Alkenes, £>selective synthesis

Mechanistic evidence indicates 450,451> that the triplet enone first approaches the olefinic partner to form an exciplex. The next step consists in the formation of one of the new C—C bonds to give a 1,4-diradical, which is now the immediate precursor of the cyclobutane. Both exciplex and 1,4-diradical can decay resp. disproportionate to afford ground state enone and alkene. Eventually oxetane formation, i.e. addition of the carbonyl group of the enone to an olefin is also observed452. Although at first view the photocycloaddition of an enone to an alkene would be expected to afford a variety of structurally related products, the knowledge of the influence of substituents on the stereochemical outcome of the reaction allows the selective synthesis of the desired annelation product in inter-molecular reactions 453,454a b). As for intramolecular reactions, the substituent effects are made up by structural limitations 449). 

Homer-Wadsworth-Emmons reactions of ketones and aldehydes with phosphono-acetate esters, (R20)2P(=0)CH2C02R1, produce E/Z mixtures of a, /Tunsaturated esters. Use of the conventional reagent, sodium hydride, gives some selectivity. The combination of tin(II) triflate and A -cthylpipcndine enhances—and sometimes also reverses—the selectivity in most cases studied.71 Six-membered oxo-coordinated tin intermediates are proposed to control the selectivities observed. A similarly selective synthesis of trisubstituted exocyclic alkenes from cyclic ketones has been reported.72... 

Imada, Y. Yuasa, M. Nakamura, I. Muraha-shi, S.-I. Copper(I)-catalyzed amination of pro-pargyl esters. Selective synthesis of propargyl-amines, l-alken-3-ylamines, and (Z)-allyl-amines./. Org. Chem. 1994, 59, 2282-2284. 

Regio- and stereo-selective synthesis of annulated arene heterocycles and carbocy-cles having a halo functionality is accomplished by Sm(OTf)3-catalysed alkylation of (S) arenes by a tethered alkene group using /V-halosuccinimide as a halogen source [e.g. 

Fig. 14.48. Ireland-CLaisen rearrangement of two 0-allyl-0-sityl ketene acetals. Trans-selective synthesis of disubsti-tuted and F-selective synthesis of trisubstituted alkenes.

Fig. 11.43. Claisen-Ireland rearrangement of two O-allyl-O-silyl ketene acetals. 7ran.v-sclective synthesis of disubstituted and E-selective synthesis of trisubstituted alkenes.

Scheme 7. Figure 19 finally provides the quantitative basis for the qualitative model (Scheme 11), which has been employed for deriving conditions that allow the selective synthesis of 1 1 products by Lewis acid induced reactions of alkyl halides with alkenes. 

Ozone, while somewhat inconvenient to use, is way qiecific in its reactions with alkenes. It is widely employed for selective synthesis, for qualitative and quantitative analysis of unsaturated compounds, and for studying the position of double bonds in macromolecules. The nature of the products obtained from ozonolysis reactions is determitted by the way in which the reaction is carried out Different workup procedures (hydrolytic, reductive or oxidative) can be used to produce alcohols, aldehydes, ketones, carboxylic acids or esters. 

The perfluorinated vinylphosphoranes were not isolated, but nevertheless are valuable intermediates in the selective synthesis of alkenes having fluoromethylene groups. Hexafluoropropene (5) was converted into (A )-l, 2,3,3,3-pentafluoroprop-l-ene (9) via the phosphorane 8 in very good yield. 

The combination of reactions described above (Sections 2.6.4.2 to 2.6.4.5) allows the selective synthesis of a large variety of alcohols, allyl alcohols, alkenes, epoxides and carbonyl compounds from p-hydroxyalkyl selenides. These products often can be obtained from two ca nyl compounds by activation of one of them as an a-selenoalkyllithium (Schemes 161-196). 

As noted in the discussion of ( )-selective alkene formation, Kishi has found that a-substituted aldehydes reacted with trimethylphosphonopropionate and KOBu to produce the (Z)-alkene selectively. A strongly dissociating base is critical to this approach. In addition to the examples already presented in the discussion of ( )-alkene formation, the (Z)-selective reaction has recently been applied to the synthesis of macrolide antibiotics. In this example, a trisubstituted alkene was formed and closed to the lactone (148 equation 33). In an application to diterpenoids. Piers encountered an example of how substrate-specific the alkene formation can be. With a-dimethoxyphosphonyl-y-butyrolactone (150), the reactions with simple aldehydes proceeded with very high selectivity [(Z) ( ) = 99 1]. On application of the reaction to the more complex aldehyde (149) the (Z) ( ) stereoselectivity dropped to 3 1 in 58% yield (equation 34). No selectivity was observed on reaction with benzaldehyde. Although for hindered substrates, strongly basic conditions with a dimethyl phosphonate can be a simple and effective method for the synthesis of (Z)-isomers, the reaction is not general. In 1983, Still and coworkers introduced methodology that used bis(trifluoroethyl)phosphonoesters (153) to provide a facile approach to (Z)-aIkenes (154) when reacted with aldehydes (equation 35). " ... 

The present work describes catalysts for the selective synthesis of alkenes and for the skeletal isomerization. 

One of several applications of the above catalysts is the production of the high octane ethers, MTBE and TAME, which are increasingly in demand as blend stocks for unleaded gasoline. The concept of the process is outlined in figure 3. Synthesis gas from natural methane would be converted by an alkene selective catalyst. The butene and pentene could then be isomerized by the... 

Dehydration of alcohols over solid catalysts can yield alkenes by intramolecular dehydration, whereas ethers are the product of an intermolecular process. The catalysts used can be acidic or basic solids or bifunctional acid-base materials. Although selective synthesis of any desired product is possible, complications can arise as a result of side-reactions-dehydrogenation and decomposition of the starting alcohol, decomposition and consecutive transformations of intermediates and products (j9-cleavage of carbocations, oligomerization of alkenes). 

When monohydric alcohols undergo dehydration, isomeric alkenes can be formed by the loss of water by f (or 1,2-) elimination. Selective synthesis of certain alkenes can, however, be accomplished-when primary alcohols are treated with appropriate solid catalysts terminal alkenes are formed. In contrast, either 1- or 2-alkenes can be produced by dehydration of secondary 2-alkanols. The reactivity of alcohols follows the sequence tertiary > secondary > primary. 

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