Charles W. Jeffordl Universitb de Gen6ve 1211 Gendve 4, Switzerland
Syntheses and Rearrangements of Cage Molecules Related to Cubane
One of the more important activities of chemists has been the elucidation and synthesis of organic structures found in nature. In many cases the appeal of the synthetic problem springs from the complexity of the target molecule. Nevertheless, the success of the planning and execution of a particular svnthesis. no matter how comolicated. can alwavs he measureh by dirict chemical compari$on as the objecti;e exists as a real entity. If the possibility of such comparison is taken as a criterion then the synthesis of "uunatural" comoounds should Dose even more of a challenee. Molecules which can he classed as unnatural are those whici fall into structural types unknown in nature and which nossess bizarre features: fb; example, molecules which appear to he seemingly overstrained or endowed with novel architecture. Molecules havine regularity of form or polyhedral shapes or displaying similarities with familiar obiects, but on a microsco~icscale. have never failed to delight and fascinate. The simplest and the earliest unnatural hydrocarbon to he reported was a derivative of tetrahedrane (I) (I).Although this claim was subsequently disproved (3), i t did serve as a point of departure for planned efforts which resulted in the synthesis of many molecules of the so-called cage type. The first series of compounds to be prepared were derivatives of birdcage hydrocarbon (111) which was freely accessible from the insecticide isodrin (11) (3). In fact, some of the synthetic steps used are similar t o those chosen for the synthesis of cubane and related molecules, which is the object of this article.
Cyclopentenone (VIII) was su'ccessively brominated allylically (IX) and electrophilically t o give the tribromo ketone (X). Elimination of two molecules of hvdroeen bromide with base gave bromocyclopentadienoue [ ~ l ) , ~ w h i cinstantly h dimerized (XII). Ketalization of the less-coniueated ketone (XIII) followed by photo-induced cyclizaii& gave the hishomocuhane dibromide (XIV). Treatment with 10% aqueous potassium hydroxide brought about the expected Favorskii reaction to yield the homocuhane carhoxylic acid (XV). Conversion of this acid to the acid chloride and thence to the t-butvl oerester (XVI) set the scene for homolvtic fragrnentati&. becarhoxylation occurred on heating to give the homncubane ketal (XVII). Successive treatment with acid and base brought about hydrolysis t o the hromoketone (XVIII) and then its Favorskii ring contraction to cuhane carboxylic acid (XIX). Once again, conversion to the t-hutyl percarhoxylate (XX)enabled the parent hydrocarbon, cubane (XXI), to he obtained by homolytic decarboxylation. b
m.
.
Cubane
The first preparation of a cubane derivative ( 4 ) , octaphenylcubane (VI) obtained by the dimerization of tetraphenylcyclohutadiene (V), turned out to he wrong ((N)(V) (VI)). X-ray crystallographic analysis (5)showed that the dimer was in fact the corresponding phenyl derivative of cyclooctatetraene (VII). a t a b o u t t h a t time some 40 erouus were interested in trying to synthesize cubane.= The First-successful tactical synthesis was essentially achieved thanks to the advantages offered by the skeleton of cyclopentadiene dimer ( 6 ) .
-
-
c ow ever,
Basketene ( 7 )
'Lecture given at Hoffmann-LaRoche, Basle, November 11,1974. This estimate is based an the rumored number of research proposals lodged with the National Science Foundation, Washington, D.C.
A few later bis-homo derivatives became freely available by the exploitation of the characteristic cycloaddition properties of cyclooctatetraene (XXII). The Diels-Alder reaction of the latter with maleic anhydride eives the wellknown adduct (XXIII) which on sensitized photolysis gives the ex~ected(2 + 2) addition uroduct (XXIVJ. Hvdrolvsis of the anhydride to the discid (XXV) followed byireatment with lead tetraacetate gave basketeue (XXVI). This Volume 53, Number 8, August 1976 / 477
simple, hut elegant, synthesis was essentially duplicated in a t least three different laboratories in 1966. Standard chemical procedures permitted the modification of the olefinic handle to give the alcohol (XXVII) by hydrohoration whence the ketone (XXVIII) by Jones' oxidation which in turn could he converted by selenium dioxide to the diketone (XXIX). The usual ring shrinkage procedure (XXX) (XXXI)) leads to the homocuhane ((XXIX) carhoxylic acid (XXXI) which by the Hunsdiecker reaction afforded the hromo derivative (XXXII) which could he reductively dehalogenated with sodium in t-hutanol to homocuhane itself (XXXIII). Apart from these traditional ma-
- -
exnosure. More interestinelv derivatives of cuhane itself eive cuneane. ~ono-substitutedcuhanes (XXXVII) give alrthe isomers of mono-substituted cuneane (XXXVIII) (9).On the other hand l,4-disubstituted cubanes (XXXIX) give only two ((XL) (XLI)) of the ten possible cuneane isomers. Initially, it was thought that silver was catalyzing a forbidden reaction by lowering the energy of the anti-aromatic transition state (10, i l a ) . Essentially, two sigma bonds are undergoing rearrangement in a type of Barton-Head reaction ((XLII) (XLIII)). Due to geometric constraints this rear-
+
-
rangement is necessarily a doubly suprafacial process, whereas theoretically i t should involve an antara component or undergo overall conrotatory motion as four electrons are implicated in the transition state. It is now known that silver acts as a Lewis acid and that hond switching simply involvea rearranging cations. This conclusion follows from the characteristic behavior of secocuhane (XLIV) (11). Treatment
nipulations which enable a wide range of derivatives of basketene and basketane to he obtained, a more profound reaction took place in one of the laboratories during the final work-up. As it turned out, the role played by the silver-coated Silica del, which was supposed to separate basketene in a pure state, was to have far-reaching chemical consequences. The basketene which Lehn and Furstoss thought they had in hand was, in fact, the rudely named snoutene (XXXIV). This chance discovery was taken up, shown to he a general reaction, and skilfully exploited (8).
ILII
ILIII
with silver fluorohorate gives cycloocta-1,5-diene (XLV), tetrahydrosemihullvalene (XLVI), and hicyclo[4.2.0]octene (XLVII). All the products can he rationalized by the initial formation of the argentication (XLVIII). Conventional migrations and eliminations separately generate the three products. Ring opening (XLVIII XLIX) followed by loss of silver cation gives (XLV). A simple 1,2 shift gives the bridged hicyclic cation (L), which can either eliminate silver cation by cyclization (L XLVI), or undergo further rearrangement (LI) and elimination (LI XLVII). Stabilization of the anti-aromatic transition state by silver cannot explain these results. Indeed, if silver were facilitating passage through an anti-aromatic transition state then a single product ought to have been observed, namely cyclopropylhicyclo[2.1.0]pentane (LII). Clearly, cubane (XLII) rearranges t o cuneane similarly (LIII LIV XLIII) although the absence of the diene (LV) is puzzling.
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- -
Silver fluorohorate or silver perchlorate with various degrees of efficiency hring about characteristic skeletal rearrangement in which two four-membered rings fused tocether by a common single hond rearrange to two cy>lopropane rings interconnected by that same single hond. Benzbasketene (XXXV) on treatment with silver fluorohorate gives henzsnoutene (XXXVI) in quantitative yield after an hour's
a
478 /
Journal of Chemical Education
A nice exploitation of parts of the foregoing chemistry is a gram scale preparation of semihullvalene (LXI) (12) which, although a familiar molecule, is scarcely abundantly available. The Diels-Alder reaction of phenyltriazolidinedione (LVI) with cyclooctatetraene (XXII) gives the expected adduct
(LVII). Light-induced closure gives the diazahasketane compound (LVIII) which in the presence of silver undergoes the now standard skeletal rearrangement to the diazacuneane derivative (LIX). Removal of the nitrogenous moiety by hydrolysis, oxidation, and thermolysis of the azo compound (LX) gives semibullvalene (LXI).
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When the symmetry of cuhane is disturhed, for example by the presence of additional bridges, then C--C bond breakages become selective. The treatment of Eaton's acid (LXII) with methyllithium gibes the corresponding methyl ketone (LXIII) which can be converted to the alcohol (LXV) by Baeyer-Villiger oxidation to the ester (LXIV) which is hydrolyzed with ethanolic hydrochloric acid (13a). However, the dioxymethylene bridge exerts precise control over the homoketonization of the alcohol. The experiment was actually done on the acetate (LXN) which on reaction with methoxide ion gave
(LXIVI
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Eaton's acid (LXII) also serves as a convenient synthesis of homocuneone (LXXVI). Heating with cuprous oxide and bipyridyl in quinoline at 230° for 2 hours results in a 93%yield of the bromoketal derivative (LXXN) of homocuneone (15). Reductive dehrominationwith lithium in t-hutanol (LXXIV LXXV) and tetrahydrofuran followed by acid hydrolysis gives homocuneone (LXXVI).
Miscellaneous Homocubane Chemistry
o-cm,
selective hond breakage is governed by product stability control. Catalytic hydrogenation of basketane (LXXI) brings about selective rupture of the C g C 4 hond ((LXXI) (LXXII)).If an additional bond is hydrogenated then twistane (LXXIII) results. In other words the hydrogenation pathway whMh is followed is the one which successively gives the thermodynamically more stable products.
n
-
n
-
-
~LXIII
ILXXIVI
(LXXV)
& ILXVII
Ill XVI Homocuneone possesses the unusual feature of being able to generate a homoaromatic anion on treatment with strong base, namely potassium t-butoxide. The first-formed oxide anion (LXXVII) opens to the homoaromatic species (LXXVIII LXXIX). Evidence for (LXXIX) was obtained on quenching the anion in aqueous acid medium and subseauentlv treatine the carhoxvlic acids obtained with diazomethane, whereupon the rearranged bicyclol3.2.l]ortadiene ester (I.XXX) and rhe rine ooened ester (LXXXIJwere iso-
-
OM
ILXV'
a single secoketone (LXVI). A deuteration study shows that the caoture of oroton or rather deuteron occurs on the inside of the cage (LXVII,. This result is incomplete contrast to that found for the homoketonization of nortricvclanol ILXVIII). Here the intermediate anion (LXIX) opens to capture deuteron on the exo side (LXX) (136).
a Snoutene
Inspection of the homocubane structure shows that there are two kinds of single bonds which are common to two cyclobutane rings. The ketonization experiment shows that the single bond situated directly under the ketal grouping does not break. From other studies (14) i t is now known that this
Apart from its remarkable name, snoutene (XXXIV) also demonstrates remarkable facility for rearrangement. The hydrocarbon skeleton possesses great thermal stability. However. on heatiue the dideuterio derivative (LXXXII) a t 500°C apparent rearrangement of the deuterium label occurs in a singular fashion (16). I t appears as if transfer is taking Volume 53. Number 8, August 1976 / 479
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place from the double bond to the oppmite orthogonal single bond ((LXXXII) (LXXXIII)). However, what is happening is an internal rearraneement of three bonds, in which two new cyclopropane rings &e created at the expense of the demolition of the two original ones in a 6 s n% + $a process.
+
Although the snoutene skeleton does not budge on heating, a subtle adjustment occurs photochemically. Irradiation of snoutene (XXXIV) is like converting a sow's ear into a silk purse. Diademane (LXXXIV) is formed (17). This is a fourelectron orocess in which interaction occurs between the double hind and the basal bond of one of the cyclopropane rines. The orocess is reversible however and is catalyzed by rho2ium (1j complexes. The mechanism of this and other rearraneements will be discussed below. Iliabemane, apart from its noble, decorative value, can nerve as a source of triauinacene (I.XXXVJ. Conversion is achieved by exposure to silver perchlorate in benzene. Triquinacene, a molecule previously prepared by a lengthy and tedious orocbdure. is redeemed bv its unusual architectural features r - - - - ~, ~ ~ ~ and its potential chemical behavior. Once again, the mechanism for this rearrangement is due t o the Lewis acid nature of silver, which simply releases the strain of the three cyclopropane rings via argenticationic intermediates ((LXXXVI) (LXXXVII)). This sequence, ((XXVI) (XXXIV) (LXXXIV) (LXXXV)), the conversion of basketene t o snoutene to diademane and thence to triquinacene, provides a oowerful illustration of how a judicious choice of catalyst, heat, or light can produce astonishing variations of bonding in a single family of valence tautomers.
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The original synthesis of triquinacene starts from isodrin (Ma). Wagner-Meerwein rearrangement of the isodrin skeleton (XC) provides the appropriate molecular framework (XCI). Oxidation of the starting alcohol followed by epoxidation gives the appropriate ketoepoxide (XCII) which on treatment with strong base undergoes a transannular nucleophilic opening of epoxide to give the ketol (XCIII). Oxidation gives the bridged ether diol (XCIV) which on oxidation with lead tetraacetate gives the olefinic anhydride (XCV). Partial esterification with methanol gave (XCVI) which was completed with diazomethane: the resulting diester was equilibrated to the sterically less hindered form (XCVII). All the subsequent steps (XCVIII) involve a modification of the ester grouping. Hydrolysis gave the d i c i d which on treatment with thionyl chloride gave the acid chloride. Heating of the corresponding acid nzide gave the diisocyanate. Subsequent conversion to the diurethane led finally w the formation of the dimethyl-N-oxide derivative (XCIX). Pyrolysis gave a small amount of triquinacene (LXXXV).
-
ILXXXVI
IXCVllll
k, LC.. 4"C.r.
ILXXXVIIII
ILXXXVI
480 / Journal of Chemical Education
ILXXXIXI
0-
0-
A
ILXXXVI
lxclxl
In contrast to this arduous approach, the method devised by Paquette is a chemical gymnastic feat (19). The starting point is 9,lO-dihydrofulvalene (CI) prepared by the dimerization of a cyclopentadienyl anion (C) with iodine at -78'C in tetrahydrofuran. The Diels-Alder addition of methylazodicarboxilate sets up the first adduct (CII) nicely for a second Diels-Alder reaction, so that the molecules join together like adduct dominoes. Hvdrolvsis resultine ~ ~ ~ ~ and oxidation ~ of the ~ ~ (CIII) gives the diazo compound (CIV) which on pKotolysis vields four comnounds. The desired triauinacene (LXXXV) is obtained in aiout 60% yield together &th a 30%yield of its rine-closed isomer (CV) and two other rearraneed comuounds, namely the fused bis-norbornene (CVI) a n d t h e bridged bicyclooctadienestructure (CVII). All these products arise from the diradical intermediate (CVIII) formed by elimination of a molecule of nitrogen. Opening of the diradical gives triquinacene. Closure creates the cyclopropane isomer. A simple 1 2 shift produces the bis-norbornene structure (CVI). Cleavage of the diradical generates the carhene (CIX) which on simple dride shift aenerates the bridged trieue (CVII). Although these strurtureslmk exotic, in faci the his-norhornene structure is already known a3 it is obtained by the thermal isomerization ~
Trlquinacene Most of the interest in triquinacene (LXXXV) resides in its predicted behavior as a precursor for more exotic structures. The introduction of an additional bond would provide a more viable homologue of pentalene (LXXXVIII). The dimerization of triquinacene, if the two molecules can be persuaded to adont the correct stance. would give dodecahedrane (1.XXXIXJ i]nfortunately, now that sewral syntheses are available IXXI. it turns out that the much wlshed for dimerization does not take place.
e
, " -
-
~
.
~
~A
~
(CVII)
The second synthesis (21) starts from cyclopentadiene (CXVII). Generation of the oxime gives the expected dimer (CXVIII) which is converted to the diketone (CXIX) with levulinic acid. Protective ketalization ensures that the conjugated ketone (CXX) can be induced by light to undergo an internal (2 2) cyclization to give the his-homocuhane derivative (CXXI). Hydrolysis yields the his-homocubanedione (CXXII). The usual conversion of the two carbonyl groups to the diol (CXXIII) permits derivatization to the mesylate by means of sulfene. Nucleophilic displacement hy iodide provides a suitable derivative (CXXIV) for fragmentation, which is carried out with sodium-potassium d o y in tetrahydrofuran (CX). to oroduce hvnostrouhene .. Having expended so much experimental effort, one immediatelv asks if the molecule is fluctional: after all this was the reason for doing the synthesis in the first place. Unfortunately, nmr experiments at room temperature indicated that the bonds did not display the desired mobility. On raising the temperature of the sample i t was observed that isomerization occurred irreversibly to the bis-norbornene structure previouslv referred to. This rearraneement uroceeds through a 1,3sibatropic rearrangement C ~ X V &I. However, more encourazinelv. the authors reported that the dideuterio der i v a t i v e i ~ k a~t -4°C ~ ~ ) i s a static structure, but that on warming to O°C scrambling appeared to occur (CXXVII).
+
Hypostrophene
Hypostrophene (CX) should be a fluctional molecule and should be a suitahle stable mate to hullvalene. As the two cyclopentene rings are face to face, bond breaking and making might occur in such a way that the sigma bond pillars would movezound the oerioherv . . . of the two pentagonal rings. The resultant fluctional process involves six electrons, tour in s honds and two in the interjoining o bond (CXI).
-
Trio-Homocubane
On looking at the structure of hypostrophene one wonders if during its oreoaration some oentaorismane tCXXVIII) could have possibly formed. beo over, it appears that the synthesis of Pettit is very similar to that previously used for the preparation of tris-homocubane. (22) Here again, cycloadditions are ingeniously exploited. Irradiation of the Diels-Alder adduct (CXXIX) obtained from p-quinone and cyclopentadiene (CXVII) gives the cage diketone (CXXX). Reduction with lithium aluminium bydride gives the diol (CXXXI) in which the two hydroxy groups are turned inwards. After conversion to the dihromide (CXXXII), the action of zinc dust produced three derivatives, a pair of diene isomers ((CXXXIII) (CXXIV)) and the homopentaprismane (CXXXV). Equilibration of the dienes favored the symmetrical one (CXXXIII). Addition of bromine to (CXXXIII) occurred in transannular fashion to put the suhstituent bromine atoms as far apart as possible (CXXXVI). Reductive debromination with lithium in t-butanol gave tris-homocubane (CXXXVII).
- .
ICXVII
ICXVI
ICX)
The two current syntheses have much in common (20,21). The first (20) takes advantage of the cycloaddition properties of the iron carbonyl complex of cyclobutadiene (CXII). Cycloaddition with p-quinone, followed by oxidation with ceric salt, gives the endo adduct (CXIII) which is nicely disposed for an internal (2 2) photo-induced ring closure. Reduction of the diketone (CXIV) to the diol (CXV) leads to the dihromo compound (CXVI) which on heating with sodium at 50°C in dioxane gives hypostrophene (CX).
+
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Volume 53,Number 8, August 1976 / 481
Catalysis by lransition Metals
Like silver, rhodium complexes can catalvze the isomerization of strained cage hydrbcarhons; however, from the heginning it was k n o w that the process pmsed through rhodium intermediates which were isolated insome c a s e s . ~ h eaction of rhodium(1) norbornadiene chloride on cuhane (XXI) gives the cis-dimer of cyclohutadiene (LV).The dicarbonyl complex effected the same process and the rhodium(I1) derivative isolated corresponds to an oxidative addition of rhodium to the carhon-carbon single bond (CXXXVIII). In similar fashion cuneane (XLIII) gives semihullvalene (LXI).
onstrated that the silver catalysis of 1,s-hishomocubane to the snoutane derivatives passes hy way of a cyclopropylcarbinyl cation 1231. Similar studies have heen carried out with homocubane derivatives 124).and the action of transition metals, in particular rhodium(1) and palladium(I11 complexes, on hishomo and homorubane derivatives has also been inveatigated ( 2 5 ) .In these rearrangements cation intermediates are ruled out and an oxidative addition pathwav has heen confirmed. Epilog The aim of this article has been to demonstrate the novelty and variety of the structure relationships in a class of cage molecules related t o cuhane. The activity in this field is intense, and fresh examples appear unabated. A ready synthesis of tris-homocubanone is now available from dicyclopentadiene which on ohotocvclization rives 1.3-bishomocubane. Carb o n y ~ a t i o ~the o f iatter is ac&eved with the diearbonyl complex of rhodium(1) (26,27). Literature Clted
It was mentioned above that diademane gives snoutene on treatment with rhodium(1) complexes. However, on permitting snoutene (XXXIV) to remain in longer contact with rhodinm(1) dicarbonyl chloride further rearrangement took place. The two cyclopropane rings are exchanged for a double bond and a new cyclopropane (CXXXIX). From this and other studies it is now known that certain structural features need to be present or latent in a molecule if isomerization is to occur. These two features are the his-cyclopropyl fragment disposed cis (CXL) and the corresponding bent-boat form of cyclohexene (CXLII). Passage between the two molecules containing these structural elements occurs via the rhodium complex (CXLI) and is reversible; however, the position of the equilibrium depends upon the relative thermodynamic stabilities of the isomers in question. Treatment of the pentacvclic decane (CXLIV) with rhodium dicarhonvl chloride a t 6 0 0 gave ~ a 4% conversion to exo-endo his-homobarrelene (CXLIII) after 16 days. However, the latter compound under the same conditions gave a quantitative yield of the pentacyclic decane in an hour. In a series of papers, Paquette and coworkers have dem-
182 1
Journal of Chemical Education
(11 6ealcy.R. M.,sndThorpe,J.F.,Ploe. Chem Soc., 29.346 l1913):il C k m . Soc., 591 ll9Ml. (21 bran, H. O.,and Woodward, R. B., Chem. Ind. (London). 193 (19591. (31 Soloway,S. B.,Damiana. A. M,Sims, J. W.,Bluestone. H..andLidov,R.E., J. A m r Cham. Soc.. 82.5377 (19601. (41 Fredman,H. H.,and Peternen.0.R. J . Amer Chom. Soe., 84,2837 (1962). (51 Lipaeomb, W. N., Tetrahedron Left., 20 (19591. (61 Eaton, P. E.. Cole, T . W., J . Amrr Chem Soe., 86,962,3157 (1961). I71 Masamura,S.,Cufa,H.,andHagben, M. C., TefmhedronLsLf.. 1017 (1966):Purnfaa*. R., and Lehn, J. M., Bull. Sor. Chim. F r , 2497 (1966):Dsuben, W. G.,and Whalen, 3743 (1966): Daubn, W. C.,Sehallhom,C.H.,and W M m , D. L., T~tmhedmnLelt.. D.L..J Amer Chem. Soc., 93.1446(1971). I81 Paquette, L. A.,Arcts. Chem. Res, 4,280 (1971): Paguette, L. A., Backley, a. %,and MeReadie, T.. Tetrahedron Lett., 775 (1971). (91 Cassar, L., Eaton, P. E., and Halpern, J.. J . Amer Chem. Sm., 92.6366 (19701. I101 Jeff0rd.C. W.,andBurger, U., Chimio, 25, 297 (19711. (11) (a1 Wristem, J.,Brcner, L., and Pettlt,R.,J Amer. Chem. Sor., 92,7499 (1970); (hl Gr1gg.R.. Chom Commun., 1247,1248 (19711. I121 Psquette, L. A., II Amer. Chem. Soe, 92.5765 l1970):A8hni, R., TstmhedronLott., 447 (19711. (131 (a) Zwsnonbwg,B.,and K1under.A. J. H., TefmhedmnLetL., 1717,1721 (1971): ibl Nickon, A,, Frank, J. J., Covey. D. F., end Lin, Y., J . Amer Chem. Sm.. 96,7575 (1970. (141 Sesaki, W. A,, Zunker, R., and Musao. H.. Chrm. Bor., 106.2992 11973): Osawe, E., Seh1eyer.P.v. R,Chang,L. K.,snd Kane,V.V.,TetrnhedronLert.. 1189(1974). t t . ,(i974). 1151 ~ a u b nw. . ~ . , a n d ~ w i e gJ.,. ~~. e t r o h ~ d r o n ~ o531 (161 Paquette, L. A,, and Stowell, J. C.. J . Amer. Chem. Soc.. 93,2459 (19711. (171 de Meijem,A., Tetrahedron Left., 1845 (1974):de Meijere, A,, and Meyo?. L. U.,TotD.. and S c h b e r , O..Angelu rnhsdmn LeLL, 1849 (1974Lde Meijere,A., K d - n . Chom.Int. Ed.. 10,417 (19711. I181 (a) Wmdward, R. B., Fukunsga, T . ,Kslly,R C., J Amen Chom. Soc, 86.3162 (1964): ibl Jambon.l. T . , A c h Cham. Scond., 21,2235 (1967): 26,2477 11972): Chom. Scr, '5.134(19741. rt, ~ o r surprixingly s (191 w w a t t , M. ~ . , s n d~aquette,L. A., ~ e t ~ a h o d m n ~ e2433(1974l. simple preparation see Mereier, C.. Soucy, P., R-n W.. and Doalongchamp, P., Syn. Cclmm. 3 121, 161 119731For yet another convenient ryntheslsaee Jacobson. I. Tarbjam.Acro Chem. Seond.. 21,2235 (19671 end26.2477 (19721. (201 McKennis, J. S.,Brener, L., Ward, J. S., and Pettit, R . , J . Amer Chem Sac.. 93.4957 (19711. (211 Paquettc.L. A..Davis,R. F.. Jame8.D.R.. TetrohodmnLeLt., 1615 (1910. (221 Undewaad. G. R.. and Ramamoorthy, B., Chsm. Commun., 12 11970): Tofmhodmn Lett., 4125 i1970);Godleaki.S.A.,Schleyer.P. u.R.Oeaws,E.,and K e n f G . L . C k m . Commun.. 976 11974): Eaton, P. E.,Hudson,R.A.,and Ciordano,C.,Chmn Cammum.,978 (19741. (231 Paquette. L. A., and Beckley, R. S.. J . Amer Chem. Sm.. 91.1084 (1975); Paquettc. L. A,, Beekley, R. S.,andFarnhsm, W. B., J. Amen Chem. Soc., 91,1089 (19751. (241 Paqwtte. L. A.. Ward, J. S.. Boggs. R. A.,and Farnham. W. B., J . A m m Chem. Sm.,
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0rk. C h m , dl; 1445 (19761.
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