Synthesis, Structure, Strain Energy, and Excess Strain of a Phospha[3]triangulane

Abstract

(7-Phenyl-7-phosphadispiro[2.0.2.1]heptane)pentacarbonyltungsten (8), a phospha[3]triangulane, was synthesized from bicyclopropylidene. Its single-crystal X-ray structure determination is reported. Comparison of the crystal structure data with those of the related phosphaspiropentane 7 and phosphirane 6 complexes suggests that the phosphirane ring tightens when the number of spiro atoms is increased. This is supported by the B3LYP and MP2/6-31G* computed geometries of the uncomplexed parent systems. Ab initio calculated heats of formation and strain energies (SE) are reported for the parent phosphirane 11, phosphaspiropentane 12, and phospha[3]triangulane 13 using both G2MP2 theory and ring separation reactions. Our best estimates for the ΔH f of 11, 12, and 13 are 18.3, 48.4, and 78.2 kcal/mol, respectively, with corresponding SE values of 21.3, 54.7, and 87.9 kcal/mol. For comparison, the slightly modified G2MP2 method was also applied to cyclopropane 1, spiropentane 2, and [3]triangulane 3 to give respective ΔH f values of 12.6, 44.3, and 75.3 kcal/mol, with corresponding SEs of 28.0, 64.6, and 100.5 kcal/mol, all of which are in excellent agreement with reported experimental data. These strain energies suggest that the excess strain per spiro atom is 5.3 kcal/mol for phospha[n]triangulanes, which is smaller than the 8.6 kcal/mol determined from the heat of combustion measurements for the [n]triangulanes.

Linear and Branched Phospha[n]triangulanes

“…The syntheses of phospha [n]triangulanes (n = 3-5) was achieved by adding Ph-P=W(CO) 5 to terminal and non-terminal double bonds of spirocyclopropanated methylenecyclopropanes and bicyclopropylidenes, respectively. The influence of the CuCl catalyst on the addition reaction will also be addressed.…”

“…Spirocyclopropanated methylenecyclopropanes, that is, terminal alkenes: Reaction of Ph-P=W(CO) 5 , generated in situ by the CuCl-catalyzed cycloreversion [12] of 7-phosphanorbornadiene complex 8, [9,13] with methylenespiropentane (9) in toluene at 55 8C (1 h) gave in 78 % yield only the crystalline terminal phospha [3]triangulane complex 12 (Scheme 1) in a 5:4 ratio of the anti (a) and syn isomer (b). Figure 1 shows the X-ray crystal structure for the less congested anti isomer 12 a that has the P-W(CO) 5 group anti to the terminal cyclopropane ring.…”

“…Figure 1 shows the X-ray crystal structure for the less congested anti isomer 12 a that has the P-W(CO) 5 group anti to the terminal cyclopropane ring. The bond lengths of the phosphirane ring compare well with the values of the W(CO) 5 -complexed smaller phospha [2]triangulane 4 b [4] ( Table 1).…”

“…[2] Compounds such as 1-phosphaspiro [2,n]alkanes 4 a-e, [4] the 1-phosphadispiroalkanes 5 a-d, [5,6] and 1,4-diphosphaspiropentane 6 [7] with a phosphorus atom, that is, a phosphirane ring, [8] became accessible by the [1 + 2] cycloaddition of the in situ generated carbene-like electrophilic phosphinidene complexes R-P= W(CO) 5 to the respective alkenes. [9,10] Abstract: Novel, highly stable, linear and branched mono-and diphospha[n]-triangulanes were synthesized in high yields by the CuCl-catalyzed phosphinidene addition to spirocyclopropanated methylenecyclopropanes and bicyclopropylidenes.…”

“…Steric factors play a role in the addition reaction when the substrate alkene carries a second sphere of spirocyclopropane rings and causes the formation of 2-phosphabicyclo[3.2.0]heptenes in small amounts. These by-products most probably result from addition of the [PhP(Cl)W(CO) 5 ]-Cu-L (L = alkene or solvent) reagent to the spirocyclopropanated bicyclopropylidene to give an intermediate s-complex, which subsequently, facilitated by steric factors, undergoes a cyclopropylcarbinyl to cyclobutyl ring expansion followed by a [1,3]-sigmatropic shift.…”