Regioselectivity phenyl substituted alkenes

The regioselectivity in the fluorination of phenyl-substituted alkenes 9 has been reported and in the case of an electron-withdrawing group (C02Me, Ac) and a donating group (4-OMe) a more stable carbenium ion is formed and complete regioselectivity is observed, but it is completely lost in the reaction with the chloro derivative.2... 

The reaction rate of the addition decreased on going from phenyl-substituted alkenes to mono-, di-, and trisubstituted alkenes. A preference for E double bonds over Z double bonds was exhibited by the monoimido complex 1. Complete regioselectivity was observed with mono-and trisubstituted alkenes and gmi-disubstituted alkenes, since the C-N bond is formed exclusively at the least substituted alkene carbon, for example of 1-methyl- and 1-phenylcyclo-hexene. However, for 1,2-disubstituted alkenes the regioselectivity depended on the substrate70,71. Unexpectedly, benzylic amination was predominant in the reaction mixtures derived from 1-phenylpropenes and 1,2-dihydronaphthalene (Tabic 5). 

Scheme 9 that both the molybdenum and iron complexes can catalyze the allylic amination of nonfunctionalized alkenes with an ene-like transposition of the double bond, but also that the yield of the allyl amine formed, 113, is moderate to high. It is generally found that higher substituted alkenes tend to give the best yields, and un-symmetrical alkenes (trisubstituted) react with virtually complete regioselectivity, as only one isomer is detected. The byproducts are primarily azoxybenzene and aniline, which arise from condensation of nitrosobenzene with phenyl hydroxylamine and reduction of phenyl hydroxylamine, respectively. 

Fig. 6.36 Regioselectivity for phenyl azide reacting with a Z-substituted alkene...

The Julia olefination reaction is highly regioselective and ( )-stereoselective, providing a valuable alternative to the Schlosser reaction for making rrans -disubstituted olefins. The reaction involves condensation of a metalated alkyl phenyl sulfone with an aldehyde to yield a P-hydroxysulfone, which is then subjected to a reductive elimination to produce the alkene. There are limitations to the preparation of tri- and tetra-substituted alkenes via the sulfone route because the P-alkoxy sulfones derived from addition of the sulfone anion to ketones may be difficult to trap and isolate or they may revert back to their ketone and sulfone precursors. 

Treatment of MBH addcut 1 with carboxylic adds under Mitsunobu conditions gave almost exclusively the S 2 products 4, rather than S 2 product 5. Weak and bulky carboxylic acids and low temperatiues favor S 2 addition. Although the reaction conditions were effective for alkyl substituted derivatives, the addition of EtsN to the Mitsunobu conditions was necessary to improve the 5n2 S 2 ratios for the vinyl and phenyl derivatives (Scheme 3.3). " More recently, it was found that the nucleophilic substitution reaction mediated by triphenylphosphine linked to non-crosslinked polystyrene 6 led to a significantly more regioselective transformation. Tri-substituted alkenes 4 were obtained almost quantitatively via a highly regioselective 5n2 Mitsunobu reaction (Scheme 3.3). ... 

Only the head-to-tail adducts were obtained in the [2+2] photoaddition of 4-hydroxy-l-phenyl[l,8]naphthyridin-2(l//)-one with various alkenes in methanol (Scheme 3). The photolysis of the hypoiodites generated by the in situ reaction of the cycloadducts with excess mercury(ll) oxide-iodine reagent in benzene induced a regioselective scission of the non-ring junction bond of the alkoxyl radical to give substituted 3,9-dihydro-9-phenylyfuro[2,3- ][l,8]naphthyridin-4(2//)-one and/or 3,5-dihydro-5-phenylfuro[3,2-f][l,8]naphthyridin-4-(2//)-ones <1996T6125>. 

With sulfides as intermediates, alkenes can be used as precursors to organolithiums with regioselectivity in the formation of 56 determined by whether a radical11 or polar73 thiol addition is employed. Easy lithiation of phenyl benzyl sulfide 57 makes substituted benzyllithiums such as 58 readily available.73 Reductive C-S cleavage is probably the best way of making benzylic organolithiums. 

The alkylation of acyclic imines with electrophilic alkenes such as acrylonitrile, methyl acrylate or phenyl vinyl sulphone is also sensitive to steric effects and again, as a consequence, only mono-alkylation occurs398. The regioselectivity of the reaction in methanol varied from 100% attack at the more substituted a-position to 70% attack at the less substituted a -position depending upon the steric inhibition manifested and the stabilization of the competing secondary enamine tautomers (vide infra) (Scheme 204). In contrast, the reaction of butanone and other methyl ketone imines with phenyl vinyl ketone occurs twice at the more substituted a-position but this is then followed by a double cyclization process (Scheme 205). Four carbon-carbon bonds are formed sequentially in this one-pot synthesis of the bicyclo[2.2.2]octanone 205 from acyclic precursors399,400. 

Monosubstituted alkylidenecyclopropanes show lower regioselectivity. While /i-hexylidenecyclopro-pane (82) and the corresponding phenyl analog (83) cycloadd to cyclopentenone, acrylate or crotonate to produce mostly alkylidenecyclopentanes, a mixture of adducts is observed with diethyl fumarate (equations 74-77). Nonetheless, compared to the parent compound (79), these mono- and di-substituted alkylidenecyclopropanes often give better cycloaddition reactions with a wider range of alkenes, and are also less susceptible to self-oligomerization. 

We invoke 7c-stacking of the alkene with a phenyl moiety on the silicon protecting group (since this high degree of selectivity was only observed for TBDPS and not with TBS), in the precursor to explain this remarkable selectivity (Figure 1). Lewis acid induced reduction of the epoxide with sodium cyanoborohydride led regioselectively to the 1,3-diol (11) the hydride attacks the more substituted position via an S 2 mechanism. ... 

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