Fernando Castañeda

Preferred conformations of stabilized phosphorus ylides

The stabilized phosphorus ylides, Ph3P=C(CO.R′)CO.OR; 1, R=Et, R′=CH2P+Ph3; 2, R=R′=Me; 3, R=Et, R′=Me; 4, R=Pri; R′=Me; 5, R=But; R′=Me, adopt a near planar conformation in the crystal which allows extensive electronic delocalization. The keto and alkoxylic oxygens are oriented and align favorably with the cationoid phosphorus. These conformations bring methyl hydrogens in the ester residue into proximity with the face of a phenyl group and lead to π-shielding and upfield shifts of the 1HNMR signals of 3 over a wide temperature range (-50–95°C) in (CD3)2CO, CDCl3 and DMSOd-6. Geometries of 2 and 3, optimized by using the HF 3-21 (G*) or 6-31 (G*) basis sets, are very similar to those in the crystal, but semiempirical treatments generate structures in which either the ester or keto moiety is twisted out of plane. The authors thank the CEPEDEQ, Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile for instrumental facilities.

3-(Triphenylphosphoranylidene)pentane-2,4-dione and diethyl 2-(triphenylphosphoranylidene)malonate.

The title ylides, 3-(triphenylphosphoranylidene)pentane-2,4-dione, C23H21O2P, (I), and diethyl 2-(triphenylphosphoranylidene)malonate, C25H25O4P, (II), differ in the conformations adopted by their extended ylide moieties. In (I), one carbonyl O atom is syn and the other is anti with respect to the P atom, the ylide group is nearly planar, with a maximum P-C-(C=O) angle of 18.2 (2)degrees, and the P-C, C-C and C=O bond lengths are consistent with electronic delocalization involving the O atoms. In (II), both carbonyl O atoms are anti and the ester groups are twisted out of the plane of the near trigonal ylide C atom, reducing delocalization, the largest P-C-(C=O) angle being 30.2 (2)degrees.

Comparison of Conformations of Diesters of Stabilized Phosphonium Ylides in Solution and in the Crystal

Computed bond lengths and angles of methyl ethyl and dimethyl triphenyl phosphonium ylidic diesters, 1b, c , respectively, are similar to those in the crystal, as for the diethyl ester, 1a , where both acyl oxygens are anti to phosphorus. The 1 H and 13 C NMR spectra of the methyl ethyl diester, 1b , where one acyl oxygen is syn and the other anti to phosphorus, are as expected in terms of the conformation in the crystal, but the dimethyl ester, 1c , in the crystal is an equimolar mixture of conformers. For a given ylidic diester the different conformers have similar energies from B3LYP//6-31G(d) computations, interconversions of conformers should not be slow at ambient temperatures and 1 H and 13 C NMR signals in solution are sharp. Estimation of Natural Atomic Charges indicates significant cationoid character on phosphorus and the acyl carbons, and anionoid character on the ylidic carbon and the ester oxygens depending on orientations towards phosphorus.

Conformational analysis of 3-triphenylphosphoranylidene-2,4-pentanedione by NMR spectroscopy and geometrical optimization

The diacyl ylide, 3-triphenylphosphoranylidene-2,4-pentanedione (2) can adopt conformations stabilized by ylidic resonance with both acyl oxygens syn- or with one oxygen syn- and the other anti- to cationoid phosphorus, 2a, Z, Z and 2b, Z, E respectively. In solution the CH3 groups give single 1H and 13C NMR signals indicating either that 2a is dominant, or that conformational equilibration is fast on the NMR time scale. However, two sets of 13C signals of CH3 and CO are observed in the solid and comparison of the chemical shifts shows that 2a is the dominant conformer in solution and 2b in the solid. Calculations with ab initio HF basis sets indicate that the heat of formation of 2a is slightly more favorable than of 2b, and that the rotational barrier for interconversion is low. These energetic differences are much smaller than for the otherwise similar monoacyl ylides and are related to differences in ylidic resonance in conformational equilibration in mono- and diacyl ylides.

A Common Conformation of Stabilized Triphenyl Phosphonium Ylidic Diesters with Bulky Alkoxy Groups

The phosphonium ylidic diesters, methyl and ethyl isopropyl and, methyl and ethyl t-butyl triphenylphosphoranylidene malonates, 1a,b and 2a,b , respectively, have the syn-anti conformation in solution, as in the crystal, and the bulkier alkoxy group is oriented towards phosphorus. The 1 H NMR spectra show that in 1a,b , the isopropyl group is oriented towards the face of a phenyl group, consistent with π shielding in the 1 H signals, and examination of the 1 H coupled 13 C NMR spectra allows assignment of the acyl carbon signals. Computed bond lengths and angles for isolated molecules are similar to those in the crystal, and the geometry and the NMR spectra indicate extensive ylidic resonance. Estimated partial atomic charges on the ester oxygens are more negative when they are oriented towards, rather than away from, phosphorus.

Thermal Decomposition of Triphenylphosphonium Alkyl Ester Salts

In thermolyses of molten triphenylphosphonium alkyl ester bromides and chlorides, alkyl = methyl, ethyl, isopropyl, at 130 and 225°C, initial attack of the halide ion on the methyl group gives the methyl halide and ylid 1 , Ph 3 P = CH 2 , which can be methylated, or is protonated by the phosphonium salt with transylidation giving Ph 3 P+-CH 3 X−, X = Br, Cl. The initial reactions of the ethyl or isopropyl esters are with the halide ion, X−, as a base giving ylid, 1 , which can be protonated by HX or by transylidation. The t-butyl ester generates Ph3P+-CH 3 X−but no products of transylidation. The first-formed ylid 1 , can be trapped by reactive alkyl and acyl halides, and the transient ylidic esters decompose thermally to triphenyl phosphine oxide, Ph 3 P = O, react further with unreacted phosphonium ester, or are trapped by added aldehyde in a Wittig reaction. The final product compositions are affected by a decrease in pressure, due to escape of volatile intermediates, and by replacement of the X− halide ion by the less nucleophilic and basic tosylate ion. Reactions under reflux, in solution in chloroform, or in suspension in benzene, are similar to those of the molten salts, but yields are generally lower at the lower temperatures.

Cyano Triphenyl Phosphonium Ylides Stabilized by Ester or Keto Groups: Computed Conformations and Crystal Structures

The mixed stabilized triphenyl phosphonium ylides Ph3P = C(CN)CO-R and Ph3P = C(CN)COOR, R = CH3, C2H5, CH(CH3)2, 1a,c and 2a,b, respectively, have sharp 1H and 13C NMR signals, except for 2b in some conditions. Ab initio geometrical optimizations indicate that in the preferred conformer the acyl oxygen is syn- to phosphorus, with barriers to rotation about the ylidic bond of 15–18 kcal. mol−1, and differences between the syn- and anti-conformers of ca. 4 and 11 kcal. mol−1 for the keto and ester derivatives, respectively. These energetic differences are not very sensitive to changes in the alkyl group, R, or the level of computation. The computed geometries indicate that the ylidic moieties are approximately planar with extensive electronic delocalization, as in crystalline 1a and 2a. The 13C NMR signals of the ylidic carbons are shifted strongly up field, consistent with their anionoid character.

Conformational Differences in Stabilized Diketo and Diester Phosphonium Ylides in a Solid and in Solution

The diacyl ylid, 3-triphenylphosphoranylidene-2,4-pentanedione 2 can adopt conformations with one oxygen syn- and the other anti- to phosphorus 2a as in the crystal, or with both oxygens syn- to phosphorus 2b . In solution, 1 H and 13 C NMR signals are sharp, but 13 C signals in the solid confirm that the two acyl groups are in different positions, and a comparison of the 13 C signals in solution and a solid shows that conformations differ. Ab initio geometrical optimizations give, for 2a , a geometry similar to that in a crystal, and structures of both conformers permit extended ylidic resonance through near planar ylidic and acyl moieties. In crystalline diethyl 2-triphenyl phosphoranylidene malonate 3 , the torsional angles of the ester moieties to the plane of the near trigonal ylidic moiety are −149.2° and 170.4°, due to packing forces and interactions with adjacent molecules, which limit ylidic resonance. The 13 C NMR spectrum in the solid differs from that in solution, with the two ester groups in different positions, but in solution, 13 C and 1 H NMR signals are sharp. Ab initio geometrical optimizations indicate that for an isolated molecule, the conformer with torsional angles as in the crystal is energetically unfavored but energetic barriers to interconversion are small, and rapidly equilibrating conformers probably exist in solution.

A STUDY OF REACTIONS BETWEEN CARBOETHOXYMETHYLENETRIPHENYL-PHOSPHORANE AND ALKYL DIBROMIDES

Abstract Carboethoxymethylenetriphenylphosphorane 1 reacts with 1,4-dibromobutane and 1,5-dibromopentane under transylidation conditions in refluxing anhydrous benzene affording the cyclization products, 1-carboethoxycyclopentyl and 1-carboethoxycyclohexyltriphenylphosphonium bromides, 6, which by alkaline hydrolysis produce the corresponding cycloalkylcarboxylic acids and esters. Phosphorane 1 and its phosphonium salt 5 formed in the transylidation step also lead to 3-carboethoxy-3-triphenylphosphoranyliden-2-oxopropanetriphenylphosphonium bromide (9) as a secondary reaction product.

Favoured conformations of methyl isopropyl, ethyl isopropyl, methyl tert-butyl, and ethyl tert-butyl 2-(triphenylphosphoranylidene)malonate

The conformations of organic compounds determined in the solid state are important because they can be compared with those in solution and/or from theoretical calculations. In this work, the crystal and molecular structures of four closely related diesters, namely methyl isopropyl 2-(triphenylphosphoranylidene)malonate, C(25)H(25)O(4)P, ethyl isopropyl 2-(triphenylphosphoranylidene)malonate, C(26)H(27)O(4)P, methyl tert-butyl 2-(triphenylphosphoranylidene)malonate, C(26)H(27)O(4)P, and ethyl tert-butyl 2-(triphenylphosphoranylidene)malonate, C(27)H(29)O(4)P, have been analysed as a preliminary step for such comparative studies. As a result of extensive electronic delocalization, as well as intra- and intermolecular interactions, a remarkably similar pattern of preferred conformations in the crystal structures results, viz. a syn-anti conformation of the acyl groups with respect to the P atom, with the bulkier alkoxy groups oriented towards the P atom. The crystal structures are controlled by nonconventional hydrogen-bonding and intramolecular interactions between cationoid P and acyl and alkoxy O atoms in syn positions.