Valence Isomerization of 2‐Phosphabicyclo[1.1.0]butanes

Abstract

The unique electronic properties of the strained bicyclo-[1.1.0]butanes [1] are enhanced by the heteroatoms in the molecular frame. Illustrative is the bond-stretch isomerization of the P 2 C 2 and P 2 B 2 bicycles.[2] However, very few systems are known with a single heteroatom, [3] probably because of their high reactivity, which is only moderated when the heteroatom occupies a bridgehead position as in the 1-aza derivatives. [4] The increased reactivity of the hetero systems is due to the valence isomerization to which the bicyclo[1.1.0]butanes are prone. We now report on the first 2-phospha derivatives.The carbene-like phosphinidene Ph-P = W(CO) 5 [5] was generated in situ by cheletropic elimination from 1 at 110 8C in toluene and then allowed to react with cyclopropene 2 a [6] (Scheme 1). This led to the desired W(CO) 5 -complexed 2-phosphabicyclo[1.1.0]butanes exo-3 a (d 31 P = À85.1 ppm) and endo-3 a (d 31 P = À36.7 ppm) [7] in a 10:9 ratio, which were isolated in 69 % yield as colorless solids. The remarkably

Fused Tricyclic Phosphiranes—Analysis of Phosphorus Chemical Shieldings

“…The data in Table 1 Phosphorus chemical shielding differences: The Z isomers of 4 a and 4 b are as much as % 60 ppm less shielded than the E isomers. Similar shielding differences between the Z and E isomers have been observed for the phosphinidene adducts 2, [7] phosphabicyclobutanes 3, [8] and 7-phosphanorbornenes 1.…”

“…This C1 À C2 bond length of 1.592(4) is elongated in comparison to those in typical phosphiranes (1.47-1.52 ) [7, 17b, 19] and is even longer than those in phosphabicyclobutane (Z)-3 (1.550 ). [8] The C3 À C6 bridgehead bond is also elongated (1.588(4) ), which is, however, common for cyclobutenes.…”

“…The phosphorus atom in (Z)-3 is less pyramidal than that of the E isomer (the sum of the C-P-C angles: 273.9 vs. 259.98, respectively), and, accordingly, is 48.4 ppm less shielded (Z: d = À36.7 ppm, E: d = À85.1 ppm). [8] In complexes 4, the metal fragment, which is both a s acceptor and a p donor, lowers the energy of the P lone pair and raises that of the empty phosphorus p orbital.…”

“…[2a, 4, 5] Chesnut et al [4] related the variations in shielding to the magnitude of the HOMO-LUMO energy gap E g and addressed the underlying principles to qualitatively account for the variations, but noted that nonadditive a-, b-, and g-substituent effects dominate the overall shielding. The 1,2-addition of carbene-like electrophilic phosphinidenes to alkenes gives direct access to both the Z and E isomeric phosphiranes, [6][7][8][9] providing suitable test systems for 31 P NMR analysis. For example, the isomers of strained phosphiranes 2 [7] and 3 [8] show a difference in 31 P chemical shielding analogous to 1.…”

“…The 1,2-addition of carbene-like electrophilic phosphinidenes to alkenes gives direct access to both the Z and E isomeric phosphiranes, [6][7][8][9] providing suitable test systems for 31 P NMR analysis. For example, the isomers of strained phosphiranes 2 [7] and 3 [8] show a difference in 31 P chemical shielding analogous to 1. Here, we report on phosphinidene addition to hexamethyl Dewar benzene (9) to give the novel Abstract: 1,2-Addition of transient W(CO) 5 -complexed phosphinidenes exo to hexamethyl Dewar benzene affords the novel 3-phosphatricyclo[3.2.0.0 2,4 ]hept-6-ene complexes.…”