Cyclic tetramers of a five-membered palladacycle based on a head-to-tail-linked isocyanate dimer and their reactivity in cyclotrimerization of isocyanates

Abstract

Reactions of [Pd(styrene)(PR3)2], generated from trans-[PdEt2(PR3)2] and styrene, with 2 equiv. of benzyl isocyanate in THF at room-temperature afforded unusual cyclic Pd-tetramers of five-membered rings consisting of organic isocyanate dimers and palladium, [Pd(PR3){-C(O)N(R)C(O)N(R)-}]4 (PR3 = PMe3, ; PR3 = PMe2Ph, ). Additionally, a cyclic trimer, (RNCO)3, (R = benzyl) was produced as a catalytic product. Treatment of the cyclic tetramer () with 4 equiv. of chelated phosphine, such as (1,2-bis(diethylphosphino)ethane) (DEPE) or (1,2-bis(dimethylphosphino)ethane) (DMPE), readily caused conversion to a metallacyclic cis-form, [Pd{N(R)C(O)N(R)C(O)}(P ∼ P)] (P ∼ P = DEPE, ; P ∼ P = DMPE, ) in quantitative yields. In contrast, reactions of Pd(0)-PR3 with 2 equiv. of Ar-NCO (Ar = Ph, p-tolyl, p-ClC6H4) afforded metallacyclic complexes having a dimeric isocyanato moiety, cis-[Pd{C(O)N(Ar)-C(O)N(Ar)}(PR3)2] (PR3 = PMe3 Ar = C6H5, ; p-MeC6H4, ; p-Cl-C6H4, ; PR3 = PMe2Ph, Ar = p-Cl-C6H4, ). Treatment of the palladacyclic complex () with an equimolar amount of chelated phosphine such as DEPE readily caused conversion to a palladacyclic cis-form, [Pd{N(Ar)C(O)N(Ar)C(O)}(DEPE)], in quantitative yield. The catalytic cyclotrimerization of benzyl isocyanate to [Pd(styrene)(PMe3)2] was achieved by varying the molar ratio of R-NCO (R = benzyl). In addition, catalytic cyclotrimerization was performed from the five-membered palladacyclic complexes or the Pd(0)-PR3 complex with excess Ar-NCO.

Aluminium-catalysed isocyanate trimerization, enhanced by exploiting a dynamic coordination sphere

“…9 However, isocyanurates' principal application is in the construction industry; they are the key component in rigid polyurethane foams which are used ubiquitously as insulation boards and are responsible for their mechanical and thermosetting properties. 10 Isocyanates can be trimerized by amines, 11 phosphines, 12-15 N-heterocyclic carbenes, 16 inorganic salts, 17 phosphides, 18,19 and main group and transition metal complexes, [20][21][22][23][24][25][26][27][28] but existing catalysts often require high catalyst loadings, high temperatures (e.g. Z100 1C for amines and halides), 11,17 can give a mixture of oligomers, and/or fail to trimerize both alkyl and aryl isocyanates.…”

A Well‐Defined Calcium Compound Catalyzes Trimerization of Arylisocyanates into 1,3,5‐Triarylisocyanurates

“…[7,8] Besides, isocyanurates are essential units for the synthesis of microporous (poly)isocyanurate, which has been applied as a support in heterogeneous catalysis. [9] Examples of catalysts that have been used for this important transformation include transition metals, [10][11][12][13][14] rare earth metals, [15][16][17][18] and Lewis bases like Nheterocyclic carbenes, [19] phosphines, [20] and N-heterocyclic olefins. [21] However, less attention has been paid to employing main-group elements, though there are some recent reports on the cyclotrimerization of isocyanates by K, Al, Ge, and Sn compounds.…”