Apicophilicity of the benzoyl group in five-co-ordinate phosphoranes

摘要

1220 J.C.S. Perkia IApicophilicity of the Benzoyl Group in Five-co-ordinate PhosphoranesBy Stuart Trippett and Peter J. Whittle, Department of Chemistry, The University, Leicester LEI 7RHEvidence i s presented from the variable-temperature l H n.m.r. spectrum of 5- benzoyl-2.2.3.3.7.7.8.8-octamethyl-1,4,6,9-tetraoxa-5-phosphaspiro4.4 nonane (9) that the apicophilicity of the bentoyl group is comparable tothose of the phenoxy- and phenylthio-groups. Benzoyldifluorodimethylphosphorane is pieDared from benzoyl-dimethylphosphine and xenon difluoride. Its low temperature l@F and l H n.m.r. spectra give some evidence forrestricted rotation round the equatorial P-benzoyl bond.ACYL groups are electronegative and x-acceptors. Interms of the analysis by Hoffmann, Howell, and Muet-terties of the factors determining the apicophilicity ofgroups in five-co-ordinate phosphoranes they wouldtherefore be expected to be highly apicophilic.As part1 R. Hoffmann, J . M. Howell, and E. L. Muetterties, J . Amer.Chem. SOC., 1972, 94, 3047.a E.g. S. Bone, S. Trippett. and P. J. Whittle, J.C.S. Perkh I ,1974, 2126; R. K. Oram and S. Trippett, ibid., 1973, 1300.of our work 2 aimed at defining the relative apicophilici-ties of groups attached to phosphorus in five-co-ordinatephosphoranes we now report on the benzoyl group.There are two previous reports on acylphosphoranes. Thebenzoyldifluorophosphorane (1) was tentatively identi-fied3 in solution (19F and 31P n.m.r. spectra) from theC. Brown, hl. Murray, and R.Schmutzler, J . Chem. SOC. (C),1970, 8781975 1221reaction of fluorodiphenylphosphine with benzoyl fluor-ide, whereas the spirophosphorane (4) was isolatedFPhC0-LMe I PhFfrom the reaction of dimethylketen lactone dimer (2) withthe phosphorodiamidite (3).Addition reactions of acylphosphetans did not lead toacylphosphoranes because of migration of the acyl groupsin dipolar intermediates. Thus the acylphosphetan (5), /Me(5)CICl(710p.p.m. to high field of 85 H,PO,), carbonyl absorptionat 1659 cm-l, and mass spectrum agree with this struc-ture. The proton n.m.r. spectrum at room temperatureconsists, in the methyl region, of two signals each equiva-lent to 12 protons, showing that, as expected, pseudo-rotation between the topomeric trigonal bipyramids (10)and (11; R = COPh) is rapid on the n.m.r.time-scale atthis temperature. This pseudorotation makes A 3 B andC = D. The two signals coalesce reversibly at 140 "C in1-bromonaphthalene ; this corresponds to a free energy ofactivation for the process leading to equivalence of themethyls of 20.9 kcal mol-I. The process involves pseudo-rotation to the phosphorane (12; R = COPh) having anapical benzoyl group and a diequatorial five-memberedIi, RLiii,PhCOCIq p ,COPh0-PO'ring. The analogous pseudorotation (1 1) 4 (12) whenR = H (ref. 5 ) has AG* 18.4 kcal mol-l indicating that inthis system a hydrogen atom is more apicophilic than abenzoyl group. As hydrogen is known2 to be moreapicophilic by 3 4 kcal mol-l than phenoxy- and phenyl-thio-groups this implies that the benzoyl group is similarin apicophilicity to these.Hoffmann predicted that with an equatorial n-acceptor group the preferred orientation would be withthe acceptor orbital in an apical plane and that thiscould, as with sr;-donor group^,^,^ lead to a considerablebarrier to rotation round the equatorial bond.We havesought this effect in benzoylphosphoranes.r 4a mixture of geometrical isomers, with hexafluoroacetonegave a product mixture whose acetyl signals appeared assinglets in the proton n.m.r. spectrum indicating that theywere no longer attached to phosphorus. Similarly fromtetrachloro-o-benzoquinone and (5) a mixture of isomeric2 : 1 adducts was isolated which are formulated as thephosphoranes (7) formed via migration of the acetylgroups in the 1 : 1 intermediates (6).Metallation of the tetraoxyspirophosphorane (8) withN-lithio-N-cyclohexylisopropylamine followed by treat-ment with benzoyl chloride gave the benzoylspirophos-phorane (9) in high yield.The 31P chemical shift (39.5W. G. Bentrude, W. D. Johnson, and W. A. Khan, J . Amer.D. Houalla, R. Wolf, D. Gagnaire, and K. B. Robert, Chem.Chem. SOC., 1972, 94, 923.Comm., 1969, 443.P A(131 (12)Treatment of phenyl dimethylphosphinite in trichloro-fluoromethane with xenon difluoride at low temperaturegave, after vigorous reaction, as the major product,E. L. Muetterties, P. Meakin, and R. Hoffmann, J. Amev.Chem. SOC., 1972, 94, 66741222 J.C.S.Perkin Idifluorodimethylphenoxyphosphorane,7 identified by its19F (doublet of septets, JPp 745, Jm 13 Hz, at +3-3p.p.m. relative to CFCl,) and lH (doublet of triplets, Jp=18, J ~ H 13 Hz, at 7 8.22) n.m.r. spectra. A similar reac-tion with benzoyldimethylphosphine gave one majorproduct (85 as judged by the 19F n.m.r. spectrum of theresulting solution). This was identified as the benzoyl-phosphorane (13) from its 31P (25 p.p.m. to high field of85 H,PO,), lsF (doublet of septets, JPF 627, JHF 13 Hz,at +29.3 p.p.m. relative to CFCl,), and lH (doublet oftriplets, JpH 18, J F H 13 Hz, a t T 8.0) n.m.r. spectra at roomtemperature.The lSF n.m.r. spectrum of (13) was unchanged at-100 "C but the methyl signals in the lH spectrumshowed considerable broadening at this temperature.Similarly the methyl signals in the lH n.m.r.spectrumof (9) were broadened considerably at -100 "C whilethe signals due to the benzoyl protons remained sharp.These results suggest that rotation round the equatorialP-benzoyl bond is slowing at -100 "C on the n.m.r.time-scale. with the benzoyl group preferring to be inthe equatorial plane. However the barrier to rotationmust be less than 8 kcal mol-l.EXPERIMENTALlgF N.m.r. spectra were obtained a t 56-4 M H z and 31Pn.m.r. spectra at 24.3 MHz. Positive shifts are upfield fromthe reference in each case.l-AcetyZ-2,2,3,4,4-~entamett~yZ~hos~~~tan.-Acetyl chlor-ide (1.2 g) in light petroleum (15 ml) was added over 15 mint o a stirred solution of 2,2,3,4,4-pentamethylphosphetan 8(2.2 g) and triethylamine (1.55 g) in ether-light petroleum(1 : 1 ; 30 ml) and the suspension was refluxed for 4 h.Filtra-tion and distillation then gave l-acety1-2,2,3,4,4-penta-methylphosphetan (76) as a 1.8 : 1 mixture of isomers (fromintegration of n.m.r. spectra), b.p. 63-65' at 0.8 mmHg,vmx. 1655 cm-l, T (major isomer) 7-47 (lH, m), 7.75 (3H, d,J 5 Hz), 8.62 (6H, d, J 11 Hz), 8-68 (6H, d, J 14 Hz), and9.10 (3H, dd, J 1 and 7 Hz), 31P 6 (CHCl,) -64 p.p.m. Theminor isomer had identifiable peaks at 7 7.67 (3H, d, J 6 Hz),and 9.18 (3H, dd, J 1 and 7 Hz), the others being obscured bythose of the major isomer, andReaction of l-Acetyl-2,2,3,4,4-pentamethyZ~hosphetan withTetrachloro-o-benzoquinone.-A solution of the quinone(2.36 g) in ether (26 ml) was added over 0.5 h to a stirredsolution of the acetylphosphetan (0-8 g) in ether (20 ml), andthe solution was refluxed for 1 h.Evaporation of solventS. C . Peake, M. Fild, M. J. C. Hewson, and R. Schmutzler,Inorg. Chem., 1971, 10, 2723.6 -73 p.p.m.and crystallisation of the residuc from hexane gave the 2 : 1adduct (7) as a 3 : 1 mixture of isomers (from integration of 1Hn.m.r. spectrum), m.p. 158-163", vmx. 1794 cm-l, T (ben-zene; major isomer), 8.02 (3H, s), 8.72 (GH, d, J 22 Hz),8.95 (6H, d, J 22 Hz), and 9.32 (3H, dd, J 1.5 and 7 Hz),6 - 15 p.p.m., m/e (3sCl) GO4 ( M - 70), 561, 387, 344, antl275 (Found: C, 39.2; H, 2.0; C1, 41.6; P, 4-8. C,,HlgC18-0,P requires C, 38.9; H, 2.8; Cl, 41.0; P, 4.6).phosphaspiro4.4nonane (9) .-A solution of the tetraoxy-phosphorane (8) (1-32 g) in tetrahydrofuran (10 ml) wasadded to a stirred solution of N-Iithio-N-cyclohesylisopro-pylamine (0.735 g) in tetrahydrofuran (20 ml) a t -78 "C andthe mixture was kept a t this temperature for 0.5 h.Benzoylchloride (0.7 g) in tetrahydrofuran (10 m1) was then addedand the solution allowed to warm to room temperature andthen refluxed for 0-6 h. Solvent was removed under reducedpressure and the residue estracted with hot ether-lightpetroleum (1 : 1 ; 30 ml). Evaporation of the extract andcrystallisation of the residue from light petroleum gave thespirophosphorane (9) (86), m.p. 158-160", T 1.50-1.57(2H. m), 2-20-2.65 (3H, m), 8-66 (12H, s ) , and 8.92 (12H,s), 31P 6 (CH,Cl,) +39.6 p.p.ni., inle 353 ( M - 15), 310,263, 181, 147, and 105 (Found: C, 61.2; H, 7.8; P, 8.3.C,,H,,O,P requires C, 61.0; H, 7.9; P, 8.4).BenzoyZdiraetkyZphosphine.-I-'h enyl d imethylphosphini t e(12 g) in toluene (10 mi) was added dropwise to a stirredsuspension of lithium aluminium hydride (5 g) in toluene(50 ml).The resulting dimethylphosphine was carried in aslow stream of nitrogen into a stirred solution of benzoylchloride (10.95 g) and triethylainine (7-9 g) in light petro-leum (60 ml). After 6 h the niisture was filtered and thefiltrate distilled to give benzoyldimethylphosphine (46),b.p. 68-72" a t 1 mmHg, vmLXp. 1643 cni-l, T 2.05-2.27 (2H,m), 2.50-2.70 (3H, m), and 8.75 (6HJ d, J 2 Hz).Reaction of PI1= Cornpowads with Xenon Diflzcorkde.-Xenon difluoride (< 1 mol. cquiv.) was sublimed in vacuoonto the inner wall of a Kel-F (polytrifluorochloroethylene)n.m.r. tube above a solution of the PIr1 compound in tri-chlorofluoromethane a t - 197 'C. The solution was al-lowed to warm until mobile antl brought into contact withthe xenon difluoride, when an immediate and vigorousreaction occurred. When reaction was complete the solu-tion was warmed to room temperature and the tube sealed.We thank Dr. J . H. Holloway for providing the xenondifluoride and for help in its use and the S.R.C. and CIBA-GEIGY (U.K.) Limited for a studentship.5/047 Rrceived, 8th January, 19763J. R. Corfield, R. K. Oram, D. J. H. Smith, and S. Trippett,5-BenzoyZ-2,2,3,3,7,7,8,8-octanietl~yZ-1,4,6,0-tetvaoxa-5-J.C.S. Perkin I, 1972, 713