methyl iodide have also been used for hydroborations. For example, the system lithium tetrahydroborate-ethyl acetate provides a simple route to dialkylboryl compounds (equation 12). Alkynes provide alkenylboron compounds. 

Arylboron compounds appear to be comparable in reactivity to alkenylboron compounds, but comparative data are not available. Alkynylboranes are fairly readily hydrolyzed, as are benzylboranes, particularly when substituted by electron-withdrawing groups on the benzene ring. However, none of these groups are available via hydroboration and so further discussion here is inappropriate. 

The additions proceed regioselectively in favor of terminal boron adducts to produce (Z)-l-alkenylboron compounds through a syn addition of the X-B bond to 1-alkynes. The mechanism is fundamentally different from the uncatalyzed process and is postulated to proceed through the oxidative addition of the X-B bonds (X = H, RS, Y2B) to the transition-metal complex [M(0)] to form X-M-BYj species (28), followed by the migratory cis insertion... 

Cross-coupling of 1-alkenylboron compounds with 1-halo-1-alkynes leads to conjugated enynes, which can serve as precursors for conjugated dienes. ... 

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