Casey, Scheck, Shusterman
1 Mechanism of Rearrangement of Platinacyclobutanes
4233
for a period of several hours. Upon irradiation at 313 nm, however, the (10) In addition to the predominant 313-nm Hg line, small amounts of the 297-, solutions gradually change from colorless to yellow. Attempts to charac302-, and 334-nm lines are transmitted by this filter combination. terize the photoproducts spectrally have proven inconclusive. (11) C. G. Hatchardand C. A. Parker, Proc. R. SOC.London, Ser. A, 235,518 (27) M. L. Kaufman and C. E. &iff in, Tetrahedron Lett., 12, 769 (1965). The major (1956). products resulting from the homolysis of an aryl-phosphorus bond are di(12) S.J. Lippard and K. M. Melmed, Inorg. Chem., 8, 2223 (1967). phenylphosphine and biphenyl. (13) J. T. Gill and S. J. Lippard, Inorg. Chem., 14, 751 (1975). (28) We encountered difficulty in obtaining reproducible quantum yields, pre(14) F. Cariati and L. Naldini, Gaz.?. Chim. Ita/., 95, 3 (1965). (15) T. J. Marks and J. R . Kolb, Chem. Rev., 77, 263 (1977). sumably because of adventitious traces of oxygen which can cause a facile photooxidation of PPh3 to its oxide: G. L. Geoffroy, D. A. Denton, and C. (16) C. Kutal, P. A. Grutsch, J. L. Atwocd, and R. D. Rogers, Inorg. Chem., 17, W. Eigenbrot, Jr., Inorg. Chem., 15, 2310 (1976). To the extent that this 3558 (1978). competing process occurs, our reported disappearance quantum yield will (17) J. C. Bommer and K. W. Morse, J. Chem. SOC.,Chem. Commun., 137 (1977). be high. (29) F. H.Jardine, Adv. Inorg. Chem. Radiochem., 17, 115 (1975). Some novel (18) F. Klanberg, E. L. Muetterties, and L. J. Guggenberger, Inorg. Chem., 7, five-coordinate Cu(l) compounds have been reported recently, but they 2272 (1968). contain a rigid, tetradentate macrocyclic ligand: R. R. Gagne, J. L. Allison, (19) The absorption spectra of C U ( P P ~ ~ ) ~ Cu(diphos)BH4, BH~, and the corresponding free phosphine molecules are displayed in ref 6. R . S.Gall, and C. A. Koval. J. Am. Chem. Soc., 99, 7170 (1977). (30) At the suggestion of a referee, we measured the 'H NMR spectra of nor(20) W. R. Cullen and R. M. Hochstrasser, J. Mol. Spectrosc., 5, 118 (1960). (21) G. Shaw, J. K. Becconsail, R. M. Canadine, and R. Murray, Chem. Commun., bornadiene in the presence and absence of Cu(PPh3)2BH4.The two spectra 425 (1966). were found to be experimentally indistinguishable, in accord with other (22) B. I. Stepanov, A. I.Bokanov, and V. I.Svergun, J. e n . Chem. USSR(€ngI. evidence that thermally induced complex formation between NBD and the Trans/.), 41, 526 (1971). Cu(l) sensitizer (e.g., paths A and B in Scheme I) plays no role in the (23) M. Kasha and H. R . Rawls, Photochem. Photobiol., 7, 561 (1968). mechanism of sensitization. (24) E. J. Halpern and K. Mislow, J. Am. Chem. Soc., 89, 5224 (1967). (31) Both pathways have been reported for the quenching of aromatic carbonyl triplets by norbornadiene and substituted analogues: A . J. G. Barwise, A . (25) Recent X-ray structural studies of Rh(diphos)(1.5-cyclooctadiene)+(personal A. Gorman, R. L. Leyland, P. G. Smith, and M. A . J. Rogers, J. Am. Chem. communication from W. S.Knowles) and [Cu(diphos)(PPhp)12 2CsHs (T. Greiser and E. Weiss. Chem. Ber., 111, 516 (1978)) reveal that the two Soc., 100, 1814 (1978). phenyl rings attached to each P atom of the coordinated diphos ligand (32) G. Ferraudi, Inorg. Chem., 17, 1370 (1978). possess nonequivalent environments. (33) D. D. Davis, K . L. Stevenson, and C. R. Davis, J. Am. Chem. Soc.. 100.5344 (26) Degassed benzene solutions of Cu(PPh&BH4 or C U ( P P ~ ~ M ~and )~BH~ (1978). excess NBD undergo no discernible spectral changes when kept in the dark (34) B. -T. Ahn and D. R. McMiilin, Inorg. Chem.. 17, 2253 (1978)
Mechanism of Rearrangement of Platinacyclobutanes Charles P. Casey,* Daniel M. Scheck, and Alan J . Shusterman Contributionfrom the Department of Chemistry, Unicersity of Wisconsin, Madison, Wisconsin 53706. Receiced December 22, I978
crossover products were observed when the rearrangement of dichlorobis(pyridine)( I -phen>lpropane-l.3-diyl)platinum(1V) (la) to dichlorobis(pyridine)(2-phenylpropane- I ,3-diyl)platinum(lV) (2a) was carried out in the presence of either p-tolylcyclopropane or p-methylstyrene. Preparation of dichlorobis(pyridine)(cis- 1 -phenqlpropane-3-d,-I .3-diyl)platinum(lV) (5a) from cis-phenylcyclopropane-2-dl and rearrangement of 5a to the P-phen~lplatinacyclobutane6 both procceded with complete retention of stereochemistry Abstract: N o
The olefin metathesis reaction' has been proposed to proceed via interconversion of metal-alkene-carbene complexes and metallacyclobutanes.* This proposal is supported both by studies of the reactions of metal carbene complexes with alk e n e and ~ ~ by labeling experiments which show that the olefin metathesis reaction proceeds in a nonpairwise manner.4 Our interest in the metathesis reaction has led us to study the rearrangement of platinacyclobutanes which was first discovered by P ~ d d e p h a t t Puddephatt .~ found that the initially formed tu-phenylplatinacyclobutane l a formed by ring opening of phenylcyclopropane rearranges to a mixture of a- and /3-phenylplatinacyclobutanesl a and 2a on heating. Initially it seemed possible that this rearrangement proceeded by a mechanism closely related to that proposed for olefin metathesis. Here we present the results of mechanistic studies of the rearrangement of platinacyclobutanes. Results The possibility that the rearrangement of the a-phenylplatinacyclobutane l a to the /3-phenyl isomer 2a proceeded via elimination and readdition of phenylcyclopropane was seriously considered since thermal decomposition of l a or 2a gives some p h e n y l c y ~ l o p r o p a n eand ~ ~ ~since reaction of l a or 2a with P( ChH 5 ) 3 gives a high yield of phenylcy~lopropane.~ Rearrangement of l a to a 1 : 2 equilibrium mixture of la:2a in the presence of p-tolylcyclopropane led to no formation of a- or /3-(ptolyI)platinacyclobutanes l b or 2b. Similarly, rear0002-7863/79/1501-4233$01 .OO/O
2a
Y
8 rangement of a-07-tolyl)platinacyclobutanel h in the presence of phenylcyclopropane gave a 1.4 equilibrium mixture of lb:2h and no phenylplatinacyclobutanes l a or 2a. Heating a mixture of dichlorobis(pyridine)( I -hexylpropane- 1,3-diyl)platinum(I V ) (IC) and the corresponding 2hexyl derivative, 2c, in the presence of phenylcyclopropane at 50 "C for several hours gave no l a or lh. Puddephatt has reported that the rearrangement of alkyl-substituted platinacyclobutanes is substantially faster than that of aryl-substituted platinacyclobutanes.8 Our experiment helps establish that coordination of the phenyl ring to platinum is not the means of maintaining the intramolecularity of the rearrangement. The possibility that the rearrangement of l a to 2a proceeded by fragmentation to styrene and a Pt=CH? species was
0 1979 American Chemical Society
Journal of the American Chemical Society /' 101:15 f July 18, 1979
4234 Scheme I fsH5
t runs- 3
c6HS
h H 5
11
cis4
u
trans.4
w_
Scheme I1
Sa
v
11
eliminated by a similar crossover experiment. Rearrangement of l a in the presence of p-methylstyrene led to an equilibrium mixture of l a and 2a; nop-tolylplatinacyclobutane l b or 2b was observed. Puddephatt has reported that the trans-2,4-diphenylplatinacyclobutane, trans- 3, obtained from trans- 1,2-diphenylcyclopropane rearranges to a trans-2,3-diphenylplatinacyclobutane, t r ~ n s - 4 The . ~ formation of only trans-4 might be due to either a stereospecific rearrangement or to the initial formation of a cis-2,3-diphenylplatinacyclobutane,cis- 4, followed by isomerization to give the more stable trans- 4. Puddephatt has noted that models of cis- and trans-4 indicate great steric crowding in the cis i ~ o m e rEquilibration .~ of cis-4 and trans-4 could have resulted from rearrangement via a metal-carbene-alkene complex as shown in Scheme 1. Unfortunately, platinacyclobutanes could not be prepared from cis- 1,2-diphenylcyclopropane(or other cis-disubstituted cyc l o p r o p a n e ~ ) .This ~ , ~ ~precluded studies of the stereochemistry of the platinacyclobutane rearrangement starting with cis3. To determine the stereochemistry of the platinacyclobutane rearrangement we have examined the rearrangement of cis2-phenyl-4-deuterioplatinacyclobutane(5a) to its 3-phenyl isomer 6. If the reaction proceeds through metal-carbenealkene complex 7 shown in Scheme I I , rotation of the alkene ligand which is required for rearrangement would place the phenyl and deuterium substituents in a trans orientation, 6b. I f rotation of the carbene ligand also occurs then some cisphenyl, deuterium-substituted material, 6a, would also be obtained. The key point is that some loss of stereochemistry would be expected to accompany a reaction proceeding through metal-carbene-alkene complex 7. Reaction of cis-phenylcyclopropane-2-dl with [(CHl= CHz)PtC12]2 followed by treatment with pyridine gave deuterated a-phenylplatinacyclobutane (5a, 5b). The cis stereochemical relationship between phenyl and deuterium in 5a and 5b was demonstrated by reaction with triphenylphosphine,' which produced Pt[ P(ChH5)3]2C12 and cis-phenylcyclopropane-2-di. The 270-MHz ' H N M R spectrum of phenylcyclopropane
shows clearly separated multiplets a t 6 0.88 due to trans hydrogens and 6 0.64 due to cis hydrogens. Integration of the 270-MHz spectrum of the cis-phenylcyclopropane-24, used to prepare 5a and 5b gave a cistrans ratio of 1.02 f 0.03:2.0, indicating that all (98 f 3%) of the deuterium is cis to the phenyl. Integration of the ' H N M R spectrum of deuterated phenylcyclopropane recovered from P(CbH5)3 treatment of the 5a and 5b mixture gave a cistrans ratio of 1.OO f 0.03:2.0, indicating that within experimental error no loss of stereochemistry had occurred. Rearrangement of deuterated a-phenylplatinacyclobutane (5a and 5b) to an equilibrium mixture of 1:2 a-:@-phenylplatinacyclobutanes6 was accomplished by heating to 50 O C for 15 h in CDC13. The mixture of isomers was treated with P(C6H5)3 to give Pt[P(CbH5)3]2CI2 and phenylcyclopropane. Analysis of the phenylcyclopropane by 270-MHz ' H N M R demonstrated that all (98 f 3%) of the deuterium label was cis to the phenyl group. In addition, the *H N M R spectrum of the deuterated cyclopropane recovered from the equilibrium mixture of platinacyclobutanes 5 and 6 gave a single peak due to a deuterium cis to the phenyl group; the amount of deuterium trans to phenyl was 510%.The 2H NMR spectrum of a mixture of cisand trans-deuterated phenylcyclopropanes, prepared by reaction of an isomeric mixture of 2-bromophenylcyclopropanes with n-butyllithium followed by D20, gave two well-resolved resonances for the cis and trans deuterium atoms. Thus the platinacyclobutane rearrangement proceeds with complete retention of stereochemistry. This result rules out the mechanism shown in Scheme 11.
D)4c6HS H
Pt
pD
k
P t y c 6 H s
Discussion The observed retention of stereochemistry in the rearrangement of trans-3 to trans-4 and of Sa to 6a can be explained with Puddephatt's postulate of a concerted rearrangement as depicted in transition state 8.' Alternatively, the rearrangement can be thought of as arising from formation of an edge-metalated cyclopropane which then undergoes an edge to edge isomerization and ring opening. A way of looking a t the rearrangement which is closely related to Puddephatt's proposal of 8 is to consider the defor-
Casey, Scheck. Shusterman
/
Mechanism of Rearrangement of Platinacyclobutanes
4235
with Pt satellites, Ji95pt-H = 102 Hz. Complex multiplets at 6 8.9-6.9 due to aromatic hydrogens and a t 6 3.4-2.4 due to the remaining ring hydrogens were observed. Platinacyclobutanes l a , l b , and l c were prepared in the same manner as previously reported.5.'o-16 Cyclopropane Crossover Experiments. A mixture of l a (7 1 mg, 0. I3 mmol) and p-tolylcyclopropane (87 mg, 0.65 mmol) was sealed in a glass tube in CHC13. The tube was heated to 52 & 1 "C for 21 h. The tube was then opened, volatile components were pumped off, and the Pt complex was washed with pentane and pumped on under high vacuum to remove the last traces of free cyclopropane. The N M R spectrum of the recovered complex showed an equilibrium mixture of a- and /3-phenylplatinacyclobutanes, and no aromatic methyl resonance, indicating that only l a and 2a, and no l b or 2b, were present. A CHCI3 solution of l b (88 mg, 0.16 mmol) and phenylcyclopropane (94 mg, 0.80 mmol) was heated to 52 "C for 22 h, and the platinum complex was isolated as above. The N M R spectrum showed a 1 :4 mixture of a- and 6-p-tolylplatinacyclobutaneswith no evidence for formation of l a or 2a. Experimental Section A CDC13 solution o f 2c and IC (46 mg. 0.084 mmol) and phenylcyclopropane (19 mg, 0.16 mmol) was heated to 50 "C and N M R Phenylacetylene-dl. Phenylacetylene (45 g, 0.44 mol) and 25 mL spectra were run after 2.5 and 6.5 h. Gradual decomposition of the of D2O containing 0.1 g of BaO were stirred for 2 days. The phenylplatinum complex took place, but no formation of l a or 2a, which acetylene was separated and three additional exchanges with D l O would have given rise to characteristic benzylic proton multiplets at were carried out in an identical fashion. Distillation gave phenyl6 4.9 and 4.05, was visible. acetylene-dl [30 g, 66% recovery, bp 35-40 " C (15 mm),
