Trans-Selective functionalizations, alkenes,

The first step in both cases is an ene reaction with the Se=0 bond (pp. 1270-1). The electrophilic selenium attacks the less substituted end (largest HOMO coefficient) of the alkene and a proton is removed from the methyl group trans to the main chain. Then a [2,3]-sigmatropic rearrangement puts the double bond back where it was (trans selectively) and functionalizes the old methyl group with an oxygen atom. 

The catalyst activity decreased with increasing polymer crystallinity. A high regioselectivity of the catalyst in the hydrosilylation of alkenes towards formation of the linear products was achieved due to the favorable microporous structure of the polyamide supports with pore size of 10-20. The stereoselectivity of the reaction can be reversed by a proper choice of donor functions in a polymer support, for example the traditional cis-selectivity of Rh catalysts in hydrosilylation of phen-ylacetylene was changed to trans-selectivity by use of a 2,5-py instead of a 2,6-py moiety. The polyamide-supported catalysts showed high stability through 6-9 synthesis runs [25]. 

The trans stereoselectivity may easily be rationalized as a result of sterics, although direct irradiation of the 1-phenylcyclohexene gives a similar ratio of trans cis stereoselectivity. The singlet reaction presumably does not involve a biradical intermediate of significant lifetime. Thus, the trans selectivity may be enhanced by sterics in the case of the triplet reaction. The preference for trans isomer may be inherent to the approach of the alkenes or, perhaps, it is a function of selective reversibility of biradical intermediates (see Scheme 1). Caldwell has published a formula for predicting the potential for photocycloaddition of alkenes and arenes in the singlet excited state [42], His analysis implicates an exciplex. 

The sulfone derivative is metallated 12 and attached to the carbonyl 13, followed by functionalization 15, and reductive elimination, to produce the alkene 3. The diastereoisomeric erythro- and f/irco-acetoxy sulfones 15 could be separated and both isomers were converted to the /rans-alkene. It was hypothesized that the (jE)-selectivity is derived from the reductive removal of the arylsulfonyl group, generating an anion 17 that assumes the low energy tra/is-configuration before loss of the acylate anion. As demonstrated by numerous examples, the mechanism for reductive elimination is consistent with the frnding that the alkenes obtained are the thermodynamic mixture and that increased branching at the site of elimination should, for steric reasons, increase the trans selectivity. 

In this paper we will report on using a series of modifiers to enhance selectivity during 1-phenyl-1-propyne over a Pd/alumina catalyst. The modifiers, trans-cinnamaldehyde, trans-cinnamonitrile, 3-phenylpropionitrile, and 3-phenylpropylamine, were chosen to have a functionality that potentially could adsorb more strongly than an alkene and to be unreactive under the reaction conditions. 

The products from alkene hydroamination are inherently lightly functionalized. To address this possible deficiency. Professor Marks also reported (J. Am. Chem. Soc. 125 15878, 2003) the cyclization of amino dienes such as 5. The cyclizations proceed with high selectivity for, the cis-2,6-dialkyl piperidines, and with a little lower selectivity for the trans 2,5-dialkyl pyrrolidine. The product alkenes are -95% E, the balance being a little Z alkene and the terminal alkene. 

Additions to functionalized three-carbon olefins have been studied extensively. We have used methyl acrylate as a standard olefin since it always reacts only at the terminal carbon and the a,/3-double bond in the product is always trans. The stereospecificity of its reactions with vinylic halides varies with structure. The simple 1-halo-l-alkenes with methyl acrylate under normal conditions give mixtures of E,Z- and E,E-dienoates. The reaction is more selective with the bromides than with the iodides and the stereoselectivity increases with increasing triphenylphosphine concentration. This occurs because the excess phosphine displaces the hydridopalladium halide group from the diene 7r-complex before readdition to form the ir-allylic species occurs (see Equation 6). The disubstituted vinylic bromides react stereospecifically with methyl acrylate (4). 

Organocopper Addition (Alkene to -Functionalized Product). Tri-n-butylphosphine-stabilized organocopper reagents add in a conjugate fashion to trans-cnoaic derivatives of the 10-dicyclohexylsulfonamidoisobomeol auxiliary from the less hindered C(a)-s ( r-face with excellent selectivity (eq 2) (Table 1). This type of reaction has formed the basis of several natural product syntheses. ... 

The CT-allyl intermediate may be transformed to a ir-allyl structure with either the syn or anti configuration (relative to the methynyl hydrogen) as in equation (31), see also Scheme 2. The union with a second hydrogen leads to adsorbed alkenes 1-butene and either trans-2-buXtnt from the iyn-allyl intermediate or cw-2-butene from the anti form (Scheme 2). The desorption of the alkene competes with the further addition of hydrogen to form the alkane (equation 32). The reaction of a cr-allyl structure with hydrogen can yield the unbound alkene directly. The selectivity may depend upon the relative importance of these competitive reactions, which are likely to be a function of the metal as well as the reaction conditions. 

An aldehyde with a-ether functionality was used in the synthesis of N-acetylneuraminic acid by Danishefsky (equation 37). A series of cis- and trans-alkenes with a-oxygen functionality were synthesized by Cinquinin and coworkers. Of the examples generated, the HWE reaction proceeded in the expected ( )-selective manner (164), while the trifluoroethyl phosphonate was used to form the (Z)-al-kenes (163) selectively (Scheme 26). Interestingly, 18-crown-6 was not used for (Z)-alkene formation. 

While rare-earth metals are proven powerful olefin polymerization catalysts [21-24], there are only limited reports on controlled olefin oligomerizations or selective olefin dimerizations utilizing these elements [204,207,208], An ansa-scandocene [207] and the bis(indenyl)yttrium complex 41 (Fig. 25) [204] were reported to produce head-to-tail dimers from monosubstimted aliphatic alkenes (57). Complex 41 produces predominantly the tail-to tail adduct with styrene. The codimerization of an aliphatic alkene (including substrates containing various functionalities) with styrene affords tran -tail-to-tail dimers, apparently as a result of 1,2-insertion of the a-olefin followed by 2,1-insertion of styrene directed by the phenyl group (58). 

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