Tri-o-biphenyl phosphite

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. 

Complex 9 and [CpFe(o-BPXCO)CH 2C1 ] [o-BP = tri(o-biphenyl)-phosphite] were also prepared by the reaction of HC1 with [CpFe(L)-(CO)CH2OEt] (L = PPh3, o-BP) (47). Alkylation of the two complexes with sodium /crt-butyl acetoacetate and pyrroline cyclohexanone enamine yielded six of the eight possible alkylation products. The two products where L was o-BP and the nucleophile was tort-butyl acetoacetate did not form, presumably because of excessive steric hindrance. The excess of one diastereomer over the other ranged from 10 to 64%. 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

Although the Diels-Alder cycloaddition is a highly versatile process for six-membered ring synthesis, the lack of reactivity of various dienophiles, such as unactivated alkynes, has limited its synthetic scope. Several examples of Diels-Alder reactions involving unactivated alkynes have been reported for which the reaction proceeds extremely slowly or is entirely unsuccessful. To avoid elevated temperatures, the feasibility of using nickel(O) to catalyze the cycloaddition under milder conditions was explored. The results of this investigation have been fruitful. For example, in the presence of 10 mol% Ni(COD)2 and 30 mol% tri-o-biphenyl phosphite in THF at 55 C, 11 undergoes efficient cycloaddition to provide 12 in 85% yield (Eq. 54). In contrast, in the absence of catalyst, 11 reacts only at... 

Significant asymmetric induction (18-60% e.e.) is observed in the alkylation of the chiral alkyl complexes [Fe(T7-C5H5)(CO)(L)(CH2Cl)] [(5) L = PPh3 or tri(o-biphenyl)phosphite] by the prochiral nucleophiles sodium r-butyl acetoacetate and pyrrolidine cyclohexanone enamine. The product diastereomer ratio in the former case was shown to be thermodynamically controlled, while kinetic control was assumed in the latter reaction. 

Analogous reactions with the chiral ethylene cations [(6) L = PPhs or tri(o-biphenyl)phosphite)] also proceed with appreciable asymmetric induction (10-64% e.e.) (Scheme 1). As with the alkyl substrate (5), increased steric bulk of the phosphorus ligand L increases the extent of induction. 

The nickel(0)-catalyzed stereoselective intramolecular [4+ 2] cycloaddition between dienes and unactivated allenes or alkynes has been shown to be an efficient complement to the uncatalyzed concerted Diels-Alder reaction that often requires stereoelectronic restrictions. In a typical reaction, treatment of dienyne with 10 mol% Ni(COD)2 and 30mol% of tri-o-biphenyl phosphite at room temperature... 

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