Preferred Conformations of Stabilized Phosphorus Ylides

Infrared spectroscopy and ab initio computation in conformer determination of keto ester and diketo triphenylphosphonium ylides

“…[6], [7] and [22], and Section 2).These conclusions agree with those from theoretical treatments. For the syn-anti diketone and diester ylides, the syn acyl group has a lower stretching frequency than the anti, because favorable interaction between an anionoid syn acyl oxygen and the cationoid phosphorus slightly increases the CO bond length, reducing the double bond character and therefore the force constant in the stretching vibration.…”

“…The reaction is an acid-base interaction with a second equivalent of ester ylide, as described for keto ester ylides 1-4 by Castañeda et al [7]. Elemental analyses confirmed the structures of ylides 5-8.…”

“…We cannot transpose these groups in diesters where the smaller ester group is always syn [16], and were unable to isolate diester ylides with two bulky alkoxy groups, but it is possible to make these structural changes in keto ester ylides where earlier evidence from X-ray crystallography and NMR spectroscopy showed that the keto acyl group is always syn and the ester group is anti, regardless of the size of its alkoxy residue [6,7,18]. Experimental and predicted stretching frequencies had been compared for some keto esters with a small (methyl) syn-keto group and a variable anti ester group (Me, Et, isoPr, t-Bu) [16].…”

“…Ylides with two identical acyl groups can adopt three conformations and with different acyl groups four conformers could form, but in most of the stabilized ylides with alkyl acyl groups examined to date one conformer is dominant, usually with syn-anti acyl, but sometimes anti-anti groups, [6][7][8][9][10][11]. These generalizations only apply to ylides with alkyl keto or ester groups and Aitken et al in considering ylides with phenyl or very bulky groups in the ylidic moiety observed a number of systems where two acyl groups adopted the syn-syn conformation [12].…”

Synthesis of new phosphorus ylides: Spectroscopic and X‐ray structural studies

“…X-ray structures of stabilized phosphonium ylides possessing a substituent that conjugates with the P C double bond have been reviewed [21]. Ylidic resonance is important in phosphonium ylides stabilized by electron-withdrawing substituents due to electronic delocalization between the P-C bond, the ylidic bond, and an acyl group [22,23]. The three ylides share a number of common features, in par- (2) 116.246 C(8)-P(1)-C (11) 110.09 (15) 109.167 C(8)-P(1)-C (18) 113.51 (16) 116.280 C(8)-P(1)-C (25) 112.44 (15) 111.646 C(9)-C(8)-P (1) 113.20 (2) 113.271 C(11)-P(1)-C (18) 102.45 (16) 101.525 C(11)-P(1)-C (25) 109.12 (15) 108.228 C(25)-P(1)-C (18) 108.71 (14) 109.295 Torsion angle O3-C7-C8-P1 27.10(4) 30.34…”