218
Organometallics 1986, 5, 218-222
inversion of configuration at carbon supports SN2 and SH2 mechanisms for the reactions involving C O ( I )and ~ ~Co~ (II)2””pgb, respectively. In either case, alkyl transfer is much more facile than in the rhodium system. Given the structural similarities among these macrocyclic complexes, this rate difference can best be accounted for by the higher metal-carbon bond strength in the case of rhodium (c.f. ref 4g). Furthermore, methyl transfer to Rh(1) complex l b from methyl-Co(II1) complex 5 proceeds quantitatively on mixing.’O Thus, the cobalt macrocycle appears to be r
1
+ B
5-
b R=Et a better leaving group than the rhodium macrocycle, presumably owing to the difference in metal-carbon bond strengths. However, the “intrinsic” kinetic barriers for SN2-like processes involving Co(I/III) and Rh(I/III) macrocycles cannot yet be ascertained precisely, as no activation parameters or overall free energy changes (for transfers between the two different metals) have been determined.
The greater efficiency of chloride-bridged vs. methylbridged metal-metal exchange reactions has also been noted for Co(I1)-Co(II1) e ~ c h a n g e . ~ ~Endicott * g * ~ et al.4f attribute this acceleration to the greater electron affinity of the chlorine radical compared to the methyl radical, leading to more favorable bonding in the transition state. The millionfold rate difference observed by these authors can be compared to the result for Rh(1)-Rh(II1) exchange, where the factor is no more than lo5. This smaller acceleration probably results from the requirements of the two-electron transfer. Furthermore, the observed lability’ of coordinated halides in the Rh(II1) complexes may reduce their efficiency as bridging ligands. Summary The scope of the Rh(II1)-Rh(1) transfer reaction (eq 3) has been defined, and kinetic measurements confirm its SN2-likenature. It is considerably slower than related Co(1)-Co(II1) exchange reactions, probably owing to the difference in carbon-metal bond energies. Alkyl transfer from Co(II1) to RhIII) is quantitative. Further comparisons within a given triad, combined with the measurement of activation parameters for “degenerate” exchange reactions, should serve to characterize the “intrinsic” barriers to the formation and cleavage of metal-carbon bonds via two-electron processes.
Acknowledgment. This work was supported by the NSF Grant CHE78-09443. Alex M. Madonik was the recipient of an NSF graduate fellowship.
(10)Collman, J. P.; Finke, R. G.; Sobatka, P. A., unpublished observations. Prepartion of cobalt complexes: Finke, R. G.; Smith, B. L.; McKenna, W. A.; Christian, P. A. Znorg. Chem. 1981,20, 687-693.
Registry No. la,41707-60-2; lb, 53335-25-4; 2a (R’= Me, X = I), 99355-04-1; 2b (R’= Bz, X = Cl), 99355-15-4; 3a,99355-03-0; 5, 57255-98-8. 4b,99355-24-5;
Reactions of a Rhodium( I ) Macrocycle with Organic Dihalides: Oxidative-Addition and ,6-Elimination Pathways James P. Collman,’ John I . Brauman, and Alex M. Madonik Depat?ment of Chemistry, Stanford University, Stanford, California 94305 Received March 18, 1985
The reduction of organic dihalides by low-valent metal species may occur via several mechanisms. We have examined the reactions of a macrocyclic rhodium(1) supernucleophile,1, with a variety of vicinal and 1,3-dihalidesubstrates. Unlike typical one electron reductants, 1 does not induce elimination of 1,3-dihalopropanes to give olefins or cyclopropanes; rather, normal alkyl-Rh(II1) oxidative-addition products are isolated. (When the two halides are identical, the expected statistical mixture of ”mono” and ”bis” adducts is obtained, although reaction in the presence of undissolved 1 can skew the result in favor of the “bis”adducts.) In contrast, the reaction of 1 with vicinal dibromides invariably gives olefiis and the Rh(II1) dibromide, the only partial exception being 1,2-dibromoethane, from which a “bis” adduct is formed in ca. 50% yield. Reduction rates are comparable to those found for the oxidative addition of monofunctional alkyl bromides to 1, except in the case of trans-l,2-dibromocyclohexane, whose reduction to cyclohexene occurs on mixing with 1 (the cis isomer is reduced at least lo5 times more slowly, although it still reacts about 10 times faster than bromocyclohexane). The olefinic products probably result from decomposition of Rh(III)-@-bromoa&ylintermediates, except in the case of trans-1,2-dibromocyclohexane, where a concerted elimination process is proposed. Introduction The reduction of organic dihalides by metals is a wellknown route to olefins1 and cycloalkanes.1a~2Competing
one- and two-electron processes frequently limit the stereospecificity of olefin formation by this route, especially from simple vicinal dihaloalkanes. In contrast, iodidepromoted elimination is highly stereo~pecific.~ More re-
(1) (a) House, H.0. “Modern Synthetic Reactions”, 2nd ed.; W. A. Benjamin: Menlo Park, CA, 1972; p 220 ff. (b) Singleton, D. M.; Kochi, J. K. J. Am. Chem. SOC.1967, 89, 6547-6555 and references therein.
(2) Kochi, J. K.; Singleton, D. M. J. Org. Chem. 1968, 33, 1027-1034 and references therein.
0276-7333/86/2305-0218$01.50/00 1986 American Chemical Society
Organometallics, Vol. 5, No. 2, 1986 219
Reactions of Rhodium(I) Macrocycle with Organic Dihalides cent studies have focused on soluble transition-metal species as reductant^.'^^^^^ The modest stereoselectivity exhibited by the one-electron reductant Cr(1I) is attributed to stabilization of radical intermediates by “neighboring group” interactions and to the formation of transient metal-alkyl s p e ~ i e s . ~ ”These . ~ reagents also reduce monohalides, but at much lower rates-further evidence for “neighboringgroup” effects in reactions with the dihalides. We have previously examined the oxidative addition5 (1) of a wide range of alkyl halides to Rh1(PPDOBF2)6,7 and find that this Rh(1) macrocycle behaves as a supernucleophile,*giving Rh(II1)-alkyl adducts according to eq 1. Its reactions with a,w-dihalides were also studied in
Table I. Product Yields in the Reaction of 1 with 1,3-Dibromopropane (2a) % yieldsb
concn,mM solventltemp;
1“
3a
4 (“mono”)
5 (“bid’)
30 30 30 30 30 30 15 15 30 30 30
45 45 45 45 45 180 22.5 22.5 45 45 45
28 22 60 61 25 55 55 58 91 92 92
66 75 42 40 57 35 40 37 9 13 7
O C
DME/28-30 DMEj28-30 DMEl28-30 DMEl28-30 DME/reflux DME/reflux DME/28-30 DME/ 28-30 acetone128 acetone128 MeCNl28-30
Concentration assuming complete dissolution of the Rh(1) reagent. Yields by UV-visible assay after chromatographic separation (accurate to within &5%). ‘The ambient temperature of the inert-atmosphere box was 28-30 OC.
U 1
pound, when the reactions are carried out in THF or DME with 50% excess of substrate.
?
order to probe for radical pathways. The principal products were again found to be Rh(II1)-alkyl adducts; moreover, in many cases only the “bis” adduct was isolated, seemingly the result of “neighboring group” activation of the second halide.’” We have since examined these reactions under a variety of conditions to determine what factors dictate the ratio of “mono“ and “bis” adducts. In general, the so-called “neighboring group” effect was found to be an artifact of reactions carried out in the presence of incompletely dissolved Rh(1) complex. However, certain vicinal dibromides do display enhanced reactivity toward 1, although in most cases the products are olefins and Rh111(PPDOBF,)(Br)2. In this paper, we discuss the mechanismb) of Rh1(PPDOBF2)oxidative additions in light of these results.
-
-
Y = Br
I
CI
OMS CI
Br
3
I
"mend' 2
“bis“
2
The yields of the two products 4 and 5 were studied in detail in the case of 1,3-dibromopropane (3a) (Table I). Increasing the substrate excess to sixfold approximately reverses the proportion of “mono” and “bis” adducts. However, we also noted disconcerting variations in the product ratios in supposedly identical experiments. Furthermore, when MeCN or acetone is substituted for Results9 the ethereal solvents, the “bis” to “mono” ratio declines to 1:9.Diluting the reactions in ethereal solvents also leads Reactions with 1,3-Dihalopropanes. We investigated to decreased yields of the “bis“ adduct. The solubility of the reactivity of 1 with a spectrum of 1,3-disubstituted the Rh(1) reagent was one factor that had not been carepropanes 3a-f. No propene or cyclopropane could be fully examined, and we found that it depends strongly on detected by lH NMR when reactions were carried out in the solvent. In MeCN, the solubility of 1 is no less than acetone-$ or acetonitrile-d3 ([Rh(I)] = 20 mM, 1equiv of 40 mg/mL (100 mM), while for acetone the figure is 8-10 3a-e). With X # Y (3c-f), we found only “mono” adducts, mg/mL. Its solubility in THF is much less, approximately as determined by TLC analysis of the reaction mixtures 2 mg/mL, and in DME it is lower still, little more than and isolation of the products. For X = Y (3a,b),we found 1mg/mL. Since preparative reactions had been conducted that the previously reported “bis” adducts7”are formed in with 10 mg of 1 per mL, reactions in the ethereal solvents conjunction with about half as much of the “mono” comcould not have been homogeneous. The deep purple color of the Rh(1) complex in solution is deceptive, as it conceals any undissolved material suspended in the reaction mix(3) Stevens, C. L.; Valicenti, J. A. J. Am. Chem. SOC. 1965,87,838-842 and references therein. ture. (4) (a) Kray, W. C., Jr.; Castro, C. E. J. Am. Chem. SOC. 1964, 86, Vicinal Dibromides: 1,2-Dibromoethane. We reex4603-4608. (b) Kochi, J. K.; Singleton, D. M. J. Am. Chem. SOC. 1968, amined the addition of this substrate to 1 by lH NMR and 90,1582-1589. (c) Chock, P. B.; Halpem, J. J. Am. Chem. SOC. 1969,91, 582-588. (d) Wegner, P. A.; Delaney, M. S. Inorg. Chem. 1976, 15, TLC and found a roughly 1:l mixture of a poorly soluble 1918-1921. Rh(II1)-alkyl species and the Rh(II1) dibromide 8, ac( 5 ) Collman, J. P.; Roper, W. R. “Reviews of Oxidative-Addition”;Adu. companied by a significant quantity of ethylene. The Organomet. Chem. 1968,7,53-94. Stille, J. K.; Lau, K. S. Y. Acc. Chem. Rh(II1)-alkyl compound proved difficult to characterize, Res. 1977,10, 434-442. ( 6 ) This ligand, [difluoro[N,N’-bis(3-pentanon-2-ylidene)-1,3-di- but treatment with AgBF, precipitated AgBr and left a aminopropane)dioximato]borate], is also known in the literature as more soluble BF4- salt, which ‘H NMR showed to be de[Cz(DO)(DOBFz),,I. rived from the “bis” adduct 7 consistent with earlier (7) (a) Collman, J. P.; MacLaury, M. R. J . Am. Chem. SOC. 1974,96, 3019-3020. (b) Collman, J. P.; Brauman, J. I.; Madonik, A. M. Organofindings‘” and elemental analysis of 7. metallics, preceding paper in this issue. (8) Schrauzer, G. N.; Deutsch, E. J. Am. Chem. SOC.1969, 91, 3341-3350. (9) Abbreviations: DME = l,2-dimethoxyethane; EtOH = ethanol; MeCN = acetonitrile; THF = tetrahydrofuran.
Br-eh-Br
6
Br-@h+@h-Br “mono”
2 “bis”
Br-@h-Br
t!
220 Organometallics, Vol. 5, No. 2, 1986
l&Dibromopropane. The only products identified by 'H NMR in the reaction of this substrate with 1 were propene and the Rh(II1) dibromide 8 (which was also isolated and analyzed). The reaction rate was similar to that for 1,2-dibromoethane. l&Dibromocyclohexane. A startling observation was the extremely rapid reaction between 1 and trans-1,2-dibromocyclohexane, resulting in elimination to form cyclohexene and dibromide 8 (which was isolated and analyzed). The reaction was complete on mixing, so the rate constant could not be less than about 10 M-l s-l (i.e., considerably faster than the addition of 1-iodobutane to 17b). Reaction with cis-1,2-dibromocyclohexanegave the same products but required a period of days for completion. A rate constant of approximately 2.5 x M-ls-l (acetonitrile-d,, 28.5 "C, statistical factor of 2) was deduced for this reaction from a series of 'H NMR spectra recorded over this time. Kinetic Data Rh(1) disappearance was monitored by UV-visible spectroscopy in the presence of a large excess of the substrate, as described elsewhere.% The reactions are second order, first order in both Rh(1) and alkyl halide. The rate constants for addition to 1 are, for 1,2-dibr0moethane,~" 4.6 X M-l s-l (THF, 25 f 2 "C, statistical factor of 2), and for l,&dibromopropane, (3.5 f 0.1) X M-' s-' (30.5 f 2 "C,THF, statistical factor of 2). For comparison, the rate of addition of 1-bromobutane to 1 is (1.67 f 0.05) X 10-2 M-1 s-l (THF, 30.5 f 2 oC).7b Discussion Reactions with 1,3-Dihalopropanes. The absence of elimination products in these reactions is strong evidence in support of a two-electron, SN2-likemechanism for oxidative addition to 1 (see ref 7b for details of our kinetic and mechanistic studies). Furthermore, the unexceptional rates of these reactions (when compared to reactions of 1 with simple alkyl halides), is in striking contrast to the large accelerations (25-50 times) observed in the case of one-electron reductants such as Cr(IU2and C O ( I I ) . ~ ~ The formation of surprisingly high proportions of "bis" adducts (eq 2) was encountered at the outset of this in~estigation.~" We have since shown that this so-called "neighboring group" effect is a phenomenon resulting from reaction in the presence of undissolved Rh(1) complex.1° When the concentration of 1 is adjusted so as to ensure its complete dissolution, the expected "mono" adducts predominate. Since all kinetic experimentswere conducted in this more dilute regime (with a large excess of substrate), a simple 1:l stoichiometry holds. The rate constant for 1,3-dibromopropane has been divided by a statistical factor of 2 (owing to the presence of two equivalent leaving groups). If formation of the "bis" adduct were predominant, a factor of 4 would be necessary, owing to the change in ~toichiometry.~" Vicinal Dibromides. Since the reduction of vicinal dibromides to olefins can occur by either one- or twoelectron mechanisms, kinetic and stereochemical information is necessary in order to distinguish among the possibilities. One plausible route to olefiis is via a normal oxidative-addition reaction, followed by decomposition of the "mono" adduct (such as 6) to the olefin and Rh"'(PPDOBF2)(Br),. No "mono" adducts could be isolated, and (10) A similar phenomenon may explain the formation of 'bis" adducts in the reaction of Co(1) macrocycle vitamin B,,, with a,@-dihalides: Smith, E. L.; Mervyn, L.; Muggleton, P. W.; Johnson, A. W.; Shaw, N. Ann. N.Y. Acad. Sci. 1964, 112, 565-574.
Collman et al. Kochi" states that no stable P-bromoalkyl metal complexes are known. Kinetic data for the reaction of 1,2-dibromoethane are consistent with a normal oxidative-addition process. While the deceleration typical of nucleophilic reactions with vicinal dibromides12 is absent, the rate differences between this substrate and l,&dibromopropane or 1-bromobutaneare minimal compared to those observed with, for example, Cr11(en)22+, a powerful one-electron reductantS2 Formation of the "bis" adduct 7 from 1,Zdibromoethane is unlikely to occur via an electron-transfer mechanism; more reasonable is trapping of the "mono" adduct 6 by Rh'(PPDOBF2) in a second SN2-likeprocess. Thus, oxidative addition must compete successfully with the decomposition of 6. That it is able to do so may reflect an activating influence of the adjacent Rh(II1) macrocycle on the &bromo group. The failure of other vicinal dibromides to give "bis" adducts is not at all surprising, since secondary bromides add to 1 some 500 times slower than primary Turning to cis- and trans-1,2-dibromocyclohexane,each of these substrates reacts with 1 faster than does bromoM-l s-l, THF, 30.5 f 2 cyclohexane ((1.25 f 0.25) X "C).ITbFor the trans dibromide, the accleration is on the order of 106-fold,which is unprecedented either in Cr(I1) reductions (where the acceleration is roughly 1500 times)lb or in iodide-induced elimination (which is actually somewhat slower than nucleophilic substitution in bromocyclohexane).13a Iodide-promoted elimination is wellknown to require a trans diaxial conformation,, and the trans dibromide is a particularly favorable substrate for the process because it actually prefers the diaxial conformation in s01ution.l~ However, the trans diaxial geometry is also the most favorable one for bromine atom abstraction, owing to "neighboring group" stabilization of the free radical intermediate.1b*4a,b A proper choice of mechanism for the Rh(1)-inducedelimination can only be made after examination of the results with other substrates. The rate difference between the cis and trans dibromide is also very large, approaching a factor of lo5. Nonetheless, the cis compound still reacts some 20 times faster than bromocyclohexane. If one allows for the difference in solvent polarity between MeCN and THF, this rate ratio is reduced to ten.7b While vicinal dibromides are normally poor substrates for nucleophilic attack at carbon,12we have noted a modest acceleration in the reaction of 1 with 1,2dibromoethane, relative to other 1-bromoalkanes. Thus, cis-1,2-dibromocyclohexane could add to 1 more rapidly than does bromocyclohexane. Furthermore, bromocyclohexane prefers an equatorial conformation in solution, inhibiting nucleophilic displacement, while one of the bromine atoms in the cis dibromide is obliged to occupy an axial position in either of the possible chair conformations. Displacement of bromide by the Rh(1) macrocycle in an SN2-likeprocess would convert the cis dibromide to a trans-substituted cyclohexane, favoring subsequent concerted elimination. Thus, a two-step, two-electron pathway appears feasible for the Rh(1)-in(11) Kochi, J. K. "Organometallic Mechanisms and Catalysis"; Academic Press: New York, San Francisco, London, 1978; p 176 ff. Note that the data auoted in the table on D 176 for Rh'PPDOBF, are actuallv 103k; this factor of lo3 was omitted: (12) Streitweiser, A., Jr. "Solvolytic Displacement Reactions"; McGraw-Hill: New York, 1962; p 17. (13) (a) Goering, H. L.; Espy, H. H. J . Am. Chem. SOC.1955, 77, 5023-5026. (b) Goering, H. L.; Sims, L. L. J . Am. Chem. SOC.1955, 77, 3465-3469. (14) Bender, P.; Flowers, D. L.; Goering, H. L. J. Am. Chem. SOC.1955, 77, 3463-3465.
Reactions of Rhodium(I) Macrocycle with Organic Dihalides
X Br
equiv used 1.5X
solv DME
I(CH2)3 I CH3S03(CH2)3Br
1.5X 1.5X
THF THF
Cl(CHJ3 Cl(CHz13
Br I
1.5X 1.5X
THF THF
Br (CH2 )3
I
2X
THF
R Br(CH2)3
Organometallics, Vol. 5, No. 2, 1986 221
Table 11. R-Rh1"(PPDOBF2)-X "Mono"Adducts 4a-f reaction time temp yield, % spectral data formula anal. 45 reflux 15' 6 1.17 (m, 4 H),b3.22 (br t, J = C16H28BBr2F2Nz02RhC, H, N, Br 6.3 Hz, 2 H) 31' 6 1.16 (m, 4 2.81 (m, 2 H) C16Hz8BFzIzN40zRh C, H, N, Id 5 room 88 v ( S 0 ) 1340 (s), 1180 ( 8 ) cm-' C17H31BBrFzN40zRhSC, H, N, Br; S 90 reflux 6 1.16 (m, 2 H)8 1.5-1.8 (m, 2 H), 2.90 ( 8 , 3 H), 3.98 (m, 2 H) 88 6 1.16 (m, 4 H),f 3.3 (m, 2 H) Cl6HZ8BBrClF2N4O2RhC, H, N, Br, C1 5 reflux 44 6 1.12 (m, 4 H), 3.29 (br t, J = Cl6Hz8BC1F21N4o2Rh C, H, N, C1, I 45 room 6.5 Hz, 2 H)g 80 6 0.9 (m, 2 H), 1.16 (m, 2 H), C16Hz8BBrFzIN402Rh C, H, N, Br, I 15 room 3.22 (br t, 2 H)b
All analyses were within *0.4% of the calculated values unless otherwise noted. * NMR spectrum in CD2C12. The major product under these conditions is the "bis" adduct. dI: calcd, 35.52; found, 34.87. eBr: calcd, 12.58; found, 13.99. fNMR spectrum in CDC13. gNMR spectrum in acetone-de
duced elimination from the cis dibromide. Returning to the trans isomer, the large trans/& rate ratio observed here is also inconsistent with a one-electron pathway (for Cr(I1) the ratio is only 150).lb In contrast, the corresponding ratio for iodide-induced elimination is3 at least 322. The Rh(1) macrocycle is quite sensitive to steric hindrance, so it is not surprising that attack at bromine is much more facile than attack at carbon.7b It remains necessary to account for the comparative lack of activation in reactions of acyclic substrates such as 1,2dibromoethane. Accelerations of 1000-fold are observed with typical one-electron reductant^,^,^^ relative to their reactions with monobromoalkanes. Iodide-induced elimination, on the other hand, is essentially an E2 process13a and as such should be comparatively slow in 1,2-dibromoethane, relative to the more highly substituted 1,2dibromocyclohexane. Indeed, the iodide-promoted elimination of 1,2-dibromoethane is relatively inefficient, requiring a higher temperature than the corresponding reaction of either 1,2-dibromocyclohexane.15 For 1,2-dibromoethane, then, the oxidative-addition pathway predominates, leading to either the "bis" adduct 7 or to elimination via the "mono" adduct 6. In summary, the reactions of 1 with organic dibromides are best explained in terms of two-electron mechanisms and thus represent a straightforward extension of the chemistry observed with monobromides. This Rh(1) complex resembles conventional nucleophiles except for the modest "neighboring group" accelerations observed in its reactions with 1,3-dibromopropaneand 1,2-dibromoethane. Formation of a "bis" adduct from the latter substrate is not well understood but may involve "neighboring group" activation of bromine by the Rh(II1) macrocycle in an intermediate P-bromoalkyl adduct (6).
Experimental Section Materials a n d Methods. Techniques for the synthesis and manipulation of highly oxygen-sensitive 1 are described elsewhere,'b along with details of the kinetics procedure used to study (15) Urata, Y.; Bunya, K.; Kakihana, H. Nippon Kuguku Zusshi 1969, 82,1403-1406 (Chem.Abstr. 1969,59,3753). While it remains true that iodide ion reacts more rapidly with bromocyclohexanethan with either of the 1,2-dibromocyclohexanes,13a the relative rates of the substitution and elimination processes are undoubtedly influenced by their respective free energy changes. In the case of 1, formation of a Rh(III)-carbonbond (via nuclophilic displacement) is a small driving force compared to the formation of a carbon-carbon double bond and the reduction of the second bromine to bromide ion (in the elimination process). Thus, elimination is probably much more favorable thermodynamicallyin the case of Rh(1) than in the case of iodide. However, this driving force is only manifested in the case of the trans dibromide, where concerted elimination generates a transition state resembling the final products.
its oxidative-addition reactions. UV-visible spectra were recorded on a Cary 219 spectrometer using 1-and 10-mm quartz flow cells supplied by Hellma Cells. Infrared spectra were recorded on a Beckman Acculab spectrometer. 'H NMR spectra were recorded at 100 MHz on a Varian XL-100 instrument equipped with a Nicolet Technology pulse generator and data system.ls Elemental analyses were performed by the Stanford Microanalytical Laboratory. The solubility of Rh'(PPDOBF2) in various solvents was determined by saturating a small volume of the solvent with excess reagent, filtering the solution, and removing a carefully measured volume via syringe. The solvent was then removed under vacuum and the sample diluted to a known volume in T H F for assay by UV-visible spectroscopy (Amm 560 nm (t 21 500)). S u b s t r a t e s f o r Oxidative-Addition Reactions. Commercially available substrates were purified by passage over activity I neutral alumina, followed by three cycles of freeze-pump-thaw degassing and storage in dark bottles over copper turnings in the glovebox. The mesylate 3d of 3-bromo-1-propanol was obtained by the method of Crossland and Servis.17 While trans-1,2-dibromocyclohexanewas commercially available (Aldrich), the cis isomer was prepared by photochemical hydr~bromination'~~ of 1-bromocyclohexene? It was recrystallized from pentane a t -78 "C and then distilled (bp 83-85 "C (5 torr) (lit.'3b 50.5-51.5 "C (0.13 torr))). Gas chromatography (10% OV-101,150 "C) indicated that less than 1% of the trans isomer was present. 1,3-Diiodopropane (3b). Treatment of 1,3-dichloropropane (Aldrich) with NaI (2.1 equiv) in dry acetone at reflux overnight provided (after filtration, evaporation of the solvent, and distillation) the diiodide in 67% yield (bp 109-113 "C (20 torr) (lit.18 110 "C (19 torr))). 1-Chloro-3-iodopropane (3e). A solution of 1,3-dichloropropane and 1equiv of NaI in dry acetone was stirred and heated under reflux overnight. Following filtration and removal of the solvent, distillation gave the desired product in 36% yield (bp 86-88.5 "C (45 torr) (lit.19 60.8 "C (15 torr))). 1-Bromo-34odopropane (3f). This compound has been prepared by allowing 1,3-dibromopropane to react with 1equiv of NaI in solution.4c Accordingly, 100 mmol of the dibromide (Aldrich, 20.2 g) and NaI (Baker, 100 mmol, 15.0 g) in 100 mL of dry acetone were stirred and heated under reflux overnight. Filtration and removal of the solvent, followed by distillation provided an impure product boiling a t 82-97 "C (24 torr), which was redistilled to give 5.34 g (21%) of product, bp 86-94 "C (25 torr). Gas chromatography (10% OV-101, 85 "C) showed that it contained less than 10% each of 1,3-dibromo- and 1,3-diiodopropane: 'H NMR (CDC13)6 2.33 (d oft, J = 7 Hz, 2 H) 3.30 !16) All 'H NMR data are reported relative to internal Me,Si and were calibrated relative to the residual solvent peak: CD,HCN, 6 1.93; CHDCl,, 6 5.27; CDZHCOCD,, 6 2.05; CHCl,, 6 7.25. (17) Crossland, R. G.;Servis, K. L. J.Org. Chem. 1970,35,3195-3196. (18) "Handbook of Chemistry and Physics", 46th ed.: CRC Press: Cleveland, OH, 1965. (19) "Dictionary of Organic Compounds", Oxford University Press: New York, Oxford, 1965; p 641.
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Organometallics, Vol. 5, No. 2, 1986
(t, J = 7 Hz, 2 H), 3.51 (t, J = 7 Hz, 2 H). Oxidative-Addition Products. Typical synthetic reactions used 0.25-0.50 mmol of the Rh(1) reagent and a 1.5-fold excess of the alkylating agent in 8 mL of solvent (light was excluded). When necessary the reaction mixture was heated under reflux until the purple color of the Rh(1) complex had disappeared. After a purity check by TLC20a (eluting with acetone or 30% MeCN/CH,Cl,) the adducts were usually recrystallized from CH,Cl,/EtOH. All adducts were characterized by IR and 'H NMR spectroscopy and by analysis. (Except where otherwise noted, analyses for the elements listed agreed with the calculated values to within f0.4%). Their IR spectra exhibit bands assigned to ligand stretching modes (KBr pellet): v(CN) 1605 (m), 1530 (m), v(B0) 1170 (s), 820 (s), v(BF) 1000 (s), v(N0) 1120 (s) cm-'. The macrocyclic ligand also exhibits a relatively unvarying set of 'H NMR resonances: 6 1.08 (t, J = 7.6 Hz, 6 H), 2.15 (m, 2 H), 2.30 (t, J = 1.2 Hz, 6 H), 2.76 and 2.78 (q, J = 7.6 Hz, 4 H total), 3.7-4.3 (br m, 4 H). See Table I1 for specific reaction conditions and further characterization of the "mono" adducts derived from 3a-f. trans ,trans-(Br)Rh(PPDOBF,)(CH,)&h(PPDOBF,)(Br) (5a) a n d trans-(Br(CH2),)Rh(PPDOBF,)(Br) (4a). A slurry in 8 mL of DME was stirred under reflux of 1 (200 mg, 0.48 "01) and treated with 1,3-dibromopropane (3a, 0.72 mmol, 145 mg). After 45 min a large amount of flocculent yellow precipitate had appeared. It was collected on a glass frit and washed with EtOH, followed by recrystallization from CH2C12/EtOHto give the "bis" adduct. TLC analysis of the original filtrate showed that it contained a second species (the "mono" adduct), which could be isolated in 15% yield (45 mg) following recrystallization from CH2C12. The "bis" adduct accounted for the balance of the Rh(1) reagent. For the "mono" adduct: UV-vis (acetone), X 362 nm (e 4700); see Table 11. For the "bis" adduct: UV-vis (acetone) A, 372 nm (e 9560); 'H NMR (CD2C12)6 0.44 (m, 2 H), 0.88 (m, 4 H), 1.15 (t, J = 7.5 Hz, 12 H), 2.31 ( 8 , 12 H), 2.55 and 2.97 (q, J = 7.5 Hz, 8 H total), 3.5-4.2 (br m, 8 H). Anal. (C29H50B2Br2F4N804Rh2)C, H, N, Br. The yields of "mono" and "bis" adducts were quantified according to the following typical procedure: A mixture of 1 (100 mg, 0.24 mmol) and 1,3-dibromopropane (0.36 mmol, 73 mg) in 8 mL of T H F was stirred under reflux for 0.5 h. The solvent was evaporated and the residue redissolved in CHzC12and absorbed onto a small portion of silica gelmbby removal of the solvent. The silica gel was applied to the top of a wet silica gel column (acetone), and the Rh complexes were eluted with acetone. Two bands were collected, concentrated, and diluted to 50 mL with acetone. Each solution was further diluted 4 1 before measurement of its W-vis absorption in a 1-mm cell. The leading band ("mono" adduct) exhibited an absorption of 0.15 a t 362 nm, corresponding to 0.8 mmol of product (33%). The yield of "bis" adduct was 66% according to its absorption (0.30) measured a t 372 nm (corresponding to 0.08 mmol of product, hence 0.16 mmol of Rh(1) reagent). The two adducts were subsequently recrystallid from CH,Cl,/EtOH: "mono", 29 mg (20%); "bis", 69 mg (55%). t r a n s ,trans -(I)Rh(PPDOBF,)( (CH2),)Rh(PPDOBF2)(I) (5b) and trans-(I(CH,),)Rh(PPDOBF,)(I) (4b). The substrate 1,3-diiodopropane (3b, 0.72 mmol, 210 mg) was added to a stirred slurry of 1 in 8 mL of THF. After 5 min at room temperature (20) (a) Silica gel plates for analytical TLC (250 pm thickness) were purchased from Andtech, Inc. (b) Silica gel (60-200 mesh, type 62) was supplied by W. R. Grace and activated at 80 "C.
Collman et al. an orange precipitate was collected on a glass frit (186 mg). It proved to be the "bis" adduct (69%). The "monon adduct was recovered from the filtrate by addition of EtOH and reduction of the volume (87 mg, 31%). Both compounds could be recrystallized from CH,Cl,/EtOH. For the "mono" adduct, see Table 11. For the "bis" adduct: 'H NMR (CD2C12)6 0.30 (m, 2 H), 0.88 (m, 4 H), 1.12 (t,J = 7.5 Hz, 12 H), 2.27 (s, 12 H), 2.40 (m, 2 H), 2.5-3.1 (m, 8 H), 3.5-3.8 (br m, 4 H), 4.1-4.4 (br m, 4 H). Anal. (C&J&F412N,O4Rh) C, H, N. Anal. I calcd, 22.36; found, 21.80. t r a n s ,trans -(Br)Rh(PPDOBF,) (CH2)Rh(PPDOBF,)(Br) (7). An NMR sample tube was charged with 0.5 mL of a 20 mM solution of 1 (0.01 mmol) in CD3CN and capped with a rubber septum. Two equivalents of 1,2-dibromoethane(0.02 mmol, 1.7 pL) were added via syringe. The purple color of 1 faded within 10 min. The 'H NMR spectrum revealed the presence of excess substrate (6 3.76 (9)) and ethylene (6 5.42 (s)) as well as two distinct Rh(II1) species. One of these was clearly the Rh(II1) dibromide 8 previously described.7bThe other species exhibited the following 'H signals: 6 1.1 (t), 2.3 (s), 2.6-2.9 (m), 3.5-4.1 (br m). A preparative scale experiment involving 105 mg of Rh'(PPDOBF2) (0.25 mmol) and 10 equiv of 1,2-dibromoethane (2.50 mmol, 470 mg) in MeCN (5 mL) at room temperature also generated two species, according to TLC analysis. One of these precipitated from the reaction mixture, and its solubility was so poor that no 'H NMR spectrum could be obtained. Treatment of 6 mg of this solid with 4 mg of AgBF, in 0.5 mL of CD&N produced an orange solution which was filtered (to remove AgBr) and examined by 'H NMR. The "bis" adduct was identified on the basis of the following spectral data: 6 0.84 (d, J = 1.25 Hz, 4 H), 1.10 (t, J = 7.6 Hz, 12 H), 2.15 (m, 4 H), 2.39 (s, 12 H), 2.75 and 2.78 (q, J = 7.6 Hz, 8 H total), 3.5-4.2 (br m, 8 H). The doublet a t 6 0.84 is diagnostic of a methylene group bound to "Rh H, N, Br. Anal. C: (spin '/,). Anal. (C28H48B2Br2F4Na04Rh) calcd. 32.84; found, 32.39. Reaction of 1 with cis- a n d trans-l,%-Dibromocyclohexane. The Rh(II1) dibromide 8 was the only product isolated when Rh1(PPDOBF2) was allowed to react with trans-1,2-dibromocyclohexane. The reaction was complete on mixing. An NMR scale experiment using 2 equiv of this substrate (0.02 "01, 2.7 pL) and 0.5 mL of the stock Rh(1) solution (20 mM in CD,CN) generated exclusively 8 as the Rh-containing product. A weak olefinic resonance a t 6 5.65 was attributed to cyclohexene. A similar lH NMR experiment using cis-1,2-dibromocyclopL) as the substrate required about a week hexane (0.01"01,113 for complete reaction. The relative amounts of Rh(1) and Rh(I1) were monitored by 'H NMR, and the progress of the reaction was as follows: 18 h, 33% complete; 44 h, 60% complete; 72 h, 90% complete. The only species remaining after 1 week were the Rh(II1) dibromide and cyclohexene, as the spectrum was identical with that obtained in the experiment with the trans isomer.
Acknowledgment. This work was supported by the NSF Grant CHE78-09443.Alex M. Madonik was the recipient of an NSF graduate fellowship. Registry No. 1, 53335-25-4; 3a, 109-64-8; 3b, 627-31-6; 3c, 109-70-6; 3d, 35432-34-9; 3e, 6940-76-7; 3f, 22306-36-1; 4a, 99494-93-6; 4b, 99494-94-7; 4c, 99494-95-8; 4d, 99494-96-9; 4e, 99494-97-0; 4f, 99494-98-1; 5a, 99494-99-2; 5b, 99495-00-8; 7, 99495-01-9; 8, 99355-26-7; 1,2-dibromoethane, 106-93-4; 1,2-dibromopropane, 78-75-1; propene, 115-07-1; ethylene, 74-85-1; truns-1,2-dibromocyclohexane, 7429-37-0; cyclohexane, 110-83-8; cis-1,2-dibromocyclohexane, 19246-38-9.
