Alkene derivatives complexes

All these features have been initially interpreted102-104 in terms of a molecular mechanism involving two successive alkene-iodine complexes of 1 1 and 1 2 stochiometries the second of which evolves by internal nucleophilic attack of the uncomplexed double bond to the diiodo derivative (equation 87). The intramolecular attack of the second double bond has been regarded as rate determining, owing to the fact that the overall rate law is second order in iodine rather than the usual third order. Nevertheless more... 

One last remark concerning the two catalysts we have discussed in more detail, cationic rhodium catalysts and the neutral chloride catalyst of Wilkinson. The difference of the catalytic system discussed above from that of the Wilkinson catalyst lies in the sequence of the oxidative addition and the alkene complexation. The hydrogenation of the cinnamic acid derivative involves a cationic catalyst that first forms the alkene complex the intermediate alkene (enamide) complex can be observed spectroscopically. 

New developments in this area from Langlois group are primarily and generally on the development of the asymmetric version of these reactions. Most of the reactions employed camphor-derived 2-alkenyloxazolines as dienophUes (Scheme 8.133). In many cases, these reactions were carried out in the presence of trifuoroacetic anhydride to activate the alkene by complexation at the oxazoline nitrogen as well as to limit the number of reactive conformations. The reader should consult the recent and extensive reviews of this chemistry by Langlois. 

Detailed studies of the hydrogenation of cinnamic acid derivatives (Fig. 6.22) by Halpern [53] and Brown [54] have revealed that the most stable intermediate of the two alkene adducts is not the one that leads to the major observed enantiomeric product, see Fig. 6.24. This means that the least stable intermediate alkene-rhodium complex reacts faster in the subsequent reaction step involving... 

Other synthetic routes to alkene-metal complexes reported in the literature involve bridge cleavage reactions of halogeno complexes (method F), facile intermolecular scrambling reactions involving metal-alkene and metal-PF3 complexes (18, 81) (method G) (Scheme 4), and the coupling together of two alkyne units to afford a metallo-cyclopentadiene derivative (method H). 

Due to the rather complex structure of terpenoid sub-strate.s, chemo- and regio.selcctivity of the reaction as well as the possibility of rearrangements in the substrate moiety are to be taken into account. Indeed, the widely invest -gated aminomethylation of alkene derivatives indicates that... 

A synthesis of a-lipoic acid and derivatives has been reported. The pendant alkene of complex (111) was transformed to a primary alcohol (112) via hydroboration-oxidation sequence. Mitsunobu coupling of (112) with thiobenzoic acids to give (113), followed by desilylation and nucleophilic substitution again employing thiobenzoic acid gave the advanced intermediate (114) (Scheme 169). 

Many nncleophiles add to one of the double bonds of chelating palladium(diene) complexes to give a chelating Pd alkyl(alkene) derivative, as exemphfied by the reaction of PdCl2(l,5-cod) with methoxide (equation 41). In most cases, the direction of attack is exo. If the nucleophile is in a form that can undergo transmetalation with the Pd l bond, such as Ph2Hg, the nucleophihc group can be delivered endo. In this case, prior formation of a Pd nucleophile bond accounts for the direction of attack (equation 42). 

Similarly, for alkenes derived from saturated methyl ketones the regioselectivity is determined by starting hydrazone ( ) (Z) ratios in some solvents but not in others. Thus 2-octanone trisylhydrazone, which is an inseparable 85 15 mixture of ( )- and (Z)-isomers, gives an 85 15 ratio of l-octene 2-octene if vinyllithium formation is carried out in THF, but a 98 2 ratio of the same products when 10% TMEDA-hexane is the solvent. The implication of this observation is that in THF the regioselectivity is determined by azomethine stereochemistry but that in TMEDA-hexane it is not. Note, however, that in this case a iyn-directing effect does not occur in TMEDA, whereas in the previous example it does. Thus more than 10 years after it was asserted that a detailed explanation of the observed solvent dependencies...await further studies owing to the complexities of the reaction system. , little headway has been made. 

If a jS-hydrogen is present, one obtains fj -CpCr(CO)3X, > -CpCr(CO)3H, and alkanes and alkenes derived from R . TpMo(CO)3, also a 17-electron complex, reacts with BrCH2CN to give TpMo(CO)3Br and TpMo(CO)3CH2CN . 

These titanium compounds can be described as an alkene 7r-complex or a metallacyclopropane, which is of practical importance. According to several computational studies, it has been concluded that the alkene titanium complexes are best represented as titanacyclopropane derivatives. The synthesis of titanium-alkyne complexes Ti(Me3SiC=CG6H13)(OR)2 from reaction between l-(trimethylsilyl)oct-l-yne with achiral or chiral alkoxo titanium compounds Ti(OR)4 has been described (Scheme 95).184 A series of organotitanium compounds (Scheme 96) are obtained by metathesis reactions.41... 

Use for resolution of cycloalkenes. W s-Cycloalkenes of intermediate size (Cg-Cjo) should be capable of existing in enantiomeric forms because of the inability of the trans double bond to rotate with respect to the remainder of the molecule. But in the absence of salt-forming groups, resolution cannot be accomplished by the usual methods of forming derivatives. However, Cope et al.s found that the strong tendency of an alkene to complex with a platinum compound provides an effective method of resolution. The complex of ethylene with platinous chloride and (+) or (-)-a-methylbenzylamine exists in only one form since ethylene is symmetrical. But addition of the base to a solution of the platinum complex of trans-cyclooctene opens the way for formation of the diastereoisomeric complexes derived from the R- and S-forms of the base. Fractional crystallization at —20° (liquid at 25°) effected separation. Liberation of the (—)-hydrocarbon from the complex with potassium cyanide gave a product of aD — 411°. 

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