Transmission mechanisms of four-bond 19F19F coupling constants in fluoropropenes: A theoritical and experimental study

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_. ..-. -. in cis- and trans-isoof I.3-difIuoro- and 1.3.3.3-retmIIuoro-propene These Four-bond F-F co&Ii& urefe m coupIings show a pronotmced dependence on ihe number of .fIuorines attached to the s$ car&m atom This dependence 3 much larger for t& cis-isothan for the -ones_ A theoreticaIanaIysisof tnnsmision mechnkms of thk F&ni cont&& orbitaI and dipolar int~cting terms is c&tied out using inner projections of the polarization propastor ai the INDO~kveI of approximatiox~_For each ikracti~ term three different coupIing~pathw~>xare consider&i. ~nameIy_throqh-space~o--and ~~Icctron contriiutions Trends for each me&au&m are d&used and resuks are compared with the experimattaI &Iuer :

_ 1, Introduction ‘of. high-resolution NMR specnotably broadened -if a detailed knOwledgeof the electronicpathways for transr& ‘_ -. ‘g the m&rect -tm spm-spm couphng constants caZ be pre&eIy ~detexmined_ As only the total coupiin& are amenable- of dir& measurement, the study of-these transmission mechanismq’is greatly improved if a theoretical approach accompanies the experimentaLmea&r&nents_With-this idea in m&d, during- the last few--yseveral methods ~-_ have been presenied p-5] which a& to.$ecom:: pose caiculated Coupling _cox&tantsaccording to 1 : the eIectronic.mechanismsinvolved in -their t.rans- : ‘don. me l&.,+tio~ ;and pO+b@& of_@ese The

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molecular orbital tbeo,$ of the tot&couplings %a difficult task by itself. The current staje of theoretic& calculatib& of $&spin cotipling Constants has been reviewed by ~owal&vski m2 r _ -. .. Iti~ this ‘paper tqnsmissi& mechaniSti Of 19F-19F c&plings tie -stu&d in cis-fluotioyr? -_

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Four-bond F- F couphss

penes. I, and trans-fluoropropene.s* JI (see scheme l)_ To this end_ experimental vahtcs of the corresponding tota couplings were determined in compounds la and Ila Transmission mechanisms are studied using the IPPP approach at the INDO lcvcl of appro_ximation_ All three interactions_ the Fermi contact (FC). spin-orbital (SO) and spin-dipohr (SD) terms are taken into account Pretiminary results for through-space components of =Ci9F [SJ and 19F-t9F (91 couplings were reponed in 1984.

Z JL-qerimental 1.3-diBuoropropene has been described previously but no NMR spectra were reported [IO]. In our work, the two isomers were prepared from I.3diftuoropropane+ol by ester&cation and ehmination, using a method described by Gerlach and Miller [ll]_ Thus. the alcohol was converted to a thionocarbonate by reaction with CiH,O-CS-CI in the presence of pyridine and this ester -was decomposed in dibutyl phthalate at ZtN”C_ Vchttik products were swept from the solution by a stow stream of nitrogen gas and trapped at 93 K_ The mixture proved difficult to separate due to the reactivity of the aIkenes_ but the ‘9F resonances were nicely separated at &t-67 &MHz and so NMR studies were performed on the mixture Spectra wcrc rccordcd for the solution in CDCI, with CFCI, as internal standard_ The spectrometer was a Bruker WH90 operating at 84.67 MHz and qxctra wcrc recorded in the pulse-Ff mode with spectra1 width 12200 Hz using 16K data points. The muItip1et.s arising from the olefinic fluorine (Fl) of the Z-isomer (ia) were identified by means of the t.rans ‘JFii of 39 Hz the corresponding cis coupling in the E-isomer (Ha) being 16 Hz These values are typical of those in a compilation of coupling constants [It]_ Fluorine-fluorine couplings were condusively identified in spectra recorded with broad-band ‘H decoupling. and ave.aged vahres were then obtained from the many obxncci spacings in the two multiplets for each isomer_ A complete anaIysis has not been performed. pending the apphcation of better instrumentation_

in fhoropropenes~

For tie Gsomer (I&z): 6, = 1216 ppm, 6 209-9 ppm J,= 7-4 Hz; Z-isomer (Ia): 8:: 123.7 ppm 6, = 216.6 ppm, JFF = 8.9 Hz The chemical shift for l-difluoropropene is 129-6 ppm (E-isomer)_ 1315 -ppm (Z-isomer) 113) and for 3-fluoropropcne 216-O ppm [14] or 216-7 ppm [15]-

3_ Theoretical approach The coupled Hartree-Fock (CHF) perturbative approach to the calculation of spin-spin couphngs was used throughout_ It employs three perturbative hamihonians, two of them named after their classical counterparts (SO and SD), being the remaining one the Fermi contact (FC) hamiltonian [16). The assumption of isotropy of the sampie yields three additive contributions to the isotropic coupling. one from each hamiltonian. This perturbative approach is easy to understand in terms of propagator theory_ where the CHF calculation of J couplings is reduced to the construction. inlrersion and contraction of a certain matrizx [9] whose indices represent electronic excitations_ Since the HF theory is the assumed background of CHF, no sensible result can depend on the particuhu choice of occupied and/or unoccupied orbitals_ Thus. a suitable unitary mixing of occupied (and of unoccupied) orbitals can always be performed_ If this mixing is chosen in order to obtain a -“local” subset of- orbit&, i-e_ certain orbitals which reflect a specific property of the system such as -to belong to a given region** or -to belong to the z-electronic systems, a “Iocal~ CHF contribution to J can be defined through inner projection of the transformed matrix_ Also. a non-local contribution can be obtained as difference between total and local J couplingsMany transmission mechanisms can be represented by specific partitions. Two common ones are (a) the through-bond/through-space partition. where the local orbitals are those belonging to a certain spatial area containing both -interacting nuclei and their envirotqnetits,~ and (b) the u-z partition (on intuitive grounds this partition is considered a sub-partition of the. through-bond coupling)), where the local orbitals are all molecu-

fax orbitr+s

excep1 l&r+ xpmsenting the ~&&u., . tronic system- -. In the. cases studied, b&low. both(a). & (b) partitions were performed and so a -qualitative. decomposition of J into two_ or three terms is discussed for the different- couplings. The proce-dure to obtain the unitary mixin+ artd subsequent inner-projections has been described elsewhere for a general problem (51 as well as for this particular study [8.9].

.

4. Results and discussion

Few clear patterns may be seen among the ‘JFF values for the compounds studied in this work_ In the l&difIuoroand 1,3,33_tetmfhxoro-propenes 1171.the cisoid coupling is greater than the transoid but the difference is small in the case of the less-fluorinated compound (see table 2 below)_ Neither isomer of 1;33-trifluoropropene has yet been s_ynthesized. The large difference observed for couplings in the tetrafluoropropenes seems to be typicA of their geometries, cf. 19.5 versus 6.7 Hz in 1.3,3.3-tetraIIuoro-2-trifIuoromethyIpropene and similar compounds shown in a compilation [12]_ The couplings in the a-fiuoro2-fluorotoluenes [18] have recently been reexamined 1191and the geometric dependence of 4JS has been elucidated_ These couplings are critically dependent on F-F distances (cf. ref- [Sl) and are laro,er where there are more fhtorines on the a-carbon. I-2. Theoretical ana&si.s In this paper only four-bond F-F couplings are anaIyzed. Because of the nature of the carbon framework, and following the notation used in H-H couplings. they can be considered to be aIlyIic [20). In I they are cisoid and in II they are transoid. Different transmission mechanisms are considered for each of the FC, SO and SD terms; for~compounds II they are the a- and 2-trans-~ mitted componeny whereas for compounds 1.the through-space component is alsO considered_ The

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&ou~&space corr&nents of non-contact : are .ver$ small_ For ‘ar~~alI-ci& conform&on:‘& : have been reported elsewhere [9] together with&‘corresponding FC term. For this reason,-.&se : values will not be discussed in de&l here:How: ever? they were not studied- before for other co& formations of the fluorinated methyl group_ In fig_ 1 the angular dependence of the through-space. component of the FC term versus the. rotation angle of the ffuoromethyl group is dispIay;ed forcompounds Ia and Ic. The corresponding plot for compound Ib is shown in fig. 2. where also the ‘5 component of the FC term and the total cisoid ~~ F-F couplinS (including alI three terms) are displayed_ It is observed that the through-space component for Ib lies between the corresponding values for Ia and Ic. It must be recaIIed that.thesame geometrical data.were used for calculations in all three compounds_ Hence. these differences arise only from an electronic substituent effect which was aheady .discussed [9]_ The angular dependence of the through-space components display& in figs 1 and 2 indicates that several throu&-space mech-. anisms are operating for F-F couplings, as previously discussed for H-H 1211 and F-H [5,22] couphnSs_ In this CaK, the main three throir&space mechanisms are: (i) the direct overlati of the electronic clouds surroundins .both interacting nuclei; (ii) a similar overlap between the rear-lobe of the C-F bond of the interacting methyl fluorine and the electronic cloud surrounding the F vinylic nucleus; and (iii) a superposition between the vinylic F orbitais with a C-X methyl bond other than that containinS the interacting nucleus. The first one is large and positive (0 = 0” -for Is Ib and Ic figs_ 1 and 2) in agreement with the experimental value of the peri F-F coupling in derivatives of peri-difluoronaphthalene ]23]- This contribution decreases rapidly when the methyl fluorine deppa.-ts from an all-& conformation. The second one is small and positive and it is completely inhibited when the other two atoti attached to C3 are also fluorine (cf. 6 = 186”. Ic.. fig- 1) This. behaviour parallels that of the corresponding C-F through-space coupling ]SJ_ The third one .is sigu@icant~~ only when X = H. and is-smaI1 and positive for 1% 6 = 120”, fig. .‘1 and becomg- more important in Ib, 0 = 240”, fig: 2. Therefore;‘ t-his path&y’->-ith

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Fig I_ IEm@-space almpOnent of the cisoid F-F coupling in I;1 (0) and Ic (in)_ e = 0’ sxamis fcr an awcis conforxEuion bauren

origin&s ,in a hyperconjugating effect $ C-F-bond and the arom+c zelectronic system [24]_ A s&iIarphenomenon dkfines the ‘; component of inter-benzylic couplings [25]_ Therefore, it can be assumed that the r-trans-. mitted component of allyiic F-F couphngs originates in a hyperconjugaating effect between .the C-F methyl bond and the vinyhc z-ekctronic system. Calculated IPPP vahxes.of these 7r component-c are shown in fig 3 for compounds Ia and Ic, whiie for compound Ib it his shown in fig 2, as already mentioned_ For IIa and IIc they are plotted in fog_ 4_ Several features of these z-Lransmitted components are worthy of coLnment_ WhiIe in compounds Ia fig 3 and IIa, fig. 4 these componems follow quite closely a sin’8 law. a slight assymmeLry is observed for other compounds. departing somewhat from this law_ This slight asymmetry can easily be explained on the following grounds: when a second C-F methyl bond is placed in all-cis conformation with respect to a vinyl C-X (X = H, F) bond, the steric repulsion be~ucen both proximate atoms enhances the abiiity to hyperconjugate of the C-F methyl bond to which the coupled flourine atom belongs. The same effect can expiain Lhe difference between both maxima in compounds Ib, fig 2, and in IIb (not shown). There is also an electronic substituent effect upon the z-transmitted components which is such that the maximum c-transmitted component couplings

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an intermediate C-H bond shows an interesting eIecLronic subsdtuent effect as it was also observed in C-F couplings [S]_ When X = F_ i-e- when the: pathway contains an intermediate C-F bond. the through-space transmission is notably inhibited. as it was ako obseLxcd in C-F couphngs [S]. MLhough the proper conformation weighing suggests (vide infra) that the TS mechanism is the less important, its consistent brhaviour stresses the predictive power of Lhe theoretical an+Gs_ The z-transmitted component of benzylic F-F

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couptiny in IIa (8) and IIc (O)_ @= 0” corresponds to a cis-tmns conformation betu-denboth interactingfluorine nuclei.

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Fig 5. a components of the Fermi contact term of tmnsoid F-F coupling in Ha (A);.IIb (m) and IIc (0). 8 = O* is taken as in fig_ 4.

oro-derivatives of isoquinoline and quintioline~ where they concluded that F-F inter-ring couattached to the methyl group. A similar subplings through an extended zig-zag path -are stituent effect was aiso observed for F-H coudominated by a z-electron mechanism. plings in fluorinated derivatives of toIuene !22]. Non-contact contributions are more important Besides, in IIa the maximum r-transmitted comin transoid than in cisoid couplings The main ponent is ‘- 1.5 Hz larger than in Ia, -showing a features of transmission mechanisms of these-SO similar behaviour tc.that of IIc as compared with and SD interactions are common to both types of IC. couplings The only difference worthy of mention The angular dependence of the cr-comionent of is the one arising in the throufi-space component .~ the FC interactions were reported before [6] for which was discussed in a previous paper [9]. In fig_ cisoid couplings (Ia and Ic), and in fig. 5 they are 6 the (r and CTcomponents of the SO term of theshown for transoid couplings (IIa, IIb and 11~). transoid coupling in_IIa is displayed_ For Ifb and The two trends are quite different. While in transIIc, trends do not differ much from that in II& oid cou$ings the u angular dependence is strongly although these contributions are less important in influenced by the number of ffuorine. atoms atthem_ In Figi 7 the c and z &mponent of thi SD tached to the methyl group, .in -c&id. couplings term are displayed_ Although this (r comI&ent is this substituent effect is only observed for a connot very important. it follows a quite interesting .. formation cis-trans (8= 1800)_ It is interesting to an extended &g-zag path Favours a trend: note that+ for cisoid couplings, the absolute value transmission of this kind. It is also observed that .. for -the all-cis conf&ration is notably her than the TZcomponent of the SD term does not. follow a. that for a cis-tranr~onzc These results indicate that plot zso smooth as that of the FC term:’ This in F.-F- couplingS the: IV%ule does -not- hold. as. _indicates that each of its tens&al components [9] lkxisely as it does: in H,H- cou$ings 1261.-This obey a diFferem angular dependence~. conclusion is experimentaIly:supported- by values to those described above . CaIcuiati~ns~similar reported-by Cassidei and Sciacqvelli [27j in-pet-flu- _ is smaller, the IarlIer the number of fluorine atoms

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magnitudes are sxnaIIer_ This smaller efficiency in each of these coupling pathways for the FC term can be understood on the following grounds: the larger aromatic C-C bond length as compared with the iinylic one renders a larger F-F distance for an aII-cis conformation, reducing the throughspace component; furthermor& the smaller s character of the aromatic C-C bond as compared with

that of the vinylic one, diminishes the efficiency The vinylic =-eIectronic 2 cr-typ2 tlans mission_ system is aIso more efficient for transmitting the for

Fermi spin information as dkussed

Fi= 6. o (I) and c [A) components of rhc orbital rcmt of the tmnsoid F-F cw!pling in 11~B = Oa is taken Y in fig_ 4. for Ia Ib and Ic were aIso cani& out for the analogous cr-fluoro derivatives of onho-ff uoro-

roluene (III) (see scheme I)_ It was found that each separate component follows a quite similar trend to the corresponding one in the analogous cis-fluoropropene (Ia Ib or Ic), ahhough the respective

before_ for

H-H couplings [28]. A few values are compared in table 1_ In table 2 the total (FC + SO i- SD) four-bond coupiings averaged assuming a free rotation of the f!uoromethyI group are compared with the corresponding cxperimen~cl vah~cs. The experimenta

results show a pronounced dependence of ?J= on the number of fluorines invoived [29], i-e_ larger J

for CF, than for CH,F_

This resuIt is not re-

produced by the average of calculated vahxes for aliphatic or aromatic compounds assuming free rotation of the fluoromethyl group_ However. there is no experimental support for this assumption. Aithough conformational studies in several halopropenes have been published, they do not refer to the same compounds studied in this workNonetheless if they are used with caution and they are complemented with an intuitive chemicaI a qualitative indication of how reasoning. spin-spin couphng calculations should be weighted to compare them with experimental values can be obtained_ For instance, to account for experimental trends in IIa and Ilc, it should be recalled thar in 3-X-propenes (X = halogen) two conformers were either observed or theoretically predicted with a C-H or a C-X bond eclipsing the double bond[30]_ On intuitive grounds a similar behaviour can be assumed for IIa_ Therefore, the main contributions to the averaged vaIue are those of conformations B = O”, 120” and 240”_ -The angular dependence of th2 total transoid F-F coupling in IIa and IIc is depicted in fig. 8: (Values. for IIb Lie somewhere in between those of IIa and IIce) For compounds IIc similar &sumptions Can be made on the preferential conformations of the_trifluor* methyl group_ As can be observed in fig. 8, confor-

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*’ AI1 vaIua arc in Hz ‘I VaIua in brackets are for B = 270’_ =’ Avenge. assuming free rotation of the fluoromethyl group. The FC ~tzrmis on& taken into xcoun~. d’ This a-or&_ =’ Taken from r&s_ [iS.19]. D Taken from rcf_ [17]_ h, Taken from ref. [29]_ mations of 120” and 240” contribute with vaiues notably lw=er than those of IIc_ This can explain the experimental result of the increase of the transoid F-F coupling when increasing the fluorination of the sp’ carbon The much larger cisoid F-F coupling in Ic than in Ia can be explained on different grounds. It can be expected that a conformation with a C-H bond eclipsing the double bond would be the preferential one in Ia. This means that in this compounds the main contribution to the weighted average may come from the 120” and %O” conformations_ On the other hand, in IIc the presence of the vinyl fluorine atom would render, as preferential, a conformation with none of the C-F methyl bonds Iying in the vinyl

plane, as reported in other halodcrivarives [31 j_ In this case the z transmission of the.Fermi contact term would be notably favoured (see fig_ 3) and the experimental trend in going from Ia to Ic c&n be accounted for at least in a qualitative b&e_ This reasoning can be applied to the (locally similar> compounds IIIa and 111~where it is also observed a dramatic increase-in ‘Jrr (see table 1). Further-

Table 2 Comparison berwccn aperimenral and rh~~~rcdcaI four-bond couplings in compounds I and II =’ Compound Ia. Ib IC IIa IIb IIC

fipuimcntal 8.9 c’ d) 173 =’ 7-4 =’ d> 8.75 =’

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afe: lb.4 Hz in VI [34]. and 11_?3 Hz in VII [35]_

Grants from the Argentine R&&UC h CouneiJ are gratefully acJcnowIedged The computational time provided -by SEYCAD (Data Processing Centre of Arpgeruine Navy) is also &anked_

intermediate observed value of compound IIIb can also be quaJitative accounted for considerino 8 = 240” and out-of-dane C-F con. formatio~as dominant_ SimiIar Izqe cisoid F-F couplings were reported in other compounds (see for notation scheme 2)_ For instant in IV a coupling of 172 Hz has been reported D21, ihilt in V it amounts to 22 Hz [33]_- AJtho& the functional groups attached to the vinyl moiety can produce an electronic substituent effect, the increase of 4-S Hz of this eisoid F-F coupling can be attributed mainly to the ti=er stcric interaction between the ttitkoromethyl group and the geminal vinyl fluorine atom in V than occurs with the corresponding H atom in IV. TJre experimental evidence points towards a similar khaviour for tJte anaJogous fJuorinated drrivatks of toluene 1181where botJi steric compression and the population of preferred rotamers can mediate the effects of substituenu_ E)ifferenees in the total averaged couplings in compounds II come from a different behaviour of the Fermi and o&&al terms. Thwz tren& are shown in table 3 where the average value of each component is sepzuxt+ shown Other experimenmore_

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(Got-donand Brzsh. New York 1985) p_ 239_ [7J J. Koa~cssski. Progr_ NMR !Qauy_ 11 (1977) 1; Ann_ [SJ Ati

NhIR Spatry_ 11(19S2) 81_ N&dIo and RH_ Contraas,

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J-P_ SIo;u5 J-M_ Tedder snd J-C Waiton. J. QKm. Sot Perkin TI (1975) 1846_ 111 J H Gahcb and W_ hKalfcr_ Hdv_ Chim Acta 55 (1972) [lOJ

2277_

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[12J J-W_ Em+-. I_ PbiIIips and V. Wxay. Progr_ NMR SpatQL10(1977) s5_ [13J RA_ Bcaudct ad J-D. ~dcschwidcr. J. twoI_ Spccuy_ 9 (1962) .-- -_ ;o_ --1141 F_I_ Wcigar. J_ org. Ghan 45 (19X8) 347’6_ [15J D-P_ Co-. I Tapinski and W_ Lwrynowi~ J. Op, Ghan 49 (1984) 3216. [16J J.E H~zmiman. Than-&cd foundations of danon spin resoluncc (Acadanic Press. Near Yor& 197s$ [17J RN_ Handdine. D-W_ Keen and A_E Tippins J_ Ghan sc_ c (1970) 416 flSJ DA Burgess =d I_D_ Ras Austsdian J_ Ghan 30 (1977) 1611_ [19J I-D_ Bx. DA Burgor S Bomb&. ML Bxon zuxi ML Wookock. AusuJlian J. Ckm 37 (l9S4) 1437. PJ XL, BarfrrM and B. Chhabx& Ghan Rev_ 69 (l&S) 757_ :

(l-4)

3536:

_ X_S.Matthews, Org. .Maga Rcson 18 (1982) 2.26:

It4JR-W~andT.Schdcfcr.Can.J.(3hem500972; PuIa&.Orgh¶a&cso~16~981)63~ -‘[m :kstandS_ [Za] E-LGkxha “d G. Jiidi. Ckm Rev_ 77 (19-n) 599. [27l L. Cassidci and 0. Sciaco+li, I Ma@- Rcson 44 (1981)

-340.

[28j J_C Face& RK Contruas, D-G. a, Kowakki. VJ_ KowaicaJki and RN. Piqaiz. J. MoL Structe 94 (1983) 163. Its] H_S_ Gkwsky zmd V.D. ~Mochel. J. Chca Pfqx. 39

D. Bhaumik and GS. Kastha. Ind@ J_’PIIF AppL.Pfiyx y : 15(197i)s20; ~_... -AY_ Meyer, J. Comsur U&L 1 ;i98p)-Zll. -. -.- : _fi. _I-1311S-H. Schci dnd K Ha8a1. J. Mel SL+CL 116311951)2ti9;. : SH Shei’and R seip, Acta ChemmSand_ 38+(198e) 3+5_._-. [321 RE Banks. W-D_ D_avi&. RN_ Ha_s#dine and D_R -_ Taylpr. J. FiuorincChcm 10 (1977)487_ : [33] R-D- Chainbcrs, A Parkin &id RS_ Mauhes~ J_ : : Sot Perkin TIZIS. I 0976) 2107__ [34] RA Efekker;Sh O_ Badanyan. G-G. Mel&“+ zind-Ii, Knuns’;iny J. Op, Ckm USSR 13 (1977) 1461. [35) DA Burgas. I-D_ Rae z&d JD_ S&&l. &swliazt J_. than 30 0977) 543_

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