Boron halides, alkyl

Chemistry of Organic Fluorine Compounds II Table 27. Alkylation of Boron Halides... 

In 6.5.2.1 it is shown that BF3 and BCI3 add to phosphine Pt complexes without cleaving the B—X bond. In contrast, aryl and alkyl boron halides are added oxidatively with phosphine Pt complexes - ... 

The organic analogues of the reactions to be discussed here are the borane reductions of aldehydes and ketones and the addition of metal alkyls across ketonic carbonyls, equation 15. In contrast to the ease of these organic reactions, qualitative data which has accumulated in our laboratory over the last decade demonstrates that the carbonyl group in organometallies is fairly resistant to addition across CO. For example, many stable adducts of organometallie carbonyls with aluminum alkyls are known, eq. lc, but under similar conditions a ketone will quickly react by addition of the aluminum alkyl across the CO bond. A similar reactivity pattern is seen with boron halides. 

The boron trifluoride alkyl fluoride (chloride) complexes gave no evidence of alkylcarbonium ion formation. It must, however, be emphasized that (a) the physical investigation of the binary system was carried out at such low temperatures (generally below —100°) that ionization of the halides could hardly be expected (with exception of highly reactive tertiary halides) (b) the methods used could not he relied on to detect a small ionization equilibrium even if it existed. 

Type I (fast homodimerization) Terminal olefins, allylsilanes" Terminal olefins, allylsilanes," 1° allylic alcohols, ethers, and esters, " allyl boronate esters, allyl halides, alkyl-substituted allenes Terminal olefms, allyl boronate esters, 1° allylic alcohols, ethers, and esters,styrenes (no large ortho substit.), " " allyl allylsilanes, allyl sulfides, allyl phosphonates, " allyl phosphine oxides, protected allylamines ... 

Linear polyethylene can further also be considered as polymethylene. Althoguh first prepared by the thermal decomposition of diazomethane,243 244 Meerwein should be credited to have prepared it by catalytic polymerization of diazomethane effected by boron compounds (esters, halides, alkyls)245-247 taking place with concomitant dediazotation ... 

The mechanisms of these exchange reactions as well as of the related exchanges with boron halides, tris(dialkylamino)boranes, and boron alkyls has been interpreted to proceed through a four-center transition state (255, 258, 260). 

Some Lewis acid-initiated polymerizations have been proposed to proceed by direct addition of the Lewis acid to the monomer s double bond. However, this is usually an exception, and has been clearly proven only for iodine [69,135] and boron halide [136,137] initiated systems. Iodi-nation and haloboration are reversible processes which produce deactivated alkyl halides due to the electron-withdrawing substituent at the neighboring carbon [Eq. (30)]. 

Initiation efficiency can also be improved by using an initiator with a leaving group better than the one in the growing species. This is the case for alkyl acetates used as initiators with boron halides as Lewis acids as activators [35,117]. The initiating acetates are ionized stronger and faster than macromolecular alkyl halides (fc,° > k"), resulting in an enhancement of the relative initiation rate ... 

Alkaline hydrogen peroxide easily oxidizes practically all alkyl- and cycloalkyl-boranes in a rapid and quantitative fashion. There is a reactivity trend of R3B > R2BX > RBX2 (X = halogen, OH, OR note that boron halides will anyway be hydrolyzed to hydroxides under the oxidation conditions), which is consistent with reduced acceptor ability of the boron atom when an electron pair of an adjacent group interacts with the vacant boron oibital. Increasing die steric hindrance around the boron atom may inhibit the reaction to the point at which it ceases altogether. - ... 

Perfluoroalkyllithium reagents react with electrophiles such as aldehydes, ketones, silicon and tin halides, alkyl halides, carbonates, esters, boron trifluoride-imine complexes, and sulfur dioxide. ... 

Exchange reactions are common between boron halides and alkyl or aryl snbstitnents, other halides, alkoxy and aryloxy gronps, and diborane. Some examples are shown in Scheme 2. 

Cleavage of a carbon-fluorine bond can be induced by reaction with a Lewis acid (see Chapter 4, Section VIC). In Friedel-Crafts alkylations, aUcyl fluorides are more reactive than the chlorides [37] with, for example, aluminium halides or boron halides as catalysts (Figure 5.17). 

Alkyl borates, boron alkyls, and boron halides have all been used to polymerize diazoalkanes. Several authors have claimed that macrozwitterions are formed56 58. ... 

One practical disadvantage of these systems lies in the fact that relatively high concentrations of Lewis acids are needed to aehieve alternation. Thus methyl methaerylate and styrene alternate perfeetly with azodiisobutyronitrile initiation at 50°C when the molar ratio of ZnCL to methyl methacrylate is 0.4. Alternation is less exact, however, when this ratio is 0.25. Alkyl boron halides like ethyl boron diehloride are effective at lower concentrations and act also as initiators if oxygen is present. 

Aluminum and boron halides are sometimes used to dealkylate alkyl aryl ethers to phenols. Boron tribromide cleaves aliphatic ethers to alcohols and alkyl halides, but the reaction has no preparative value in the aliphatic series. Aluminum halide and the ether first form a complex from which a molecule of alkyl halide is eliminated upon heating. 

Boron halides can also act as halide ion acceptors when they serve as catalysts— for example, in the Friedel-Crafts alkylation of aromatic hydrocarbons ... 

Olah and Kuhn S found that fluorochloro-, fluorobromo- and fluoroiodo-alkanes are effective chloro-, bromo- and iodo-aikylating agents, respectively, in Friedel-Crafts alkylations of arenes in the presence of boron halide catalysts (equation 86). Boron trihalides catalyzq reactions of only the C—F bonds, but not of the C—Cl, C—Br or C—I bonds. The order of reactivity of the catalysts was found to be BI3 > BBr3 > BCI3 > BF3, and that of the carbon-halogen bonds C—F > C—Cl > C—Br > C—I. 

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