Dimerization and Cycloaddition Reactions of Transient Tri-tert-butylphosphacyclobutadiene Generated by Lewis Acid Induced Isomerization of Tri-tert-butylphosphatetrahedrane

Abstract

is shown here to act as a synthon of isomeric tri-tert-butylphosphacyclobutadiene in the presence of a Lewis acid or transition-metal complex. When it is combined with a substoichiometric amount of triphenylborane, compound 1 forms a ladderane-type dimer of tri-tert-butylphosphacyclobutadiene in 72% isolated yield. Trapping of a generated intermediate was achieved by repeating the experiment in the presence of excess styrene (20 equiv) or ethylene (1 atm), and the corresponding [4 + 2] cycloadducts of tri-tert-butylphosphacyclobutadiene were isolated in 88% and 74% yields, respectively. The platinum complex (Ph 3 P) 2 Pt(C 2 H 4 ) also reacts with 1 to form an orange η 2 complex of tri-tertbutylphosphacyclobutadiene in 80% isolated yield. Additionally, we report a novel method for generating a phosphinidenoid species via fluoride-induced trimethylsilyl fluoride elimination, leading to an improved preparative procedure for 1 (182 mg, 33% isolated yield).

Triphenylborane in Metal-Free Catalysis

“…Recently, Cummins and co-workers disclosed the synthesis and reaction behavior of highly strained tri- tert -butylphosphatetrahedrane 20 ( Scheme 17 ) [ 86 ]. Thus, upon the addition of BPh 3 (20 mol%), 20 underwent rapid dimerization to form the bicyclic structure 22 in 72% yield.…”

Structure and photochemistry of di-tert-butyldiphosphatetrahedrane

“…Moreover, the present study complements recent results on the related tri- tert -butylphosphatetrahedrane ( t Bu 3 C 3 P), which was shown by Cummins and co-workers to isomerise to a planar phosphacyclobutadiene upon addition of the Lewis acid BPh 3 . 14 …”

Reactions of Tri-tert-Butylphosphatetrahedrane as a Spring-Loaded Phosphinidene Synthon Featuring Nickel-Catalyzed Transfer to Unactivated Alkenes

“…Carbon-based tetrahedranes are highly strained molecules that possess significant potential energy. − Given the tetrahedral nature of elemental white phosphorus (P 4 ), a recent approach to alleviating the strain of these “fiery” compounds has been the incorporation of phosphorus into the tetrahedrane core, and mono-, di-, and triphosphatetrahedrane derivatives have been successfully synthesized. Despite a reduction in their strain energies, phosphatetrahedranes also exhibit interesting cage-opening reactions and have led to isomeric phosphacyclobutadienes, − phosphaalkenes, and a novel polymeric material …”

“…Given the spring-loaded nature of its highly strained core (strain energy ca. 96 kcal/mol for P­(CH) 3 ), we wondered whether tri- tert -butylphosphatetrahedrane ( 1 ) , could function as a phosphinidene synthon where two of the three C–P bonds are cleaved to form a new cyclopropene ring and a phosphorus lone pair. Such behavior would be desirable given the utility of singlet phosphinidene synthons in the construction of organophosphorus compounds. − Moreover, strain energy calculations suggest that monophosphatetrahedranes are the most strained class of phosphatetrahedranes, providing significant driving force for these cage-opening reactions .…”

“…In the absence of Ni­(COD) 2 or i Pr 3 P, no reaction was observed, strongly suggesting that the reaction is promoted by a generated nickel catalyst. When the catalyst loading was lowered to less than 10 mol %, partial conversion to the known [4 + 2]-cycloadduct of tri- tert -butylphosphacyclobutadiene with styrene occurred, and when i Pr 3 P was replaced with chelating bis­(phosphine) ligands no conversion to the desired phosphirane was observed (see the Supporting Information, S.1.4.1). Conveniently, catalytic phosphiranation (55% conversion) was also observed when Ni­(COD) 2 was replaced with Ni­( 4‑tBu stb) 3 , a commercially available long-term air- and room-temperature-stable Ni(0) source.…”