Alternating diene metathesis

ROMP Ring-opening metathesis polymerization ADMET Acyclic diene metathesis polymerization ALTMET Alternating diene metathesis polycondensation MCLCP Main chain liquid crystalline polymer SCLCP Side chain liquid crystalline polymer mru molecular repeating unit... 

Closing this section, a new metathesis-based method for the preparation of strictly alternating copolymers should be mentioned. Alternating diene metathesis polycondensation (ALTMET) was used to prepare a MCLCP consisting of an alternating architecture of a calamitic (linear rod-like core) and a banana-shaped (bend-shape core) mesogen (Fig. 24). 

Perfectly alternating copolymers can be obtained via alternating diene metathesis (ALTMET) polymerisation (Scheme 5.18), a concept introduced by Slugovc and colleagues more than a decade ago [65]. This concept is based on the high CM selectivity between terminal double bonds and acrylates in the presence of a second-generation Grubbs catalyst. If a terminal diene (considered a type-I olefin) reacts with... 

Demel, S. Slngovc, C. Stelzer, F. Fodor-Csorba, K. Galli, G. Alternating diene metathesis polycondensation (ALTMET) - a versatile tool for the preparation of perfectly alternating AB copolymers. Macromol. Rapid. Commun. 2003, 24, 636-641. 

Metathesis reactions may be intramolecular and ring-closing diene metathesis (RCM, implicated in Scheme 1.13, see Chapter 12) allows disconnections in retro-synthetic analysis otherwise of little use. The normal disconnection of the macrocyclic amide in Scheme 1.13 would be at the amide but, because of the ready reduction of alkenes to alkanes, the alternative disconnection now becomes a viable option. And since any of the C—C linkages could be formed by RCM, such a disconnection allows far greater synthetic flexibility than the conventional disconnection at the functional group. 

With the advances in pro-catalyst design that have been witnessed over the last decade or so, the transition-metal-catalysed alkene metathesis reaction has now become a practical procedure that can be utilised by the chemist at the bench. Undeniably, this has added a new dimension to the repertoire of synthetic organic chemistry as it facilitates disconnections that, pre-metathesis, simply would not have been considered. Take, for example, a macro-cyclic amide where the normal disconnection would be at the amide. Now, with the ready reduction of alkenes to alkanes, a ring-closing diene metathesis (RCM), followed by hydrogenation, becomes an alternative disconnection. And, when one considers that any of the C—C linkages could be established in such a manner, the power of the RCM disconnection becomes obvious. 

The use of acyclic diene metathesis (ADIMET) provides an alternative synthetic approach to analogous Co polymers of type 144b with values of 1.3 x 10 -3.1 x 10 (PDI = 2.9-3.7) (Equation (52)). These materials... 

Subjects specifically excluded are cycloolefin polymerizations catalyzed by naked nickel catalysts, palladium-catalyzed ethylene/carbon monoxide alternating copolymerizations, metathesis polymerizations of cyclic olefins, and diene polymerizations... 

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). 

An illustrative example of an alternative strategy (cf Fig. 11c) involving the use of a novel traceless linker is found in the multistep synthesis of 6-epi-dysidiolide (363) and several dysidiolide-derived phosphatase inhibitors by Waldmann and coworkers [153], outlined in Scheme 70. During the synthesis, the growing skeleton of 363 remained attached to a robust dienic linker. After completion of intermediate 362, the terminal olefin in 363 was liberated from the solid support by the final metathesis process with concomitant formation of a polymer-bound cyclopentene 364. Notably, during the synthesis it turned out that polymer-bound intermediate 365a, in contrast to soluble benzoate 365b, produced diene 367 only in low yield. After introduction of an additional linker (cf intermediate 366), diene 367 was released in distinctly improved yield by RCM. 

The homoallylation product 16a presumably stems from oxidative cycloaddition of a Ni(0) species across the diene and aldehyde moieties of 15, leading to an oxanickellacycle intermediate 17 (path A, Scheme 5), which undergoes 0-bond metathesis with triethylsilane giving rise to a o-allylnickel 19. On the other hand, formation of 16b may start with addition of a Ni - H species upon the diene followed by intramolecular nucleophilic allylation as described in Eqs. 4-6 (path B). Alternatively, allylic transposition of the NiH group providing 20 from 19 may be related to the formation of 16b. The different reactivity between cyclohexadiene and many other acyclic dienes is also observed for the reaction undertaken under typical homoallylation conditions (see Scheme 14). 

RCM of dienes to cycloalkenes provides a useful method for the syntheses of carbo- and heterocycles and thus has been proved to be extremely effective in total synthesis of various natural products. Usually, however, mixtures of (E)- and (Z)-olefms result. In contrast, ring-closing alkyne metathesis provides a reliable route for synthesis of both (E)- and (Z)-macrocycloalkenes in a stereoslective manner taking advantage of stereoselective partial reduction of resulting cycloalkynes. A Lindlar reduction gives (Z)-cycloalkenes, whereas a hydroboration/ protonation sequence afford ( )-cycloalkenes (Equation (23)). Recently, Trost reported an alternative procedure for the synthesis of (E)-olefins from alkynes through hydrosilylation by a ruthenium catalyst. This procedure converts cycloalkyne 130, for example, to vinylsilane 131 and then to (E)-cycloalkene 132 in a stereoselective manner (Scheme 46)7 ... 

Title Synthesis of A,B-Alternating Copolymers by Olefin Metathesis Reactions of Cyclic Olefins or Olefinic Polymers with an Acyclic Diene... 

While cumulene V might evolve directly into dimethylfiimarate derivative III by proton transfer, alternatively, it might evolve into cyclobutene VI, whereby the desired C-6 methyl acetate unit is expediently formed. Such a structure has very little chance to survive in a hot reaction medium and three decomposition reactions are likely to take place (1) return to starting materials (2) a metathesis-type retro(2-l-2]cycloaddition to diene VII, something that finds... 

Reduction of TiCl4(THF)2 with Mg in the presence of excess (RNC(Ph))2 (R = Ph, C6H4Me, C6H4OMe) led to the formation of diaza-diene complexes Ti(RNC(Ph))2)3.939 Alternatively, the analogous Ti(Pr NCH)2)3 was prepared by metathesis using the corresponding Li ligand salt.940 The molecular structure of the latter octahedral complex was determined.940... 

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