Addition reactions to olefin

The photo-initiated addition process appears to have general applicability, although it can require extensive photolysis times [194-196]. Indeed, photolytic generation of RF- from RFI has been the method used to add Rf- to C60 and C70, not for synthetic purpose, but to examine epr spectra of the resulting radical species [197-199]. A good comprehensive review of the early work on thermal and photochemically-induced free radical addition reactions to olefins can be found in Sosnovsky s book [60]. 

In addition reactions to olefinic double bonds, two new sp3-hybridized C atoms are produced. Each of them can be a stereogenic center (stereocenter) and obviously is one if it possesses four different substituents. If stereocenters are produced, their configuration must be specified. Stated more accurately, the first question is which absolute configuration is produced at any new stereocenter. Then the question arises about the... 

Generation of cation radicals of enamines and their addition reactions to olefins were investigated by the use of metallic oxidants as shown in Scheme 4. The reaction of an enamine 6 and a-f-butyldimethylsiloxystyrene (4a) proceeded by oxidation with CAN, giving the addition product 7 in 63% yield. ... 

The direct photolysis and benzophenone-sensitized decomposition of methyl diazomalonate has been shown to produce two different spin states of biscarbo-methoxycarbene. In the direct photolysis experiments stereospecific addition to olefins and a high yield of insertion products in the presence of alkanes demonstrate the singlet character of the carbene in the sensitized decomposition the yield of insertion products decreases, and addition to cis- or ra 5 -4-methyl-2-pentene yields about 88 % trnwj-cyclopropane. In this case, as with most carboalkoxy-carbenes, the addition reaction to olefins can proceed by two paths ... 

Addition reactions to olefins can be used both for the construction and for the functionalization of molecules. Accordingly, chiral catalysts have been developed for many different types of reactions, often with very high enantioselectiv-ity. Unfortunately, most either have a narrow synthetic scope or are not yet developed for immediate industrial application due to insufficient activities and/ or productivities. These reactions include hydrocarbonylation [Ilf], hydrosilyla-tion [12 i], hydroboration [12j], hydrocyanation [12 k], Michael addition [11 g, 121, 12 m], Diels-Alder reaction [11 h, 12n] and the insertion of carbenes in C-H bonds [Hi, 12p, 12q, 38], Cyclopropanation [Hi, 12p, 12q] and the isomerization of allylamines [12 s] are already used commercially for the manufacture of Cilastatin (one of the first industrial processes) [12 r], and citronellol and menthol (presently the second largest enantioselective process) [12t] respectively. 

The catalytic activation (CuCl/bipyridine) of a,a,-y-trichlorinated-7-lactams has provided a simple way to introduce alkyl groups at the a-position of the lactam by the intermolecular and intramolecular addition reaction to olefins. Thus, a sequential reaction from 7V-allyltrichloroa-cetamides (e.g., 27), as depicted in Scheme 25.13, provides access to potential building-blocks (e.g., 28) for alkaloid synthesis. ... 

Thus in the dimerization reactions of allenes, or in their cyclo-addition reactions to olefins, non-concerted pathways appear to be favoured. The allene dimers are always of the head-to-head, tail-to-taU, type which suggests that 1,4-diradical intermediates are formed in these reactions. The stereospecific addition that may attend these and related reactions may merely be an indication of the lack of conformational flexibility of the diradical intermediates because of steric crowding. 

Sulfur-containing carbenes are known and review articles have been pubhshed. - The diazo precursors are prepared either by the action of weak bases on the appropriate N-nitrosocompound or by the use of diazo transfer reactions.. izs The sulfonylcarbene 241 undergoes an addition reaction to olefins and acetylenes (Scheme 46) to give 242 and 243, respectively. 

Protonated /V-chloroalkyl amines under the influence of heat or uv light rearrange to piperidines or pyrroHdines (Hofmann-Lriffler reaction) (88). The free-radical addition of alkyl and dialkyl-/V-chloramines to olefins and acetylenes yields P-chloroalkji-, P-chloroalkenyl-, and 8-chloroalkenylamines (89). Various N-hiomo- and N-chloropolyfluoroaLkylarnines have been synthesized whose addition products to olefinic double bonds can be photolyzed to fluoroazaalkenes (90). 

These singlet and triplet state species exhibit the important differences in chemical behavior to be expected. The former species, with their analogy to carbonium ions, are powerful electrophiles and the relative rates of their reaction with a series of substrates increases with the availability of electrons at the reaction center their addition reactions with olefins are stereospecific. Triplet state species are expected to show the characteristics of radicals i.e., the relative rates of additions to olefins do not follow the same pattern as those of electrophilic species and the additions are not stereospecific. 

Since electron-donating substituents at the phosphorus atom favor addition reactions over olefination reactions, addition of 9 to aldehydes leads to the exclusive formation of the silyl-pro-tected allylic alcohols 10. No reaction products arising from Wittig alkenylation could be detected. The ylides (R,S)-9 and (S.S)-9 and their enantiomers were prepared from the corresponding optically pure l-[2-(diphenylphosphino)ferrocenyl]-A,A -dimethylethanamine diastereomers 7 via the phosphonium salts 8. 

This and other work indicates that HC1 is largely undissociated in nitromethane for [HC1]>- 0.015 M and that there is little association either. There is evidence that a corresponding addition occurs to olefins in theimally degraded PVC. Results carried out in a variety of solvents (26) are consistent with elimination of HC1 occurring by a/3 -elimination of the Ex type favored by polar solvents. The same authors showed that at least in nitrobenzene containing dissolved HC1, the reverse reaction, i.e. addition of HC1, takes place. The fact that this may be interpreted as a retardation of the degradation process may have contributed to the confusion which has arisen and emphasizes the care which must be taken to disentangle the possible catalytic effect of HC1 when concurrent addition of HC1 to the polyenes is possible. 

In general, transition metal-catalyzed addition reactions to 1,3-dienes gave 1,4-adducts via 7t-allyl metal intermediates.23 The ar //-Markovnikov 1,2-addition mode of this reaction is therefore unusual (Scheme 17). It was noted that the configuration of the 3-olefin was retained with either ( )- or (Z)-1,3-dienes. The observation that the 3-olefin was unimportant for this reaction strongly suggests that the method could be applicable to unactivated alkenes. 

As illustrated in Section 2.3, enantioface differentiation occurs in addition reactions to hetero double bonds (C = X, with X = O, NR, etc.) and olefinic double bonds in achiral substrates. Thus, the enantioface differentiation can only arise from the reagent, either stoichiometrically (eq. I)18 or catalytically, e.g., from an enzymatic partner (eq. 2)19. 

In this case the absolute configuration at (at least) one of the chiral units needs to be determined. This situation is encountered, for example, in enantioselective addition reactions of olefins with known configuration when two stereogenic centers are created and the olefin geometry is translated to the product in a predictable way. For example, the enantioselective vicinal hydroxylation of (ZT)-l,2-disubstituted olefins97 98, such as tert-butyl (ZT)-3-(methoxycar-bonyl)-2-propenylcarbamate (for assignment, see p 439). 

Substituted cyclopropylidenes have been shown to participate in both inter-and intramolecular addition reactions with olefins. The resulting products are spiropentane derivatives as well as carbene dimers which are formed as side-products [99, 100]. In the absence of olefinic reaction products the latter may even become the main products [99 b],... 

Magnesium, 235 Samarium(II) iodide, 270 Titanium(IV) chloride, 304 Addition reactions to carbonyl groups—Addition of functionalized CARBON NUCLEOPHILES (see also Aldol reaction and other specific condensation reactions, Meth-ylenation, Peterson Olefination, Refor-matsky reaction, Wittig reaction, Wittig-Horner reaction)... 

New chiral centers are produced by addition reactions to other trigonal centers as well. Hydrogenation of 3-methyl-3-hexene gives 3-methylhexane. Clearly the addition of hydrogen to one face of the planar olefinic system gives one enantiomer and addition to the opposite face gives the opposite enantiomer. Likewise reaction of styrene with chlorine or bromine (X2) or potassium permanganate produces products with a new chiral center. Formation of the two possible enantiomers results from addition to either face of the olefin. 

The first issue confronted by Myers was preparation of homochiral epoxide 7, the key intermediate needed for his intended nucleophilic addition reaction to enone 6. Its synthesis began with the addition of lithium trimethylsilylacetylide to (R)-glyceraldehyde acetonide (Scheme 8.6).8 This afforded a mixture of propargylic alcohols that underwent oxidation to alkynone 10 with pyridinium dichromate (PDC). A Wittig reaction next ensued to complete installation of the enediyne unit within 11. A 3 1 level of selectivity was observed in favour of the desired olefin isomer. After selective desilylation of the more labile trimethylsilyl group from the product mixture, deacetalation with IN HC1 in tetrahydrofuran (THF) enabled both alkene components to be separated, and compound 12 isolated pure. 

The organopalladium addition reactions to produce substituted olefinic compounds are very useful laboratory syntheses since a wide variety of substituents and functional groups can be present in both the organopalladium species and the olefin. The only groups which may inhibit the reaction to some extent are ones which form stable complexes with the palladium salt, such as unhindered amines. Lower yields are generally... 

Fortunately, most of the palladium addition reactions with olefins can be carried out catalytically in the palladium compound so that large amounts of the expensive palladium compounds are not needed. As in the inorganic palladium salt additions, cupric chloride is a useful reoxidant. This, of course, limits the catalytic reaction to cases where olefin isomerization is not a problem. The cupric chloride is reduced to cuprous chloride during the reaction. As in the acetaldehyde synthesis, the reaction may be made catalytic in copper as well as palladium by adding oxygen and, in this case, hydrogen chloride also. 

Another one-step addition reaction to C=C double bonds that forms three-membered rings is the epoxidation of alkenes with percarboxylic acids (Figure 3.19). Most often, meta-chloroperbenzoic acid (MCPBA) is used for epoxidations. Magnesium monoperoxyphthalate (MMPP) has become an alternative. Imidopercarboxylic acids are used to epoxidize olefins as well. Their use (for this purpose) is mandatory when the substrate contains a ketonic C=0 double bond in addition to the C=C double bond. In compounds of this type, percarboxylic acids preferentially cause a Baeyer-Villiger oxidation of the ketone (see Section 14.4.2), whereas imidopercarboxylic acids selectively effect epoxidations (for an example see Figure 14.35). 

Stereochemical aspects are therefore an important part of the chemistry of addition reactions to the olefinic double bond. We will therefore investigate them in detail in this chapter. In fact, the content of Chapter 3 is arranged according to the stereochemical characteristics of the (addition) reactions. 

A third one-step addition reaction to C=C double bonds that forms three-membered rings is the epoxidation of olefins with percarboxylic acids (Figure 3.14). Suitable percarboxylic acids must, however, not be (too) explosive. Thus, aromatic percarboxylic adds are preferable. Until recently one epoxidized almost exclusively with mefa-chloroper-benzoic acid (MCPBA). An alternative has become magnesium monoperoxyphthalate (MMPP). In the transition state of this type of epoxidation, four electron pairs are shifted simultaneously (which is a record in this book except for the Corey-Winter elimination in Figure 4.42). 

Addition reactions to unsym metric olefins in which a reagent with the structure H—X transfers the H atom to the less-substituted C atom and the X group to the more-substituted C atom give, according to an old nomenclature, a so-called Markovnikov addition product On the other hand, addition reactions of reagents H—X in which the H atom is transferred to the more substituted C atom of an asymmetrically substituted C=C double bond and the X group is transferred to the less substituted C atom lead to a so-called anti-Markovnikov addition product. 

The most important catalytic asymmetric syntheses include addition reactions to C=C double bonds. The best known of them are the Sharpless epoxidations. Sharpless epoxidations cannot be carried out on all olefins but only on primary or secondary allyl alcohols. Nonetheless, nowadays they represent the most frequently used synthetic access to enantiomerically pure target molecules. 

Behavior of Carbethoxynitrene toward Polyisoprene Structures. After studying the reaction of a number of divalent carbon derivatives with polyenes (17, 18), attention was directed to monovalent nitrogen derivatives. Of these, carbethoxynitrene, N—C02Et, gives addition reactions with olefins (12, 13). It can be produced either by photolysis of ethyl azidoformate (12), or by a-elimination (13) (Figure 5). 

Apart from the addition reaction to electrophilic olefins, the / -carbon atom of 1,1-enediamines can also substitute a,/ -unsaturated compounds carrying a leaving group. Schafer and Gewald129 have shown that 141 and 142 react with 143 to give product 144 in moderate to good yields (equation 54). Ketene dithioacetals derived from an alkyl cyanoacetate and malononitrile behave similarly to 143130. When imine 145 is employed, the reaction results in the formation of 146 (equation 55)129. Apparently displacement of ethoxy group and cyclocondensation by attack on the nitrile moiety are the key steps in the reaction. 

Copper compounds are catalysts for the Michael addition reaction (249), olefin dimerizations (245, 248), the polymerization of propylene sulfide (142), and the preparation of straight-chain poly phenol ethers by oxidation of 2,6-dimethylphenol in the presence of ethyl- or phenyl-copper (209a). Pentafluorophenylcopper tetramer is an intriguing catalyst for the rearrangement of highly strained polycyclic molecules (116). The copper compound promotes the cleavage of different bonds in 1,2,2-tri-methylbicyclo[1.1.0]butane compared to ruthenium or rhodium complexes. Methylcopper also catalyzes the decomposition of tetramethyllead in alcohol solution (78, 81). 

Geometric features of transition states for addition reactions of olefins (Chapter 3), bimolecular substitutions (Si f3 reactions p. 36), bimolecular eliminations (E2 reactions p. loi) and many other reactions have been defined from studies of a variety of steroid systems (e.g. Fig. la). Reactions proceeding through carboniumions (e.g. Fig.Tb seep. 228) are sensitive to... 

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. 

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