Symmetrical olefins

Miscellaneous. Symmetrical olefins were obtained from reactive ylides and sulphur under fairly vigorous conditions. Yields were high when R = Ar, but the ethylidenephosphorane gave only 28% of hex-3-ene at 150 ""C in 1-methylnaphthalene. 

Although the symmetrical olefin is the main product, Scheme 21, 8 is obtained from fenchone p-toluenesulfonyl hydrazone220 (tosyl hydrazone) with t-BuOK and elemental selenium. 

A second observation was the fact that isomerization of the starting asymmetric olefin was much faster than the formation of new symmetric olefins. In fact, 40% of the initial cis olefin (Fig. 1) had isomerized to trans after only 4% conversion to new olefins. This result formally parallels the highly selective regenerative metathesis of a-olefins (60, 61), except that steric factors now prevail, because electronic effects should be minimal. Finally, the composition of the initially formed butene from r/j-4-methyl-2-pentene was essentially identical to that obtained when cA-2-pentene was used (18). When tra .v-4-methyl-2-pentene was metath-esized (Fig. 2), the composition of the initially formed butenes indicated a rather high trans specificity. 

Boelhouwer s discovery (23) prompted a flurry of activity in this area. Baker applied the Boelhouwer catalyst to the metathesis of w-olefinic esters (88). At an ester/W molar ratio of 20/1 (68°C), symmetrical olefinic diesters were formed in 34-36% yields with concomitant elimination of ethylene. In addition, Baker identified products recovered in 3-8% yield corresponding to addition of HC1 across the terminal double bond. 

The most common byproducts encountered in cyclopropanations with diazoalkanes as carbene precursors are azines and carbene dimers , i.e. symmetric olefins resulting from the reaction of the intermediate carbene complex with the diazoalkane. The formation of these byproducts can be supressed by keeping the concentration of diazoalkane in the reaction mixture as low as possible. For this purpose, the automated, slow addition of the diazoalkane to a mixture of catalyst and substrate (e.g. by means of a pump or a syringe motor) has proven to be a very valuable technique. 

Fliszar, S., and J. Renard. Quantitative investigation of the ozonolysis reaction. XIV. A simple carbonium ion stabilization approach to the ozone cleavage of un-symmetrical olefins. Can. J. Chem. 48 3002-3018, 1970. 

The preparation of -carotene [j8,/3-carotene (63)] in 50% yield by reductive coupling of retinaldehyde (67) with TiCl4-LiAlH4 has been described. Similarly )3-ionone (68) gives the symmetrical olefin (69). The enolic /3-diketone caro-... 

For carbenes or carbenoids of the type R—C—R there is another aspect of stereochemistry.1040 When these species are added to all but symmetrical olefins, two isomers are possible, even if the four groups originally on the double-bond carbons maintain their configurations ... 

In agreement with the Criegee mechanism, unsymmetric alkenes,606,607 such as 2-pentene606 [Eq. (9.108)], or a pair of symmetrical olefins, such as a mixture of CH2=CH2 and CD2=CD2,608 yield three different ozonides, including two symmetric cross-ozonation products (80, 81) ... 

While die above reactions will provide carboxylic acid products, each has problems associated with it. The cleavage of olefins to carboxylic acids [reaction (7.1)] can be carried out using potassium permanganate or by ozonolysis at low temperature followed by oxidative workup with hydrogen peroxide. Neither of diese mediods is very useful since only symmetric olefins provide a single carboxylic acid product. Unsymmetrical olefins give a mixture of two acids which must be separated. Furthermore the most useful synthetic processes are those which build up structures, whereas these reactions are degradative in nature. 

In cases where the reactant is a symmetric olefin, e.g. 2-butene, the product will also be the same as the reactant. 

Scheme 7 Cross metathesis with symmetric olefins, (a) methyl oleate with m-2-butene-1,4-d iyl diacetate [67], and (b) 10-undecenoate with diethyl maleate [68]...

Fig. 3.37. Hydration of a symmetric olefin through solvomercuration/ reduction.

Table III. Relative Rate Constants for the Ozonolysis of Symmetrical Olefins (RCH=CHR) in CCI4...

When Equation 10 is applied to symmetrical olefins, the term k°rei refers to 2-butene (R = CH3). The calculated k°Tei values are 17.5 for trans-2-butene and 6.13 for the cis-isomer. 

For symmetrical 1,2-disubstituted ethylenes unsymmetrical olefins, or twice that observed for unsymmetrical olefins or twice that calculated by Equation 12. 

The main compound released in the gas phase during TPD is the symmetric olefin (RHC=CHR). The reaction may occur via an intermediate containing both C-0 and C-C bonds such as a diolate (7.9). It is not known if this reaction occurs via two metal... 

The reaction of several a,p-butenolides (70) with two simple, fully symmetric olefins [ethylene and tetramethylethylene (TME)] revealed that the exo product was always favored (73 27 ratio exo/endo), a finding consistent with approach of the olefin from the less hindered side of the enone [123]. The size of the substituent at C5 also played an important role in controlling the de of the reaction, and changing the substituent from methyl to t-butoxycarbonyl increased the de fi-om 18% to 56% for reactions with ethylene. TME has also been reacted with uridine and related compounds (71) [124]. Endo attack was greatly favored over exo attack (9 1 in the best case). As the bulk of the sugar moiety increased, the amount of exo product increased slightly, but endo was always found to be the predominant mode of attack. 

The hydroformyiation reaction, still called the Oxo synthesis, was discovered in 1938 by Rohlen Rukrchemie It consists in adding hydrogen and the formyl (—CHO) group to the double bond of an olefin, by treating the olefin with a mixture of CO and H2 in the presence of a cobalt catalyst. For non-symmetrical olefins, this reaction leads to the... 

In a formal total synthesis of steroid brassindole, sulfide 102 was used as the substrate of the Ramberg-Backlund reaction.57 Chlorination of 102 followed by oxidation with m-CPBA, and treatment of the resulting chloro-sulfone with potassium t-butoxide, provided symmetrical olefin 103. 

The kinetics of thermolysis of 2,5-diaryltetrazoles (205) have been investigated by Hong and Baldwin428 445 in order to estimate the sensitivity of 1,3-dipolar cycloadditions to the electronic characteristics of the nitrilimine rather than the steric characteristics, which often tend to dominate such cycloadditions. For the species 205 the steric factors are constant, and the reverse of the cleavage reaction was considered to be analogous to the 1,3-cycloadditions with symmetrical olefins. A Hammett p value of +1.16 was obtained for the compounds 205 (X = H) and a value of —0.23 for the series 205 (Y = H). The p values... 

---