Chemoselectivity in cycloalkene—cycloalkene transformations via platina(IV) cyclobutane derivatives

ELSEVIER

lnorganica Chimica Acta 265 t 1997) 23-.33

Chemoselectivity in cycloalkene-cycloalkene transformations via platina (IV) cyclobutane derivatives Brian Williams ~, P.W. Jennings * Department gf Chemistry. Montana State University. Bozeman. MT 59717. USA Received 6 March 1997; revised 9 May 1997; accepted 12 June 1997

Abstract

Evidence presented in this paper suggests that translormations from simple alken:s Io homologs can be fi~cile via the pathway outlined. Use of platinum and other metals (Rh) can provide an clemen! of control to Ihe process that does not exist with other processes using acids or heat directly on the hydrocarbon substrate. The evidence derived I'rom the substrates investigated indicates that control of the reaction may be achieved so as to produce high chemoselectivity or even chemospecilicity for olelin homologation. Mechanistic evidence indicates that exocyclic methylene and methyl-cycloaikene produc|s follow a pathway Iha| involw~s an initial alpha hydride transfer step. It is important to note thai there is ample precedent suggesting thai the isolated platinacyclic complex may not be the complex that yields the observed products, but ra|her an intermediate platinacycle derived from a 'Puddephatt rearrangement' is the pre-product complex. Finally, a pathway involving a carbocaliortic inlermediate is proposed Ibr the ring-homologalion reaction Ior the acenaphthalene to Imenaiene t~ansl\}~mation. ~3 1997 Elsevier Science S.A. K~'vword.v: I~hllmacych'Jl~ulanecomplexes; ('ycloalkenes

I. I n t r o d u c t h m It is well known that platiwl( IV )t:yclol'~uiane tlcrivalives

..........................),,

),,

Ig~tC[ |IlidCF a vlfficty Of c o n t l i l m n s ai)d willt sevcrttl dil]'crcnt

reagents Io yield organic products containing alkene mt)ielics. The objective of this investigation was to detertninc if one could control the type of olclin that is derived li'otll the thcro. real reaction of plalina(IV)cyclobutane complexes. In Scheme I, a sequence is shown that describes both the overall palhway as well as the pathway for the formation of the three possible olelinic products. There is ample precedent thai the controlling factor, which determines product formalion, is the position of the metal atom (i.e., platinacycle 3 yields product 5 and platinacycle 4 may yield 6 and/or 7). One feature that is both useful and a complication to this control is that complexes 3 and 4 have been shown to bc in cquilil'wmm. Obviously, if this equilibrium can be shifted in one direction or the other, the chemoselectivity for formation of one of the olefinic products could be high. While there is * Corresptmdmg author. Presenl address: NSF. Division of Graduale Educalion, Suite 907N, 4201 Wilson Boulevard, Arlington, VA 22230, USA Fax: + I 703 306 0468; e-mail: [email protected] Present address: Department of Chemistry, King's College, WilkesBarre, PA, USA. 0020-1693/97/$17.00 ~5 1997 Elsevier Science S.A. All rights reserved PHSO020- 1693 ( 9 7 ) 0 5 7 0 8 - 3

L----t- ).

II

5

|I

6

7

Scheme I.

very little hard evidence, it is assumed thai Pt(ll) inserts (oxidalively adds) into the bond with the highest electron density. This is often the ring-juncture bond. However, duc to steric encumbrances in many cases, tiffs is an untenable position and the metal moiety rearranges to a less crowded position. This inlrarnolecular rearrangement is referred to as the 'Puddephatt rearrangement' [ 1].

B. Wilhams, P W. Jennings / lmn'ganica Chimica AcuJ 265 (1997) 23-33

24

There is one final background concept that requires elaboration prior to the discussior, of results. When Pt(II) is allowed to react with a cyclopropane containing compound in ether solvent, a precipitate is formed which we refer to as th~ "initially precipitated complex' (IPC). C! I

CI

lion (ppm) with shifts relative to solvents. High resolud'm mass spectral data were acquired on a VG 7070 mass spectrometer. Elemental analyses were performed by Galbraith Laboratories. X-ray crystallographic studies were performed using a Nicolet R3ME automated diffractometer and structural refinements were made using a SHELXTL program package.

I . c l .................. ~ P c . . _ . ~

.~--

pt-~-t--~Ci

I

2.2. Synthesis of cyclopropanes 2.2.1. Cyclopropla]acenaphthylene (9)

A

c,, CI

CI IPC

This name purposely avoids the term initially formed complex (IFC) because there most certainly are pre-lPC products that are soluble. Another name for this precipitate when dealing with cyclopropane itself is 'Tipper's' complex. The detailed chemical swucture of the IPC is unclear. It is robust and usually a light cream-colored solid which is believed to have a tbrmula of a chloride-bridged tetramer ( IPC structure) 12 I. Nevertheless, its characteristics are useful. First, due to its insolubility, it may be readily washed and cleansed of starting reagents which provides a very simple purification protocol, Furthermore, on treatment with a coordinating solvent such as THF or pyridine, it dissolves forming a monomerit platinum(IV) complex that is soluble in a variety of t)rgani¢ solvents including chloroform which |hcilifates NMR ~pcctro~copy, ~ll~ese monome|'ic complexes are well characo terized by many methods including NMR spectroscopy and Xoray crystallography 131, in contrast, the IPC ha~ not been well characterized, Solidoslat¢ t~C NMR, IR spectroscopy and mass spectrometry have provided informatioll on the gross structure hut adequate detail is missing 141, For ex,uno pie, specific structural characteristics dealing with the regio~ and stereochemistry of the organic moiety are not available, Despite this lack of intbrmation, it is possible to explore the reactions of the iPC, Thus, late in this article, it will be shown that products derived from the IPC are different from those derived from the soluble monomeric complex,

2, Experimental

2,1, General l.vcedures All solvents and reagents were purchased from commercial suppliers and used without further purilication unless otherwise stated, Diethyl ether was distilled from sodium hen~oo phenonc. Zeise's dimer was prepared i'ron~ K:PICIa which was on loan from Johnson-Matthey Corporation, Silica gel chromatography was pefforlned with grade 60, 230=400 mesh purchased from Merck, ~H NMR and ~'~CNMR were obtained on Bruker AM500, AC300, or WM250 MHz instruments, Chemical shifts are reported in units of parts per rail-

Compound 9 was prepared by reaction of acenaphthylene (5.3 g, 0.035 reel) in ether (30 ml) with diazomethane and a catalytic amount of palladium acetate at 0 °C. The diazomethane was generated and distilled by the dropwise addition of an ether ( 130 ml) solution containing Aldrich diazald ( 15 g, 0.07 reel) to a heated (65 °C) mixture of potassium hydroxide (5 g, 0.09 reel), water ( 10 ml) and 2-(2-ethoxyethoxy)ethanol ( 14 ml). The mixture was allowed to stir for ! 2 h, tbllowed by the removal of the ether by rote-evaporation. The palladium acetate residue was removed by passage through a short silica gel column using n-pentane as the eluent. Yield: 5.2 g (89%). Further purification was achieved by fractional silica gel chromatography and these yields were not optimized, tH NMR (CDCI,): 7.35-7.56(om, 6H); 3.01(dd,2H); 1.51(dt, lH); 0.77(dr, Ill) t:~C NMR (CDCI~): 146.3; 136.5; 132.0; 127.3(2C); 122.7(2C); 119.3(2C): 27.0; 23.9(2C). MS, m/e (rela|ive intensity): 167(6,2)0 166(M ~, 41,8), 165(I00), 163(14.9), 139(3.3), 82(25.7). Exact mass calculated l'or C~}I.~: 166.0783. Found: 166,077 !.

2,2.2. Mcth~lcych~proplakwemq~hOudcn¢(15 and 16~ A ,~olution of KOH (34,5 g, 0,61 reel), H;O (40 ml) and ether ( 350 ml) was placed in a 500 ml Erlcnmcycr Ihlsk and cooled to 0 °C in an ice bath, Solid portions of I oethyb3+ nilroolonitrosoguanidhle (12.85 g, 0.08 reel) were slowly added. Upon completkm of the addition, the ethereal layer was decanted and the aqueous KOH solution was washed several times with ether and the ether solutions were corn° bined. The combined ether washings were added to a cooled (0 °C) solution containing acenaphlhylcnc (2.43 g, 0.016 reel ), ether (400 ml) and palladium acetate (0.305 g, 0.001 reel). The solution was warmed to room temperature and allowed to stir Ibr several hours. After removal of the ether by rote-evaporation, the cyclopropancs were purilied by silica gel chromatogB~phy with a 2/I mixture of pcn|ane and chlorotorm as the eluent. A mixture of isomers 1~; and 16 were isolated in a ratio of 5.5 t~ !. Combined yiekl: !.8 g (63%), Compound 15: IH NMR (CDCI~): 7.60=7.35 (om,6H); 2,78(d,2H); i,20(d,3H); i.08(m,!H). ~:~CNMR (CDCI~): 146,1(2C), 137,3, 131.9, 127.3(2C), 122.3(2C). ! 18.9(2C), 35,8(2C), 32,6, 18,2. MS, role (relative intensity): 180(M ~. 26.5), 178(42,1), 166(15.3), 165(100). 151 (i0,0), 89(18,7), 82(12.0), 76(10,3). Exact mass calculated for CI,~H~: 180.0939, Found: 180.0939. Compound

B. Williams, P. W. Jemffngs / b~organh'a Chbnic~l Acre 265 (1997) 23-33

16: ~H NMR (CDCIs): 7.60--7.35(om,6H); 3.11(d,2H); 1.81(m, lH); 0.43(d,3H). ~sC NMR (CDCls): 142.4(2C), 139.1,131.3, 127.8(2C), 122.7(2C), 120.9(2C), 29.4(2C), 27.1, 7.4. MS, m/e (relative intensity): 180(M ~, 24.3), 178(40.6), 166(14.7), 165(100), 151(8.5), 89(15.9), 82(12.7), 76(9.7). Exact mass calculated for C,4H~2: ! 80.0939. Found: 180.0933.

2.2.3. htdenyl o, clopropane (23) This compound was synthesized using a modified Simmons-Smith procedure [ 12]. A mixture of zinc dust (17 g, 0.26 mol), cuprous chloride (2.58 g, 0.026 mol) and dry ether (80 ml) was stirred at reflux under a nitrogen atmosphere for 30 rain. Indene ( 11.6 g, 0.1 mol) and methylene iodide ( 10.5 mi, 0.13 mol) were added, and the mixture was maintained at reflux for 48 h. Upon completion of the reaction, water (20 ml) was added, and the ether and cyclopropane layer was decanted. The ethereal cyclopropane solution was washed several times with water. Yield: 10.7 g (82%). Further purification was achieved by fractional silica gel chromatography and these yields were not optimized, tH NMR (CDCls): 7.35-7.03(om,4H); 3.18(dd, I H); 2.93(d, IH); 2.35(m, lH); 1.85(m, IH); 1.06(ddd, IH); 0.06(m, IH). IaC NMR (CDCIs): 147.1,141.9, 125.8, 125.4, 125.3, 123.3, 35.5, 23.9, 16.7, 16.0. MS, role (relative intensity): 130(M' ,100), 129(91.7), 128(58.5), 127(29.3), 115(71.4), 102(10.2), 89(7.3), 77(14.4), 64(18.8), 63(18.0), 51 (26.6), 50( i 1.99). Exact mass calculated Ibr C,~H,~: 130.0783. Found: 130.0782.

2.2~4, {3"'h~propaphem'lene (2~)) Tllis compound was prepared as described for 9 starting li'om phenalenc (800 rag, 4.8 tnnlol ), oilier ( 10 ml), pallao dium ~cc|atc, |11c diazomethane was generated using ether (~lO ml), Ahh'ich diazald (3.1 g, 0.0!5 tool), potassium hydtx)xide (I, I g, 0.019 tool), water (2 1hi) and 2o(2oethoxy~ cthoxy)ethanol (3 ml). Yield: 466 m$ (54%). tH NMR (CDCI:~): 7.15=7.71(om,6H); 3.43(d,2H); 2.27(ddd, IH); 1,81(m, IH); 1.15(ddd, IH); 0.42(dd, IH). I:~C NMR (CDCI:~): 137.4, 133.7, 132.0, 127.8, 125.9, 125.6, 125.4, 125.1, 124.8, 123.4, 28.2, 16.4, 15.8, 13.6. MS, m/e (relative intensity): 180(M',67.2) 179(78.3), 178(28.7), 176(!1.4), 166(18.5), 165(100), 152(22.9), 151(8.4), 89(28.5), 88(!0.8), 82.5(19.4).

2.2.5. Cyclopropane (32) This compound was prepared as described for 23 starting I?om dihydronaphthalene ( 13 g, 0.1 tool), zinc dust ( 17 g, 0.26 tool), cuprous chloride (2.58 g, 0.026 tool), ether (80 ad) anrt methylene iodide ( 10.5 ml, 0.13 mol). Yield: 6.8 g (47%). Further purification was achieved by fi'actionai silica gel chromatography and these yields were not optimized. tH NMR (CDCls): 7.25(t, lH); 6.90--7.20(om,3H); 2.54(om,2H); 2.13(qt, lH); i.92(m, lH); 1.77(m, IH); 1.58(m, lH) ;0.87(dt,2H). 'SC NMR (CDCI3): 138.9, 133.8, 128.5, 128.3, 125.9, 124.7, 25.4, 19.4, 15.4, 14.2, 8.2. MS,

25

m/e (relative intensity): 144(M+,33.9), 129(100), 128 (52.2), 115(33.2), 102(4.9), 89(7.2), 77(7.7), 51(15.4), 39(19.4). Exact mass calculated for C~H~2: 144.0939. Found: 144.0938.

2.2.6. Cyclopropane (38) This compound was prepared as described for 9 starting from 4-methoxy-6,7-dihydronaphthalene (4 g, 0.025 moi), ether (30 ml), palladium acetate, the diazomethane was generated using ether (130 ml), Aldrich diazald (15 g, 0.07 mol), potassium hydroxide (5 g, 0.09 mol), water ( 10 ml) and 2-(2-ethoxyethoxy)ethanol (14 ml). Yield: 2.9 g (67%). ~H NMR (CDCIs): 7.16(d, lH); 6.69(d, lH); 6.59 (s, IH); 3.77(s,3H); 2.55(m, lH); 2.44(m, lH); 2.11(dd, IH); 1.87(m, lH); 1.71 (m,lH); 1.52(m,lH);0.80(dd, lH); 0.72(m, lH). IaC NMR (CDCis): 157.0, 135.0, 130.94, 128.9, i14.1, 111.4, 55.2, 25.6, 22.3, 19.4, 14.6, 13.7, 8.0. MS, m/e (relative intensity): 174(M',71.5), 159(100), 144(43.8), 128(35.7), !15(49.8), 91(24.1), 77(20.2), 63(17.7), 51(23.1), 39(24.6). Exact mass calculated for Ci...Hiao: 174.1045. Found: 174.1036.

2.2.7. Cyclopropane (41) This compound was prepared as described for 9 except that no further purification was required beyond a short silica gel column in which ether was the eluent as determined by GCmass spectrometry and proton and carbon NMR spectromeo try; tetrahydrophthalimide ( 4.5 g, 0.03 tool), ether ( 30 ml), palh~dium acetate, the diazomethane was generated using ether ( ! 30 ml ), Aldrich diazald ( 15 ~, ().07 tool), pott~ssium hydroxide (5 g, (i.09 tool), water ( 10 ml) and 2-(2oclhoxy o ethoxy)cthanol (14 ml). Yield: 4.8 B (97%). 'H NMR (Ci)CI:t): 8.91(bs, IH); 2.91(d,2H); 2.50(dd,2H); 1.19 (m,2H); 0.77(m,2H); 0.70(m, IH); =O.02(dd,IH). s'~C NMR (CDCI:~): 181.3(2C); 39.8(2C); 23.9(2C); 12.3; 5.4(2C). MS, m/e (relative intensity): 165(M ',43.7), 137(!1.5), 122(20.0), 105(12.9), 98(63.1 ), 97(10.6), 94 (56.3), 93(37.5), 91(18.2), 80( 17.1 ), 79(100), 78(13.2), 77(21.0), 68(57.0), 67(25.1), 66(31.6), 59(22.7), 57 (25.2), 53(24.9). Exact mass calculated tk~r C,,Ht~O2N: 165.0790. Found: 165.0800.

2.3. General procedure for the synthesis of initially precipitated complexes (IPCs) A mixture of Zeise's dimer (300 mg, 0.5 retool) and cyciopropane (1 mmol) was allowed to react in diethyi ether (5 ml) at room temperature. The reaction was allowed to continue until all of the orange-colored Zeise's dimer had disappeared and the yellow- or tan-colored IPC had been formed. The IPC was washed with several portions ofpentane and dried under high vacuum. Yields: 85-95%.

26

B, WiUiams, P, W Jrmm~gs / lm,g,mi,',t Chimic,~ Acre 265 t 1997) 23-33

2.4. General prodecure fm" the synthesis of pl.tina(IV)cyclobutane monomers 2.4./. Compoumls 10, 17, 24, 33, 39, 44 A mixture of the IPC (0.25 mmol) and pyridine (2.5 retool) was stirred in diethyi ether (5 ml) at 25 °C for 3 h. The monomeric platina(IV)cyclobutanes were precipitated by the addition ofpentane, followed by several pentane washings. The products were dried under high vacuum. Yields: 90-95%. Compound 10: tH NMR (CDCI3, Jpt-H in Hz): 9.12 (d,2H); 8.58(d,2H); 7.88(t, IH); 7.69(t, lH); 7.63(d, lH); 7.61(d, IH); 7.46(t, lH); 7.44(d,2H); 7.31(d, IH); 7.21 (d,2H); 7.19(t, IH); 6.72(dt, IH); 5.08(d, lH) (113); 4.87(m, IH); 3.41(dd, IH) (80); 3.12(t, iH) (81). ~~C NMR (CDC!~, Jvtc in Hz): 151.21, 149.8(2C), 149.3, 139.7, 138.2, 131.1,128.1,127.9, 124.6(2C). 122.5, 121.8, 119.4, 118.7, 55.3(107), 5.5(364), -4.5(366). Compound 17: ~H NMR (CDCI~, JptH in Hz): 9.02 (d,2H); 8.76(d,2H); 7.88(t, IH); 7.72(t, lH); 7.58(d, iH); 7.41(om,3H); 7.30(om,4H); 7.13(t, iH); 6.68(d, lH); 5.29(d, IH) (117); 4.56(t, IH); 3.67(t, lH) (92); {).82 (m,3H). I~C NMR (CDCI:a.Jt,t c in Hz): 153.7(2C). 151.4, 149.9(2C), 149.0, 138.2, 138.0,0.137.0, 131.5,127.8, 126.9, 125.6(2C), 125,4(2C), 124.4, 123.9, !19.5, !17.5, 64.0(107), 23.4(23), 10.7(373), 6.4(387). Compound 24: IH NMR (CDCI~, Jl,~ Jt in Hz): 9.09 (d,2H): 8.6o(d,2H); 7,88(t, IH); 7.73(t, lH); 7.45(t,2H); 7.30(t,2H); 7,17(t, IH): 7.10(d, lH); 6.86(t,IH); 6,61 (d, IH); 4,68(d, IH) ( 115); 3.70(m IH); 3.20(m,2H); ~.lS(d, IH) (75); 2.82(t, IH) (82). I~C NMR (CDCI~, .l~,, ~ in I:lz): 149.5(2C), 1487, 145.6, 138,2, 137,8, 126.3, 125.4, 125.1(2C), 124,7, 124.4, 48.2(11)2), 421, 10.7 (355), ~ 4,7(349). Amd, Calc. for telramcr C,,,H~.CI~Pt: C, 31).32: H, 2,54, F,~ottnd:C, 29.55; H, 2.58%. Compound 33: ~H NMR (CDCI~, Jp,~t in Hz): 9.11 (d,2H); 8,55(bs,2H); 7,82(t, IH); 7.69(bs, IH); 7.36 (t,2H); 7.26(bs,2H); 7,13(t, lH); 6,92(d,1H): 6.83(t, lH): 6,48(d, IH); 4,82(d, IH) (I 14); 3,40(m, IH); 3.18(dd, IH) (79); 2.93(d, lH); 2,71(dd, IH) (83); 2,41(d, IH); i,64(d, lH): 1,40(m, iH), ~C NMR (CDCI~, ,lt,~¢: in Hz): 149,4(2C), 143,0, 140,5, 137,8(2C), 129,3, 128.3, 125,6, 124.4(2C), 124,2, 42.2(103), 28.4(30), 6.1(339). = I 1,0(352), Compound 39: ~H NMR (CDCI~, J~,~ ~; in Hz): 9.07 (d,2H); 8.58(bs,2H); 7.84(t, lH); 7,77(t, lH); 7.40(t,2H); 7,30( I,2H )' 6.56(d, iH)' 6.45(dt, IH); 6,34(rid, Ill)' 4,76(d, IH) ( 115); 3,71(s,3H); 3,36(m, lH); 3.11(dd, iH) (78); 3,(~)(dd,IH); 2,68(dd, IH) (86); 2.43(d, lH); 1.63(d.lH); 1.41(m, IH). t~C NMR (CDCI:~,.1~,~c in Hz): 156,6, 149.2(4C), 141.8, 137,5(2C), 134.9, 124.5(4C), 113.0, II 1.9, 54.9, 42.2(103), 28.4(29). 27.8, 6.4(334), - 11,5(353), Compound 44: ~H NMR (CDCI~, J ~ in H~): 8.89 ( d,2H); 8.82(bs.1 H); 8.57(d,2H); 7.83(t, lH); 7.76(t, lH); 7.40(t,2H); 7.32(t,2H): 3.40=i.60(om,10H). ~aC NMR

(CDCI3, Jpt_c in Hz): 180.1,179.5, 149.4(2C), 149.3(2C), 138.2, 137.9, 125.3(2C), 125.1 (2C), 39.0(105), 39.3, 39.0, 24.8(20), 22.2(19), -3.5(383), - 10.2(353). Anal. Calc. for C~gH2~O2N.~CI:,Pt:C, 38.72; H, 3.59. Found: C, 39.1 !; H, 3.84%.

2.5. Synthesis of isotopically labeled compounds 2.5.1. Compound 63-d2 Compound 63-d2 was formed by reaction with cyclopropane 9-d~ as described for the general synthesis of platina(IV)cyclobutanes. Compound 9-d, was synthesized and purilied by the method described for 9, except that NaOD and D,O were used in place of KOH and H,O. Counpound 63-d, showed 89% deuteration per proton as determined by ~H NMR integrations. 2.5.2. Compound 53-d, Compound 3a-d, was formed by reaction with cyclopropane 23-d~ as described for the general synthesis of platina(IV) cyclobutanes. Compound 23-d, was synthesizedand purilied by the method described Ibr 9 except that NaOD and D:O were used in place of KOH and H20. Compound 53-d: showed 88% deuteration per proton as determined by ~H NMR integrations. 2.5.3. Compound 5,q-d~ Compound 58-d:~ was formed by reaction with cycloprol~ane 23°d~ as described for the general syntilesis of platina(IV)cyclobutancs. Compound 23-d;~was Ibrmed by the cyclopr,~panation procedure described for 23 except tha| indencod~ was used in place of indenc, lndenc+d~ was preo pared by coml)iniag mdcue ( i.7 B, 0,015 inol ) with THI: (4(1 ml ), sodium ri~ctal(0,4 g,.0,015 |lml) arid[)~O ( 30 tlll)mid

maialaining reliux 1or 24 h. The dculeratcd indene wa~ iso, lated by several extractions of the reaclion unixture with di° cthyl ether. Yield: 1,6 ~ (91%), Compound SSod~ showed 96% deuteration at each position as determined by alol NMR integrations.

2.6. I)ecompositiot~ of platina( IV)o,ch, s to form aikenes 2.6. i. Formation ¢g'41 Compound 10 (6(X)rag, 1.3 mmol) was dissolved ill trither chloroform or benzene (5 ml) and heated at reflux for 50 rain. Tile reaction solvent was removed by roto-evaporalion and tile remaining residue was washed wi|h several 3 ml

portions of pcntanc. The pcntane washings werc combined and the pcmanc evaporatcd. Yield: 194 mg (90%). mH NMR (CDCi~): 7.57(t, lH); 7.51(t, lH); 7.27(d, IH); 7.23(om. 2H); 6.97(d, lH); 6.59(dt,|H); 6.04(dr, Ill); 4.06(bs,2H). t~C NMR (CDCI~): 134.2, 133.6, 132.0, 129.5, 127.7, 127.6, 126.7, 126.2, 126.0, 125.0, 124.9, 122.1,32.1. MS, m/e (relative intensity): 166(M',47.4), 165(100), 164(15.1), 163(17.8), 83(9.6), 82(24.5), 69(5.0), 63(4.7). Exact mass for C~3H~o: 166.0783. Found: 166.0779.

B. Williams,P. W, ,lem~ing.~/ hu~rganh'a Chdmica Acla 265 ~1997) 23-33

2.6.2. Formation of 21 Compounds 15 and 16 (230 mg, 1.3 mmol) in a ratio of 5.5 to ! respectively were combined witli Zeise's dimer ( 370 mg, 0.63 mmol) in 5 ml of refluxing diethyl ether. AIier 10 rain, 16 was quantitatively converted to 21. Reflux was continued tbr 5 h, followed by roto-evaporation of the ether and washing of lhc residue with several portions of pentane. The combined pentane washings contained a stoichiometric amount of 21 relative to 16, and 15 was not observed. The remaining solid was presumed to be the IPC formed from the reaction of 15 with Zeise's dimer and compound 17 was generated from this as previously described. Thermolysis of 17 was complete after 50 rain in refluxing benzene ( 1 mi). The benzene was removed by roto-evaporation and the rcmaining residue was washed with several 3 ml portions of pentane. The pentane washings were combined and the pcntane evaporated. Yield: 181 mg (94%) relative to 15. ~H NMR (CDCI3): 7.55(d, lH); 7.481d, iH); 7.351t, lH); 7.26(om,2H); 6.931d, lH); 6.371d, lH); 3.931bs,2H); 1.931bs,3H). 13CNMR (CDCI3): 136.5,134.7, 133.5, 133.3, ! 26.4, ! 26. !, ! 25.9, 125.7, 125.0, 124.5, 123.2, ! 21. I. MS, role (relative intensity): 180(M',64.6), 179193.4), 178 (25.8), 17618.3), 166(14.5), 1651100), 16317.0), 152 ( 6.8 ), 89(35.0), 82.5 (14.6), 76( 17.4 ), 75 (5.0). Exact mass calculated for C laHl,,: 180.11939. Found: ! 80.0939. 2.6.3. Formation of 25, 26, 27, 28 Comp~und 24 (600 rag, 1.4 retool) was rctluxcd in chlor~fformod~ ( 3 ml) for 3 h. An aliquot was taken Ii"omthe crude reaclion mix|are and analyzed dirccily by proton and carbon NMR wilhot|l purilJcation, Crt|dc product |'altos were deter o mined by proton NMR integrations to be 11.2/!.8/2.7/I for alkcncr~ 25, 26, 27 and 28, respectively, Purilication of the product nlixttl|'c was accomplished by removal tff the chh~ ,'~fform by i~}to~cv~lpt~ratit~nand thcn washing the rcmainin~ residue with several portions of pentane. The penlane extracts were combined and run through a short silica gel cohmrmwith pcntanc as the cluent. Product ratk~s determined by GC°mass spectral analysis were in good agreement with those deter° mined by proton NMR lbr the crude reaction mixtures. Combined yiekl: 165 mg (93%). Compound 25: IH NMR (CDCla): 7.50-6.95(om,4H); 6.47(dt, IH); 6.03(dt, lH); 2.81(dt,2H); 2.34(m,2H). ~aC NMR (CDCI.~): 135.4, 134.1, 128.6, 127.8, 127.5, 126.8, 126.4, 125.9, 27.5, 23.2. MS, m/e (relative intensity): 1301M',100), 129184.3), 128154.2), 127127.4), !15 (56.2), 10219.0), 77( i 1.9), 64(19.8), 63(12.7), 51 (19.6), 50(7.6). Exact mass calculated for C ,}!olt~: 130.0783. Found: i 30.0779. Compound 26: 'H NMR (CDCI3): 7.50-6.95(om,4H); 6.451bs, lH); 3.281bs,2H); 2.131bs,3H). t~C NMR (CDCI3): 42.7, 16.7. MS, m/e (relative intensity): 130(M', 100), 129165.5), 128132.2), 127(14.8), 115171.1), 6519.1 ), 64(13.5), 63(11.8), 51 (13.4). Exact mass calculated l'or CioHto: 130.0783. Found: 130.0780.

27

Compound 27: IH NMR (CDCI3): 7.50 6.95(om,4H); 5.51(m,2H); 3.72(t,4H). ~3C NMR (CDCI3): 107.8, 39.4. MS, m/e (relative intensity): 130(M~-,100), 129172.9), 128146.6), 127121.3), 115(63.2), 77(12.1), 64114.3), 63113.1), 5118.8). Exact mass calculated for C,,H,o: 130.0783. Found: 130.0784. Compound 28: IH NMR (CDCI3): 7.50--6.95(om,4H); 6.221m,1H); 3.331t,2H); 2.20(m,3H). ~3C NMR (CDCI3): 37.6, 13.0. MS, role (relative intensity): 130(M+,100), 129(60.9), 128(31.2), 127(17.3), 11618.4), 115(74.4), 64114.1), 63(9.7). Exact mass calculated for C~oH~o: ! 30.0783. Found: 130.0782. 2.6.4. Formation of 29, 30, 31 Formation of the IPC by reaction of 29 with Zeise's dimer was accomplished as previously described. The 1PC (500 rag, 0.3 mmol) was added to a solution of chloroform (5 ml) and pyridine (212 mg, 2.7 mmol) at 25 °C and the mixture was stirred tbr 2 h. The chloroform was removed by rotoevaporation and the remaining residue was washed with several 3 ml portions of pentane. The pentane washings were combined and the pentane evaporated to give compounds 29, 30 and 31 in a ratio of 2/1.5/1 respectively. Yield was not detcnnincd for this reaction. Compound 31: tH NMR (CDCI3): 7.80-7.20(om,6H); 5.121s,2H); 3.81 (s,2H). ~C NMR (CDCI3): 108.5, 39.4. MS, m/e (relative intensity): 1801M',67.2), 179178.3), 178(28.7), 176111.4), 166 (18.5), 1651100), 152(22.9), 15118.4), 89(28.5), 88 (10.8), 82(19.4). 2.6.5. Fornuttion +~f34 am135 The thcrmolysis of 33 was performed and analyzed a~ described for tlJe formalion of 26, starting Ii"om 33 (601)rag, 1.3 retool) in chloroformodt (3 ml) and 3 h at rcllux. Crude prt)duct ralit)s for 34 and 35 were determined by proton NMR intc[~rations to bc 1.4 t~ I. Combined yicld after purification: 157 mg (84%). Compound 34: t H N M R ( CDCI ~): 7.40=6.99 ( ore,4 H ); (dt, IH); 2.75(t,2H); 2.22(m,2H); 2.05(s,3H). ~C NMR (CDCI:~): 136.2, 135.9, 132.2, 127.3, 126.7, 126.3, 125.3, 122.8, 28.4, 23.2, 19.2. Compound 35: ~H NMR (CDCI~): 7.40-6.991om,4H); 4.881s, IH); 4.841d, iH); 3.55(s,2H); 2.87(t,2H). t*C NMR (CDCla): 145.5, 137.1,136.5, 128.5, 128.4, 125.9, 125.6, 108.2, 37.2, 31.9, 31.2. 2.6.6. l'bmtation of 40 Complex 39 (600 nag, 1.2 retool) was decomposed by relluxing in chloroform (5 ml) Ibr 3 h. The chlorotbrm was removed by roto-evaporation and the remaining residue was washed with several 3 ml portions of pentane. The pentanc washings were combined and the pentane evaporated to give alkene 40. Yield: 192 mg (92%). tH NMR (CDCI~): 6.95 (d, IH); 6.71 (d, IH); 6.60(s,lH); 5.871s, IH); 5.801s, IH); 3.731s,3H); 3.441s,2H); 2.821t,2H); 2.431t,2H). ~~CNMR (CDCI3): 158.3, 150.3, 138.2, 136.1, 129.8, 114.2, Ii2.0, 110.4, 52.6, 48.8, 32.0, 30.1.

2~

B, Williams, P ~V Jenning.~'/lm,l:~anWa Chimica Acta 265 (1997) 23-33

2.6. 7. Fornmtion ~ 4 5 Complex 44 (600 rag, 1.3 stool) was decomposed by relluxing in chloroform (5 ml) for 3 h. The chloroform was removed by roto-evaporation and the remaining residue was washed with several 3 ml portions of pentane. The pentane washings were combined and the pentane evaporated to give alkene 45. Yield: 174 mg (83%). ~H NMR (CDCI3): 8.94(bs, lH); 4.83(s, lH); 4.77(s, lH); 3.01(om,3H); 2.50(d,2H); 2.24( m, I H); 1.95(m,2H). ~3C NMR (CDC13): 180.0, 179.8, 141.4, ! 11.7, 41.8, 40.8, 30.9, 27.9, 22.7.

65 8

CH2"

/ ,,i,,,\ ry N

2, Z Isotopic labeling experiments and kinetic measurements 10

13

The isotopic labeling patterns in the resulting products {as shown in Schemes 5~8) were determined by IH NMR integrations and mass spectrometry. All reaction rates were measured by tH NMR spectroscopy. In a typical experiment, 5 mg of platina(IV)cyclobutane was added to 0.5 ml of chlorotbrm~dl in an NMR tube and placed in a proton selective NMR probe equipped with a variable temperature unit. The probe temperatures were set and calibrated prior to the insertion of the sample with either a methanol or ethylene glycol sample. The decompositions were monitored by periodic automated acquisitions of proton NMR spectra. Each spectrum contained 16 000 data points and no line broadening was applied. All slopes from the acquired data were obtained by the leastosquare~ method and correlation coefficients of 0.992 or greater were observed, All experin~ents appeared to obey tir~lo~rder kinetics and each rate const,mt wa~ dctcro mined a~ the average of a minmlum of three experiments. Kinetic i~otope eft~¢ts were detertlained by direct comparison ~t +tile rate c o n s t a n t s l'rotll the deconlpo~itiotl~ o f tile dculer:+

ated ¢on|potinds with those fl+om their non+deutcratcd atm.+ Io~s, Activation parameters were obtained ITom plols of In(k/T) versu~ I/T, where the enthalpy of activation was calculated tiros the slope of the line and the entropy was determined from the intercept.

3. Results and discussion

3, I, Acemq~hthaletw substl~ttes Fs~m the relatively simple substrate, acenaphthalene, we hoped to determine: ( I ) which of the three possible products would dominate; (2) which platinacycle would be isolated; and (3) which platinacycle would dominate the product lbro marion pathway. Scheme 2 exhibits the potential pin|inncyclic inten~aediates and anticipated olelinic products of the reaction. The reason for choosing this system was that steric encumbrances are relatively small for 13 because of the overall planar structure. Furthermore, an alternative synthesis of phenalene could be useful as existing methods tend to yield a signiiicant amount of the by-product, phenalenone. From

¢ 63 14

II Scheme 2.

12

8, compound 9 was readily prepared, purified and reacted with Pt(lI), Zeise's dimer, to ibrm the IPC. Attempts to derive a substantial yield of products 11, 12 or 14 from tile IPC were not successful as polymeric products were formed. Thus° monomerization of the IPC with pyridine produced complex 10, exclusively. It was thoroughly characterized by NMR spcc|roscopy and Xoray crystallography j 5 J. Since tile position of Ihe platinunl is Ihought to determine the reaction cou~',~e,products 11 and !2 were anlicipaled &ore |he thermal decomposilion of iO H~)wever, on heating in either chioroo f~m or benzene, phenalene 14 was the only t~action pn~ducl I'~nued, Clearly, chemospccific produc|ion of the rmg~ homolo~aled produc| was achieved. While it is possible to write a pathway from 10 to |4 that involves carbon~ocarbon bond migralions derived from heterolytic cleavage of the platinum~arbon bond to yield a carbocation, we prefer to invoke a 'Puddephatt rearrangement' of 10 to 13 with subsequent reaction to form 14. The mechanism of this reaction ( 13 to 14) will ~ discussed later in this paper. This lype of sequence wherein one platinacyele rearranges to another platinacycle prior to olcfin formation is not novel and has been proved by labeling experiments [ 3 ]. It is interesting that the present results ,ndicate that tile energy ban'ier for the reaction 10 to 13 and on to 14 is less than the barrier for 10 to 12. An additional probe using this substrate series was conducted. It was designed to analyze the steric effect of a propitiously placed alkyi substituent on the reaction. Also, the aikyl group (methyl) would act as a label for detecting unanticipated re,~aTangements (Scheme 3). Thus, acenaphthalene was cyclopropanated with diazoethane to yield 15 and 16 which were subsequently separated. Reaction of 15 with Pt(II) and then pyridine gave 17 in high yield after a protracted reaction time. Subsequent refluxing in benzene gave 21, exclusively. Presumably, a 'Puddephatt rearrangement'

B, Williams, P, W. Jem+ings/ lm,~rg+~+n&:'+++['himica Act++265 ¢1997) 23-33

c~

~

"C[I,,"

CI+Ia 22

/

~

PI.C|,zPY+,

7

/

' 23

16

15

~

29

1

24

PtCIzPy 2

2Sa 19

17

26-27a

l

1

/'-,.

'iS (11.2)

PtCi2Py2

28a

26 11.81

27 12.71

28 111

Scheme 4.

20

,s

21 Scheme 3.

occurred it> form 18 which decomposed to the obser:,ed product. In contrast, 116 reacted rapidly with Pt(ll) in refluxing ether to produce 21 without precipitating either phi+ thin( IV)cych~butane i9 or 20. As you will see i,1 the mecho anislic discussion, this relative ,';tie difference argues lbr a mel~d+medi:t|ed hydrogen tnmsl~r step.

an 'a' behind the corresponding number. Compounds 26 and 27 are formed in the ratio shown throughout the reaction, but it should be mentioned that they could be in equilibrium. It appears reasonable to conclude that the added feature relative to acenaphthalene ( i.e., one sp ~center in the immediate reaction vicinity) shifts the energetics enough to produce other anticipated alkenes. However, since 25 is dominate, it is reasonable to conclude that platinum still prefers to react from the ring-juncture bond position. The next preference exhibited in this reaction is that the platinum prefers to react from the position in which it is bonded to a benzylic carbon (26++ 27a). +~.+~. (,yclohexenyl derivatives ~',:| I1,~

3,2, Iml+'m,/ sM, strates (

Indenyl subs,rates were chosen for tile following reasons: ( I ) They offer ;! degree of i,creased complexity over the acenaphthalene system in that one center that previously had sp a hybridization in the acenaphthalene series now has sp "~ hybridization. This difl'erence should provide additional steric hindrance to the platinacycle in which the platinum has inserted into the ring-juncture bond. (2) There is asymmetry with regard to all three of the cyciopropane bonds as well us with the two cyciopropyl carbuns at the ring juncture ( i.e., one carbon is benzylic and the other is ailylic). Products of the reaction, if formed in reasonable yield and chcmoselectively, could serve as useful models in future synthetic strategies. Results using this substrate are shown in Scheme 4 with product ratios in parentheses. Only one platinacycle was observed, 24, which was characterized by NMR spectroscopy and X-ray crystallography 151. Although there are four products lbrmed, compound 25 is dominant. The platinacycles fl'om which each product is proposed to be derived are shown below the products with

~

%+

+

I

1. PI[II)

2.~'~ gX

I

I

+

a9

30

(!) al

|! H

"%

I(X.Y 1 33'

34

Two six-membered ring systems were investigated and the results ale shown in Eqs. ( 1) and (2). Obviously, the ringhomologation pathway to a seven-membered ring product is too high in energy relative to the pathway for the other types of olefins. As observed with the previous subs,rates, only one platinacycle is isolated (33), but products are derived from it as well as from an unobserved platinacycle (33'). As a result of Eq. (2) and the ready availability of the precursor to 38, we investigated the question as to whether or not it

]0

B. Williams, P. IV. Jennings I lnorganica Chimica Acta 265 (1997) 23-33

would be possible chemoselcctively to control the reaction so as to produce only one major product. Compound 38 was synthesized with the presumption that the methoxy moiety would stabilize (and thus give preference to) platinacycle 33. This assumption is based on the fact that Pt(IV) bound to carbon polarizes the electron density of the sigma bond so as to give the carbon atom some cationic character. Thus, platinacycle 39 would result in cationic charge being developed on a benzylic carbon. The result of reaction from this substrate is shown as Eq. (3). Compound 40 was formed

$$t

39

40

(3)

exclusively and rapidly in high yields. Thus, it is reasonable to conclude that (a) an electron-directing functionality can provide control of a reaction that otherwise produces a mixture, and (b) the assumption that carbocationic character is produced at the carbon attached to a Pt(IV) center is further co~roborated.

0I'~~/PIClaPy: 0 ~ ~np

0 P~I

~

~.

__

.,0! ~

~

Z~

.

(4) 0

,ts

,,

i

~t!

~~1,,c,,I 0

1144|t

~-~

-. 0

"

ll

ix

H

(8)

benzene

0

0

49

50

Prior to discussing the mechanistic pathways for the abovementioned reactions, there are three reactions derived from IPCs that show significant results (Eqs. (6)-(8)). In all three examples, a methyl cycloalkene derivative is the exclusive product and reflects on the structure of the organic moiety in the IPC. Comparison of these chemospecific processes with the results from the monomeric complex analogs is interesting. Product 26 in reaction (6) is a minor product in the mixture that is derived from the monomeric complex 24 (Scheme4). Compound 48 is not among the products derived from complex 33 (Eq. (2)) and 50 is not the product from 44 (Eq. (5)). Thus, use of the IPC yiekts chemospecific product formation (methylcyclohexene derivatives) and that product is different or is formed in different concentration from those formed via the analogous monomeric complex. One caveat of caution here is that the structure of the IPC that is shown has not been established. They are proposed to be the precursor because the carbon-carbon connectivity is correct for the products. Also, it is assumed that a Puddephatt rearrangement does not occur with tile IPCs prior to olefin tbrmation. 3,3, Reaction mcchanism results

II

y,

o

(5)

41

Attempts to coerce the metal into the riug.,junctur¢ bond to yield the ringoexpanded product 43 using a succinamide derivative were unsuccessful as the cyclopropane derivative 41 had the opposite stereochemistry to that needed to tbmt 42 (Eq, (4)), Thus, neither 42 nor 43 are Ibrmed in the reaction of 41 with Pt(tl), However, complex 44 which was the product of reaction between 41 and Pt(ll) nicely decom. posed to form 45 exclusively (Eq, (5)), The stereochemical statement made above was garnered from NMR and X-ray analyses of 44 15]. Further work on this concept is continuing.

Expcrhnenls designed to elaborate IIIc p'lh of olelin trans~ tbrmation t}om platina(IV)cyclobutancs involve measurco mcll ofaclivalion paramelers and deuterium labelm Bfor bo|h kinetic isotope offeels aid atom rearrangcmcn|s, 3.5. I, Atom tvarrangement results

Translbrmations from platinacyclcs tO alkcncs requires hydrogen rearrangements. Two pathways are commonly brought into question in this regard [61. They arc labeled as either an alpha or a beta hydride transfer pathway. The name refers to the position, relative to platinum, from which the initial hydrogen atom is transferred, Previous results IYom our laboratory (Eq. (9)) as well as from Puddephatt's lab° oratory conclusively lbund th:~t tile alpha hydride transfer pathway prevailed [ 71.

3,4. Reactions of t h e / P C It

,~ll(1)} ~I!C!~L:

PlUlth $1

bl~lllll£ 4~

(6) 16

btnl~ne 4T

(7) 411

~ t~¢n~¢ne

If(D) x~w_j" "IIID)

(9)

$2

For this investigation, tile mdenyl substrates were labeled uniquely and were subjected to typical reaction conditions. Schemes 5 and 6 show the theoretical prtxlucts with tile corresponding labeling patterns. From the theoretical products shown in Schemes 5 and 6, it is clear that the pT-'. . . . . . the exocyclic methylene product is readily distin~ dm ladling pattern shown in Scheme 5. Since this scheme has

B, Williams. P, W, Jemungs / hu~rganica Chimica Acu~265 {1997) 23-33

~

~

CltDa S6a

~

P.tCI2La~

3!

~

D CDii 54

PtCl:Py2

67

~

1

-~CDz

CHD2 $6b

D ~

PtClzPy~' D

D ~ alpha

55

Scheme5. 70

D

!1

""~ta

62 D

is not entirely unexpected for this substrate, as an initial alpha hydride shift in intermediate 68 would yield a bridgehead carbocation. Although this cation would be benzylic, it is still too constrained. Thus, it appears that the beta hydride transfer pathway could be operational for the ring-expansion process. However, we suspected that this was not correct and proceeded with two additional studies that included kinetics.

D

60 D

Scheme6. no value in discerning the pathway for methylcycloalkene, the labeling pattern shown in Scheme 6 was used. From NMR spectroscopy and mass spectrometry analyses, it was readily concluded that the alpha hydride transfer pathway was operative in the formation of products 26 and 2"/ (Scheme 4). Similarly, product 28 was determined to result from an initial alpha hydride transfer pathway. Study of the pathway leading to the ring-expanded product 25 was not so fruitl'ul~ In this case, the label was lost to an extent that con° clusions were tenuous. For I1|e ringqumlologatit)n pathway, better results were gm~nct~¢d with the acenaphthalene subsh'ate. Appropriately I~d~elcd cyc!opropane derivatives were prepared and reacted with Pt(il), The reactions shown in Schemes 7 and 8 show the theorc|ical pathways with labeled intermediates and products. Actual reaction results for complex 63, which were garo nered I?oln NMR spectroscopy, clearly indicate that the beta hydride tran,~fer pathway is operative. This conclusion was corroborated li'om the results of reacting complex 67. Obviously this conclusion is in stark contrast with those found for the indenyl system and others reported in the literature. This

~

I) PICIzPy~

63

D

|)~~|C'IzPYz )

66

D

~.6. Kinetic investigations

Previous work in our laboratory with substrate 51 (Eq. (9)) had shown that the alpha hydride transfer pathway was accompanied by a significant kinetic isotope effect. It had a magnitude of 3.6 (57 °C) I8]. One would presume that a similar result would emanate from the beta hydride pathway. Results from complex 63 were surprising. The measurement gave a klllkD el'l~ct of 1.07 which is a secondary effect, at best. A similar, but expected, result (km~/ka) .... 1.05) was derived for complex 67. Thus, neither alpha nor beta hydride shifts are rate determining in the ringoexpansion pathway. Exchange reaction kinetics using deuteriumo[ab¢lcd pyridine revealed that the loss of lhe lisand, pyridine, was much faster than ring homologation and thus was not responsible lbr the rateodetermining step. It is possible that the rearrange° ment 63 to 64 or 67 to 68 is the rateodetermining step. if so, one might expect a minor perturbation or a negative inllucncc on the entropy ot" activation and a low enlhalpy el' activation. Thus, we embarked on a kinetic investigation to determine the activation parameters. 3.7. Activation parameters

A plot ol'the data Ibr substrate 9 ( via complex |0) is shown in Fig. I and reveals an activation entropy of - 0.9 ± 0.5 e.u, and enthalpy of 24.6±2 kcal/mol. Clearly the entropy ci'~ange is minor which could be consistent with a Puddephatt rearrangement. However, the enthalpy change is too large tbr the rearrangement process. It is more consistent with a bond heterolysis pathway.

alphtt

64 Scheme7.

69

68

Scheme8.

(Ojp)2Hh(CO)Ci cnlalyli¢ 65

9

(i0)

14

32

B. Williams, P. W. Jennings / Inorgamca Chimic. Acta 265 (1997) 23-33

- 1 2 1 =~......................... ~ ' ~

1S

~

~

~

D

D

y = 23.32 - 1.24e÷4x

PtCI2Pyz

PtClzPy

63

&,S # = -.9eu

D I +PY

In(k/T)

19

-i

0.0023

~

i ........

0.0028

0.0033

,

--

64

1

0.0038

CI~Py

I/T Fig, I, Eyring plot for the reaction of 9 with Pt(ll). -I1

y .- 34.41 - l.S6e+4x ~

-13

/kH ~-31.0Kcal/mole

-1S,

- 21.2eu

6S

Scheme 9.

In(k/T) =17"

=19

, 'Z h(

oZ!

)

o,oo3o

o.oo3s

0.0040

1/I' i~i~= 2 ~y|ittg plol G~f the r~liOt~ ot'9 wttl~ RIl(I),

Using another metaili¢ reagent, rhodium, 'similar' results were obtained (see Fig. 2 and Eq. (10)). Previous results obtained with this reagent were interpreted by the author as direct insertion of metal into the ring-juncture bond with subsequent hydride re~angements [ 9]. From other evidence in our laboratory on a different substrate, we support the idea that rhodium inserts into the ring-juncture bond. Moreover, we believe that it does not undergo a Puddephattotype rearrangement. As in the case for platinum, the rhodium-facil= itated reaction results ap~ar to support a bond dissociation process. Furthermore, the entropy of activation is consistent with bond heterolysis. The lower entropy of activation for the platinum reaction might be derived from a negative entropy from the Puddephatt rearrangement that is offset by a positive entropy of the bond-breaking process. As a result of the studies to date, the mechanistic pathway sht~wn as Scheme 9 is l×~stulated. While the~ is no direct evidence for complex 64, two similar structures ?1 and 72 exist that have been isolated and characterized 1101.

~t

~

CI

~a

Thus. it is not unreasonable to suggest 64 as an interlnediate. There is growing evidence and ample precedence for heterolytic cleavage of the Pt(IV)-carbon bond to yield a carbocation [11]. Furthermore, Ihe benzylic resonance stabilo ization fi~ctorshould fiwor the tbnnation of 73~ In proceeding beyond 73 to 65. there are two viable allernatives~ One involves a prolon transt~r |o fi~rm a metal hydride h~terme.0 diate (73 to 74 to 65) and the other iuvolves a hydride |ranstk:r from carbon |o carbon (73 t~ 75 |o 65)~ 'illcl~ is ~,e piece of data thai suggests thai the metal hydride hltcrmedh~le sequence may be operative. That evidence comes fia)m the reaction of the methyl derivatives I$ and 16. In the transtbrmation of 15, a platinacycle is formed slowly and is isolated. Furthermore, it is necessary to heat 17 for a protracted length of time to produce 21. In contrast, 16 proceeds to :21 rapidly in boiling ether without the precipitation of platina{ |V )cyclobutanes 19 or 20. From intermediate 18, heterolysis would yield a carbocation in which the methyl substituent is syn to the platinum moiety. Therefore. the metal could not assist in the transfer of the hydride. However, from structure 20, ionization would produce an intermediate cation with the methyl group anti to the platinum moiety and the requisite hydrogen would be syn 1o the platinum moiety. This would provide the correct ste~eochemistry for the sequence 73 to 74 to 65. h is clear to us that the transformation of 16 could be facile via the metal-mediated proton transfer pathway and that formation of 17 would be retarded due to steric factors. However. it is not obvious that the relative rate of hydride transfer via 75 would be slower than the metal-mediated transfer. Thus, the pathway via intermediate 74 remains as a postulate.

B. William.,. P.W. Jemlings / lm}rganh~a Chimica Acta 265 (1997) 23-33

4. C o n c l u s i o n s Evidence presented in this paper suggests that transformations from simple alkenes to homologs can be facile via the pathway outlined in Scheme 1. Use o f platinum and other metals ( r h o d i u m ) can provide an element of control to the process that does not exist with other processes using acids or heat directly on the hydrocarbon substrate. The evidence on the substrates investigated indicates that control of the reaction may be achieved so as to produce high chemoselectivity or even chemospecificity for olefin homologation. Mechanistic evidence indicates that exocyclic methylene and methyl cycloalkene products follow a pathway that involves an initial alpha hydride transfer. Use of the IPC looks particularly interesting Ior the specific production of methylcycioalkenes. It is important to note that there is ample precedence suggesting that the isolated platinacycle may not be the complex that yields the products but rather an intermediate platinacycle derived from a Puddephatt rearrangement is necessary. Finally, a pathway involving a carbocationic intermediate is proposed for the ring-homologation reaction for the acenaphthalene to phenalene transformation,

Acknowledgemen|s The autllors wish to express their gratitude lbr support of |his investigation to The Petroleum Research Foundation adluhlislcrcd by the American Chemical Society, The

33

National Science Foundation Chemistry Division and Montana State University.

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