Boron-Wittig reaction

The acidity of such R2B—CH2—R systems has been utilized as the basis for developing the so-called boron-Wittig reaction for organic synthesis.52-54 One of the distinct advantages of such reagents, the direct conversion of an aliphatic aldehyde to a ketone, is illustrated in Eq. (19).55... 

Boron-Wittig reaction (12, 12-13). The direct reaction of the anion of an alkyldimesitylborane at -78° with an aromatic aldehyde followed by oxidation results in an (E)-alkene in low yield. The intermediate adduct can be isolated in about 80% yield as the silyl ether of a iyn-l,2-diol by addition of CISi(CH,), to the reaction, and this product on desilylation (HF, CHjCN) affords (E)-alkenes with high selectivity. Somewhat lower (E)-selectivity obtains in a one-pot reaction. In contrast, addition of trifluoroacetic anhydride (slight excess) to the reaction at -78° to -110° results in a (Z)-alkene with almost comparable selectivity (Z/E 9 1). 

The boron-Wittig reaction has been carried out by bis(boryl)methyllithium 309 generated in situ from tris(boryl)-methane 308 (Equation (91 )).469 The geminal dichromium reagent 3 12470 was found to be an excellent alternative yielding 1-alkenylboronic esters 313470-472 with high //w/ -selectivity (Equation (92)). 

Boron-Wittig reactions." The reaction of anions of Mes,BR with nonenolizable aldehydes or ketones results in an adduct that spontaneously eliminates MesiBOLi to give an alkene in good yield. 

Scheme 6 Stereoselective alkene formation using the boron Wittig reaction...

Anion (51) protonates and methylates at the a-position but reacts with TMS-Cl and acetone (equation 49) specifically at the -Y-position. No product derived from a boron-Wittig reaction was noted in the reaction with acetone, which upon work-up with propionic acid gave the corresponding alkene (52). ... 

For the preparation of 1-alkenylboronates from ketones or aldehydes, the boron-Wittig reaction is the method of choice when the the starting carbonyl compounds are readily available. An efficient route to (E)-1 -alkenylboronates [39] from carbonyl compounds is achieved by the reaction with lithio(boryl)methanes. The (E)/ Z) isomeric ratio is reported to be 20/1 (eq (26)). Lithio(triboryl)methane with aldehydes or ketones yields diborylalkenes [40] (eq (27)). 

A new synthetic approach to polycyclic aromatic compounds has been developed based on double Suzuki coupling of polycyclic aromatic hydrocarbon bis(boronic acid) derivatives with o-bromoaryl aldehydes to furnish aryl dialdehydes. These are then converted to larger polycyclic aromatic ring systems by either (a) conversion to diolefins by Wittig reaction followed by photocyclization, or (b) reductive cyclization with trifluoromethanesulfonic acid and 1,3-propanediol (Eq. (12)) [30]. 

The G-B bond of boronic esters is inert for functionalization of remote sites other than the B-G bond by metal-catalyzed reactions (Equations (98)-(100)). Gross-coupling reactions of 320478 and 321,479 and titanium-Wittig reaction on polymer resin 323480 have been studied. 

The dialkoxyboiyl-substituted Wittig reagent (45) also loses boron upon reaction with benzophenone to yield another l ttig reagent that has been used in the synthesis of allenes (equation 48). ... 

Four-membered heterocyclic systems have been postulated to be involved as intermediates along the reaction pathway in a number of systems allowing olefin synthesis. This is the case of the Wittig reaction (phosphorus) and a family of related reactions the boron-Wittig (boron), the Peterson (silicon) 44 and the Peterson-type reactions (germanium, tin, lead). 5,l46 11 these systems,... 

Finally, boron has now joined the group of elements which stabilize anions capable of undergoing the Wittig reaction. ... 

Fluorous DEAD was prepared and used in a Mitsunobu reaction.Fluorous triaryl phosphine was used for a Wittig reaction.Fluorous boronates were prepared and used in functional transformations and a Suzuki-Miyaura coupling reaction. 

One of the serious drawbacks of the Wittig reaction is the unavoidable production of triphenylphosphane oxide in stoichiometric quantities. Whilst its direct reduction with boron, aluminium or silicon hydrides would be possible, these reagents are too expensive for a viable process. In fact, distilled triphenylphosphane oxide is reacted with phosgene, generated in situ, to give the corresponding dichloride, which is then reduced with metals, like aluminium. [55]... 

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