Unsubstituted terminally

In Entry 4 the silyl group appears to introduce a controlling steric factor, leading to the observed stereoisomer. The unsubstituted terminal alkyne, which reacts through the dianion, gives the alternate isomer. 

The yields for reactions of unsubstituted terminal alkenes were lower than for substituted alkenes but they were still reasonable and could be increased further by increasing the aldehyde alkene ratio. Total conversions of substrate were reported with epoxide selectivity as high as 95% in some cases. The FBC system allows for a much higher substratexatalyst ratio (1000 1) than the non-fluorous epoxidation reported (20 1) previously. Recycling the fluorous layer once showed no reduction in conversion or selectivity. 

Only unsubstituted terminal double bonds can be prepared by using the strategy described in Scheme 18 (only stereoselective preparation of 39). 

The mechanism snfficiently explains how prodnctive condensation metathesis occms since terminal alkenes are contained in every step of the productive cycle (Scheme 5). In this kinetically controlled regime (high concentration of terminal alkene present), alkylidene reactions with unsubstituted (terminal) alkenes are favored over interchain redistribution reactions or other degenerate eqnilibria. However, when terminal alkene concentration decreases near the end of the polycondensation reaction, competing... 

As more active members of the ruthenium catalyst family were developed, more complex systems could be prepared. For example, the first generation of ruthenium catalysts were very selective for less-substituted double bonds, and would not dose tri-substituted double bonds in medium-ring systems. As demonstrated below, Ru-1 would only react with the unsubstituted terminal double bond. However, the newer catalyst will convert the intermediate into the desired ring system containing a tri-substituted double bond (Eq. 6.5) [23]. 

Although few examples of acylations of 1,3-butadienes have been described, Friedel-Crafts acylations of diene complexes, in particular iron tricarbonyl derivatives, can give synthetically useful yields. In acylations of iron tricarbonyl complexes with the Perrier reagent from acetyl chloride and aluminum chloride, acylation occurs only at unsubstituted terminal carbons (Scheme 18). ° The primary product is... 

Either separately or as a mixture, isomers XIII and XIV undergo nucleophilic attack by l-pyrrolidino-2-methylpropene on the unsubstituted terminal carbon of the allyl ligand to yield, after hydrolysis, a single product, XV ... 

As an example, the suprafacial [1,7] sigmatropic H-shift will be considered that occurs on irradiation of 1,3,5-CHT with formation of bicyclo[3.2.0]hepta-2,6-diene (BHD) as a minor product. In 1-substituted CHT this [1,7]-H shift is regiose-lective in 1-cyano-cycloheptatriene (CN-CHT) the hydrogen atom moves exclusively to the unsubstituted terminal carbon of the heptatriene moiety, whereas in 1-methyl-cyclohep-tatriene (Me-CHT) only 2% of the product exhibits this regio-chemistry and 98% corresponds to a hydrogen shift toward the substituted carbon, as shown in Scheme 6.1. Different models have been proposed to explain this regioselectivity. ... 

Mercuration exhibits a carbocation-like pattern, but with the superposition of a large steric effect. For unsubstituted terminal carbons, the rate increases from ethene to propene to 2-methylpropene. This trend also holds for internal alkenes, as 2-methyl-2-butene is more reactive than 2-butene. However, steric effects become dominant for 2,3-dimethylbutene. This incursion of steric effects in oxymercuration has long been recognized and is exemplified by the results of Nelson and co-workers, who found separate correlation lines for mono- and disubstituted alkenes. Hydroboration by 9-BBN (structures) shows a different trend steric effects are dominant and reactivity decreases with substitution. Similar trends apply to rates of addition of dibromob-orane and disiamylborane. The importance of steric factors is no doubt due in part to the relatively bulky nature of these boranes. However, it also reflects a decreased electron demand in the hydroboration TS. 

The reactions in Scheme 38 with allenes as coupling components proceed regioselec-tively with C—C bond formation at the central carbon and carbon-heteroatom bond formation at the more highly substituted terminal carbon atom t i Reactions of this type have recently been performed enantioselectivelyJ " Remarkably, for the macrocyclization in Scheme 39 the carbon-heteroatom bond formation takes place at the unsubstituted terminal carbon atomJ ... 

The octahydrophenanthrene skeleton is also formed from 2-bromotetradeca-l-ene-7,13-diynes with a trialkylsilyl-substituted terminal triple bond (Scheme 46, Eq. 1). With an unsubstituted terminal acetylene, the same bromoenediynes yield bisannelated ful-venes by a completely different cascade carbopalladation mode involving a 5-exo-trig cy-clization and a [l,5]-sigmatropic shift of the CH2PdBr group or a 3-exo-trig cyclization... 

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