Dilithium phenylphosphide reacts with 1,3-dichloropropane or
1,2-dichloroethane to give
1-phenylphosphetane (1) or 1-phenylphosphirane
(2), respectively, both of which can be
isolated by distillation in vacuo. The phosphetane rapidly
polymerizes when neat but is
stable in benzene wherefrom the polymer can be selectively and
quantitatively separated
from 1 by the addition of
trans-dichlorobis(diethyl
sulfide)palladium(II). Four-membered
1
has a remarkably low-field 31P NMR chemical shift (13.9
ppm), and 2, a remarkably high-field shift (−236 ppm). The crystal and molecular structures of
the potential cyclotrimerization precursor complexes
fac-[Mo(CO)3(1)3]
(7),
fac-[Mo(CO)3(2)3]
(8), and
[(η5-C5H5)Fe(2)3]PF6 (9) have
been determined. Both molybdenum complexes have
C
3 symmetry in the
solid state, and the iron complex has C
1
symmetry. An interesting feature of the three
structures is that the phenyl groups of the small phosphorus
heterocycles in each case are
arranged in groups of three syn or anti to the auxiliary ligands.
Ab initio molecular orbital calculations at the G2 level of theory have been used to characterize the
potential energy surface for the reactions of phosphirenium or phosphiranium ions with acetylene or ethylene.
Comparisons between two competing pathways for π-ligand exchange are discussed along with comparisons
between the lower energy exchange pathway and a competing insertion reaction. The lower energy exchange
reactions have barriers (4−39 kJ mol-1) much smaller than those of the insertion reactions (215−234 kJ
mol-1), despite the fact that the insertion products are strongly preferred thermodynamically. This is consistent
with the exclusive observation of exchange experimentally. The transition structures for the preferred exchange
reactions contain PH2
+ moieties that are rotated so that they are almost parallel to the CC bond of the ring and
the CC bond of the incoming hydrocarbon, leading to the prediction that the reactions are stereospecific with
respect to the configuration at phosphorus.
A us t ra lia 1 -P h enyl p hosp h i ra ne reacts with methyl trif late to give 1 -methyl-1 -p hen yl p hosp h i ra n i u m t riflate, which reacts with acetylenes to give the corresponding phosphirenium salts.
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