Sterically encumbered systems for two low-coordinate phosphorus centers.

Tetraarylphenyls of the form 2,3,5,6-Ar4C6 (Ar = p-tert-butylphenyl) are investigated as sterically demanding ligands for the syntheses of compounds having two p-phenylene-bridged phosphorus centers. The precursor to such materials, 1,4-diiodo-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (1), is readily obtained via a one-pot procedure in 68% yield. Compound 1 is then used to provide the bis(dichlorophosphine) 1,4-bis(dichlorophosphino)-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (2) and the derived bis(phosphine) 1,4-bis(phosphino)-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (3) in yields of 56 and 94% respectively. These materials provide access to novel materials containing two low-coordinate phosphorus centers bridged by a sterically encumbered phenylene unit. Compound 2 reacts with benzaldehyde and 2,6-dichlorobenzaldehyde in the presence of excess trimethylphosphine and zinc to produce the new pale yellow crystalline bis(phosphaalkenes) (E,E)-PhC(H)=PAr4C6P=C(H)Ph (4a; 42%) and (E,E)-Ar'C(H)=PAr4C6P=C(H)Ar' (4b; 46%; Ar' = 2,6-dichlorophenyl). The crystal structure of 4a shows a P=C bond length of 1.676(5) A. Compound 2 is also used to provide the unusual red-orange bis(diphosphene) DmpP=PAr4C6P=PDmp (5; 55%; Dmp = 2,6-Mes2C6H3). Compound 5 is structurally characterized, and a P=P bond length of 2.008(2) A is ascertained.

Structural determination of a dimeric side-product accompanying dihydropyrazine preparation.

The identity of a major side-product of attempted 2,3-dihydropyrazine dehydrogenation has been elucidated using a geminal dimethyl analog [2,2,5,6-tetramethyl-2,3-dihydropyrazine (1)] which cannot aromatize. 1H, 13C NMR and GC-MS analyses were consistent with the formulation of the product as a symmetrical dimer of 1, but did not allow unambiguous distinction between two possible isomers, each of which could exist as either syn or anti diastereoisomers. X-Ray diffraction studies identified the product as the anti isomer of 3,3,5a,8,8,10a-hexamethyl-1,2,3,5,5a,6,7,8,10,10a-decahydropyra zino [2,3-g] quinoxaline (2). Compound 2 crystallizes in the tetragonal space group I41/a (No. 88) with a = 12.090(1), c = 22.007 (3) A, V = 3216.8(7) A3 and Z = 8. The solid-state structure also displays extensive hydrogen bonding between molecules of 2. A likely mechanism for the formation of 2 is presented.