Ethenes alkene/alkyne reactions

The application of alkene [1] - and, more recently, enyne [2] and alkyne - metathesis to the synthesis of natural products has been triggered by the development of powerful catalysts that allow metathesis reactions to be carried out under mild conditions. Scheme 1 outlines two important cases of alkene and alkyne metathesis of particular interest to the synthesis of natural products (together with the general scheme of enyne metathesis, not discussed in this review). The metathesis products can be obtained in high yields, since ethene/2-butyne are formed as volatile products. After the alkene/alkyne metathesis, the substituents (R) of the alkenes/alkynes are located on the same multiple bond. Enyne metathesis can be considered as the more general case of alkene metathesis, because two new double bonds are again formed, albeit now connected by a single bond. 

We dealt with [4+2]-cycloadditions very briefly in Section 3.3.1. As you saw there, a [4+2]-cycloaddition requires two different substrates one of these is an alkene—or an alkyne—and the other is 1,3-butadiene or a derivative thereof. The reaction product, in this context also called the cycloadduct, is a six-membered ring with one or two double bonds. Some hetero analogs of alkenes, alkynes, and 1,3-butadiene also undergo analogous [4+2]-cycloadditions. In a [2+2]-cycloaddition an alkene or an alkyne reacts with ethene or an ethene derivative to form a four-membered ring. Again, hetero analogs may be substrates in these cycloadditions allenes and some heterocumulenes also are suitable substrates. 

These reactions are found to be promoted by electron-donating substituents in the diene, and by electron-withdrawing substituents in the alkene, the dienophile. Reactions are normally poor with simple, unsubstituted alkenes thus butadiene (63) reacts with ethene only at 200° under pressure, and even then to the extent of but 18 %, compared with 100% yield with maleic anhydride (79) in benzene at 15°. Other common dienophiles include cyclohexadiene-l,4-dione (p-benzoquinone, 83), propenal (acrolein, 84), tetracyanoethene (85), benzyne (86, cf. p. 175), and also suitably substituted alkynes, e.g. diethyl butyne-l,4-dioate ( acetylenedicarboxylic ester , 87) ... 

As with the transition state of the [4+2]-addition of butadiene and ethene (Figure 15.8) both HOMO/LUMO interactions are stabilizing in the transition state of the [2+2]-addition of ketene to ethene (Figure 15.13). This explains why [2+2]-cycloadditions of ketenes to alkenes—and similarly to alkynes—can occur in one-step reactions while this is not so for the additions of alkenes to alkenes (Section 15.2.3). 

Although alkali metal/liquid ammonia reductions (Birch reductions) of simple alkenes is difficult, presumably as a result of the very high energy of an ethene type LUMO, the corresponding reduction of non-terminal alkynes to trawi -alkenes is an efficient and useful synthetic tool for accessing trans-alkenes [116]. The mechanism for this reaction (Scheme 69), involves the homogeneous reduction of the alkyne to the corresponding anion radical by the solvated electrons present in liquid ammonia solutions of alkali metals. 

Iron complex (55) also reacts with H2 to produce methane and ethene to afford propene <80JA1752>. Both reactions appear to involve insertion into a metal-carbon bond followed by elimination. When osmium complex (56) adds ethene, the diosmacyclopentane which results from ethene addition is isolated. When terminal alkynes react with (55), an alkene-substituted ring carbon results... 

Experimental evidence and computational analysis point to a mechanism in which the alkene (or alkyne) carbons and the M-H bond must be nearly coplanar to react. Once the metal alkene complex has achieved such geometry, 1,2-insertion can occur. During insertion, the reactant proceeds through a four-center transition state. 14The reaction involves simultaneous breakage of the M-H and C-C n bonds, as well as the formation of an M-C a bond and a C-H bond at the 2-position of the alkene (or alkyne). The result is a linear compound, L M(CH2CH3), in the case of ethene insertion. The reverse reaction, (3-elimination, follows the same pathway starting from a metal-alkyl complex with an open coordination site. 

Type Ilbd cyclocondensation reactions between isocyanide-based reagents and electron deficient alkenes or alkynes provides a convenient S3mthetic approach to highly functionalized pyrroles. The cyclocondensation between tosyl methyl isocyanide (TosMic) and 1,2-bis(benzenesulfonyl)ethene provided 3,4-bis(benzenesulfonyl)p)nTole 37 <05JHC333>. 

Thermolysis of (44) in the presence of alkynes results in 1,4-disilanaphthalenes <9lOM3l73>. As already mentioned, they can be formed in higher yields during the nickel <92JOM(439)l9) and platinum catalyzed reactions <930M4987> of the (44) with diphenylalkyne and ethene respectively. Platinum-complex catalyzed dehydrogenative double silylation of alkynes, alkenes, and dienes with... 

Recent additions to the family of alkene complexes are fullerene derivatives such as Rh(CO)(q -Cgo)(H)(PPh3)2 Pd(q -Cgo)(PPh3)2 (Figure 23.17b) and (q -Cp)2Ti(q2-Cgo). The Cgo cage (see Section 13.4) functions as a polyene with localized C=C bonds, and in Cgo Pt(PEt3)2 6, six C=C bonds (remote from one another) in the Cgo cage have undergone addition. Reaction 23.73 illustrates CgQ-for-ethene substitution (the 16-electron centre is retained), and reaction 23.74 shows addition to Vaska s compound (a 16- to 18-electron conversion). Equation 23.75 shows the formation of the first fullerene complex of titanium, by fullerene displacement of a coordinated alkyne. 

Cyclopropene can also be used as the alkene component and affords bicyclo[3.1.0]hexen-2-ones upon reaction with alkyne dicobalt octacarbonyl complexes in the presence of NMO (Scheme 250). Vinyl ethers and vinyl esters serve as ethene equivalents in Pauson-Khand reactions. For example, reaction of vinyl benzoate with complex (169) furnished cyclopentenone (170) (Scheme 251). This reactionwas used in a synthesis of (+)-taylorine and nortaylorine. Allenes participate in intermolecular Pauson-Khand reactions affording alkylidene-substituted cyclopentenones (Scheme 252). ... 

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