Synthesis and properties of air-stable 1,3-diphosphacyclobutane-2,4-diyls and the related compounds

A kinetically stabilized phosphaalkyne bearing a bulky Mes^∗ (2,4,6-tri-t-butylphenyl) group is useful compound to prepare an enormous number of highly stable 1,3-diphosphacyclobutane-2,4-diyls through reactions with a lithium reagent and an electrophile. By utilizing this synthetic protocol, we prepared several non-symmetrical 1,3-diphosphacyclobutane-2,4-diyls in which the substituents on the phosphorus are different. Furthermore, we succeeded in preparation and characterization of novel air-tolerant symmetrical 2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyls bearing the identical alkyl substituents on the phosphorus atoms. Structures and properties of the 1,3-diphosphacyclobutane-2,4-diyls indicate characters as singlet ground-state carbon centered biradicals. In addition to those biradicals, we succeeded in preparation and isolation of a novel P-heterocyclic air-stable neutral radical as well as a P-heterocyclic cation radical.     

Preparation of 1,3‐diphosphabuta‐1,3‐dienes from sterically encumbered phosphaalkyne and phosphaethenyllithium

Kinetically stabilized 2‐lithio‐1‐(2,4,6‐tri‐t‐butylphenyl)‐1‐phosphapropene was allowed to react with a bulky phosphaalkyne Mes*CP (Mes* = 2,4,6‐t‐Bu₃C₆H₂) followed by quenching with iodomethane or benzyl bromide to give the corresponding 1,3‐diphosphabuta‐1,3‐dienes. The presence of the bulky Mes* group on the 1‐phosphorus atom prevents intramolecular [2+2] cyclization and gave the PC PC skeleton, whereas Mes*CP reacted with half an equivalent of nucleophile to afford the PCPC four‐membered ring compounds. X‐ray crystallography of 4‐benzyl‐1,3‐diphosphabuta‐1,3‐diene confirmed the molecular structure showing conjugation on the 1,3‐diphosphabuta‐1,3‐diene moiety. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:357–360, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20104     

Dissociation of 2,4-bis(2,4,6-tri-tert-butylphenyl)-cyclo-1,3-dipnicta-2,4-diazanes (pnict = P, As, Sb) imposed by substituent steric strain: a cyclobutane/olefin analogy

Three 2,4-bis(Mes)-cyclo-1,3-dipnicta-2,4-diazanes, 2PnX, have been synthesized as 1,3-dichloro- (Pn = As; X = Cl) or 1,3-bis(triflato)- (Pn = P, Sb; X = OTf) derivatives (Mes = 2,4,6-tri-tert-butylphenyl; OTf = triflato = trifluoromethanesulfonoxy). The compounds have been structurally characterized as cyclodimers, but spectroscopic characterization of melts and solutions indicates facile dissociation of the diphosphadiazane 2POTf derivative to give the corresponding iminophosphine 1POTf and of the diarsadiazane 2AsCl derivative to give the corresponding iminoarsine 1AsCl. 1,3-Bis(triflato)-2,4-bis(Mes)-cyclo-1,3-distiba-2,4-diazane (2SbOTf) does not dissociate in the melt or in solution. The presence of the sterically bulky Mes substituent does not preclude association of N-Pn olefin analogues to give single bonded cyclobutane analogues. The facile dissociation of 2POTf and isolation of 1POTf implicates a relatively high degree of substituent steric strain in the dimer. In comparison, dissociation of 2AsCl is only apparent in the melt and in solution, likely the result of the lower substituent strain in the larger N(2)As(2) framework. The largest N(2)Pn(2) framework in 2SbOTf provides sufficient space for the Mes substituents, and the monomer is not observed under the conditions examined.     

Chemistry of Several Sterically Bulky Molecules with P=P,P=C,and C≡P Bond

Several sterically protected, low-coordinate organophosphorus compounds with P=P, P=C, and C≡P bond are described in this study. Molecules such as diphosphenes, phosphaalkenes, 1-phosphaallenes, 1,3-diphosphaallenes, 3,4-diphosphinidenecyclobutenes, and phosphaalkynes are stabilized with an extremely bulky 2,4,6-tri-t-butylphenyl (Mes*) group. The synthesis, structures, physical, and chemical properties of these molecules are discussed, together with some successful applications in catalytic organic reactions.     

New Aspects of 1,3‐Diphosphacyclobutane‐2,4‐diyls

1,3‐Di(tert‐butyl)‐2,4‐bis[2,4,6‐tri(tert‐butyl)phenyl]‐1,3‐diphosphacyclobutane‐2,4‐diyl was formed from [2,4,6‐tri(tert‐butyl)phenyl]phosphaacetylene and t‐BuLi. In addition, the X‐ray diffraction analysis was carried out, together with theoretical calculations of the structure and NMR data.     

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.     

Substituent effect on low coordination phosphorus chemistry

It has been shown by means of computational chemistry that the successful synthesis of some of the most remarkable phosphorus containing molecules in the last decades (e.g., 1,3,5-triphosphinine or 1,3-isodiphosphinine) was greatly facilitated by the presence of the bulky tBu group. This substituent is needed to stabilize tBuCP, which is used to be the starting material in many reactions. In a six-membered ring the repulsion of the neighboring tBu substituents brings a destabilization of about 22-25 kcal/mol for each tBu unit, this amount of energy is comparable to the aromatic stabilization in benzene. The minimization of this steric repulsion between neighboring tBu moieties in the possible isomers plays a crucial role in determining the stability of the product. The effect of the steric repulsion on the kinetics of the cycloaddition reaction between tBuCP and diazomethane has also been shown to be important, accounting for the observed regioselectivity in the reaction.     

Tetrasila-1,3-butadiene and its transformation to disilenyl anions

The reaction of dilithiosilane, ( ^t Bu₂MeSi)₂SiLi₂ (2), with 1,1,2,2-tetrachloro-1,2-dimesityldisilane produced the tetrasila-1,3-butadiene derivative, ( ^t Bu₂MeSi)₂Si=Si(Mes)–(Mes)Si=Si(SiMe ^t Bu₂)₂ (3, Mes = 2,4,6-trimethyl-phenyl), which was isolated as reddish-purple crystals. The structure of 3 was determined by both spectroscopic and crystallographic methods; the Si=Si double bond lengths are 2.2003(12) and 2.1983(12) Å, and the length of the central Si?Si single bond is 2.3376(11) Å. The tetrasilabutadiene moiety is highly twisted, the torsional angle of Si1=Si2?Si3=Si4 being 72^°. The reaction of 3 with ^t BuLi or KC₈ in THF gave the disilenyllithium 4 and disilenylpotassium 5, which contain an sp ²-type silyl anion, by the reductive cleavage of the central Si–Si bond in 3.     

Valence Isomerization of a 1,3-Diphosphacyclobutane-2,4-diyl: Photochemical Ring Closure to 2,4-diphosphabicyclo[1.1.0]butane and Its Thermal Ring Opening to gauche-1,4-Diphosphabutadiene

The bond stretch isomer 1,3-diphosphacyclobutane-2,4-diyl 1 was transformed photochemically to give the previously unknown 2,4-diphosphabicyclo[1.1.0]butane 2 , which itself can be converted thermally into gauche-1,4-diphosphabutadiene 3 . The crystal structures of these three energy-rich valence isomers of 1,2-diphosphete have been determined. R=SiMe₃; Mes*=2,4,6-tBu₃C₆H₂.     

Free Radical Chemistry of Phosphasilenes

Understanding the characteristics of radicals formed from silicon‐containing heavy analogues of alkenes is of great importance for their application in radical polymerization. Steric and electronic substituent effects in compounds such as phosphasilenes not only stabilize the Si=P double bond, but also influence the structure and species of the formed radicals. Herein we report our first investigations of radicals derived from phosphasilenes with Mes, Tip, Dur, and NMe₂ substituents on the P atom, using muon spin spectroscopy and DFT calculations. Adding muonium (a light isotope of hydrogen) to phosphasilenes reveals that: a) the electron‐donor NMe₂ and the bulkiest Tip‐substituted phosphasilenes form several muoniated radicals with different rotamer conformations; b) bulky Dur‐substituted phosphasilene forms two radicals (Si‐ and P‐centred); and c) Mes‐substituted phosphasilene mainly forms one species of radical, at the P centre. These significant differences result from intramolecular substituent effects.     

Concise synthesis of 1,3-di-O-substituted tetrahydropyran derivatives as conformationally stable pyranose mimetics

A practical synthetic route to 1,3-di-O-substituted tetrahydropyrans has been developed. An important feature of these obtained glycomimetics is the bulky t-butyl functionality at the C-4 position, which imposes a chair conformation as the lowest energy conformer, making these compounds ideal pyranose mimetics.     

Anionic polymerization of methyl methacrylate and tert-butyl acrylate initiated with the YCl3/lithium amide/nBuLi systems

The anionic polymerization of methyl methacrylate (MMA) was initiated with a mixture of lithium amide of various secondary amines and nBuLi in the presence of YCl₃, where an Y-ate complex was formed and an amide ligand on Y attacked MMA nucleophilically. In THF at −78 °C, PMMAs with narrow molecular weight distributions were obtained in high yields. The presence of a secondary amino group derived from the initiator at the polymer chain end was confirmed by MALDI-TOF-MS analyses. The initiating system using indoline as a secondary amine was effective for block copolymerization of MMA with tert-butyl acrylate (tBA), giving poly(MMA-b-tBA)s with narrow molecular weight distributions.     

π‐Excess Aromatic σ2P Ligands: Formation of a Heterocyclic 1,2‐Diphosphine by the Addition of tBuLi and Subsequent Inverse Addition of the Product at the P=C Bonds of Two Molecules of 1‐Neopentyl‐1,3‐benzazaphosphole

The reactivity of tBuLi (pentane) toward the N‐neopentyl‐substituted π‐excess P=CH–N heterocycle 1 depends on the solvent (tetrahydrofuran, diethyl ether, hexane, and toluene) and reaction conditions. Trapping of the resulting organolithium compounds with CO₂/ClSiMe₃, ClSiMe₃, or EtI led to various products indicating CH lithiation ( 1a , b ), normal addition of tBuLi at the P=C bond (E/Z ‐2a , b ), inverse addition of the primary addition product 2_Li at the P=C bond of a second molecule 1 , affording 3‐tert‐butyl‐2,2’‐bis(1,3‐benzazaphospholines) 3 , or inverse addition of tBuLi ( 4b,c ). The formation of 3 demonstrates a novel route to asymmetric heterocyclic 1,2‐diphosphine ligands. The structure elucidation of the new compounds is based on their ³¹P and ¹³C NMR data with conclusive chemical shifts and P–C coupling constants, that of the isolated PH‐functionalized diphosphine 3 on crystal structure analysis.     

σ-P Ligands: convenient syntheses of N-methyl-1,3-benzazaphospholes

A convenient three-step route to 1,5-dimethyl-1,3-benzazaphosphole via Cu- or Pd-catalyzed phosphonylation of 2-iodo-4-methylaniline, reduction to 2-phosphino-4-methylaniline, and disproportionative cyclization with excess formaldehyde is reported. N-Methylbenzazaphospholes can be functionalized in the 2-position via α-CH-lithiation with tBuLi and are π-acidic σ²P-ligands.     

Mitt. XV: Struktur und reaktivität von isoanellierten heterocyclischen systemen mit 4nπ- und (4n + 2) π-elektronen. 2-t-butyl-4-methyl-2,4-dihydropyrrolo[3,4-b]indole. reaktionen mit aktivierten CC-dienophilen

Reactions of 2-t-butyl-4-methyl-2,4-dihydropyrrolo[3,4-b]indoles with N-aryl-maleimides have been investigated. The formation of 1:1-cycloadducts is influenced by substituents at 1,3-position. The consecutive transformations of these intermediates can be controlled by nucleophilic solvents or reagents. Ring opening reactions occur at the labile N,O-acetal group of the cycloadducts followed by eliminaton or addition reactions.     

Cleavage of C-Si bond by intramolecular nucleophilic attack: lithiation-promoted formation of siloles from 1-bromo-4-trisubstituted silyl-1,3-butadiene derivatives

Lithiation of 1-bromo-4-trisubstituted silyl-1,3-butadiene derivatives with t-BuLi afforded substituted siloles in high yields. Cleavage of one of the three C-Si bonds took place via intramolecular nucleophilic substitution. Selective cleavage was observed when the silyl group possessed different substituents. Results showed that vinyl and phenyl substituents on Si were substituted much more easily than methyl groups, whilst a methyl group was exclusively deleted from an i-Pr-SiMe₂ moiety.     

Iodoenolcyclisation of 2-(1,3-disubstituted-1-allyl)-1,3-dicarbonyl compounds: diastereoselective synthesis of tetrasubstituted dihydrofurans

The I₂-induced cyclisation of 2-alkenyl-1,3-dicarbonyl compounds with the mono- and di-substituted double bond occurred with good diastereoselectivity. A study of stereochemical aspects for different substituents on the allyl side chain was carried out. When the substituents were alkyl groups, the trans isomers formed preferentially, in the case of aromatic substituents the reaction lead instead to cis isomers.     

Reactions at the B–B Bond of Tri‐tert‐butylazadiboriridine NB2R3: Ring Extensions

The OCO carboxylate unit of pivalic acid adds to the B–B bond of the azadiboriridine NB₂R₃ ( 1 a , R = tBu) to give the chiral heterocyclohexadiene 2 a ; the enantiomers of 2 a are transformed into one another by a [1,3] sigmatropic hydride transfer along the B–N–B ring fragment. The azadiboracyclopentanes 3 a – e are formed from 1 a and the alkenes ethene, propene, isobutene, (trimethylsilyl)ethene, and 2,3‐dimethyl‐1‐butene. Only one double bond of cyclopentadiene and 1,3‐butadiene reacts in the same way to give 3 f , g , respectively, and both of the double bonds of 1,3‐butadiene react with an excess of 1 a to give 3 h , which is obtained in a 9 : 1 mixture of racemate and meso‐isomer; the meso‐isomer crystallizes in the space group P2₁/n. The corresponding diazadiboracyclopentane 3 i and the triazadiboracyclopentane 3 j are formed from 1 a and N‐phenyl benzaldimine or azobenzene, respectively. Ethyne and 1 a give either the azadiboracyclopentene 4 a (1 : 1) or the diazatetraborabicyclo[3.3.0]octane 3 k (1 : 2). The phosphaalkyne P≡C–tBu and 1 a  analogously yield the heterocyclopentene 4 c . The insertion of SitBu₂ into 1 a to give the azasiladiboracyclobutane 5 a is achieved by applying Li powder and tBu₂SiCl₂. The hitherto unknown azadiboriridines BN₂R₂R′ (R = tBu; R′ = 1‐iPr, 2‐Mes, 2‐CMe₂Et: 1 b – d ) were synthesized by the chloroboration of the iminoboranes RB≡NiPr and RB≡NR with RBCl₂, MesBCl₂, and (EtMe₂C)BCl₂, respectively, and subsequent dechlorination of the isolated and characterized diborylamines Cl–BR–NiPr–BR–Cl ( 6 a ), Cl–BR–NR–BMes–Cl ( 6 b ), and Cl–BR–NR–B(CMe₂Et)–Cl ( 6 c ), respectively, with lithium (Mes = mesityl).The azadiboriridine 1 b dimerizes to give the diaza‐nido‐hexaborane 7 a , whereas 1 c and 1 d are storable at room temperature. The product 1 c crystallizes as a racemate in the space group P2₁/c; its ring geometry differs from that of the known N‐mesityl isomer.     

Bulky Substituent Effect on Reactivity of Localized Singlet Cyclopentane‐1,3‐diyls with π‐Single Bonding (C‐π‐C) Character

Thorough investigation of key intermediates, such as long‐lived singlet diradicals, is essential to understand the homolytic bond cleavage reactions. In this study, we evaluate the effect of bulky substituents at the meta‐position of the phenyl ring on the bond formation process in singlet 2,2‐diethoxy‐1,3‐diarylcyclopentane‐1,3‐diyls. The bulky groups have significant influence on the diradical lifetime, as such, when the triisopropylphenyl group was used, the lifetime was 45 times longer than that of the parent diradical in benzene at 293 K.     

Palladium-catalysed carbonylation of 4-substituted 2-iodoaniline derivatives: carbonylative cyclisation and aminocarbonylation

2-Iodoaniline derivatives were used as bifunctional substrates in palladium-catalysed carbonylation. Depending on the substituents, two types of compounds were synthesised: having methyl or hydrogen in 4-position 2-aryl-benzo[d][1,3]oxazin-4-one derivatives have been formed, chloro, bromo, cyano or nitro groups in the same position resulted in the formation of dibenzo[b,f][1,5]-diazocine-6,12-dione derivatives. In the presence of various primary and secondary amines (tert-butylamine, amino acid methyl esters) as N-nucleophiles 2-ketocarboxamides were obtained as major products in aminocarbonylation reaction with double carbon monoxide insertion.