2.6- dichloro-, halogen exchange

Bayer and Siebert reported the reaction of 3,3-dimethyl-l-butyne with -BuLi in pentane, followed by the addition of boron trichloride to afford the intermediate dichloro(3,3-dimethylbut-l-ynyl)borane that was hydroborated with dichloroborane formed in situ to give l,l-bis(dichloroboryl)-3,3-dimethylbutene 51 (Scheme 6) <2002ZN1125>. When the latter was subjected to halogen exchange with boron triiodide, it was converted into the corresponding tetraiodide 52, which underwent a redox reaction with hex-3-yne to give the 2-(2,2-dimethylpropylidene)-l,3-diborole 22 in 73% yield. 

Lithium 4,4 -di-t-butylbiphenylide (LDTBB). 13, 162-163 16, 195-196 17, 164 Lithium-halogen exchange. The reagent has been proven particularly effective for chloro compounds, as shown in the Barbier-type reactions involving chloromethyl ethyl ether, l,4-dichloro-2-butyne and -2-butenes. Vinylic chlorides also readily undergo exchange, and actually 2,3-dichloropropene is converted to the dilithio reagent. ... 

G. L. Johnson, J. C. Bailar, Jr., and R. H. Herber, Kinetics of the Isotopic Halogen Exchange Between trans-Dichloro-bis-ethylenediamine-platinum(IV) Ion and Aqueous Chloride, J. Inorg. Nuclear Chem. 26 1061 (1964). 

The hitherto unknown iodocyclopropenes (160) and (161) can now be obtained in high yield by halogen exchange with 1,2-dichloro- and l,2-dibromo-3,3-difluoro-cyclopropene. ... 

When 1,1-dichloro- and 1,1-dibromoalkenes with different /1-substituents (R 7 R ) are submitted to a halogen-lithium exchange, the problem of stereoselectivity arises. The substituents R and R may differ in their steric demand and in their ability to chelate the lithium atom. For the former case, there is a tendency in the sense that the bulky group seems to favor lithiation in the Z position - - , as illustrated by equation 26 . 

The halogen-fluorine exchange with an HF-amine complex allows the transformation of a gem-dichloro goup into a difluoro group. While this reaction is difficult to initiate in nonactivated positions, it is efficient in the benzylic position and in the a position of functional groups (e.g., esters, phosphonates) (Figure 2.10). ... 

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