A cyclodimerization

A general type of stabilization for iminoboranes is a cyclodimerization, which yields diazadiboretidines (RBNR )2 that are isoelectronic with cyclobutadienes. 

Irradiation of Z-but-2-ene 8 initiates a cyclodimerization reaction, even without a photosensitizer." This cycloaddition proceeds from a singlet state and is likely to be a concerted, one-step reaction. Bond formation occurs suprafacial with respect to both reactants, whereupon only the tetramethylcyclobutanes 9 and 10 can be formed ... 

Treatment of diallenyl sulfone 354 with n-butyllithium resulted in a cyclodimerization to afford 2,6-dithiaadamantane derivative 356. This dimerization is considered to be initiated by formation of the a-sulfonyl carbanion 355 and to proceed through a carbanion walk or carbanion tour process426. 

Although hole-catalyzed (cycloaddilions involving radical cation intermediates) and PET (photochemical electron transfer) mixed [2 -I- 2] cycloadditions have been reported from electron-rich alkenes, the only report of a cyclodimerization is that of (E)-4-(prop-l-enyl)anisole which gives stereoisomeric mixtures of the head-to-head dimers 1 and 2.12... 

Silirenes (140, equation 32) could also be involved in the transition-metal catalyzed decomposition of bis(diazoketones) 139 which provides the electron-rich [4]radialenes 14266,67. While the formation of 142 directly from silirene 140 cannot be excluded a priori, it is more reasonable to assume that 140 undergoes twofold ring-expansion to form the cyclic cumulene 141, which then provides 142 by a cyclodimerization reaction. The intermediacy of 141 is corroborated by the isolation of the Diels-Alder product 14366. 

The cyclotrimerization reaction described above can be converted into a cyclodimerization reaction by blocking one of the vacant coordination positions around the nickel atom (77, 66, 83). Phosphines and phosphites in a ligand-to-nickel ratio of 1 1 have been found to be particularly effective. 

When thioketones are irradiated alone, a cyclodimerization may occur to give a 1,3-dithietane (4.98). In the presence of an alkene, different cycloadducts are found, usually thietanes. If visible radiation is used, electron-rich alkenes are especially effective as addends, and the products can be rationalized on the basis of a two-step mechanism involving the more highly stabilized biradical intermediate (4.99). Sometimes a 1,4-dithiane accompanies foe thietane (4.100) as a result of foe trapping of the biradical by a further molecule of ground-state thioketones (unlike ketones, thioketones react with radicals quite readily, which is one of foe causes of... 

A cyclodimerization, analogous to that of methyl anthranilate, was exhibited by 2-aminothiophenes bearing a carboxylic portion in position 3 to give the dithienodiazocines. In that patent, the procedures for the synthesis of unsymmetrical dithienodiazocines, thienobenzodiazocines and tetracyclic diazocines were also described <2004W0010136>. 

The DA dimer obtained in the aminium salt reaction differs from that obtained by the PET method and from that obtained using the aminium salt/hindered base method, but is the same as that obtained by the Bronsted acid catalyzed reaction of this diene. The addition of insoluble bases hke sodium carbonate is not sufficient to suppress the extremely facile, acid-catalyzed cyclodimerization of this particular diene, which yield a highly stabilized tetramethylallyl carbocation intermediate. Nevertheless, this results demonstrates the necessity for caution in assigning a cation radical mechanism to a cyclodimerization reaction observed under aminium salt conditions. 

Dilution s. High dilution Dimerization (s. a. Cyclodimerization, Self-condensation) 16, 947 17, 838... 

Cyclodimerization or trimerization of straight chain mono-olefins using metal complexes have not been reported. However, norbornadiene was found to give a cyclodimerization product, equation (7-23). 

On the other hand, a cyclodimerization of aUenylboronates was performed with catalytic amounts of Cp RuCl(cod) to produce 1,3-dialkylidenecyclobutane derivatives by [2-1-2] cychzation of the terminal double bonds [112] [Eq. (50)]. 

The synthesis of p-sulfonic acid-a,/ ,/ -trifluorostyrene was patented by the Ballard Power System Company [78] (Fig. 2.15). First, the aetion of ehlorosulfonic aeid onto iodobenzene was achieved from the Sanecki s synthesis [79] and the chlorosulfonate or fluorosulfonate trifluorostyrene compound was obtained via the Burton reaction [77]. The corresponding sulfonic acid monomer was obtained by hydrolysis of the p-halogenosulfonate-a,/ ,/ -trifluorostyrene. It has been shown that dimers or dienes can be prepared by a cyclodimerization reaction of TFS [80-82]. 

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