Synthesis, Structure, and Reactivity of Novel 3,4-Diphosphacyclobutenes Bearing Silyl Groups

[reaction: see text] Copper-mediated homocoupling of sterically hindered 2-(2,4,6-tri-tert-butylphenyl)-1-trialkylsilyl-2-phosphaethenyllithiums afforded 1,2-bis(trialkylsilyl)-3,4-diphosphacyclobutenes (1,2-dihydrodiphosphetenes) through a formal electrocyclic [2+2] cyclization in the P=C-C=P skeleton as well as 2-trimethylsilyl-1,4-diphosphabuta-1,3-diene. Reduction of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes followed by quenching with electrophiles afforded ring-opened products, (E)-1,2-bis(phosphino)-1,2-bis(trimethylsilyl)ethene and (Z)-2,3-bis(trimethylsilyl)-1,4-diphosphabut-1-ene. The structures of the ring-opened products indicated E/Z isomerization around the C=C bond after P-P bond cleavage of 5, and the isomerization of the P-C=C skeleton. Ring opening of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes affording (E,E)- and (Z,Z)-1,4-diphosphabuta-1,3-dienes was observed upon desilylation.

Solvent Free Amination Reactions of Aryl Bromides at Room Temperature Catalyzed by a (π-Allyl)palladium Complex Bearing a Diphosphinidenecyclobutene Ligand

An air- and moisture-stable (pi-allyl)palladium complex bearing a unique diphosphinidenecyclobutene ligand effectively catalyzes amination reactions of aryl bromides with amines, where the reactions proceed under mild conditions without solvent, with 2 mol % of catalyst and 1 equiv of t-BuOK at room temperature. Under these conditions the amination products were obtained in good to excellent isolated yields.

Preparation and Properties of Phosphaethynes Bearing Bulky Aryl Groups with Electron-Donating Substituents at the Para Position

Phosphaethynes bearing a 2,6-di-tert-butyl-4-(dimethylamino)phenyl, 2,6-di-tert-butyl-4-methoxyphenyl, or 2,6-di-tert-butylphenyl group were prepared. A (31)P NMR spectroscopic investigation of the chemical shifts indicated that electron-donating groups at the para position cause shifts to a higher field. Bathochromic shifts caused by the electron-donating groups were apparently observed in UV-vis spectra. The structure of 2-[2,6-di-tert-butyl-4-(dimethylamino)phenyl]-1-phosphaethyne was analyzed by X-ray crystallography.

Preparation and Reactions of 1,3-Diphosphacyclobutane-2,4-diyls That Feature an Amino Substituent and/or a Carbonyl Group

The preparation and properties of a 1-amino-1,3-diphosphacyclobutane-2,4-diyl and a 1-benzoyl-1,3-diphosphacyclobutane-2,4-diyl, which can be regarded as functionalized cyclic biradical derivatives, were investigated. Hydrolysis of 1-diisopropylamino-3-methyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl (7), which is formed by reaction of Mes*C[triple chemical bond]P (4; Mes*=2,4,6-tBu(3)C(6)H(2)) with lithium diisopropylamide and iodomethane, resulted in ring-opening of the 1,3-diphosphacyclobutane-2,4-diyl skeleton, as well as de-aromatization of one of the Mes* rings. 3-Oxo-1,3-diphosphapropene 8 and 7-phosphabicyclo[4.2.0]octa-1(8),2,4-triene 9 were the resultant products, and these were subsequently characterized. Isomerization and oxidation of 7 occurred in the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinoxy) to give the first example of a cyclic dimethylenephosphorane derivative, namely 3-oxo-1,3-diphospha-1,4-diene 10. 1-Benzoyl-3-tert-butyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl (12) was isolated and characterized from the reaction of 4 with tert-butyllithium and benzoyl chloride. Compound 12 was subsequently heated and underwent rearrangement of the benzoyl group and ring-expansion to afford 1-oxo-1H-[1,3]diphosphole 13. Reaction of 4 with lithium diisopropylamide and benzoyl chloride afforded the 2H-[1,2,4]oxadiphosphinine 15, which was probably formed through the 1,3-diphosphacyclobutane-2,4-diyl intermediate 14. Thermolysis of 15 afforded 1-oxo-1H-[1,3]diphosphole 16 in an Arbuzov-type rearrangement.

Formation and Electrochemical Properties of a 1,4-Diphosphafulvene Including Formal Dimerization of Phosphaallene

Reactions of sterically protected 2-bromo-3-phenyl-1-phosphapropenes with bases such as tert-butyllithium and potassium tert-butoxide successfully afforded a bulky 1,4-diphosphafulvene (Mes = 2,4,6-tBu(3)C(6)H(2)) through a novel and formal dimerization pathway of 1-phosphaallene, together with small amounts of 3-phenyl-1-phosphaallene and 3,4-diphosphanylidenecyclobutene. The structure of the 1,4-diphosphafulvene was confirmed by X-ray crystallography indicating somewhat flattened phosphorus atoms due to the bulky Mes groups. The electrochemical properties of the 1,4-diphosphafulvene were investigated to show promising suitability as an electron donor, and indeed, it afforded a charge-transfer complex with TCNQ. Preparation and structural elucidation of an alkoxy-functionalized 1-phosphaallene was also performed, and the effect of the alkoxy group on the 1-phosphaallene moiety was characterized.

Preparation and Coordination Properties Including Catalytic Activities of a Bulky 2-Methyl-3-thioxo-1,3-diphosphapropene

[reaction: see text] 2-Methyl-3,3-diphenyl-3-thioxo-1-(2,4,6-tri-tert-butylphenyl)-1,3-diphosphapropene, which bears a P=C-P=S skeleton, was prepared and used as a ligand of transition-metal complexes. The palladium complexes containing the ligated 3-thioxo-1,3-diphophapropene were stable and used for catalytic reactions such as cross-couplings and direct conversion of allyl alcohol to allylaniline.

Application of a diphosphinidenecyclobutene ligand in the solvent-free copper-catalysed amination reactions of aryl halidesElectronic supplementary information (ESI) available: details of general procedure for the solvent amination reaction; characterisation data of the products listed in Table 1 and Table 2. See http://www.rsc.org/suppdata/cc/b4/b408232j/

1,2-Diphenyl-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene was used as an additive ligand in copper-catalysed amination reactions of halobenzenes with amines in the presence of base to afford the corresponding secondary or tertiary amines in good to excellent yields.