Cycloadditions tris phosphite

Cyclodimerization and Cycloaddition. Tris(l,l,l,3,3,3-hexafluoro-2-propyl)phosphite serves as a ligand in Ni-catalyzed cyclodimerization reactions as well as Rh- or Ni-catalyzed [4-f2] cycloaddition processes. The first example is Ni-catalyzed dimerization of isoprene with tris(l,l,l,3,3,3-hexafluoro-2-propyl) phosphite ligand. The main product is l,4-dimethyl-4-vinyl-cyclohexane (eq 4). 

Distal ring-opening is also observed in the intramolecular 3+2 cycloaddition reaction of a 3-pentenone diphenylmethylenecyclopropane derivative in the presence of bis(diben-zylideneacetone)palladium/tri-isopropyl phosphite catalyst, affording regioselectively a diphenylmethylene pentalenone system (equation 360)424. Similarly, thermal cyclization of... 

Alkynes are poor dienophiles in the Diels- Alder reaction decomposition occurs by an attempted thermal intramolecular Diels-Alder reaction of dienynes at 160 °C. In contrast, the Ni-catalysed [4+2] cycloaddition of the dienyne 50 proceeded smoothly at room temperature using tri(hexafluoro)isopropyl phosphite to give 51, which was converted to the yohimbine skeleton 52 [15]. The same reaction is catalysed by RhCl(Ph3P)3 in trifluoroethanol [16]. Intramolecular Diels-Alder reactions of the 6,8-dieneyne 53 and the 1,3,8-triene 55, efficiently catalysed by [Rh(dppe)(CH2CH2)2]SbF6 at room temperature, gave 54 and 56 [17],... 

Ni- and Rh-catalyzed intramolecular [4+2] cycloadditions of the diene-allene 65 using tri(o-phenylphenyl) phosphite gave the different products 66 and 67 [21],... 

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. 

When bis(cycloocta-l,5-diene)nickel/tris(2-phenylphenyl)phosphite (1 1) was used as the catalyst a [3 -f 2] cycloaddition with cleavage of the three-membered ring was observed. However, when l-methylene-2-trimethylsilylcyclopropane was reacted with l-phenylpyrrole-2,5-dione, traces of a product (9%) that derived from a [2 -h 2] addition were isolated. 

The [3 -I- 2] cycloadditions of substituted methylenecyclopropanes 1 to alkynes usually proceed more efficiently than the reactions with the parent MCP. In the reactions catalyzed by nickel(O)/ tris(2-phenylphenyl) phosphite a variety of 4-methylenecyclopentenes, 2 and 3, can be prepared. The product distribution is highly variable, depending on the specific substituents employed. ... 

The use of alkynes in transition metal catalyzed reactions is often complicated by their tendency to undergo cyclo-trimerization and -tetramerization. Thus, it is useful to note that a phosphite-modiHed catalyst, Ni(COD)2/tris(o-phenylphenyl) phosphite (TOPP), promotes codimerization of alkynes with methylenecyclopropane and its alkylidene analogs. Both electron-rich and electron-poor alkynes participate in cycloaddition with moderate regioselectivity. Opposite regiochemistry is sometimes observed with disubstituted alkylidene systems (equations 97-99). 

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. 

Similarly, intramolecular [4 4- 2] cycloaddition of unactivated dienynes is also catalyzed and dramatically accelerated by low-valent rhodium complexes, e.g., Wilkinson s catalyst [chloro-tris(triphenylphosphane)rhodium] and phosphite analogs, under mild conditions46. Thus, ( , )-l-(2-propynyloxy)-2,4-hexadiene (3, Z = O) and similar dienynes, with 5 mol% of chlorotris(tri-phenylphosphane)rhodium in 2,2,2-trifluoroethanol for 30 minutes at 55 C. give up to quantitative yield of the cycloadducts with excellent to complete diastereoselection. According to control... 

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... 

Alkylidenecyclopropanes have shown an interesting reactivity, which was exploited to develop several metal-catalyzed cycloadditions [13], Among them, the nickel-catalyzed [4+3] cycloadditions between ethyl cyclopropylideneacetate and 1,3-dienes is an efficient synthetic procedure for the preparation of cycloheptenes (Scheme 8.4) [14]. Up to 70% cycloheptene has been obtained with a catalytic system combining Ni(cod)2 and tri-o-biphenylyl phosphite. A key parameter of the reaction is the slow addition of both substrates otherwise, yields dramatically dropped. In the case of dissymmetric 1,3-dienes, the cycloaddition did not show any selectivity except for isoprene (R = H, = Me, Scheme 8.4), where the ii-isomer is predominately obtained (5 1 ratio). 

With a common nickel(O) catalyst, prepared by the reduction of Ni(acac)2 with Et2A10Et in the presence of tris(l,1,1,3,3,3-hexafluoro-2-propyl)phosphite, the dienyne undergoes stereo-controlled cycloaddition (eq 10). Nickel(O) also catalyzes intramolecular [4+2] cycloadditions of nitrogen-containing dienynes, providing a novel method for the synthesis of hydroisoquinolines (eq 11). A typical Ni(0) catalyst derived from Ni(cod>2 and tris(l,l,l,3,3,3-hexafluoro-2-propyl)phosphite at room temperature. 

It has been found that dialltyl phosphites reacted with difluoro- or tri-fluoroacetonitriles (378) in the presence of a catalytic amount of EtgN at room temperature to form a-iminophosphonates (379), existing as an equilibrium mixture of F/Z-isomers. The synthetic potential of electron-deficient iminophosphonates (379) has been demonstrated by their easy functionalisation to afford various acyclic and heterocyclic derivatives with an aminophosphonic fragment by the use of reductive nucleophilic addition reactions, direct aminoalltylation of electron-rich heterocycles, cyclocondensations, and dipolar cycloaddition reactions (Scheme 113). ... 

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... 

---