Norbomadiene homo-Diels-Alder cycloaddition

Nonetheless, there are a small number of systems that do mediate such [2 -i- 2 -t- 2] cycloadditions. With allenes as the alkene , cycloaddition with both acetylene and terminal alkynes proceeds regio-selectively to give 3,5-dimethylenecyclohexenes using Ni catalysts, and mostly 3,6-dimethylenecyclo-hexenes using Ni° catalyst precursors (equation 19). Norbomadiene undergoes so-called homo-Diels-Alder cycloaddition with both alkenes and a ynes in the presence of nickel catalysts. Further elaboration of this chemistry with alkynes but not alkenes has been described using a Co/Al catalyst system (equation 20). Attempts to produce cyclohexenes via all-intramolecular [2 + 2 + 2] cycloaddition of l,13-dien-7-ynes or 1,1 l-dien-6-ynes have been unsuccessful. ... 

In addition to the assembly of various carbocyclic and heterocyclic six-membered ring assembly, nickel-catalyzed [2+2+2] homo-Diels-Alder cycloadditions have been illustrated using norbomadiene derivatives (Scheme 3-30). Electron-deficient dienophiles undergo cycloadditions with norbomadiene using catalysts prepared from Ni(cod)2 and PPI13. Chemical yields and exo-selectivities are both generally high in these processes. 

Our overall objective is to explore the utility of the [2ti + 2tc + 2%] (homo Diels-Alder) cycloaddition reactions of norbomadiene as a route to synthesizing linearly and angularly fused polycyclic natural products... 

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

The Homo Diels-Alder Reaction of Norbomadiene with Acetylenes. [2 + 2+2] Cycloadditions of dienes such as norbomadiene with the double bonds in 1,4-position are called homo Diels-Alder reactions. Using an in situ catalyst (consisting of Co(acac)3-Et2AlCl-bis(diphenylphosphino)ethane) the products obtained with monosubstituted acetylenes, such as phenyl, i-propyl-, n-butyl-, t-butyl-, and trimethylsilylacetylene, are 4-substituted deltacyclenes. - In the formation of the polycyclic deltacyclene skeleton, six new stereo centers are generated in one step. Thus enantiocontrol by using optically active phosphine ligands as cocatalysts allows the synthesis of optically active cycloadducts, as shown for the reaction of norbomadiene with phenylacetylene to give 4-phenyldeltacyclene (eq 1). 

Cycloaddition of benzyne occurs in two cases of homo-Diels-Alder additions (e.g., to norbomadiene) reported in Ref. 1. 

The cycloaddition of norbomadiene with al-kynes to give deltacyclenes has also been described as a homo-Diels-Alder reaction ... 

Cycloaddition Reactions with Norbomadienes (The Homo Diels-Alder Reaction).55... 

In this article, we focus the discussion on our investigations on [27t + 2n + 2ti] (homo Diels-Alder, HDA) and [2 i + 2 t] cycloaddition reactions of norbomadiene that were carried out during the past seven years. 

We have investigated the scope of the cobalt- and nickel-catalyzed [2 + 2 + 2] (homo Diels-Alder) and [2 + 2] cycloaddition reactions with norbomadienes. Excellent chemo-, regio- and stereoselectivities and high levels of enantioselectivities are achieved. 

An example of Cp RuQ(cod)-catalyzed homo Diels Alder [2+2+2] cycloaddition of alkynylphosphonates with norbomadiene providing the corresponding phosphonate substituted deltacyclenes was also described [133] [Eq. (63)]. This reactivity was found to be dependent on the presence of the phosphonate moiety. 

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