Further Reactions with Olefins

The preceding discussions have clearly shown that type II photosensitized oxygenation of olefins is a powerful tool for the preparation of allyl hydroperoxides and allyl alcohols. The introduction of a hydroxyl group into an olefin occurs under mild conditions and can even be achieved at very low temperatures if the instability of the starting material or the expected allyl hydroperoxides requires such conditions. In many cases a stereoselective introduction of a hydroperoxy group occurs which makes this method even more valuable, especially since hydroperoxides can easily be reduced with retention of configuration. 

The method of photosensitized oxygenation was successfully applied in the preparation of alcohols 265-270 from sylvestrene (264),207 and seems to be the most simple and successful method for the preparation of optically active rose oxides (272,273) from (+)- or (—)-citronellol C271).177 It may also be used for the preparation of certain organo-metallic hydroperoxides. Thus, the triphenyl-tin derivative of tri-methylethylene (274) undergoes a photosensitized oxygenation reaction with a rate similar to that of tetramethylethylene, giving rise to the hydroperoxides 275 and 276 219 

After-reactions such as Hock-Cleavage or dehydration may become predominant at room or higher temperatures. While, for example, hydroperoxide 278 is cleaved to two molecules of cyclohexanone (280) only in the presence of acid,206 photosensitized oxygenation of allo-ocimene (281),220 homoverbenene (286),198 sorbic acid methyl ester 

Chlorine eg Hematoporphyrin Methylene Blue Rose Bengal Erythrosin B Sulforhodamin B Acridine Orange Eosin Y Fluorescein Riboflavin 

Dehydration products have been observed from photosensitized oxygenation reactions with cycloheptatriene (304),220 AMialin (307) 222 3/1- and 3a-hydroxy-A4-cholestenes (312a and 312b, respectively),226,227 and 2-hydroxy-10-methyl-1,9-octalin (316).226 

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Fluonnated ylides have also been prepared in such a way that fluonne is incorporated at the carhon P to the carbamonic carbon Vanous fluoroalkyl iodides were heated with tnphenylphosphine in the absence of solvent to form the necessary phosphonium salts Direct deprotonation with butyUithium or hthium dusopropy-lamide did not lead to yhde formation, rather, deprotonation was accomparued by loss of fluonde ion Flowever deprotonation with hydrated potassium carbonate in thoxane was successful and resulted in fluoroolefin yields of45-S0% [59] (equation 54) P-Fluorinated ylides may also be prepared by the reaction of an isopropyli-denetnphenylphosphine yhde with a perfluoroalkanoyl anhydnde The intermediate acyl phosphonium salt can undergo further reaction with methylene tnphenylphosphorane and phenyUithium to form a new yhde, which can then be used in a Wittig olefination procedure [60] (equation 55) or can react with a nucleophile [6/j such as an acetyhde to form a fluonnated enyne [62] (equation 56)... 

Some typical examples of this useful transformation are shown in Table 3 [63], These olefinic products can be transformed into l-aryl-3,3,3-trifluoropropynes via further reaction with sodium rerf-butoxide [64],... 

For straight chain and cycloalkanes, RoCek et al. prefer a mechanism involving hydride ion abstraction to give a partly-developed carbonium ion which suffers further reaction with the Cr(IV) portion before it can become free to give acetate or olefin... 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

Thus, reactions affording either cyclopropanes or propylenes would most likely represent forms of termination of metathesis activity. As a corollary, any catalytic conversion of cyclopropanes to metathesis olefins via Eq. (26) would seem to require decomposition of the metal-carbene species in order to regenerate a naked metal species (M ) capable of further reactions with cyclopropanes. Of course, bimolecular carbene decomposition to yield an olefin as in Eq. (11) (e.g., ethylene from 2M=CH,) is one accepted process which could account for regeneration of M ... 

Triazoline-3,5-dione 184 underwent an ene reaction with olefins 183 to yield trialkylated allylic urazoles 185, which were further elaborated into allylic amines 186 . DBU has been found to be a mild and convenient base for the alkylation of 1,2,4-triazole with various alkyl halides in the high yielding syntheses of 1-substituted-1,2,4-triazoles <00TL1297>. 

The scheme of reactions proposed to explain the products obtained is shown, after small modifications, in Scheme 8. Primary radicals 12 formed at the anodes produce with added 30 or 36 (equation lOe) the substituted benzyl or allyl radicals 38, which can dimerize to 39 or can couple with the added olefin to form radicals 40 or 41. For allyl radical (38) a 1,1 - or l,3 -coupling is possible yielding 41 and 40, respectively. Further couplings of 40 and 41 with the primary radical 12 produce 39 and head-to-tail dimer 42, respectively. It was evident from the products obtained that the coupling of 38 in the 1-position occurs 5 to 11 times faster than in the 3-position. However, for readily polymerizable olefins, rather polymerization occurs, in particular at graphite electrodes. At Pt electrodes both dimers 39 and 42 are formed, but for Cu electrodes exclusively dimers 39 were obtained with moderate yields. Thus, an indirect electrolysis including the oxidation of copper to Cu+ ions and their further reaction with 5 yielding intermediate RCu was considered, but not proved . ... 

A different experimental approach to the study of chlorine atom reactions with olefins will be mentioned briefly. Wijnen(l06) has studied the photolysis of phosgene as a source of chlorine atoms in the presence of ethylene, and Guercione and Wijnen (49) have carried out similar experiments with propylene. The features of these processes are quite different from those encountered in photochlorination in the presence of molecular chlorine, since the chain propagating reaction (3) cannot occur. Although in the photolysis of phosgene Cl and COC1 are initially formed, it appears that all COC1 radicals further decompose into CO and Cl. 

The protected methyl glycoside 3 is converted to the corresponding aldehyde by Swern oxidation using oxalyl chloride activated DMSO. Further reaction with triethyl phosphonoacetate and sodium hydride -known as the Horner-Wadsworth-Emmons reaction - provides selectively the trans et /Tun saturated ester 4 in 72 % yield. This valuable alternative to the Wittig olefination protocol uses phosphonate esters as substrates which are readily available from alkyl halides and trialkyl phosphites via the Arbuzov rearrangement.9 co2Et Reaction of the phosphonate with a suitable base gives the... 

The mechanism indicates that intermediate formation of a carbene, but tests to confirm the presence of the carbene by possible addition reactions with olefins were negative. The formation of both cyclohexene and cyclohexene are depicted as having the carbene as a common precursor. The experimental data do not, however, rule out the formation of these molecules from diffei ent precursors. For example, cyclohexend could be formed directly from the excited diazirine molecule. Again, further consideration of this point will be deferred until later. 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

Phenylthiocarbene, generated by phase-transfer catalysis from (phenylthio)chloromethane adds stereospecifically to ( )-1,2-diphenylethene to provide, 2-trans-2,3-cis-, 2-diphenyl-3-(phenylthio)cyclopropane1. The simple diastereoselectivity of reactions with olefins such as cyclohexene and phenylethene is moderate to good, favoring the endo- or cis-products. For further examples of sulfur-substituted carbenes, see Houben-Weyl Vol. E 19b, pp 1682-1745. 

Adams et al reported that the activation of Fe-Re complex 14 by Me3NO followed by the addition of Me02GG=CG02Me afforded the (Z)-dimetalated olefin complex 15 in good yield. Further reaction with Me02GG=GG02Me in heptane at reflux gave the metallacyclic complex 16, in which the two alkynes coupled to each other and to the Re atom to form a metallacycle that is 7r-coordinated to the iron center. 

There are several processes for producing the olefines or chloroparaffins required for the further reaction with benzene from these n-paraffins. The classical process is the clorination of paraffins to give chloroparaffins and the further reaction with AICI3 according to Friedel-Crafts [9]. 

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