The effects of substituents at phosphorus on the mode of decomposition of phosphonium betaines in protic solvents

摘要

1976 2529The Effects of Substituents at Phosphorus on the Mode of Decompositionof Phosphoniurn Betaines in Protic SolventsBy David W. Allen," Phillip Heatley, Batrie G. Hutley, and Malcolm T. J. Mellor, Department of Chemistryand Biology, Sheffield Polytechnic, Pond Street, Sheffield S1 1 WBA delicate balance of steric and electronic effects controls the course of decomposition of phosphonium betainesgenerated in protic solvents. Elecron-withdrawing heteroaryl substituents (e.g. 2-fury1 and 2-thienyl) promoteintramolecular betaine collapse to form the normal Wittig products, as also does enclosure of the phosphorus in thedibenzophosphole ring system. Except in such cyclic systems, both electron-donating and bulky groups (e.g.o-tolyl and t-butyl) reduce the rate of intramolecular collapse and allow a dehydration step to occur, with theformation of a vinylphosphonium intermediate (IV).The decomposition of (IV) leading to abnormal productsdepends on the carbanionic stability of the group cleaved from phosphorus. Carbanions of moderate stability suchas phenyl, p-methoxyphenyl, or m-chlorophenyl require the additional stabilisation of negative charge afforded inthe transition state of the reaction leading to the rearrangement product (V). For carbanions of greater stability,such as 2-heteroaryl and m-trifluorophenyl carbanions, simple cleavage of the leaving group can occur, with form-ation of the vinylphosphine oxide (VI). The presence of an aryl substituent on the carbon cc to phosphoruspromotes betaine collapse to the normal Wittig products even when the phosphorus bears a t-butyl group [as in thereaction of the salt (XIII) with benzaldehyde in the presence of ethoxide ion].THE Wittig reaction between a phosphonium salt and acarbonyl compound in the presence of alkoxide ion inprotic solvents may occur by two competing mechanisticcourses (see Scheme).On the one hand, the initiallyformed betaine (I) may undergo rate-determining collapseto the normal Wittig products (Le. olefin and phosphineoxide) via an oxyphosphorane (11) .l Alternatively,protonation of the betaine may occur to give the P-hydr-oxyalkylphosphonium salt (111) , which subsequentlymay eliminate water to form the vinylphosphonium salt(IV). Hydrolysis of the latter may then occur by one ormore of the following routes: (i) with cleavage of thevinylic substituent to give the normal Wittig products;(ii) with migration of an aryl group from phosphorus toadjacent carbon to form the rearrangement product (V) ;and (iii) with loss of a substituent other than the vinylicgroup to form a vinylphosphine oxide (VI).4s5The preferred course has been shown to depend on thenature of the substituents R1, R2, and R3,2*39697 on theelectron-withdrawing character of the substituents a tphosphorus,s and on the steric constraints placed on thephosphorus atom by its incorporation into a small ringsystem.B When R1 = H, and either R2 or R3 is anelectron-withdrawing group such as phenyl or methoxy-carbonyl, then the rearrangement products (V) areformed preferentially when the substituents at phos-phorus are phenyl, m-chlorophenyl, or P-methoxyphenyl,that group migrating which is the more stable as a car-banion.3.7 However, when R1 = Ph or Me, the normalWittig products (olefin and phosphine oxide) are formedfrom collapse of the phosphonium betaine uia the oxy-phosphorane (II).697 If the electrophilicity of the phos-G. Wittig and U.Schollkopf, Chem. Ber., 1954, 87, 1318.E. E. Schweizer, D. M. Crouse, T. Minami, and A. T. Weh-E. M. Richards and J. C . Tebby, J. Chem. SOC. ( C ) , 1971,J. W. Rakshys and S. V. McKinley, Chem. Comm., 1971,5 Preliminary communication, D. W. Allen, B. G. Hutley,man, Claem. Comm., 1971, 1000.1059.1336.and M. T. J. Mellor, Tetrahedron Letters, 1974, 1787.phonium centre in the betaine is increased by attachmentof the electron-withdrawing 2-fury1 substituent, theneven when R1 = H, the normal Wittig products areformed preferentially.8 In addition, betaine collapse viathe oxyphosphorane is also facilitated even when R1 = Hby enclosure of the phosphorus in the small, straineddibenzophosphole ring system, owing to the formation ofAr3b-C HR' Ar, P-CHR'A~,P=CHR'+ RZR~C=O 1 - 1 1 L6-CR2R3 0- C R ~ RI1Ar3P-C=CR2R3 1- Ar,F-C=CR2R3 2% Ar3 + P=O(Iv) ,:I C!i=CRZR3 t OH I R' I-OH h OH 1 I O H -Ar O h 0I-* 0 2 3 ROH 11 I I I0- R' fV\R Ar2P-C-CHR2R3 ArZP-C=CR2R3I IR1i V I ) i n 1SCHEMEa strain-free spirophosphorane in which both four- andfive-membered rings span apical-equatorial positions.9We have continued to investigate steric and electronic191.D.J. H. Smith and S. Trippett, J.C.S. Chem. Cornm., 1972,S . Trippett and B. J. Walker, J . Chem. SOC., 1966, 887.D. W. Allen, B. G. Hutley, and T. C. Rich, J.C.S. Pevkin 11,D. W. Allen, B. G. Hutley, and K. Polasik, J.C.S. Perkin I,1973, 820.1975, 6192530 J.C.S. Perkin Ieffects of substituents at phosphorus on the mode ofdecomposition of phosphonium betaines in protic sol-vents, and now report as follows.The reaction of methyltriphenylphosplionium iodideand benzaldehyde in ethanolic sodium ethoxide [oralternatively the reaction of triphenylphosphine andstyrene oxide, which leads directly to the betaine (I;Ar = Ph, R1 = R2 = H, R3 = Ph)] yields predomin-antly the rearrangement product (V; Ar = R3 = Ph;1 0I MeR1 = R2 = H) via a vinylphosphonium interrne~liate.~.'In contrast, the analogous reactions of the phosphoniumsalts (VII and VIII; X = 0) yield the normal Wittigproducts of styrene and the phosphine oxides (IX and X ;X = 0, respectively).8 On the basis o€ earlier work onthe chemistry of heteroarylphosphonium salts,1° it wassuggested that the 2-fury1 substituents act as electron-withdrawing groups which favour intramolecular collapseof the betaine (I).We now find that the presence of the2-thienyl substituent at phosphorus has the same result.Thus the betaines (I) derived from the reactions of tri-2-thienylphosphine or diphenyl-(2-thienyl)phosphinewith styrene oxide in ethanol collapse to give styreneand the phosphine oxides (IX and X; X = S, respec-tively).Although on the basis of our earlier work lo the2-thienyl group is less strongly electron-withdrawing than2-fury1 when attached to a second-row element, it isnevertheless still appreciably more electron-withdrawingthan phenyl, and this is reflected in the increased rate ofbetaine collapse via the oxyphosphorane (11).The presence of bulky groups at phosphorus has beenshown to have a marked influence on the course ofalkaline hydrolysis of phosphonium salts ; in suchcrowded systems, loss of the substituent which forms themost stable carbanion may not necessarily occur.11Since both competing routes for the Wittig reaction in aprotic solvent involve nucleophilic attack at a phos-phonium centre, we have studied the effect of introducingbulky groups at phosphorus both on the preferredmechanistic course and on the products of such reactions.10 D. W.,411en, B. G. Hutley, and M. T. J. Mellor, J.C.S.Perkin 11, 1972, 63.Introduction of a t-butyl group at phosphorus has amarked effect on the course of the Wittig reactions ofmethylphosphonium salts. Thus the major product ofthe reaction of the salt (XI) with benzaldehyde in ethan-olic sodium ethoxide is the vinylphosphine oxide (XII),arising by loss of furan from a vinylphosphonium inter-mediate. However, the Wittig reaction of the benzyl-(2-furyl)-t-butylphosphonium salt (XIII) with benz-aldehyde proceeds normally to give stilbene and di-(2-fury1)-t-butylphosphine oxide, again reflecting the dom-inant role of the substituent R1 in the decomposition ofthe betaine, as established for phenylphosphorus sys-t e m ~ .~ * ' In contrast, the reaction of t-butyldiphenyl-phosphine with styrene oxide gives a complex mixtureof products which are difficult to separate and identifywith certainty. The only phosphorus-containing prod-uct fully characterised was the normal Wittig product,diphenyl-t-butylphosphine oxide, which however wasisolated in only very small yield. The complexity of theproducts indicates that betaine collapse is slow, owing toboth the steric effect of the t-butyl group and the reducedelectron-withdrawing effects of the substituents atphosphorus, leading to the vinylphosphonium inter-mediate.However, the betaine derived from thereaction of 5-t-butyldibenzophosphole (XIV) with sty-rene oxide undergoes decomposition to form only thenormal Wittig products, styrene and the dibenzo-phosphole oxide, in a clean reaction. In this case (as forthe related reaction of 5-phenyldibenzophosphole 9, theBU'(XIV)B u ty - 4 2'l &ti Ph0I IPh,P-CH=CH PhH(XYIII)0Pk II(XIXIdominant factor seems to be relief of steric strain informing the spirophosphorane (XV), in which both four-and five-membered rings span apical-equatorial positions,the t-butyl substituent occupying an equatorial position.Replacement of the 2-fury1 substituents in the salts(VII and VIII; X = 0) by the l-methylpyrrol-2-ylsubstituent also reduces the rate of betaine collapse and11 J.R. Corfield, N. J. De'ath, and S. Trippett, J . Chem. Soc.(C), 1971, 19301976 2531leads to abnormal products. Thus the Wittig reactionsof the salts (VII and VIII; X = NMe) with benzalde-hyde give the vinylphosphine oxides (XVI) and (XVII),respectively, together with l-methylpyrrole. The 1-methylpyrrol-2-yl substituent may reduce intramolecularbetaine collapse in two ways : (i) by its reduced electron-withdrawing efiect as compared with the 2-fury1 and2-thienyl groups l2 and (ii) by a possible steric effect ofthe l-methyl group.13 In order to investigate theseeffects more fully, we have studied the reaction betweendiphenyl(pyrro1-2-y1)phosphine (XVIII) and styreneoxide in ethanol.This reaction also leads to the vinyl-phosphine oxide (XVI) (together with pyrrole) as themajor product (although substantial quantities of thenormal Wittig products are also formed). It seems,therefore, that it is the overall electronic effect of thepyrrole ring system on the electrophilicity of the phos-phonium centre which is largely responsible for thecourse of the reaction, the steric effect of the N-methylsubstituent playing a minor role. Additionally, therelative ease of carbanionic cleavage of the pyrrol-2-ylgroup from the vinylphosphonium intermediate isdoubtless of importance. We have shown previouslythat the l-methylpyrrol-2-yl substituent is cleaved inpreference to phenyl on alkaline hydrolysis of mixed( 1 -methylpyrrol-2-yl) phenylphosphonium salts .12We have also studied the effects of various substitutedphenyl groups (o-tolyl, o-chlorophenyl, o-methoxyphenyl,and m-trifluoromethylphenyl) on the mode of betainedecompositions by investigating the reactions of eitherthe pliosphines Ph2PC,H4X with styrene oxide inrefluxing ethanol, or the methiodides XlePh2P-C,H4X I-with benzaldehyde in ethanolic sodium ethoxide.Pre-vious work has shown that reactions of this type whichinvolve m-chlorophenyl or fi-methoxyphenyl substituentsat phosphorus result in the rearranged oxides (V), withthe group most stable as a carbanion migrating preferen-tially.7y9 The reaction of diphenyl-(o-to1yl)phosphinewith styrene oxide (and that of the corresponding meth-iodide with benzaldehyde) gives a mixture containing twoisomeric rearrangement products (V) in which eitherphenyl or o-tolyl groups have migrated from phosphorusto adjacent carbon, and also two vinylphosphine oxides,(XVI) and (XIX), corresponding to cleavage of phenyl oro-tolyl groups from a vinylphosphonium intermediate.The reaction of (o-chloropheny1)diphenylphosphine withstyrene oxide gives predominantly the vinylphosphineoxide (XVI), together with chlorobenzene. The majorproducts from the reaction of (o-methoxypheny1)di-phenylphosphine with styrene oxide are the vinylphos-phine oxide (XVI) and a mixture of rearrangementproducts (V).In addition, both (o-methoxypheny1)-diphenylphosphine oxide and methyldiphenylphosphineoxide are formed, the latter arising from dissociation+12 D.W. _411en, B. G. Hutley, and 31. T. J. Mellor, J.C.S.l3 D. W. Allen, J. R. Charlton, and B. G. Hutley, Phosphorus,l4 K. Tssleib and A. Brack, Z. anorg. Chem., 1957, 292, 245.Perkin II, 1974, 1690.1976, 6, 191.of the betaine to the ylide and benzaldehyde, protonationof the ylide being followed by hydrolysis with loss of theo-methoxyphenyl group. Both styrene and anisole arepresent in the reaction mixture.The most significant result of this group of substitutedphenyl compounds, however, is that from the Wittigreaction of the rnethiodide of diphenyl- (m-trifluoro-methylpheny1)phosphine with benzaldehyde, which givespredominantly the vinylphosphine oxide (XVI) togetherwith trifluoromethylbenzene.No rearrangement prod-ucts appear to be formed. This reaction thus resemblesthat of the pyrrol-2-ylphosphine with styrene oxide, andthis suggests that the key factor is the stability of theforming carbanionic leaving group. In the case of theabove ortho-substituted phenyl groups, it could beargued that in addition to the relative stability of theo-chlorophenyl and o-methoxyphenyl carbanions ascompared with phenyl, the steric effect of the orfho-substituent is also likely to be of importance; in the caseof the nzeta-trifluoromethyl substituent, the steric effectshould be much reduced.Clearly a delicate balance of steric and electroniceffects controls the course of Wittig reactions conductedin protic solvents. The ultimate fate of the phosphon-ium betaine (I) is determined by the relative rates of theintramolecular collapse and dehydration steps.Elec-tron-withdrawing substituents at phosphorus (e.g. 2-fury1and 2-thienyl, but not m-trifluoromethylphenyl) promoteintramolecular betaine collapse, as also does enclosure ofphosphorus in a small ring system. In contrast (exceptin such cyclic systems), both electron-donating andbulky groups (e.g. o-tolyl and t-butyl) reduce the rate ofintramolecular collapse and allow the dehydration stepto occur, with the formation of a vinylphosphoniumintermediate (IV). The decomposition of (IV) leadingto abnormal products depends on the carbanionic stabilityof the group cleaved from phosphorus. Carbanions ofmoderate stability such as phenyl, 9-methoxyphenyl,and m-chlorophenyl require the additional stabilisationof negative charge afforded in the transition state of therearrangement reaction.For carbanions of greaterstability, such as the 2-heteroaryl and meta-trifluoro-methylphenyl carbanions, simple cleavage of the leavinggroup can occur, with formation of the vinylphosphineoxide.EXPERIMEXTALlH N.m.r. spectra were recorded a t 60 MHz with a JEOLspectrometer (ille,Si as internal standard). Mass spectrawere recorded a t 70 eV with an A.E.I. MS30 spectrometer.Operations involving organolithium intermediates ortertiary phosphines were conducted under nitrogen.Synthesis of Phosplaines and Derivatives.-Tri-( 2-thieny1)-p h ~ s p h i n e , ~ ~ diphen yl-( 2- thien yl) phosphine, l3 5-t-bu tyld i-ben~ophosphole,~~ dipheny1-t-butylphosphine,l6 l-methyl-pyrr01-2-yldiphenylphosphine,~~ tris-( l-methylpyn-ol-2-y1)-phosphine,12 diphenyl(pyrr0l-2-yl)phosphine,~~ (o-methoxy-l5 D.W. ,411en, F. G. Mann, and 1. T. Millar, J . Chem. SOC. (C),1971, 3937.S. 0. Grim, W. McFarlane, and E. F. Davidoff, J . Uvg.Chem., 1967, 32, 7812532 J.C.S. Perkin Iphenyl) diphenylphosphine, l7 (o-chlorophenyl) diphenylphos-phine,le and diphenyl-(o-toly1)phosphine lS were preparedand, where appropriate, converted into phosphonium saltsand phosphine oxides, as described in the literature.Bis-( Z-furyZ)-t-butyZphosphine.-To an ice-cold solution of2-furyl-lithium (0.2 mol) [prepared from n-butyl-lithium(0.2 mol) and furan (0.2 mol) in ether (100 cm3)] wasadded, dropwise with stirring, a solution of t-butylphosphon-ous dichloride (12 g, 0.075 mol) in benzene (40 cm3).Theresulting solution was heated under reflux for 1 h beforecooling and hydrolysing with aqueous ammonium chloride(10% w/v; 100 cm3). The organic layer was separated,dried (Na,SO,), and evaporated. The residue was distilledunder reduced pressure to give the phosphine (4.8 g, %yo),b.p. 78-82" at 0.6 mmHg, z (CDCI,) 2.3 ( 2 H, m), 3.2 ( 2 H,m), 3.7 (2 H, m), and 8.95 ( 9 H, d, 3JpcH: 13 Hz); themethiodide (XI) had m.p. 185" (from EtOH-EtOAc-Et,O)(Found: C, 43.2; H, 5.15. C13H1810,P requires C, 42.85;H, 4.95%); T (CDCI,) 2.05 ( 4 H, m), 3.20 ( 2 H, m), 7.15Oxidation of the phosphine with hydrogen peroxide inacetone gave the phosphine oxide, m.p.67", as a hygroscopicsolid, M+ 238, 7 (CDCI,) 2.15 ( 2 H, m), 2.70 ( 2 H, m), 3.37(2 H, m), and 8.72 (9 H, d, 3JpCH 16 Hz). Treatment of thephosphine with benzyl bromide gave benzyldi-( 2-furyZ)-(t-butyZ)phosphonium bromide (XII), m.p. 21 1-212" (fromEtOH-EtOAc-Et,O) (Found: C, 58.2; H, 5.8. C,,H,,-Br0,P requires C, 58.0; H, 5.65%), T (CDCl,) 1.74 (2 H, m),1.90 (2 H, m), 2.73 ( 5 H, s ) , 3.10 ( 2 H, m), 4.98 ( 2 H, d,2JpCH 12 Hz), and 8.45 (9 H, d, 3 J p c ~ 19 Hz).Dipheny 2- (m-trifluoromethyZPhenyZ)phosPhine .-To theGrignard reagent prepared from m-bromotrifluoromethyl-benzene (9.0 g) and magnesium (1.5 g) in ether (90 cm3) wasadded a solution of diphenylphosphinous chloride (4.5 g) inether (40 cm3), and the resulting solution was heated underreflux for 1 h before being cooled in ice and hydrolysed withammonium chloride solution (10% w/v; 50 cm3) and dilutehydrochloric acid (2M; 100 cm3).The organic layer wasseparated, dried, and evaporated to give the crude phosphine,which resisted crystallisation from a range of solvents. Themethiodide had m.p. 180-181" (from EtOH-EtOAc) (Found:C, 51.0; H, 3.5. C,,H,,F,IPrequiresC, 50.85; H, 3.65%);the oxide had m.p. 115-116" (from EtOH-hexane) (Found:C, 65.95; H, 4.05. Cl,H,,F,OPrequiresC, 65.9; H, 4.05%).Reactions of Phospkines with Styrene Oxide.-Generalprocedure. The phosphine mol) and styrene oxide(2 x mol) were heated together in ethanol ( 2 cm3) for24 h. The solution was then analysed by g.1.c.for thepresence of styrene and appropriate hydrocarbons arisingfrom cleavage of aryl or heteroaryl groups from phosphorus.It was then evaporated and the residue subjected to prepar-ative t.1.c. on 20 x 20 cm preparative plates coated ( 1 mm)with Kieselgel HF 256 (solvent 1 : 1 hexane-ethyl acetate).Individual bands were extracted with methanol to yield thephosphine oxide products.( A ) Diphenyl-(2-thieny1)phosphine and styrene oxidegave styrene (41 yo) and diphenyl-(2-thienyl)phosphineoxide, m.p. 117', identical with an authentic specimen.( B ) Tri-( 2-thieny1)phosphine and styrene oxide gavestyrene (40%) and tri-(2-thieny1)phosphine oxide, identicalwith an authentic specimen.l7 W. E. McEwen, W. I. Shiau, Younn-Ing Yeh, D.N. Schulz,R. U. Pagilagan, J. B. Levy, C. Symrnes, jun., G. 0. Nelson, andI. Granoth, J . Amer. Clzem. SOC., 1975, 97. 1787.I R F. A. Hart, J . Chem. SOC.. 1960, 3324.( 3 H, d, , J p c ~ 13 Hz), and 8.59 (9 H, d, 3 J p ~ ~ 18 Hz).(C) G.1.c. analysis of the product from diphenyl(pyrrol-2-y1)phosphine and styrene oxide showed the presence ofstyrene (26%), pyrrole, benzaldehyde, and unchangedstyrene oxide. T.1.c. gave diphenyl-( P-styry1)phosphineoxide ( 3 2 y 0 ) , m.p. 168" (lit.,Z0 168-169") (Found: C, 7 9 . 1 ;H, 5.85. Calc. for C,,H,,OP: C, 78.95; H, 5.6y0), identicalwith an authentic specimen, and diplzenyZ(pyrro2-2-yZ)-phosphine oxide (24%), m.p. 184-185O (from EtOH-hexane)(Found: C, 72.55; H, 5.45. C,,H,,NOP requires C , 7 1 .9 ;H, 5.3%), identical with a sample prepared by oxidation ofthe phosphine.(D) G.1.c. analysis of the product from (o-methoxypheny1)-diphenylphosphine and styrene oxide showed the presenceof styrene (15%) and anisole (15%). T.1.c. gave diphenyl-(P-styry1)phosphine oxide (15%), a mixture of rearrangedphosphine oxides, (1,2-diphenylethyl)-(o-methoxyphenyl)-phenylphosphine oxide and [ 1-(o-methoxypheny1)-Z-phenyl-ethylldiphenylphosphine oxide (30% total) ; m/e 412 (M+)232 [(o-MeO.C,H,) PhPOH] , 2 10 [PhCH=CH-(o-MeO*C,H,)],202 (Ph,POH), and 180 (PhCH=CHPh), in accord with thevery specific fragmentation pattern established for suchcompounds.21 In addition, (0-methoxyphenyZ)diphenyl~hos-fihine oxide (15y0), m.p. 165-166" (from hexane-EtOH)(Found: C, 7 4 .1 ; H, 5.55. C,,H,,O,P requires C, 7 4 . 0 ;H, 5.55%), M+ 308, identical with a sample prepared byoxidation of the phosphine, and methyldiphenylphosphineoxide were isolated and identified.(E) G.1.c. analysis of the product from (o-chloropheny1)-diphenylphosphine and styrene oxide showed the presenceof styrene ( 12y0), chlorobenzene (55%), and benzaldehyde.T.1.c. gave diphenyl(methy1)phosphine oxide (20%), to-gether with diphenyl-(P-styry1)phosphine oxide and (o-chloropheny1)diphenylphosphine oxide, which were in-completely separated.( F ) G.1.c. analysis of the product from diphenyl-(o-toly1)-phosphhe and styrene oxide indicated the presence ofstyrene (8%), together with both benzene and toluene.T.1.c. gave a mixture of the isomeric rearranged phosphineoxides, diphenyl- C2-phenyl- 1- (o-tolyl) ethy1Jphosphine oxideand ( 1,2-diphenylethyl)phenyl-(o-tolyl)phosphine oxide(20% total) (Found: C, 81.7; H, 6.5.Calc. for C,,H2,0PC, 81.8; H, 6.35%); m/e 396 (Mf), 216 [Ph(o-MeC,H,)POH],202 (Ph,POH), 194 [PhCH=CH(o-MeC,H,)], and 180(PhCH=CHPh), in accord with the specific fragmentationpattern found for such compounds.21 Also isolated was amixture of substituted vinylphosphine oxides, phenyl-(o-tolyl) -( P-styry1)phosphine oxide (M+ 318) and diphenyl-(P-styry1)phosphine oxide (M+ 304), which could not beseparated (20% total).(G) G.1.c. analysis of the product from diphenyl-(t-buty1)-phosphine and styrene oxide indicated the presence ofstyrene (14%) and benzene ( ( 5 % ) .T.1.c. revealed thepresence of a number of phosphine oxides, which wereincompletely separated from each other. The only productidentified with certainty was diphenyl-t-butylphosphineoxide, m.p. 131" (lit.,22 131"), M' 258.( H ) G.1.c. analysis of the product from 5-t-butyldibenzo-phosphole and styrene oxide showed the presence of styrene,19 M. A. Bennett and P. A. Longstaff, J . Anzer. Chem. SOC.,1969, 91, 6266.20 A. M. Aguiar and D. J. Daigle, J . Org. Chem., 1965, 30,2826.21 D. TV. -4llen, J. C. Tcbby, and D. H. Williams, TetrahedronLetters, 1965, 2361.22 D. Seyfertll, 33. A. Eisert, and J. I. Heeren, J . Organo-metallic Chem., 1964, 2, 1011976 2533and t.1.c. separation gave 5-t-butyldibenzophosphole 5-oxide,m.p.159-160" (from MeOAc-hexane) , identical with aspecimen prepared by oxidation of the phosphine (Found :C, 75.1; H, 6.75. C,,H,,OP requires C, 74.95; H, 6.7%),M f 256.Wittig Reactions of Methylphosphoraium Salts with Benz-aldehyde.-General procedure. To a solution of the phos-phonium salt ( mol) in ethanol (2 cm3) containing sodiumetlioxide ( lo-, mol) was added freshly distilled benzalde-hyde mol), and the resulting solution was kept atroom temperature under nitrogen for 7 days. The solutionwas then analysed by g.1.c. for the presence of styrene andother hydrocarbons, and the phosphine oxide products wereseparated by t.1.c. as in the case of the reactions of phosphineswith styrene oxide.G. 1 .c.analysis showed the absence of styrene, but the presence offuran (g.1.c.-mass spectrometry, M+ 68).T.1.c. followedby fractional sublimation gave as the major product(2-furyl) -( p-styvyl) - (t-butyl)phosphine oxide, m.p. 102-104"(after sublimation) (Found: C, 69.5; H, 7.05. C,,€€,,PO,requires C, 70.05; H, 7.0%), M+ 274, T (CDC1,) 2.1-3.9 (10H, m, aromatic, olefinic, and furan ring protons) , 8.8 (9 H, d,3JpCH 16 Hz, ButP) ; also isolated as a hygroscopic oil (theminor product from fractional sublimation) was (2-furyZ)-metl~yZ-(t-butyZ)phosphine oxide, mle 186 (M+), 129 (M -But), and 114 (M - But - Me), 7 (CDC1,) 2.35 (1 H, m),2.95 (1 H, m), 3.55 (1 H, m), 8.3 (3 H, d, 2 J p c ~ 12 Hz,PMe), and 8.8 (9 H, d, 3JpCH 15 Hz, €'But).(A) Di- ( 2- fury Z) methyl- (t-butyZ)phosphonium iodide.(B) Methyl (diphenyl) - (m-trij7uorophenyl)phosphonium iod-ide. G.1.c. analysis showed the presence of styrene (16%) ,together with trifluoromethylbenzene (g.1.c.-mass spectro-metry M+ 146). T.1.c. gave diphenyl-(p-styry1)phosphineoxide, together with diphenyl-(m-trifluoropheny1)phosphineoxide, identified by comparison with authentic materials.(C) Methyl- ( l-unetlzylpy~~oZ-2-yI)di~henyZ~hos~honiu iod-ide. G.1.c. analysis showed the presence of styrene ( 5%)and 1-me thylpyrrole . T. 1. c. gave diphenyl- ( p-s tyr yl) phos-phine oxide (53%).(D) Methyltris-( 1-methylpyrrol-2-y1)phosphonium iodide.G.1.c. analysis showed the presence of styrene (5%) and1-methylpyrrole. T.1.c. gave bis-( l-methylpyrrol-2-yl)-P-styrylphosphine oxide as an oil which resisted crystallisation,M+ 310, T (CDCl,) 2.20-3.25 (9 H, m), 3.47-3.95 (4 H, m),and 6.14 (6 H, s).Wittig Reaction of Benzyldi-( 2-furyl) -(t-butyZ)phosphoniumBromide (XIII) with Ben2aldehyd.e.-The salt (XIII)(0.5 x lo-, mol), benzaldehyde mol), and sodiumethoxide (0.5 x lo-, mol) in ethanol (2 cm3) werekept at room temperature for 7 days. The mixture waspoured into dilute hydrochloric acid (10 cm3) and theresulting mixture extracted with hexane (3 x 5 cm3), andthen with chloroform (3 x 5 cm3). On evaporation, thedried hexane extract gave trans-stilbene (95y0), and thechloroform extract gave di-(2-furyl)-(t-butyl)phosphineoxide, identical with an authentic specimen.[6/1133 Received, 14th June, 1976