Phosphabarrelene

Recently, a new class of phosphabarrelene/rhodium catalysts has been developed, which for the first time allows for hydroformylation of internal alkenes with very high activity and which proceeds essentially free of alkene isomerization [36-38]. Two examples, results of hydroformylation of an acyclic and a cyclic internal alkene substrate, are depicted in Scheme 2. 

Scheme 2 Position-selective hydroformylation of internal alkenes with a rhodium(I)/-phosphabarrelene catalyst...

Markl, Lieb and Martin were also able to add arynes 112 to 2.4,6-tiiphenyl-X -phosphorins the yields are better with 2.4.6-tri-tert-butyl-X -phosphorin. Here again 1,4 addition takes place with the formation of the 1-phosphabarfelenesiii. The arynes were generated either from 2-fluorophenylmagnesium bromide or penta-chlorophenyl-lithium. The reaction of the more nucleophilic 2.4.6-tri-tert-butyl-X -phosphorin with benzene-diazonium carboxylate also leads to 1,4 addition. The structure of the benzo-phosphabarrelenes 113a-d is supported by analytical and spectroscopic data (Table 16). 

Benzo-phosphabarrelenes have been obtained from the addition of cyclooctyne (14) to 2-phenyl-l-phosphanaphthalene 226>. Cyclooctyne (14) has recently been used to synthesize a [6]-paracyclophane (68) the first step being a Diels-Alder addition of (14) to a substituted furan 220 222). 

Di-r( rt-butyl-4-methyl-2,3-bis(trifluoromethyl)-l-phosphabarrelene (11) Typical Procedure. ... 

Thermolysis of (19) in absence of a phosphine generates 1,2-thiaphosphole 2-sullides (21) which can be trapped with suitable dienophiles such as acrylonitrile, styrene, butyl vinyl ether, and norbornene. The [4-1-2] cycloadducts such as (22) are formed regiospecifically <85BCJ667>. The reaction with dimethyl ethynedicarboxylate or phenyl ethyne affords 1-phosphabarrelene 1-sulfides (23) (Scheme 3) <86Ba2047>. 

The 1,2/l -azaphosphinines (58a-c) react with DMAD to form A -phosphinines (66), presumably via the phosphabarrelene intermediate (65) (Scheme 7) <85CL1459>. 

The synthesis of tri-/-butyl-lP,32, 5A -triphosphinine arose from studies of the behavior of t-butylphosphaethyne (206) in the coordination sphere of transition metals. The first indication of the transition metal template catalyzed trimerization of (206) was the isolation of the vanadium-complexed 1,3,5-triphospha-Dewar benzene (207) <87AG(E)908>. The formation of the tetra-phosphabarrelene complexed to zirconium (208) implied the intermediate formation of the com-plexed tri-(-butyltriphosphinine derivative. The free barrelene could be obtained by oxidative decomplexation using C2CI6 <95AG(E)8i>. 

Abstract Recent developments in the chemistry of phosphinines are reported. This chapter presents and discusses the most important breakthroughs achieved during the last decade. New synthetic approaches allowing the synthesis of polyfunctional phosphinines as well as improvements of well known methods are reported. The use of phosphinines in coordination chemistry is also presented with particular emphasis on their use in the stabilization of low valent and highly reduced transition metal complexes. Another aim of this review is to discuss recent applications of phosphinines, and structures derived directly from phosphinines such as phosphacyclohexadienyl anions and phosphabarrelenes, as ligands in homogeneous catalysis. 

Phosphaalkynes also play a prominent role as dienophiles in Diels-Alder reactions hence the X -phosphinines 13 [23] are formed from cyclic 1,3-dienes such as a-pyrone or cyclopen-tadienones by way of extrusion of CO2 or CO, respectively reactions with anthracene provide an access to the phosphabarrelene series [24]. This type of reaction is also of significance for the construction of phosphorus-carbon cage compounds. The same is true for homo-Diels-Alder reactions which, with 2-phosphabicyclo[2.2.2]octa-2,5-diene as reaction partner, lead to the diphosphatetracyclodecenes 14 [25]. Last but not least, ene reactions with phosphaalkynes as enophiles [25] are also valuable for the construction of polycyclic phosphorus-carbon compounds. The reactions of 9 with 2,3-dimethyl-2-butene (- 15) [26, 27] emphasize this behavior. 

Relatively little has appeared in the past year on the synthesis and reactivity of the six membered, potentially aromatic, phosphinine ring system. A route to the first C2-asymmetric phosphinine (147), derived from (+ )-camphor, has been developed from the reaction of a pyrylium salt precursor with P(TMS)3. The chiral phosphinine is a crystalline, air-stable solid and forms complexes with metal ions, therefore having some potential as a ligand in homogeneous eatalysis." A pyrylium salt-P(TMS)3 final step was also used in the synthesis of a series of 2-(2 -halo)triarylphosphinines (148), subsequently shown to form a series of tungsten(0) and rhodium(I) complexes involving coordination of the phosphorus sp lone pair to the metal." Interest has continued in studies of the coordination chemistry of phosphabarrelenes, derived from the established reactions of phosphinines... 

A cycloaddition between phosphabenzene 137 and benzyne has also been reported by Breit and coworkers, giving rise to phosphabarrelene 138 in moderate yields (Scheme 12.43) [77]. 

A cychc associative transmetallation of 69 to 70 through a low-energy transition state was proposed based on density functional theory (DPT) calculations for the Negishi couphng using [ Pd(allyl)Cl 2] and a bulky monodentate phosphabarrelene hgand (Scheme 1.37) [290],... 

Phosphabenzenes can be converted into phosphabarrelenes by Diels-Alder reaction (Scheme 2.10) [36]. 

Usually imidazoles are less prone to electrophilic substitution in comparison to pyrroles. Nevertheless, Muller and colleagues [8] achieved cyclization of an aldehyde formed as an intermediary by employing a rhodium catalyst based on a specially designed phosphabarrelene ligand (Scheme 5.126) (see also Chapter 2). A prehminary trial for comparison with P[0(o-rBuPh)]3 as a ligand exhibited no conversion, whereas the reaction with a rhodium catalyst based on PPhg ceased at... 

A new class of phosphabarrelene (13)-rhodium catalysts was described that allow the hydroformylation of internal alkenes with very high activity and proceed free of alkene isomerization [36]. 

A new class of phosphabarrelene/rhodium catalysts was found to display very high activity toward hydroformylation of internal alkenes with unusually low tendency toward alkene isomerization (Equation 7.6) [54]. 

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