Norbomadienes 2 + 2 + 2 cycloaddition reactions

Having established the validity of the Huckel analysis of these molecules, it is useful to extend the method to the study of the cycloaddition reaction of the RjPSjNj molecules with norbomadiene. Thus the additidn of a 2e-olefin to the R PS Nj system can be regarded as a novel example of a thermally allowed 8 + 2 reaction. As in the addition of olefins to the preference for S,S-addition over... 

In an analogous reaction of (2) with norbomadiene, the tetracyclic adduct (116) is formed which on heating undergoes a retro Diels-Alder reaction with formation of the thiophene derivative (117). Further cycloaddition reactions of mesoionic 1,3-dithiolones have been carried out with cyclopentadiene, 1,3-cyclohexadiene and 1,5-cyclooctadiene (78CB3037). 

Itoh and coworkers carried out tandem [2 - - 2 - - 2]/[4 - - 2] cycloadditions catalyzed by a ruthenium catalyst. The reaction of diyne 147 with excess norbomene 148 in the presence of ruthenium catalyst 153, for example, afforded 149. Adduct 150 either dissociated from the catalyst or reacted with anotoer equivalent of norbomene. In the latter case, a ruthenium catalyzed Diels-Alder reaction occurred, affording hexacyclic adduct 152 via 151 (equation 43). Compounds 150 and 152 were obtained in yields of 78% and 10%, respectively. Both cycloaddition reactions proceeded with complete stereoselectivity. When 1,6-heptadiyne was used instead of 147, only trace amounts of a cycloadduct were obtained. Replacing norbomene by norbomadiene, which was expected to result in polymer formation, did not afford any adduct at aU. 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

In the case of the chromium carbonyl compounds the intermediacy of norbomadiene-Cr(CO) 4 has been demonstrated. 1,4-diphenylbuta-diene-Fe(CO) 3 and [(C0H5)3P]2Ni(CO)s have been recovered to >90% showing their catalytic function. UV irradiation of norbomadiene in the presence of Ni(CO)4 gives the following product resulting from a formal (2 + 2 + 2) cycloaddition reaction 571> ... 

Several iodonium- and bis(iodonium) norbomadienes and other polycyclic adducts have been synthesized by [2-1-4] cycloaddition reactions of alkynyliodonium triflates with cyclic 1,3-dienes [458,460-464]. In particular, the bis-iodonium acetylene 331 undergoes Diels-Alder reactions with cyclopentadiene 329, furan... 

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. 

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... 

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. 

Metallacycles are also featured in Chapters 2 and 3 of this volume. Cobalt- and nickel-based metallacycles are key cataljrtic intermediates in Transition-Metal-Catalyzed Cycloaddition Reactions of Bicy-clo[2.2.1]heptadienes by Professor Mark Lautens and Dr. William Tam of the University of Toronto (Chapter 2). The high sttain energy of norbomadiene makes possible many of the interesting processes covered in this informative chapter. It is followed by a comprehensive review of the State of the Art in Selective Hetero- and Carbocyclic Synthesis Mediated by Cyclometallated Complexes ... 

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. 

One important use for ruthenium catalysts is in cycloaddition reactions. A Ru-catalyzed cascade reaction of norbomadiene 12 with diynes 13, furnishing nor-bomene derivatives 14, was reported by Cheng et al. (Scheme 5.6) [13]. The process involves Ru-catalyzed [ 2+2)+2] cycloaddition and transfer hydrogenation. 

A catalytic asymmetric cycloaddition reaction between norbomadiene and methylenecyclopropane can also be achieved in the presence of a [Ni(cod)2]-(—)-benzylmethylphenylphosphine catalyst to give the cycloadduct (72) in an optically active form. This reaction may proceed via a metallocyclopentane intermediate. The reactions of methylenecyclopropane with [Ni(cod)2l-phosphine systems do not appear to involve cleavage of the three-membered ring. However, the bis(acrylonitrile)nickel-catalysed cycloaddition reaction of methylenecyclopropane with methyl acrylate, which yields 3-methoxy-carbonylmethylenecyclopentane (73), does involve C—C bond cleavage. Reaction with the deuterium-substituted compound CHD=CDC02Me gives the cyclopentane derivative (74). An intermediate of the type (75) may be involved in this reaction. 

Figure 9.8 Endergonic and reversible electrocyclic reactions obeying Woodward-Hoffman rule, (a) Valence isomerization, (b) cycloaddition, (c) sigmatropic effect, and (d) norbomadiene to quadricyclene conversion.

The final example of the intramolecular 1,3-dipolar cycloadditions of nitrile oxides is the formation of the norbomadiene-derived tetracyclic adducts 146, described by Tam and co-workers (240,241). The nitrile oxide 145, formed from 144 by dehydration, can in principle give rise to four different cycloaddition products (three [2,3]-cycloaddition products). In practice, only diastereomer 146 was obtained. The reaction was used on substrates with a variety of different substituents (R=H, Me, hexyl, Cl, Br, C02Me, CH2OMe), and in these cases, yields ranging between 66-89% were obtained (Scheme 12.48). 

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. 

Thermodynamically unfavorable cycloaddition products can be formed photochemically. A striking example is the photochemical conversion of norbomadiene to quadricyclene. The reverse of this reaction can occur with almost explosive violence in the presence of appropriate metal catalysts or on simple heating ... 

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). 

A variety of alkenes can participate in the cycloaddition. Simple alkenes such as ethylene and allene will react to form methylenecyclopentane adducts. Facile cycloadditions with strained alkenes are also observed. For example, norbomene reacts smoothly with (79) to give only the exo adduct (80) in good yield (equation 64). Electron-deficient alkenes, having an ester, ketone or sulfone activating group, are also substrates. However, the methylenecyclopropane cycloaddition does not appear to be very chemoselective. This is demonstrated by the Pd-catalyzed reaction of (79) with 2,3-dimethoxycar-bonylnorbomadiene, where both double bonds of the norbomadiene react to an almost equal extent (equation 65). ... 

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