Transition acyclic linear

The reaction of ( )-crotylsilane with RCHO in the presence of TiCl4 proceeds with high 5yn-selectivity. On the other hand, the corresponding (Z)-silane furnishes syn-anti mixtures of homoallylic alcohols. It has been shown that the reaction proceeds via an acyclic linear and not via a cyclic six-member transition state. As shown below for the (E)-crotylsilane, transition state A leading to the jyn-alcohol is sterically favored over the diastereomeric transition state B, which has a gauche Me-Et interaction. ... 

To explain the observed selectivities, acyclic linear transition structures are invoked. In these transition structures the double bonds are placed in an antiperipla-nar relationship. The observed diastereoselectivities are proposed to result from a minimization of steric interactions in the transition structures (Scheme 10-16). Because the enantiomeric excess of the product matches that of the starting material, this reaction is also selective for the anti Sg pathway [28 c],... 

For the first time, a simple and efficient method for the preparation of optically active allylsilanes using catalytic asymmetric Grignard cross-coupling has been reported. These allylsilanes react in a highly efficient and stereocontrolled fashion with aldehydes, in which the reaction is thought to proceed via acyclic linear diastereomeric transition states (Scheme 21). ... 

Among the recently published works, the one which showed that the cyclic structures of water clusters open up to form a linear structure above a certain threshold electric field value a was a systematic ab initio study on the effect of electric field on structure, energetics, and transition states of trimer, tetramer, and pentamer water clusters (both cyclic and acyclic) [36], Considering c/.v-butadiene as a model system, the strength and the direction of a static electric field has been used to examine the delocalization energy, the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations [37]. Another recent work performed by Rai et al. focused on the studies using the DFT and its time-dependent counterpart of effects of uniform static electric field on aromatic and aliphatic hydrocarbons [38],... 

When Lewis acids such as SnCU and TiCU are used to promote additions of allylic trialkyltin reagents to aldehydes several reaction outcomes are possible, depending on stoichiometry and the mode of addition. If the Lewis acid is added to the aldehyde followed by the allylic stannane, the typical product (syn for crotylstannanes) derived from an acyclic transition state is formed. If, however, the stannane and Lewis acid are premixed and left to equilibrate, metathesis can occur forming the allylic halome-tal compound which reacts with the subsequently added aldehyde to give products (anti for crotyl) consistent with a cyclic transition state (Eq. 22). The initially formed allylic halostannane gives rise to the linear adduct, but if aldehyde addition is delayed, this initial secondary allylic metal halide can equilibrate to the primary isomer which then reacts with the aldehyde to afford the branched product. 

Attack of an atom or free radical upon alkanes generally involves attack on hydrogen atoms via a more or less linear transition state (1). Attack at carbon in acyclic alkanes is excluded by a number of experimental observations including the absence of rupture of carbon-carbon bonds. Reactions proceeding via transition state 2 are excluded by the observed primary deuterium isotope effects which vary considerably with the nature of the attacking radical or atom. Moreover, when X = D , no hydrogen-deuterium exchange is observed by EPR spectroscopy for the radicals formed by attack of D upon... 

Acyclic diene metathesis (ADMET) [75] is the process by which a transition metal catalyst leads to a stepwise condensation polymerization of diene monomers, characterized by loss of gaseous ethylene and the production of linear polyolefins containing regular unsaturations along the polymer backbone (Scheme 1.8). In fact, many of the polymeric structures accessible by ADMET can be made by alternate mechanisms (e.g., 1,4-polybutadiene made by ADMET polymerization of 1,6-hexadiene is more commonly made by the anionic polymerization of 1,4-butadiene). 

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