Halo-olefins addition

For olefins with Ji-substitucnts, whether electron-withdrawing or electron-donating, both the HOMO and LUMO have the higher coefficient 021 the carbon atom remote from the substituent. A predominance of tail addition is expected as a consequence. However, for non-conjugated substituents, or those with lone pairs (e.g. the halo-olefins), the HOMO and LUMO are polarized in opposite directions. This may result in head addition being preferred in the case of a nucleophilic radical interacting with such an olefin. Thus, the data for attack of alkyl and fluoroalkyl radicals on the fluoro-olefins (Table 1.2) have been rationalized in terms of FMO theory.16 Where the radical and olefin both have near neutral philicity, the situation is less clear.21... 

It is noteworthy that the use of these strong nucleophiles results in the addition-elimination route for the unactivated halo-olefins. 

The monosubstitution of vinyl bromide (Truce et al., 1956b) probably occurs via elimination-addition owing to the low reactivity of the halo-olefin. Acetylene is the only product formed with alkoxide ions or aniline. 

Olefins, addition—Continued of diazoacetic ester, 498 of halo ethers, 232 of halogen, 106 of hydrogen cyanide, 603 of hydrogen halide, 105 of hypohalous acid, 109 of polyhalides, 107 to cyanides, 571 allylic bromination, 36, 104 condensation, with halides, 108 with phenols, 179 conversion to amides (Wi 11 gerodt),... 

The methods described thus far represent only a small subset of the available methodologies for the formation of C-furanosides and C-pyranosides. Other methods shown to be useful include cyclizations of halo olefins and ene-ynes. As shown in Scheme 7.6.1, Lee, et al.,29 prepared halo olefins from suitable alcohols and acetylenic esters. Subsequent application of free radical conditions thus effected cyclizations. All reactions proceeded in yields exceeding 95%. Furthermore, where the formation of diastereomers was an issue, selective cis formation was observed. Thus the ease of preparation of the vinyl ether substrates required for these cyclization reactions makes this methodology an extremely useful addition to the technology surrounding the preparation of C-glycosides. 

TrialkyIboranes or alkyl 9-borabicyclo[3,3,1]nonanes, obtained by appropriate olefin addition reactions, may be used in Sjj2 displacement reactions upon a-halo esters, i r s 2 a-haloketones, or a-halonitriles to effect alkylation of these substrates in high yield. In many cases 2,6-di-t-butylphenoxide, a base of very high steric requirement, is essential. 

Carbamates. Lower alkyl A/-halo- and W,W-dihalocarbamates are distillable Hquids (70,112). A/-Halo-A/-metallocarbamates are crystalline hygroscopic soHds. A/-Chloro-A/-sodiourethane [17510-52-0] C2H OCONQNa, does not decompose on heating to 250°C (113), but violent decompositions have occurred at room temperature (114). A/-Halocarbamates react with a variety of organic substrates, eg, the free-radical addition of W-chlorourethane [16844-21 -6] C2H OCONHCl, and A,A-dichlorourethane [15698-16-5], C2H OCONCl2, to olefins provides a convenient route to... 

Ion 24 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a (3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 5-34). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HC1 to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical olefins, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 750). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2S04, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1-methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-l-methylcyclohexene.198... 

This method, which is applicable to mono-, di-, tri-, and tetraalkyl as well as phenyl-substituted olefins, gives almost complete Markovnikov addition. Hydroxy, methoxy, ace-toxy, halo, and other groups may be present in the substrate without, in general, causing difficulties.147 When two double bonds are present in the same molecule, the use of ultrasound allows oxymercuration of the less-substituted one without affecting the other.148... 

The groups R2N and Cl can be added directly to olefins, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-N-chloroamines and acids.663 These are free-radical additions, with initial attack by the R2NH + radical ion,664 N-Halo amides RCONHX add RCONH and X to double bonds under the influence of uv light or chromous chloride.665 For an indirect way of adding NH2 and I to a double bond, see 5-32. 

With some olefins, the initial p-halo nitroso compound is oxidized by the NOCI to a p-halo nitro compound.667 Many functional groups can be present without interference, e.g., COOH, COOR, CN, OR. The mechanism in most cases is probably simple electrophilic addition, and the addition is usually anti, though syn addition has been reported in some cases.668 Markovnikov s rule is followed, the positive NO going to the carbon that has more hydrogens. 

The same result can be achieved in one step with m-chloroperoxybenzoic acid and water.719 Overall anti addition can also be achieved by the method of Prevost. In this method the olefin is treated with iodine and silver benzoate in a 1 2 molar ratio. The initial addition is anti and results in a 3-halo benzoate (71). These can be isolated, and this represents a method of addition of IOCOPh. However, under the normal reaction conditions, the iodine is replaced by a second PhCOO group. This is a nucleophilic substitution reaction, and it operates by the neighboring-group mechanism (p. 308), so the groups are still anti ... 

Additions to functionalized three-carbon olefins have been studied extensively. We have used methyl acrylate as a standard olefin since it always reacts only at the terminal carbon and the a,/3-double bond in the product is always trans. The stereospecificity of its reactions with vinylic halides varies with structure. The simple 1-halo-l-alkenes with methyl acrylate under normal conditions give mixtures of E,Z- and E,E-dienoates. The reaction is more selective with the bromides than with the iodides and the stereoselectivity increases with increasing triphenylphosphine concentration. This occurs because the excess phosphine displaces the hydridopalladium halide group from the diene 7r-complex before readdition to form the ir-allylic species occurs (see Equation 6). The disubstituted vinylic bromides react stereospecifically with methyl acrylate (4). 

However, a respectable alternative to the direct [2 + l]-routes (a)-(c) is the variant using halo- or dihalocyclopropanes as precursors for the desired target molecules (path d). The cyclopropane ring is formed by addition of halocarbenes to the olefin and subsequent change of functionalities is achieved by treatment with nucleophiles. It is very unlikely that a direct substitution incorporates the donor-substituent. Instead an elimination/addition sequence with the intermediacy of a cyclopropene has to be assumed. 

March76 has provided a valuable summary of the work on additions of halogen azides to olefinic double bonds to provide / -halo azides (e.g. equation 52) the summary is based... 

In recent years, we have been investigating easy and economical functionalization of widely nsed carbon based polymers snch as polybntadienes. The preliminary results of these studies have led our group to discover a highly selective and mild synthetic route to silyl-functionalization of 1,2-polybutadienes (PBD) via Pt-nanocluster catalyzed hydrosilylation of olefin bonds. Unlike other catalytic systems, our system was found to be equally effective with all varieties of functional silanes such as halo-, alkyl-, aryl- and alkoxy- silanes affording high yields and selectivities. In addition, all the hydrosilylation reactions were found to be very clean with the ease of product separation and purifications (Scheme 2). 

In a similar manner, Grignard reagents react with cyclic a,/3-dihalo ethers derived from 3,4-dihydro-l,2-pyran and tetrahydrofuran to form the corresponding 2-alkyl-3-halo derivatives. Thus, addition of 2,3 dibromotetrahydropyran to methylmagnesium halide at 0° followed by hydrolysis gives a 65% yield of 2-methyl-3-bromotetrahydropyran. These materials are valuable intermediates in the synthesis of olefinic alcohols (cf. method 99). 

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