Alkenes boron halides

Boron halides react with norbornadienes to produce 1,3-insertion products. As with insertion into simple alkenes, such reactions need forcing conditions that result in elimination and reduce the potential of the insertion reaction... 

Lewis acids have long been used in both polymerizations and copolymerizations to enhance the reactivities of monomers. The addition of ZnC, alkyl aluminum compounds, or boron halides has been shown to increase both the rate and degree of polymerization of monomers such as acrylonitrile and methyl methacrylate [8]. The use of Lewis acids to enhance electrophilicity of acrylate monomers has also been exploited to enhance alternation in copolymerizations with electron-rich alkenes such as isobutylene [9], Systems that would never produce alternating copolymers can be induced to do so with as little as 0.1 equivalents of an appropriate Lewis acid. This section focusses on efforts to utilize Lewis acids to both alter reactivity and control stereochemistry in copolymerizations. 

Treatment with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid. This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles. The R group does not rearrange, and this reaction is a step in the hydro-boration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving a rearrangement from boron to oxygenr ... 

A similar type of catalyst including a supported noble metal for regeneration was described extensively in a series of patents assigned to UOP (209-214). The catalysts were prepared by the sublimation of metal halides, especially aluminum chloride and boron trifluoride, onto an alumina carrier modified with alkali or rare earth-alkali metal ions. The noble metal was preferably deposited in an eggshell concentration profile. An earlier patent assigned to Texaco (215) describes the use of chlorinated alumina in the isobutane alkylation with higher alkenes, especially hexenes. TMPs were supposed to form via self-alkylation. Fluorinated alumina and silica samples were also tested in isobutane alkylation,... 

As an example of the selective reactivity of borazirconocene alkenes, their hydrolysis was examined [1]. The carbon—zirconium bond is more reactive than the carbon—boron bond towards various electrophiles, and so hydrolysis can be expected to occur with preferential cleavage of the former bond. Since hydrolysis of alkenylzirconocenes is known to proceed with retention of configuration [4,127—129], a direct utility of 45 is the preparation of (Z)-1-alkenylboronates 57 (Scheme 7.17) [12]. Though the gem-dimetalloalkenes can be isolated, in the present case it is not necessary. The desired (Z)-l-alkenylboronates can be obtained in a one-pot procedure by hydrozirconation followed by hydrolysis with excess H20. The reaction sequence is operationally simple and is compatible with various functional groups such as halides, acetals, silanes, and silyloxy protecting groups [12]. 

Organoborane intermediates can also be used to synthesize alkyl halides. Replacement of boron by iodine is rapid in the presence of base.150 The best yields are obtained with sodium methoxide in methanol.151 If less basic conditions are desirable, the use of iodine monochloride and sodium acetate gives good yields.152 As is the case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is opposite to that observed for direct ionic addition of hydrogen halides to alkenes. Terminal alkenes give primary halides. 

The metal halide thus functions in similar manner to the proton and may be considered to be an acidic catalyst (cf. Luder and Zuffanti, 19). The catalyst-olefin complex differs in one significant respect from the product formed by the addition of the proton (or the corresponding acid) to the olefin the halide catalyst is a neutral but electronically deficient molecule and combines with the pi electrons of the double bond to form a coordinate bond between the carbon atom and the aluminum or boron. On the other hand, the addition of the positive proton to the double bond results in the formation of a true (covalent) link between carbon and hydrogen. In other words, the complex, while it contains an electron-deficient (hence, positive) carbon atom, is in itself electronically neutral the product of the addition of a proton to the alkene contains a similar carbon atom but is itself electrically positive. It has been suggested (Whitmore and Meunier, 20) that this difference is related to the fact that metal halide catalysts tend to yield much higher polymers than do the acid (proton) catalysts. 

Halo boronic esters have long been known to undergo anti elimination of boron and halide to form alkenes on treatment with bases as mild as water13, l4. Thus, synthetic schemes involving bases in the presence of boronic esters must avoid any structure incorporating a /(-halogen. 

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

Other catalysts which may be used in the Friedel-Crafts alkylation reaction include iron(m) chloride, antimony pentachloride, zirconium tetrachloride, boron trifluoride, zinc chloride and hydrogen fluoride but these are generally not so effective in small-scale preparations. The alkylating agents include alkyl halides, alcohols and alkenes. 

Trialkylboranes, including 9-alkyl-9-BBN, underwent cross-coupling with 1-alkenyl or aryl halides or triflates.750,751 The reaction was limitedly used for primary alkylboranes thus, hydroboration of terminal alkenes with 9-BBN was the most convenient to furnish the desired boron reagents in the presence of a base and PdCl2(dppf)750,751 or PdCl2(dppf)/2Ph3As (Equation (200)).752... 

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