A Mono-enes

One of the best examples of this type of transformation is illustrated in Fig. 1 showing spectra determined after intervals of irradiation of 45 at 404 nm 55) the new maximum at 515 nm due to 46 appears with isosbestic points (381, 463 nm, two 

A more typical example is shown in Fig. 2 which presents the results62) of 404 nm 

The first bridged cyclohexadiene shown to undergo 1,3-acyl migration was bicyclo-[2.2.1]heptenedione (50) and its photochemistry has been investigated extensively. The results obtained are quite general and will be discussed in some detail. Irradiation 66 of 50 at 404 or 436 nm in benzene solution at room temperature to high 

In addition to chemical reaction, weak fluorescence was detected from 50 at room temperature (acxc 460 nm, Xem 552 nm, cj)f = 0.04). Temperature effects on reaction and fluorescence from 77-310 K have been studied 68). A steady decrease in quantum yield for reaction ( j r) and a complementary increase in fluorescence quantum yield ( )f) were observed down to about 150K where a sharp increase in j f occurred. Photochemical reaction was negligible at 77 K (436 nm). The fluorescence lifetime at 77 K was a few nanoseconds and the estimated value at room temperature is on the order of 60 ps. Detailed analysis of the data showed that two thermally-activated processes are involved (1) chemical reaction of the singlet state with an Arrhenius activation energy of 1.5 kcal/mol and (2) radiationless decay of the singlet with Eact =1.1 kcal/mol. Both processes would appear to be associated with certain vibrational modes of the excited state which become progressively less populated with decreasing temperature. 

These temperature effects were also shown to be influenced by wavelength. Compound 48 showed temperature dependent r (436 nm) and f (450 nm) analogous to the results described above for 50. However, from a comparable study at 404 nm it was found691 that for chemical reaction at 300 K cj) 04/ j)f36 = 1 but at 150 K t) 04/ t)f36 — 30 while for fluorescence t 06/4 r5° — 0.6 and at 150 K this ratio was 

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The first metal-catalyzed [4 +2]-reaction of tethered dienes with 7r-systems was reported by Wender and Jenkins using alkynes initially as the two-carbon component.21 This study was based on the earlier observation by Wender and Ihle that in the [4 + 4]-cycloaddition of bis-dienes a competing side-reaction is the [4 + 2]-cycloaddition of the diene with a mono-ene portion of a second diene. The extension of this reaction to the synthesis of seven-membered rings by trapping the metallacycloheptadiene with CO, a formal [4 + 2 + l]-cycloaddition, has been shown in preliminary studies to be feasible. For example, tethered diene-yne 160 can be converted to cycloheptadienone 163 in an Rh(l)-catalyzed [4 + 2 + l]-reaction with CO, albeit the [4 + 2]- and [2 + 2 + l]-reaction products dominate (Equation (29)). The mechanistic scheme (Scheme 44) illustrates the possible metallacyclic intermediates leading to the observed products and provided the conceptual basis for the realization of three novel reaction types ([4 + 2], [2 + 2 + 1], and [4 + 2 + 1 ]).1... 

The Diels-Alder reaction is the reaction of a diene with a mono-ene to form a cyclohexene derivative, an important reaction for the construction of organic intermediates. One of its attractions is the atom efficiency of 100%, no by-products being formed. The mono-ene, or dienophile which may also be an alkync, has a LUMO of low energy while the diene is usually electron rich with a high lying HOMO. The interaction of these two orbitals starts the reaction between the two molecules (Figure 2.33) [26],... 

The results of the reductions of some steroidal a,)3-unsaturated ketones have been summarized by Brown. " The carbonyl group is usually reduced to the hydrocarbon, but the behavior of the double bond depends on the structure of the compound undergoing the reduction. Cholest-4-en-3-one gives chol-est-4-ene. Addition of aluminum chloride to a solution of a 4-ene-3,6-dione followed by treatment with LiAIH4 gives the 4-ene-6-one. Steroid 4,6-dien-3-ones yield mixtures of dienes. When the ketone and double bond are in different rings the results become even more complex dienes as well as mono-enes are obtained. 

The isotopic purity of the product is usually about 48-62%, the rest of the material being mainly undeuterated. (An alternate preparation of a-mono-deuterio ketones of high configurational and isotopic purity is the mild oxidation of cis- or tra 5-deuterated alcohols under Jones conditions, see sections V-D and VII-A.) Treatment with zinc in acetic acid-OD has also been applied to the deiodination of 2a-iodoandrost-4-ene-3,17-dione. In a slightly modified version the iodine in 19-iodocholesterol acetate has been replaced with tritium by using tritium oxide as the isotope source/... 

In the next step, one of the borane-hydrogens is transferred to a sp -carbon center of the alkene and a carbon-boron bond is formed, via a four-membered cyclic transition state 6. A mono-alkyIborane R-BH2 molecule thus formed can react the same way with two other alkene molecules, to yield a trialkylborane R3B. In case of tri- and tctra-substituted alkenes—e.g. 2-methylbut-2-ene 7 and 2,3-dimethylbut-2-ene 9—which lead to sterically demanding alkyl-substituents at the boron center, borane will react with only two or even only one equivalent of alkene, to yield a alkylborane or mono alky Iborane respectively ... 

Allylic carboxylation. Diethyl oxomalonate (1) undergoes a thermal ene reaction with mono-, di-, and trisubstituted alkenes at 145 180°. The reaction is also subject to catalysis with Lewis acids, which can lead to a different ene product. The products are a-hydroxymalonic esters. The corresponding malonic acids are converted to carboxylic acids by bisdecarboxylation with NaI04 and a trace of pyridine- or with ceric ammonium nitrate (CAN). Diethyl oxomalonate then functions as an cnophilic equivalent of C02. 

Two other Ni(CO)4 substitutes, Ni(CO)3PPh3 and Ni(COD)2/dppe, prove to be appropriate for the catalysis of tandem metallo-ene/carbonylation reactions of allylic iodides (Scheme 7)399. This process features initial oxidative addition to the alkyl iodide, followed by a metallo-ene reaction with an appropriately substituted double or triple bond, affording an alkyl or vinyl nickel species. This organonickel species may then either alkoxycar-bonylate or carbonylate and undergo a second cyclization on the pendant alkene to give 51, which then alkoxycarbonylates. The choice of nickel catalyst and use of diene versus enyne influences whether mono- or biscyclization predominates (equations 200 and 201). 

These Nazarov approaches add three extra carbon atoms (usually CH2CH2CO) onto a ketone to form the five-membered ring. It is also possible to add the three atoms CH2CO.CH2 onto an alkene to form a cyclopentenone of different structure. The oxyallyl cation 87, which can be made by the action of a metal, often zinc, to a,a -dibromoketones 85, might provide the three carbon atoms. Unfortunately it is a two electron system and so adds to dienes 88 rather than mono-enes giving cycloheptenones 89 in good yield.22... 

If iron carbonyl is used to generate the oxyallyl cation, the iron supplies two extra electrons in the complex 90 which now adds23 to mono-enes to give cyclopentanones 91. We shall see more examples of transition metal complexes altering selectivity later in the book. To get a cyclopentenone 92 we need an extra degree of unsaturation, best provided by using the enamine of a ketone instead of a simple alkene. 

A related process involves the condensation of ketones with the mono-IV,IV-dimethylhydrazone of glyoxal. Base-catalyzed condensation gives the monohydrazone of a 2-ene-1,4-dione and reductive cleavage of the N—N bond affords 2,3-disubstituted pyrroles (Scheme 8) <7 CB49i>. 

Monomer purity is vital in ADMET polymerization primarily for two reasons (i) any mono-ene impurity will function as an end-capping agent in the polymerization, and (ii) some impurities may promote unwanted side reactions. Cationic addition was an issue when classical catalysts were used, but with the introduction of well-defined catalysts, however, this became less of a problem. Still, the elimination of unwanted side reactions is of paramount importance in ADMET polymerization. 

The addition of singlet oxygen to isoquinoline-1,3-diones (132) yields the triketone (133) via a Schenck ene reaction and the benzoisofuranone (134) via a 4 - - 2-cycloaddition (Scheme 51). The Diels-Alder addition of singlet oxygen to cis-3,8-dihydroheptalene-3,8-dicarboxylate (135) yields a mono-addition product (136), which on further addition of a second singlet oxygen produces jyn-bis(norcaradiene) bis(endoperoxide) (137) (Scheme 52). ... 

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