Disubstituted norbornadienes

The tungsten analogue of 149a adds to 139 to form a transoid square-pyramidal metallacyclobutane eomplex, which can be isolated and characterized at low temperature. Its rearrangement to the propagating metal carbene complex can also 

and on irradiation of these protons, the signal collapses to the expected AB quartet with a coupling constant of 10 Hz characteristic of cis C=C. It seems safe enough to carry over these conclusions to the polymers of 140a but to do so for the polymers of 139 is less secure (McConville 1993, O Dell 1994). 

The all-CIS polymers in the series 140 (R = Me, Et, i-Pr, t-Bu), initiated by 149c in toluene, have an isotactic bias ( r 0.78, 0.84, 0.81, 0.97, respectively), as judged by the C-7 fine structure in the C NMR spectra, assuming that the line order is the same in each case (Schrock 1995b). A remarkable observation has been made for the system 140c/149g. The GPC of the polymer (M — 28 200) shows two sharp peaks (Fig. 13.7), each of which has 1.06, and taken together, 1.13. 

This has been attributed to the fact that the initiator is a 1 1 mixture of non-interconvertible enantiomers and that highly regular all-cw isotactic chains grow separately from each enantiomeric metal site. The chirality at the metal site, interacting with the chiral monomer, controls the rate of propagation, while the chirality of the chain end , i.e. the previously added monomer unit, independently controls the stereochemistry of the next monomer addition. This effect is not observed with 140b as monomer, where the rates of propagation at the two types of site are presumably not sufficiently different (O Dell 1994). 

The ROMP of the dicyano analogue of 139, also the tricyclic monomer having [CH2C(CN)2C(CN)2CH2] attached to the 5,6-positions, can be initiated by 149a, the latter giving a 97% trans polymer (Feast 1995). 

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Scheme 1 ROMP of a 2,3-disubstituted norbornadiene, a 2-substituted norborn-5-ene and polymerization of a 1-alkyne via a- and /3-addition, respectively. A and B are initiator-and termination-derived endgroups, respectively...

Norbornadiene is the most reactive and extensively used diene in these reactions. The reactivity of other 1,4-dienes has been investigated under atmospheric and high-pressure conditions. Free CH groups at both ends of the 1,4-diene appear to be necessary for homo-Diels-Alder reactivity 2,3-disubstituted norbornadienes were totally unreactive with a variety of dienophiles. ... 

SCHEME 20.21 Proposed chain end control mechanism producing trans-syndiotactic polymers in ROMP of 2,3-disubstituted norbornadienes (Ar = 2,6-i-Pr2C6H3, P = polymer chain R = CF3, C02Me, or chiral substituent R arrow indicates monomer approach) (Reference 85). 

A detailed study of ROMP of disubstituted norbornadienes (e.g., 2,3-dicarbo-methoxynorbornadiene or 2,3-bis(trifluoromethyl)norbornadiene (NBDF6) [124] showed that they are polymerized by Mo(f-butoxide) initiators in a well-behaved living manner to give essentially monodisperse homopolymers that are... 

Metal-Catalyzed Formal [4+2] CycUzations of Conjugated Enedienes and Dienynes Enantioselective cyclizations of enedienes or dienynes lead, respectively, to optically enriched bicyclic cyclohexenes or cyclohexadienes. Importantly, only intramolecular approaches have afforded satisfactory results so far. For these transformations, the metals of choice were found to be rhodium and iridium in the presence of various chiral ligands. Main examples were reported with DIOP, BINAP, bidental diphosphane such as BDPP and DuPHOS, and/or dienes [22] as the ligands. Thus, dienynes can lead to chiral bicyclic cyclohexadienes in high yields and enantioselectivities, by treatment with cationic rhodium in the presence of a disubstituted norbornadiene as the chiral ligand (Scheme 7.15). 

Disubstituted acetylenes or olefins, including cyclohexene, norbornadiene, styrene, and a-methylstyrene, do not react with metal or metalloid amides. However, powerfully electron-withdrawing groups conjugated with the acetylenic or ethylenic bond encourage addition to occur ... 

Optically active polyacetylene derivatives 97 were synthesized through ring-opening polymerization of the corresponding cyclooctatetraene derivatives.25 A twisted conformation of the main chain was proposed on the basis of CD and UV absorptions. Various optically active polyacetylenes have also been prepared from chiral monomers.24,25,263,177-183 The examples include a phenylacetylene derivative (98),26a alkylacetylenes 99,24 propionic esters such as 100,177,178 a Si-containing monomer (101),179 and disubstituted monomers such as 102.180 Poly-(A)-98 synthesized using a [RhCl(norbornadiene)]2 catalyst shows intense CD bands in the UV—vis region, probably based on a predominant helical sense of the main chain.263 This polymer effectively resolves several racemic... 

Obviously, substitution of the norbornadiene double bond with two electron-withdrawing groups lowers the activation energy barrier for valence isomerization even more. A summary of examples for the preparation of 1,5-disubstituted quadricyclanes 4 is given."... 

The reaction of norbornadiene with various Z- and f-disubstituted dienophiles is highly regio-and stereospecific, giving products 2 implying a concerted cycloaddition and giving evidence for homoconjugation in this diene. ... 

The reactivity of the catalyst is enhanced by azeotropic drying of the cobalt complex before use, thus allowing the reaction of sterically hindered and silyl-substituted alkynes. Good reactivity between norbornadiene and disubstituted alkynes (yields in homo-Diels-Alder adducts ca. 85%) is also achieved by adding zinc powder to cobalt iodide/triphenylphosphane catalysts. " ... 

An early use of the norbornadiene reaction in synthesis employed conjugated addition followed by retro-Diels-Alder to deliver 4,5-disubstituted cyclopentenones [Eq. (42)] [102]. More recently, this approach was adopted with considerable methodological improvement (including use of dimethylsulfoxide (DMSO) as a promoter/solvent) to a preparation of substituted cyclopentadienyl ligands for use in transition metal chemistry [Eq. (43)] [103]. 

However, beside PI, P2 and the perfluorated poly(norbornadiene) P9, the oxygen permeabilities of most ROMP polymers, measured on films of 20-100 pm thickness [11], are relatively low, not sufficient for applications as contact lenses. The values are comparable to saturated hydrocarbons like low density polyethylene (LDPE) which has an oxygen permeability of 2.6 barrer, measured under similar conditions [11]. The situation is different for mono- and disubstituted poly(acetylenes), which will be discussed in part 2. 

Amir-Ebrahimi, V. Corry, D. A. K. Hamilton, J. G Rooney, J. J. Determination of the tacticities of ring-opened metathesis polymers of symmetrical 5,6-disubstituted derivatives of norbornene and norbornadiene from the NMR spectra of their hydrogenated derivatives. J. Mol. Catal. A Chem. 1998,133,115-122. 

Cycloocta-1,3-diene merely rearranges to the 1,5-isomer, norbornadiene is polymerised, and no reaction occurs with 2,5-dimethylhexa-2,4-diene. Disubstituted alkenes, RCH=CHR, afford isomerised terminal 1 1 adducts. Similarly, terminal alkenes RCH CH2 gave RCH2CH2SiX2 (X = Ph, OEt) or a mixture of... 

The Ni -catalysed (2+2) cycloaddition of activated olefins such as acrylonitrile or acrylic acid esters to quadricyclane which gives endo- and exo-substituted products is complicated particularly in the case of 1,2-disubstituted olefins by a competing isomerization of quadricyclane to norbornadiene. Norbomadiene gives identical cycloadducts under these conditions with a similar isomer ratio. Kinetic studies of the cycloaddition reactions of quadricyclane gave a plot of log [quadricyclane]t o/ [quadricyclane]i=t against time which exhibited a marked deviation from linearity due to the retarding effect of norbomadiene which is also formed and in turn coordinates to the Ni catalyst. Scheme 11 accounts for these observations. 

Several syntheses of aldehydes and ketones based on the insertion of carbon monoxide have been reported. Reaction of norbornadiene with monosubstituted acetylenes in the presence of octacarbonyldicobalt gives the fused cyclopen-tenone (35) in high yield conjugate addition of a cuprate and vacuum pyrolysis completes a simple synthesis of 2,3-disubstituted cyclopent-4-enones (36) (Scheme 30)." Conjugated dienes with methyl iodide in the presence of the same cobalt reagent yield conjugated dienones (37) [equation (8)]." In a review of carbonyl insertion reactions, the same reagent is reported to catalyse the reaction... 

HC=CR and RC=CR denote mono- and disubstituted acetylenes, respectively, acac, acetylacetone nbd, 2,5-norbornadiene. 

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