Tri-substituted olefins

This model prediets that tri-substituted and tetra-substituted olefins would also be poor substrates. Thus it was not until 1994 that a study in the epoxidation of higher substituted olefins appeared. Indeed Jaeobsen revealed that tri-substituted olefins, and even tetra-substituted olefins ean be excellent substratesA new model was put forth that encompasses a skewed side-on approach of tri-substituted olefins to the Mn-oxo eomplex. The observation that certain tetrasubstituted olefins undergo epoxidation with good enantioseleetivity suggests that further studies are needed in order to fully understand the transition state geometry of the catalyst and substrate. 

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

For example, substituents in terminal olefins (43) in the resulting nitronates (35) are generally adjacent to the C-6 atom. The alkoxy and siloxy substituents from vinyl ethers and silyl enolates respectively, as well as the amino group from enamines, are oriented in the same positions. In the case of tris-substituted olefins, C-6 is the most crowded atom in the resulting nitronates. 

In turn, the propensity of 1 to respond to steric hindrance can be used to control the site of initiation of an RCM reaction in a polyene substrate (Scheme 9) [20]. Thus, dienyne 25 reacts with the catalyst regioselectively at the least substituted site the evolving ruthenium carbene 26 undergoes a subsequent enyne metathesis leading to a new carbene 27, which is finally trapped by the disubsti-tuted olefin to afford the bicyclo[4.4.0]decadiene product 28. By simply reversing the substitution pattern of the double bonds, the complementary bicyclo [5.3.0] compound 32 is formed exclusively, because the cyclization cascade is then triggered at the other end of the substrate. Note that in both examples tri-substituted olefins are obtained by means of a ruthenium based metathesis catalyst [20] ... 

The configuration of the product in diastereoselective hydrogenation -whether 1,2-syn or 1,2-anti - is related to the substitution pattern of the starting alkene. The allyl alcohol with a 1,1-disubstituted olefin unit affords the antiproduct, while the syn-product is formed from the allyl alcohol with a trisubsti-tuted olefmic bond (Table 21.8, entries 6-9). The complementarity in diastereoselective hydrogenation of di- and tri-substituted olefins may be rationalized based on the conformation analysis of the intermediary complex (Scheme 21.1)... 

Fig. 10.6. Structure of jamaicamide A (2) showing a key NOE defining the geometry of a tri-substituted olefin.

These catalysts, 11-13, show good enantioselectivity ranging from 80 to 95% ee in the epoxidation of conjugated cfs-di- and tri-substituted olefins. Epoxidation of "good substrates such as 2,2-dimethylchromene derivatives proceeds with excellent enantioselectivity (>95% ee). Since the results obtained with these first-generation Mn-salen catalysts have been reviewed [21,33], only typical examples are shown in Table 6B.1. These reactions are usually carried out in the presence of donor ligand [34] such as 4-phenylpyridine A -oxide with terminal oxidants such as iodosylbenzene and sodium hypochlorite as described above. However, the use of some other terminal oxidants under well-optimized conditions expands the scope of the Mn-salen-... 

On the other hand, the addition of a quaternary ammonium salt to the reaction medium accelerates the isomerization of the radical intermediate [36]. Thus, the epoxidation of c/j-stilbene in the presence of A -benzylquinine salt gives rranr-stilbene oxide with 90% ee as major product (Table 6B.1, entry 24). This protocol provides an effective method for the synthesis of trans-epoxides. In contrast to the epoxidation of c/s-di- and tri-substituted olefins for which complexes 11-13 are the catalysts of choice, the best catalyst for the epoxidation of tetra-substituted conjugated olefins varies with substrates (Table 6B.1, entries 27 and 28) [37]. The asymmetric epoxidation of 6-bromo-2,2,3,4-tetramethylchromene is well-promoted by complex 14 and that of 2-methyl-3-phenylindene, by complex 12a. 

The trimethylsilylated ylides (1), easily generated from trimethyl chlorosilane and ylides, react with aldehydes 2 to form vi-nylsilanes 3 (2,3). The vinylphosphonium silanolates 4 are also formed. Compounds 3 are versatile reagents for further reactions (4). The ylide J (with R1 =H) reacts with aldehydes 2 to give the dienes j). The oxidation of with the adduct 6, from triphenyl-phosphite and ozone, gives access to a generaT synthesis of acyl-silanes (trimethylsilylketones) (2). The silylated ylides react to form phosphonium salts 7 with halogen compounds. The salts 7.can be desilylated by fluorine ions. The disubstituted ylides JO Tormed can be converted in statu nascendi with aldehydes V[ into the tris-substituted olefin J2 (2,3). In the case of R3-I, vinyl... 

The relevant use of BT and PT sulfones in arranging prenyl [121] and other tri-substituted olefin patterns has been recently reported [122,123]. 

Catalyst Ru-4 exhibits overall superior activity and improved substrate scope relative to catalyst Ru-2. For example, Ru-4 completes simple metathesis reactions, such as the RCM of diethyl diallylmalonate or the ROMP of cyclooctadiene, at rates several orders of magnitude greater than with Ru-2. In addition, whereas catalyst Ru-2 is unreactive toward sterically congested or electronically deactivated substrates, Ru-4 successfully mediates the formation of tetra-substituted olefins in five- and six-mem-bered rings systems [9], as well as CM to form tri-substituted olefins and products containing electron-withdrawing substituents [10]. 

Because ruthenium catalysts are relatively unreactive for alkene hydrogenation and they are poor for double-bond isomerization, these catalysts are particularly effective for the selective hydrogenation of monosubstituted alkenes in the presence of di- and tri-substituted olefins at ambient temperature under 2-3 atmospheres of hydrogen (Eqn. 15.28). Water in the reaction medium... 

The P-2 Ni(B) is an effective catalyst for the saturation of double bonds.68> With this catalyst, terminal alkenes are hydrogenated with greater than 95% selectivity in the presence of di- and tri-substituted olefins in ethanol solution at room temperature and atmospheric pressure.69 Solvent Effect... 

Markownikoff hydration of olefins (2, 265-267). Brown and Geoghegan1 have reported a detailed study of the oxymercuration-demercuration procedure. Yields of alcohols in the case of terminal olefins, RCH=CH2 and R2C=CH2, and disubstituted internal olefins, RCH=CHR are practically quantitative. Tri-substituted olefins of the type R2C=CHR show a wide variation in reactivity. 1 -Phenylcyclopentene and 1 -phenylcy clohexene, for example, are unreacti ve. 

Carbon-carbon double bonds in oxabicydic systems are cleaved by ozonolysis. Moreover, tri-substituted olefins generate cyclic ethers bearing side chains with differentiated ends upon ozonolytic cleavage, thus allowing subsequent selective elaboration of each appendage. Naked sugars were used extensively by Vogel as furanosides and C-nucleoside derivatives [11a]. 

In order to show the quality of the updated mnemonic device, the authors compared the experimental results with those obtained computationally for a set of tri-substituted olefins with one large substituent and two methyl groups. By comparing the reactivity of these olefins, the interactions in each of the positions of the mnemonic device can be elucidated. The obtained results agree with the computational predictions therefore, the new mnemonic device can rationalize the experimental results from aU alkene classes, and improves the predictivity for tri-substituted alkenes. 

Deamination via sulphonamide intermediates allows the conversion of the carbon-nitrogen bond into the carbon-oxygen bond. Conditions have been established such that nitrates may be obtained from the deamination of primary amines with dinitrogen tetroxide at low temperature rather than alcohols/ The oxidation of amines to nitriles has been carried out using copper(l) chloride in pyridine under an oxygen atmosphere/ which is a milder reagent than those previously used to perform this transformation. Cleavage of tertiary amines with ethyl chloroformate affords a simple preparation of di- and tri-substituted olefinic synthons (Scheme... 

In the NMR spectrum, the tetra-substituted olefinic carbon gives rise to the peaks from 130 to 140 ppm. The tri-substituted olefinic carbon accounts for the peaks from 118 to 128 ppm, and the aliphatic methyl carbon results in peaks from 20 to 25 ppm. This latter set of peaks can be used to distinguish c/s-l,4-polyisoprene from trans-, 4- and... 

Osmium-mediated cw-addition of nitrogen and oxygen moieties to mono-, di- and tri-substituted olefins to yield vicinal amino or amido alcohols ... 

This work demonstrates the first case where both a non face-selective and a face-selective addition to the tri-substituted olefinic portion of DMAPP has occurred within the same molecule. A related observation with respect to the loss of stereochemistry in the reverse prenylated indole alkaloids echinulin and roquefortine have been reported and are discussed elsewhere in this article. 

Oxidative addition of alkyl halides to vinylrhodium(i) complexes gives isolable alkylvinylrhodium(m) species (75). Heating the latter gives a tri-substituted olefin whose stereochemistry can be controlled by judicious choice of conditions. Derivation of (76) by addition of rhodium hydride to the acetylene results in an overall conversion of an acetylene into a tri-substituted olefin (Scheme 2). ... 

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