Vinylboronates

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

The utility of the Suzuki reaction in the challenging arena of natural product total synthesis has been explored. The constitution of bombykol (106) (see Scheme 26), a well-known pheromone, lends itself to a Suzuki coupling. Indeed, in a short stereospecific synthesis of 106, Suginome et al. demonstrated that ( )-vinylboronic acid ( )-104 can be smoothly cross-coupled with (Z)-l-pentenyl bromide [(Z)-105] 44 the configurations of both coupling partners are preserved in the C-C bond forming process. 

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... 

The stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

The adducts derived from catechol borane are hydrolyzed by water to vinylboronic acids. These materials are useful intermediates for the preparation of terminal vinyl iodides. Since the hydroboration is a syn addition and the iodinolysis occurs with retention of the alkene geometry, the iodides have the -configuration.214... 

The synergistic action of these four effects may explain why trivinylborane (24) shows no sign of B—C 7r-bonding but vinylboron fluorides (e.g., 25a) do. 

Through the use of arenediazonium salts, the straightforward transformation of amines into cross-coupling products can be realized. Whenever the diazonium salts do not tolerate bases and strong nucleophiles (e.g., phosphines), base- and phosphine-free protocols have to be used. Heterocyclic carbene ligands serve well in cross-coupling of Aryl- and vinylboronic acids, or alkylboronates with arenediazonium salts.369,370 Several convenient phosphine-free protocols have been developed for the same purpose.371-373... 

A recently reported vinylation of nitrones was accomplished by using ZnMe2 and vinylboronic esters of pinacol 256. The optimal conditions for the addition were 3.5 h at 60 °C in DMF. Yields of the products, A -allylic hydro-xyamines 257, varied significantly from 14% to 92% (Scheme 137).359... 

However, most asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides with alkenes are carried out without Lewis acids as catalysts using either chiral alkenes or chiral auxiliary compounds (with achiral alkenes). Diverse chiral alkenes are in use, such as camphor-derived chiral N-acryloylhydrazide (195), C2-symmetric l,3-diacryloyl-2,2-dimethyl-4,5-diphenylimidazolidine, chiral 3-acryloyl-2,2-dimethyl-4-phenyloxazolidine (196, 197), sugar-based ethenyl ethers (198), acrylic esters (199, 200), C-bonded vinyl-substituted sugar (201), chirally modified vinylboronic ester derived from D-( + )-mannitol (202), (l/ )-menthyl vinyl ether (203), chiral derivatives of vinylacetic acid (204), ( )-l-ethoxy-3-fluoroalkyl-3-hydroxy-4-(4-methylphenylsulfinyl)but-1 -enes (205), enantiopure Y-oxygenated-a,P-unsaturated phenyl sulfones (206), chiral (a-oxyallyl)silanes (207), and (S )-but-3-ene-1,2-diol derivatives (208). As a chiral auxiliary, diisopropyl (i ,i )-tartrate (209, 210) has been very popular. 

A series of remarkable stoichiometric CH functionalization reactions have been employed, including transmetalla-tion with a vinylboronic acid moiety, to synthesize the core of teleocidin B-4 (Equation (200)).1... 

Alkylative aldol reactions with aryl- or vinylboron reagents are also catalyzed by rhodium complexes.404 4043 4041 Equations (54) and (55) show examples of enantioselective reactions. 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

Vinylboronates are generally less reactive than vinylzirconocenes towards various electrophiles and hence selective reactions of the latter should be possible. It was found that selective cleavage of the carbon—zirconium bond in 45 by N-halosuccinimides provides (a-haloalkenyl)boronic esters 53 in excellent chemical yields and with complete re-gioselectivity (Scheme 7.17) [54], An X-ray crystal structure determination of 45 confirmed the configuration of the four-coordinate Zr complex, with two cyclopentadienyl rings, Cl, and C(sp2) as the four ligands (Fig. 7.5) [54,126]. 

The Mannich reaction of secondary amines with paraformaldehyde and vinylboronic acids gives excellent yields of pure (E)-allylamines, e.g. equation 63181. 

Fig. 2.11 Hydrogenation of vinylboronates. Conditions (i) 3 mol%, R-BINAP, -20°C, CICh2CH2CI, 65%, 7 days, then HjOj, NaOH ...

Kabalka and co-workers reported the direct cross-coupling of cinnamyl alcohols with aryl- and vinylboronic acids using simple rhodium salts in an environmentally benign ionic liquid medium (Eq. 6) [30]. The ability to utilize allylic alcohols, without activation, is significant from the viewpoint of atom economy, yet challenging due to the poor leaving group ability of hydroxide. 

When vinylboronic acids are used as reactants, bases, especially Tl(OH)3, can accelerate the reaction.143 Scheme 8.11 gives a number of examples of coupling using organoborane reagents. 

The cyclopropanation of vinyl organometallic and heteroatom substituted vinylic compounds has also been reported using zinc carbenoids. Vinylboronates (equation 37) °, -alanes (equation 38) °, -zincs (equation 39) °, -stannanes (equation 40) ° , -phos-phonates (equation 41) °, -germanes (equation 42) °, and silanes (equation 43) °° "° could be readily converted into cyclopropane derivatives. 

The capture of the palladium complex formed in the intramolecular insertion of A-acryloyl-2-haloanilines has also been exploited by Grigg. The addition of aryl-, and vinylboron reagents to the mixture of N-(2 methylacryloyl)-2-iodoaniline and a palladium catalyst led, after the closure of the five membered ring, to the transfer of the organic moiety to the formal terminal carbon atom of the acryloyl chain (3.16.). The role of the substituent in the 2 -position is presumably to block / -hydride elimination and so prolong the lifetime of the palladium complex formed in the insertion step.21... 

The Heck reactions of thiophenes, particularly in the 2-position are well documented. In a recent example 2-bromothiophene was converted into a thienyl-vinylboronic acid derivative using a conventional palladium-triphenylphosphine catalyst and tributylamine as base (6.59.),... 

Vinylboronic acids have found use in a multicomponent one-step synthesis of a-amino acids (see question 24). The HOMO and LUMO of the parent compound, CH2=CHB(OH)2, from an SHMO calculation are... 

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