Tetracyanoethene, cycloaddition reactions

Similarly small rate factors were obtained for 1,3-dipolar cycloadditions between diphenyl diazomethane and dimethyl fumarate [131], 2,4,6-trimethylbenzenecarbonitrile oxide and tetracyanoethene or acrylonitrile [811], phenyl azide and enamines [133], diazomethane and aromatic anils [134], azomethine imines and dimethyl acetylenedi-carboxylate [134a], diazo dimethyl malonate and diethylaminopropyne [544] or N-(l-cyclohexenyl)pyrrolidine [545], and A-methyl-C-phenylnitrone and thioketones [812]. Huisgen has written comprehensive reviews on solvent polarity and rates of 1,3-dipolar cycloaddition reactions [541, 542]. The observed small solvent effects can be easily explained by the fact that the concerted, but non-synchronous, bond formation in the activated complex may lead to the destruction or creation of partial charges, connected... 

Examples of the solvent-influenced competition between concerted [4 -I- 2]Diels-Alder type cycloaddition reactions and 1,4-dipolar reaction pathways with zwitterionic intermediates can be found in references [677-679], For example, in solvents of low polarity (CHCI3, CH2CI2), homofuran reacts with tetracyanoethene to form the seven-membered [4 -I- 2]cycloadduct A in quantitative yield. In solvents of high polarity (CH3CN), however, the [2- -2]cycloadduct B predominates, formed via a 1,4-dipolar activated complex and a zwitterionic intermediate [679] cf. Eq. (5-142). 

Related to Diels-Alder [2 + 2]cycloadditions are 1,3-dipolar cycloadditions, which are known to be far less solvent-dependent cf. Eq. (5-44) in Section 5.3.3. Nagai et al [169] found that the 1,3-dipolar cycloaddition reaction of diazo-diphenylmethane to tetracyanoethene (TONE) is an exception it is 180 times faster in nonbasic trichloro-methane than in the EPD solvent 1,2-dimethoxyethane cf. Eq. (7-25). The second-order... 

Mesoxalic acid esters tend to undergo cycloaddition reactions at their central carbonyl function. Treatment of complex 37 with an excess of diethyl mesoxalate furnished the oxadiphosphatricyclo[3.1.0.0 ]hexane 40 as a colorless oil in 17% yield after chromatography (Scheme 15). When a threefold excess of tetracyanoethene was added to a solution of 37 in dichloromethane at —85 °C the color of the reaction mixture changed spontaneously from red to black, and 1,6-diphos-phatricyclo[3.1.0.0 ]hexane 41 was isolated in 15% yield (Scheme 15) <1999S639>. 

Tetracyanoethene oxide, which behaves like a carbonyl ylide in cycloaddition reactions, gives the cycloadduct 31 with benzo[f>]thiophene.178... 

Tetracyanoethene undergoes [2 + 2] cycloaddition with cis- and trans-1-methoxypropene. The following facts are known about these reactions. 

The last factor often is the one that determines the reaction rates of [4+2]-cycloadditions. This factor allows one to understand, for example, why the cycloadditions of ethene or acetylene with butadiene (cf. Figure 15.1) occur only under rather drastic conditions, while the analogous cycloadditions of tetracyanoethene or acetylenedicarboxylic acid esters are relatively rapid. As will be seen, a simple orbital interaction between the reagents at the sites where the new a bonds are formed is responsible for this advantageous reduction of the activation energies of the latter two reactions. 

A prototype cycloaddition involving a zwitterionic intermediate is the reaction of -butyl vinyl ether with tetracyanoethene, as shown in Eq. (5-33) [94-98]. 

Tetracyanoethene is one of the most active and used dienophiles its cycloadditions, including homo-Diels-Alder reactions, have been reviewed in detail. Preparative homo-Diels-Alder reactions using this dienophile are presented in Table 1. 

A [2 + 2] cycloaddition occurred on reaction of 4-phenyl-4//-l, 2,4-triazoline-3,5-dione with vinylcyclopropane (1) or 2-oxabicyclo[3.1.0]hex-3-ene (3). The yields of products 2 and 4 were excellent, and the cycloadduct 4 was formed as a single (cw-a t/-< w)-isomer. Formation of both adducts is, in fact, noteworthy since 4-phenyl-4//-l,2,4-triazoline-3,5-dione (PTAD) reacts in different ways with other alkenes, and 2-oxabicyclo[3.1.0]hex-3-ene undergoes [(2 + 2J + 2J cycloadditions with maleic anhydride and tetracyanoethene. " ... 

In contrast, isolable t/ -transition-metal complexes of trimethylenemethanes, e.g. the iron compounds 8 or 9, usually exhibit almost no tendency to undergo cycloadditions due to their extraordinary stability. Only low yields of cycloaddition products are obtained, even at elevated temperatures and prolonged reaction times, and with highly reactive alkenes such as tetracyanoethene. As in all of these cases, the complexes have to be oxidatively destroyed in order to accomplish any cycloaddition, so that catalytic pathways cannot be generated using these reactants. 

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