Novel, Highly Stereoselective Rearrangements of Chiral Dihydropyranones

Abstract

Bi,C1i-[61 and as structural unit in the SbS;-layer [']. Here again the apical ligands are trans to each other ['].As expected from electron pair repulsion the phosphorus atom lies below the basal plane of the pyramid (Fig. 1). The basal PCN groups are bent in such a way that their N atoms too are below this plane. The apical PCN group on the other hand is linear and leans somewhat towards the linking edge. Whereas in +-octahedra the apical bond is normally shorter than the equatorial ones, the PC-distances here do not differ significantly. In accordance with its synthesis the P2(CN)6X$-ion can be regarded as two P(CN)3 molecules linked by a halide double bridge. Actually the geometries of the P(CN)3 units therein, for X = Br as well as for X = I, deviate only slightly from free P(CN)3 itself (Table 1). Thus the Table 1. Bond lengths [pm] and angles ["I of the anion in [C,2H2406Na(OC4Ha)2]2[P2(CN)6X2] and of P(CN), (average values) for comparison 191, deviations [pml of P and of N from the basal C2X2-plane (opposite to the apex direction). PX PC basal apical CN basal apical PXP XPX XPC basal apical CPC basal apical PCN basal apical P N X=Br 305.8(1) 182.3(6) 112.5(8) 112.8(tO) 99.9(1) 84.4(2) 83.5(1) 91.0(4) 92.9(2) 179.8(2) 8.1 7.6 1 81.0(8) 80.1(1) 173.5(5) . , 183.5(13) 178.8(28) 184.6( 18) 112.7( 13) I 10.6( 16) 114.9(45) 82.0(1) 98.1(1) 85.2(4) 84.3(3) . , 93.5(20) 171.6(30) 91.3(8) 92.7(5) 172.3(11) 179.2( 15) 6.6 10.7CPC-angles shrink by only about 1". On average the PC bonds lengthen by 4 pm and the CN bonds shorten by 3 pm, indicating a weaker bond between P and CN. The distances from P to X are considerably longer than for every known PX bond, being approximately the average of the single bond length in PX3 (X = Br: 222, X = I: 243 pm) and the sum of the van-der-Waals radii (X = Br: 385, X = I: 405 pm). The coordination number 5, unexpected high for P"', is thus apparently a result of the small reorganization necessary, rather than of strong PX bonds (the +-octahedral structure of the dimer with ideal PCP angles of 90" should be more favorable than the +-trigonal-bipyramidal one of the monomer with two ideal angles of 90" and one of 120"). For the solid P(CN),X salts we find Ej3'P= -177 (X=Br) and -168 (X= I); in solution these shifts are almost the same, indicating the dimeric anion to be preserved in solution. independently synthesised tetraalkylammonium salts of anions of the same composition (personal communication). As their S"P data in solution and in the solid state agree with those of our salts, they should also correspond in the structure of the anions. 141 Bromo derivative: a = 1367.9(1), b=2063.5(1), c=2227.6(2) pm, pLaLL=l 31 g cm I: 1853 reflections (CuKc.. 3.5" 5 2 0 5 135.0°, empirical absorption correction). R = O.O64.--Iodo derivative: a = 1385.0(2), b = 2068.0(2), c=2238.8(3) pm: pC,,,=1.32gcm~': 1314 reflections (Mo~,.. 3°5 2 0 5 5 0 0 ) , R = 0.059. 151 W. S. Sheldrick, J. Kroner, E Zwaschka. A. Schmidpeter, Angew. Chem. 91, [6] A . Hershafr, J. D. Corbett, Inorg. Chem. 2, 979 (1968)...