Norbomadiene adducts

This reaction, which is allowed orbitally, is contrasted with the formation of a norbomadiene adduct by a symmetry forbidden reaction where the dithiene reacts presumably in its first excited state (27) ... 

Early work established that S4N4 forms di-adducts with alkenes such as norbornene or norbomadiene. Subsequently, structural and spectroscopic studies established that cycloaddition occurs in a 1,3-S,S"-fashion. The regiochemistry of addition can be rationalized in frontier orbital terms the interaction of the alkene HOMO with the low-lying LUMO of S4N4 exerts kinetic control. Consistently, only electron-rich alkenes add to S4N4. 

Dehydrobenzene or benzyne 158 can be trapped by all manner of species. 1,2-Dehydro-o-carborane 159 has been shown to undergo many of the same reactions as its two-dimensional relative, 1,2-dehydrobenzene. Although dehydroaromatic molecules can be formed in a variety of ways, synthetic pathways to 1,2-dehydro-o-carborane are quite limited. An effective procedure reported so far78 first forms the dianion by deprotonation of o-carborane with 2 equiv. of butyllithium. Precipitated dilithium carborane is then treated with 1 equiv. of bromine at 0°C to form the soluble bromo anion 160. Thermolysis of 160 with anthracene, furan, and thiophene as substrates leads to the adducts 161-164.79 80 1,2-Dehydro-o-carborane reacts with norbomadiene to give both homo 2+4 and 2+2 addition, leading to three products 165-167, in a 7 1 ratio79. An acyclic diene, 2,3-dimethyl-... 

Besides the use of porphyrins as azomethinic ylide derivatives, the porphyrin macrocycle can also be used to generate porphyrinic nitrile oxides 55 (Scheme 17) <04RCB(E)2192>. Thus, the treatment of oxime 54 with /V-bromosuccinimide in the presence of triethylamine, led to the formation of nitrile oxide 55, which was trapped in 1,3-DC reactions with dimethyl maleate and 2,5-norbomadiene to afford 56 and 57, respectively. In the reaction with 2,5-norbomadiene, if an excess of 55 was used, then the corresponding bis-adduct was obtained in good yield. 

Ejection of dinitrogen from the triazoline adducts to form the related aziridines was promoted by ultraviolet irradiation (300 nm, benzene) and usually proceeded in excellent yield. An exception was found in the irradiation of the triazoline substrate 59, where cleavage of the cyclobutane ring occurred as the dominant reaction pathway to form the pyridazino norbomadiene 61 (and secondary photoproducts derived therefrom), together with the triazole-4,5-diester 62. A role for the pyridazine ring and the 2-pyridyl substituents in stabilising the diradical intermediate 60 has been proposed for this abnormal outcome (Scheme 8). 

Reaction of norbomadiene 74 (in excess) with 7V-benzyl aziridine 67 formed exclusively the all-sy l l-adduct 77. This stereochemistry, confirmed by NOE between Ha and Hb, resulted from attack at the underface of the dipole by the exo-face of the dipolarophile. Similarly, reaction of A -benzyl aziridine 67 with the diacetoxybenzonorbomadiene 30 gave a single adduct 78 (Scheme 11), the symmetrical structure of which was clearly apparent in the ll NMR spectrum. These stereochemical outcomes demonstrated that the transition state (TSa), in which the methano-bridge was adjacent to the (V-substituent, was favoured in the A -benzyl series (X and R small), and in accord with the semiempirical calculations. 

The DMS method has not been employed yet for the generation of 117 and 123, since the dibromocarbene adducts of norbomadiene and norbornene rearrange under the usual conditions for the preparation [89]. However, they could be synthesized at -60 °C by taking advantage of tetrabromomethane and methyllithium as a source of the carbene [90] and could prove stable enough to serve as precursors of 117 and 123. On the other hand, the adducts of bromofluorocarbene to norborna-diene and norbornene having the fluorine atom in a cis-orientation should be isol-able at room temperature and hence be usable as stable precursors of 117 and 123. These variations ofthe DMS method were published on the occasion ofthe preparation of cycloadducts of l-oxa-2,3-cyclohexadiene (351) (Section 6.3.6) [35, 91], 1,2,4-cyclohexatriene (162) and 3d2-lJ-f-naphthalene (221) (Section 6.3.4) [35, 92],... 

Adducts were obtained as exclusive adducts or as by-products in the nickel mediated reactions of some substituted norbomadienes with various dienophiles. The formation of these products was considered to result from an intermediate metallocy-clopentane species built up of the metal center, the dienophilic double bond and one of the double bonds of the norbomadiene moiety. 

The regioselectivity in the reactions of 7-substituted norbomadienes with substituted acetylenes generally proved low. The reactions of 563 and 2-methoxynorbornadiene with 1-hexyne (198) did not proceed. With 2-trimethylsilylnorbomadiene (574), adducts 575 and 576 were obtained, albeit in low to moderate yields (equation 165). The best regioselectivity (575/576 = 92/8) was obtained when the reaction was performed at room... 

As noted with the reactions between terpenes and dihalocarbenes, mono-insertion adducts at the more electron-rich sites can be isolated from the reaction of non-conju-gated acyclic and cyclic dienes although, depending on the reaction conditions, the bis-adducts may also be formed. Norbomadiene produces both 1,2-endo and 1,2-exo mono-insertion adducts with dichlorocarbene, as well as a 1,4-addition product (Scheme 7.4) [67]. The mono adduct produced from the reaction with dimethylvinylidene carbene rearranges thermally to yield the ring-expanded product (Scheme 7.4) [157] a similar ring-expanded product is produced with cyclo-hexylidene carbene [149]. 

The phosphadithiatriazenes form white crystalline adducts with norbomadiene which facilitate the characterization of the less stable R2PS2N3 compounds (R = Me, PhO) " . An X-ray structural determination of PhjPSjNj C Hg has confirmed the prediction, based on NMR spectroscopic data, that the cyclo-addition occurs in a l,3-S,S-fashion... 

The endo-adducts of dihalocarbenes onto norbomadiene and norbomene are thermally so labile that they even isomerize under the conditions of their formation. 

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. 

Both readily entered into reaction with ethylene, 1-hexene and methyl methacrylate to afford cyclic 1,2-sulphates in 50-85% yield with norbomadiene a mixture of adducts were formed, depending on the conditions. 

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). 

The cyclophosphadithiatriazines (16) reversibly form white crystalline adducts with norbomadiene, where the alkene adds in a 1,3-fashion across the sulfur atoms to give the exo adduct. This adduct formation provides a convenient way of characterizing the less-stable derivatives of (16 R = F, OPh, Me). 

The diphosphadithiatetrazocines (50) and (51) contain one more R2PN units in the ring than (16). The structure of the orange-red 1,3-isomer, (50 R = Ph), consists of an essentially planar S2N3 unit with phosphorus atoms located on opposite sides of the plane. The isomer (50) forms a 1 1 adduct with norbomadiene via 1,3-addition across the sulfur atoms. 

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). ... 

Booth et al. (M3) carried out the reaction of [Mn(CH3)(CO)j] with cyclo-pentadiene and found the reaction to be so slow that dimerization of cyclopentadiene occurred before reaction with [Mn(CH3)(CO)5] [Eq. (40)]. The reaction of [Mn(R)(CO)j] (R = CH3, Ph) with dicyclopentadiene directly, however, also gave a 1 2 adduct [Eq. (41)]. The reaction of [Mn(CH3)(CO)5] with norbomadiene, which gave a mixture of the 1 1 and 1 2 adducts, was also reported in this study. Other studies with different alkenes and alkynes gave the same type of products (M4,M5). 

This diene reacts with [2.2.1]bicycloheptadiene (norbomadiene, II) to give the Diels-Alder adduct III, useful as an insecticide and named aldrin in honor of K. Alder. The reagent is the precursor of 5,5-dimethoxy-l,2,3,4-tetrachlorocyclo-pentadiene, which see. 

The radical addition of benzenethiol to norbomadiene afforded four 1 1 adducts, including 3-phenylsulfanyltricyclo[2.2.1.0 ]heptane, in a mixture that also depended on the concentration of the starting material and on reaction temperature, At 150°C, 3-phenylsulfanyl-tricyclo[2.2.1.0 ]heptane was the major compound (73%) in a mixture obtained in 63% yield. 

A stepwise sequence of nucleophilic addition to the complexed norbomadiene followed by intramolecular alkene insertion was first observed with dichloro(norbornadiene)palla-dium(II) which, upon addition of methanol, only forms the normal adduct 11. Subsequent conversion with l,2-bis(triphenylphosphanyl)ethane (dppe), however, gave the nortricyclene complex 12. Reductive displacement of the alkyl ligand with lithium aluminum hydride or oxidative cleavage with halogens gave noncomplexed nortricyclene derivatives. ... 

Under carbonylation conditions, both the homoallylic adduct and the nortricyclene system derived from norbomadiene complexed with methanol to give methyl exo-5-methoxytri-cyclo[2.2.1.0 ]heptane-e r/o-3-carboxylate (18). ... 

Lautens and colleagues found 5 mol% Ni(( ()D)2/2PPh3 to be the most efficient catalytic system for the cycloaddition between methyl vinyl ketone (100) and norbomadiene (560). The adducts 561 and 562 were obtained with 99% overall yield and with an exo/endo ratio of >95/<5 (equation 162). 

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