Boron-stabilized halogenation

The most widely used method for the synthesis of iminoboranes involves the 1,2-addition of boron-element bonds such as boron-hydrogen, boron-halogen, boron-carbon, or boron-sulfur bonds across the C=N bond of nitriles thereby producing variously substituted iminoboranes (Eq. (2)). The formation of iminoboranes as well as the stability of the products depends on the substituent on the nitrile group, the nature of the boron-element bond to be cleaved during the 1,2-addition across the C N bond, and to a lesser extent on the non-reacting boron substituents 26T... 

In the Suzuki strategy 138 was treated with trityl chloride to afford amine 139 in 84% yield (Scheme 4.29) (Todo et al., 2000). Metal-halogen exchange of 5-bromoisoindoline 139 was achieved using w-butyl lithium at — 65°C to deliver an intermediate organolithium species that was trapped with triisopropylborate to afford the boronic acid upon work-up. Further treatment of the boronic acid with diethanolamine gave diethanolamine boronic ester 140 (Hayashi et al., 2002). Presumably, the diethanolamine boronic ester was formed for stability reasons, as a given diethanolamine boronic ester tends to be less... 

The block copolymer of ethylene oxide and 3,3-dimethylthietane shows useful properties of complexing halogen and heavy metal salts (79MI51402). Thietanes can be polymerized with methylmagnesium iodide as well as with a variety of electrophiles such as methyl sulfate, trimethyloxonium tetrafluoroborate, triethylaluminum, boron trifluoride and phosphorus trifluoride (67IC1461, 67MI51400). Thietane (210) has been patented as a stabilizer for poly(vinyl chloride) (73USP3767615). 

The stability of the complexes also depends on the halogen ion in the boron compounds the fluorides are not stable, no compound being observed in the system BF3—PF3. The experimental data are too scarce to decide whether the bromides and iodides are more, or less, stable than the chlorides. 

Boron trihalide adducts of 4-methylpyridine show halogen exchange behavior similar to that of the adducts of trimethylamine 10), and isolation of individual mixed-halogen species is feasible here too. The stability of nitrogen-donor adducts of the mixed boron trihalides parallels the stability of nitrogen-donor adducts of BH2X and BHX2 which have been more extensively studied 19, 153,156). 

The requirement for an initial dissociation step implies that halogen redistribution rates should parallel the ease of dissociation of one of the halogens, and this expectation is confirmed in fluorine - heavier halogen redistributions where the rates change from slow to very fast over the series Cl, Br, I as the B—X bond becomes weaker. Rates should also be related to stabilization of the residual boron trihalide formed on dissociation. The boron trihalide with the greatest number... 

This chapter deals with the formation of new C—C bonds by alkylation of sp -hybridized carbanions stabilized by a range of heteroatoms, including oxygen, boron, silicon, phosphorus and the halogens. The properties of the same carbanions with additional stabilization provided by an adjacent double bond are also discussed. 

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