Tris cyclohexadienes

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

Physical Properties. The stmctures and the boiling and melting points of several diaLkyl peroxides are Hsted in Table 2 a comprehensive Hst is given in the Hterature (66). The melting point of 4,4 -dioxybis[2,4,6-tris(/ i -butyl)-2,5-cyclohexadien-l-one] [1975-14-0] is 148—149°C. 

The dimerization of 1,3-cyclohexadiene gives 30% adduct after 20 h at 30 °C [32]. In the presence of a catalytic amount of tris(p-bromophenyl) aminium hexachloroantimonate (ArsN SbCle Ar = /iBrC6H4) in CH2CI2 at 0°C, the cyclodimerization occurs in 15 min with 70% yield with a greater diastereos-electivity endojexo = 5 1) than that observed under thermal... 

The results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

The reaction of 1,3-cyclohexadiene with la at a temperature of — 50°C gives a 97 3 mixture of 1,4-allylsilylated product, trara-3-allyl-6-(trimethylsilyl)cyclohexene and 1,2-allylsilylated product, tra 5-3-allyl-4-(trimethylsilyl)cyclohexene, in quantitative yield. At the same temperature, the [3 -I- 2] cycloaddition product is detected only in trace amounts after 1 h. As the reaction mixture is warmed to — 10°C, the allylsilylated compounds are converted to the [3-1-2] cycloaddition product (72%). When purified tra 5-3-allyl-6-(trimethylsilyl)cyclohexene and tra i-3-allyl-4-(tri-methylsilyl)cyclohexene are treated separately under the same reaction conditions, the former compound is converted to the [3 -I- 2] cycloaddition product (major) and 3-(trimethylsilyl)propylbenzene [Eq. (11)], while the latter compound is converted to polymeric materials without giving any [3-1-2] cycloaddition product. The reaction rates of allylsilylation and [3-1-2] annulation are also accelerated by the addition of trimethylchlorosilane to aluminum chloride, as observed in other allylsilylation reactions. 

Of the many PET Diels-Alder reactions of potential synthetic utility we mention two reactions of vinylindole (103) catalyzed by 2,4,6-tris(4-methoxyphenyl)-pyrylium tetrafluoroborate. With cyclohexadiene, 103 reacts as a diene, giving rise to tetrahydrocarbazole (104) with exocychc dienes, 103 serves as dienophile generating a different tetrahydrocarbazole (105). Molecular orbital calculations provide a rationale for the regio- and diastereoselectivities of these reactions. 

In comparison with reaction in neat Et3N 3HF, benzene in a 1 mol/1 solution of Et3N 2.9HF in acetonitrile, electrolysed at 2.5 V (vs Ag/Ag+) produced 35% fluorobenzene, together with 4.8% difluorobenzene and 0.6% 3,3,6-tri-fluoro-1,4-cyclohexadiene. 

We see the same pattern, but with opposite stereochemistry in the disrotatory reactions of the hexatrienes 4.47 and 4.49. The direction the reaction takes is determined by thermodynamics, ring-closing in this case, but the stereochemistry is not, since the formation of the cis disubstituted cyclohexadiene 4.48 is counter-thermodynamic. If you try the frontier... 

Cyclization of dienynes.1 This Ni(0) catalyst in combination with a triarylphos-phite, particularly tri-o-biphenyl phosphite, permits intramolecular cycloaddition at 25° of [4+2] dienynes, in which the diene and the alkyne are separated by 3- and 4-atom units. This reaction is a useful route to products containing a cyclohexadiene group, which are oxidized to an arene by DDQ. 

Chemical Name 2,5-Cyclohexadiene-l,4-dione, 2,3,5-tris(l-aziridinyl)-Common Name Triaziquone Oncoredox Structural Formula ... 

Unsuccessful attempts to prepare stable complexes of six-membered ring allenes that succeeded for their seven-membered counterparts (Section IV,C) include reaction of 299 with KOt-Bu in the presence of tris(triphenyl-phosphine)platinum(O) [Eq. (50)]118 and abstraction of methoxy from 301 (Scheme 37)119120 (L = CO) with trimethylsilyl triflate. The latter gave products suggestive of an intermediate complex of 1,2-cyclohexadiene, but it could not be detected or trapped. Based on fluxionality studies in the seven-membered counterpart (Section IV,C), equilibration of 302 with a... 

Until recently, the triple-decker tris(i75-cyclopentadienyl)dinickel cation first made by Werner was the only example of this type of complex with all three rings composed of only carbon and hydrogen. Then in 1983 Jonas and co-workers detailed the novel [(i75-C5H5)V]2(/x,7j6-C6H6) (43), prepared from reaction of (i75-C5H5)V(7)3-C3H5)2 and 1,3-cyclohexadiene... 

Interestingly, although the reaction of 1,3-cyclohexadiene, chromium atoms, and PF3 yields bis(//4-cyclohexadiene)bis(trifluorophos-hine)chromium, the corresponding reaction with 1,4-cyclohexadiene yields the //5-cyclohexadienyl hydrido tris(trifluorophosphine)-chromium complex, which rearranges in the presence of silica gel to form [Cr( 4-C6H8)2(PF3)2] rather than (i/4-cyclohexadiene)tetrakis-(trifluorophosphine)chromium (40) (Scheme 2). 

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