7/30/2019 anie.199510211 Preparation, Structure, and Reactivity of 1,3,4- Triphenyl-4,5-dihydro-lH-l,2,4-triazol-5-ylidene, a

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COMMUNICATIONSPreparation, Structure, and Reactivity of 1,3,4-Triphenyl-4,5-dihydro-lH-l,2,4-triazol-5-ylidene,a New Stable Carbene**Dieter Enders,* Klaus Breuer, Gerha rd R aabe,Jan Runsink, J. Henri que Teles,* Johan n-Pe ter Melder,Klaus Ebel. and Stefan Brode

Carbenes are among the most intensively investigated reac-tive species in organic chemistry.['] For many decades chemistsaccustomed to the quadrivalence of carbon were intrigued bytheir divalent nature, so there were many attempts to preparecdrbenes and to "store them in bottles".[z1 Despite this effort,access to these compounds was impossible for a long time be-cause of their high reactivity. The isolation of carbenes was onlyseldom accomplished in inert matrices at low temperatures.However. Bertrand et al.[31 and Arduengo et aI.r4] recentlyreported the preparation of compounds stable at ambient tem-perature, which can be formulated as carbenes.['] Yet doubtswere raised about the extent of the carbene character ofBertrand's phosphinocarbene,[61 ince it reacts distinctly like aphosphaacetylene.['IWe wish to report a new heterocyclic carbene, the 1,3,4-triphenyl-4.5-dihydro-1 H-I ,2,4-triazol-5-ylidene (1, Scheme I) ,which is stable up to 150C-according to DSC/TG experi-ments (DSUTG =differential scanning calorimetry/ thermo-gravimetry) unspecific decomposition begins at 150 "C-andhas the typical reactivity of a nucleophilic carbene stabilized byheteroatoms in the 1,3-p0sitions.~'~~~

N"PhPh Ph Ph

1Scheme 1. Triazohnylidene 1 and its resonance structures.

Carbene 1 was prepared by reaction of 1,3,4-triphenyl-1,2,4-triazolium perchlorate (2 ) with sodium methoxide in methanol,affording 5-methoxy-I,3,4-triphenyl-4,5-dihydro-l-I ,2,4-tria-zole (3). Upon heating to 80 "C 3 decomposes endothermically(AH = 33 kJmol-' by DSC/TG) with concomitant eliminationof methanol to form carbene 1 (Scheme 2).

rh aOMe, ph 8o"C, ?l0.1 rnbar N"quant.

Ph Ph Ph2 3 I

Scheme 2. Preparation of carhene 1.

[*I Prof Dr. D. Enders, Dip1.-Chem.K . Breuer, Dr. G. Raahe, Dr. J. RunsinkInstitut fur Organische Chemie der Technischen HochschuleProfessor-Pirlet-Strasse 1, D-52074 Adchen (Germany)Telefdx: In t . code +(241)8888-127Dr. J. H. Teles. Dr. .I.-P. Melder, Dr. K, hel, Dr. S. BrodeBASF AG, AmmoniaklahoratoriumD-67056 Ludwigshafen (Germany)Telefax: I n t . code +(621)60-56116[**I This work w a s supported by the Fonds der Chemischen lndustrie and theDeutsche E'orschungsgemeinschaft SFB 380, Leibniz-Preis). We are obliged toDr. Nigel Walker, BASF AG. for the X-ray structure determination of 3.

In the 13C NM R spectrum of 1 the carbene carbon gives riseto a characteristic[41 trongly low-field shifted signal at 6 =214.[']The chemical shifts of 1 were calculated by the GIAO-SCFmethod. For the parent heterocyclic system, the 4,5-dihydro-1H-I ,2,4-triazol-5-ylidene l', C,N,H,; the three phenyl groups werereplaced by hydrogen atoms), a GIAO-MP2 calculation with anextended basis set was carried out."'] This was used to correctthe chemical shifts of the carbon atoms in the heterocyclicmoiety, since electron correlation was neglected in the calcula-tion of the chemical shifts of 1. A very good agreement betweenexperimental and calculated chemical shifts was found(Table 1).

Table 1 . Selected experimental and calculated I3C N M R chemical shifts for triazo-linylidene 1.Carbon atom 6(HF/dzp) [a] 6 (MPZ/tzp-corr.) [b] 6 (exp.)C1 238.7 213 214.6c 3 149.3 148 152.1CIA 142.8 ~ 142.5ClB 128.2 - 127.3c1c 140.5 ~ 139.8[a] The chemical shifts for 1were calculated with the GIAO method at the HF/DZPlevel on an ah initio HF-optimized geometry [101.[b] The correction was carried outby calculation of the chemical shifts of 1' with the GIAO method at the HF/DZPlevel o n a HF/DZ P-optimized geometry and a t the MP2/T ZP level using a MP2/TZP-optimized geometry [ lo]. The difference attributable to electron correlationwas subtracted from the corresponding chemical shifts of 1 calculated at the HF/DZP level of theory.

In the 'H NMR spectrum no significant line broadening ofthe signals of the aromatic hydrogen atoms was found at tem-peratures as low as - 00 "C. The rotation of the phenyl groupsappears to be unhindered at this temperature. Yet the HFjSCFoptimized geometry of 1 shows that the two adjacent phenylgroups are twisted out of the plane of the triazoline ring, whichmakes a geared rotation of these phenyl groups likely. Singlecrystals of 1 suitable for X-ray structure determination wereobtained by layering a solution of 1 in toluene with pentane;crystals of 3 were prepared by crystallization from methanol.An ab initio calculation at the MP2(fu11)/6-31+G* level oftheory['31 was carried out for the parent heterocyclic system 1'in order to gain some insight into the equilibrium structureof 1 unperturbed by the crystal field.['41 Furthermore, thestructure of 1,2,4-triazolium ion 2' (C,N,Hf ) derived from1' was determined at the MP2(fu11)/6-31 +G* level of theo-ry." ' 1The structure of 1 in the solid state (Fig. 1 ) features a bondangle of about 100" at the carbene carbon C1, which is typicalof singlet carbenes.['61This angle is significantly smaller than thecorresponding one calculated for 2' (105.0").The phenyl groupsat N3 and C2 are twisted out of the plane of the triazoline ring,while the phenyl ring at N1 is virtually coplanar with the hetero-cyclic rin .The C1 -N1 and C1 -N3 bond lengths (1.351(3) and

for single bonds, which suggests a significant interaction of theoccupied 2p orbitals of nitrogen atoms and the unoccupied 2porbital of the adjacent carbene carbon atom in 1 (see Scheme 1 ) .This stablizing interaction should be rescinded by coordinativesaturation of the carbene carbon and should cause a lengtheningof the bonds. In fact, the corresponding bonds in 3 (1.443(7) Afor both C2-N3 and C2-N1) are clearly longer (Fig. 2) thanthose in 1.On the other hand, the C2-N2 bond (I .304(3)A) in 2is only negligibly longer than a typical C-N double be-

1.373(4)R, espectively) are considerably shorter than expected

A n g e w . C hrm . I r i l . Ed . Engl. 1995, 3 4 , N o . 9 0 CH Vrrlagsgesel/schufr m b H , 0-69 451 Weinhelm, 19Y5 0570-0833~95/0909-1021 10.00+ .3XO 1021

7/30/2019 anie.199510211 Preparation, Structure, and Reactivity of 1,3,4- Triphenyl-4,5-dihydro-lH-l,2,4-triazol-5-ylidene, a

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COMMUNICATIONSp c ,

CSB

Fig. 1. Crystal structure of 1 (SC HAKAL representation) [ I f ] . Selected bondlengths [A] and angles ["I: C l- N l 1 .351(3). C1LN3 1 .373(4), C2-N2 1 .304(3),N I- N2 1.398(3), C2-N 3 1.391(2), Nl -CI -N 3 100.6(2).

Fig. 2. Crystal structure of 3 (SC HAKAL representation) [12]. Selected bondlengths [A] and angles ["I: N l- C 2 1.443(7), C2-N 3 1.443(7), N4-C5 1.296(6),N3-N 4 1.394(6), N l- C 5 1.391(6), Nl-C 2-N 3 100.1(4).

ing barely longer than the C5-N4 bond in 3 (1.296(6) A ). Fur-thermore the NI -N 2 and C2-N3 bond lengths in l are verysimilar to the corresponding bond lengths in 3. As a rule, bondlengths determined by X-ray crystallography tend to be appar-ently shorter than their theoretical values.['81Provided that thiseffect is negligible or comparable both for 1 and 3, one canconclude that the interaction of the C2-N2 bond and the NI -CI-N3 unit in 1and the corresponding unit in 3 is weak for bothsystems. Meanwhile the ab initio calculations yield a slightlylonger N2-C2 bond for 2' than for l',while the C1 -N1 and theCI-N3 bonds in 2' are shorter by 0.2-0.3 A. One should bewary of overrating these results given the positive charge of 2',yet they suggest an increased interaction of the lone pairs at thenitrogen atoms with the empty 2p A 0 a t C1. Arduengo et al.attributed the stability of similar systems to kinetic factors, name-ly , to a hindered attack of nucleophilic species due to the accu-mulation of electron density at the carbene carbon atom itselfand in its vicinity.["] Neither ou r crystallographic nor ou r theo-retical results permit us to judge the significance of these factorsfor the stability of the system examined in this work. Yet thedifferences between the bond lengths in the NI-CI-N3 unit in 1and those in the corresponding unit in 3 indicate that 2p-2pinteractions between the carbene carbon atom and the adjacentnitrogen atoms play a significant role in the stabilization ofcarbene 1 .Triazolinylidene 1 reacts with alcohols such as methanol andethanol by insertion into the 0 - H bond to form the 5-alkoxytri-

ROHquant. *

I 68%

Ph

Ph bh3 R = M e , 4 R = E t

Ph

Ph5

PhNN C02Me

PhA N K C O , M ebh6

0 2 or s830%,quant. p h q xh

7 X=O. 8 X =SScheme 3 . Typical reactions of carbene 1.

azolines 3 and 4 (Scheme 3). Insertion into the N- H bond ofamines occurs similarly. Thus the reaction of 1with morpholineyields the 5-morpholinotriazoline 5 . The carbene 1 reacts withethyl fumarate or ethyl inaleate to give methylenetriazoline 6.['01A cyclopropane system is presumably formed as the primaryintermediate of the [2+ ] cycloaddition, leading to the morestable ally1 system by ring opening and subsequent 1,2-Hshift.[21]The instability inherent in the cyclopropane spiro sys-tem can be traced back to the considerable ring strain and the"push-pull"-type substitution pattern of the cyclopropanering. AM1 calculations1221 ield a strongly negative reactionenthalpy ( A H =-76 kJmol-' ) for the rearrangement of theprimary cyclic product to the methylenetriazoline 6 and there-fore confirm the experimental results. Triazolinylidene 1 alsoreacts with oxygen with formation of the corresponding triazoli-none 7. Reaction with sulfur analogously leads to the triazo-linethione 8.The X-ray data , ab initio calculations, and the reactions pre-sented in this work show that compound 1 is a new stable car-bene displaying a marked nucleophilic reactivity.Experimental Procedure3 : Sodium methoxide (1.4 g, 26 mmol) dissolved in methanol (30 mL) was slowlyadded to a solution of 1,3,4-triphenyl-1,2,4-triazoliumerchlorate (10.0 g, 25 mmol)in methanol (150 mL) . The methanol was removed under reduced pressure at am bi-ent temperature, and the residue extracted several times with ether. The solvent wasremoved from the combined ether layers under reduced pressure at ambient temper-ature. Crystallization from methanol yielded 3 as pale yellow crystals. Yield: 5.2 g( 6 3 % ) ; m.p. 80 "C (decomp.); analysiscalculated for C,,H,,N ,O: C 76.6, H 5.8, N12.8, found C 76.5, H 5.8. N 12.6; 'HNMR (300M Hz, C,D,. 25"C, TMS):6 =7.70-6.80 (m, 15H. arom. H), 6.58 (s , IH , OC (H)N) , 3 .02 (s, 3H , OCH,) ; 13CNMR (75.4 MHz, C,D6, 25'C. TMS) [23]: 6 =145.1 (C-S), 142.7 (ipso-C). 140.7(~P su- C) , 29.5 (C-,), 129.2 (C",), 129.1 (C&,) ,128.6 (C*,), 127.8 C,,), 125.0 (Car),123.1 (C ar) , 20.5 (C J, 113.5 (C%,), 01.0 (C2), 46.9 (C7); IR (KBr): i. =3049 (m),3000(m),2952(m ). 2928(m ).2905(m), 2816(m), 1597(s) , 1567(s) , 1554(s) , 1505( s ) , 1494(s). 1458 (s), 1446(m). 140 8(s). 1382( s), 1358(s), 1287 (m) , 1261 (s), 1160(m). 069 (m), 1053 (s). 1038 s ), 101 1 (s).91 4 (s).773 (s),747 (s), 693 (s) cm- ;MS( 70eV ) : m/ r (%): 329 (0.21) [ M ' ] , 298 (2.73) [M+-CH,OH] , 83 (100).1: 3 (1.0 g, 3. 0 mmol) was heated at 80C in vacuo fo r 16 h. Yield: 0.9 g (100%);m.p. 150'C (decomp.); analysis calculated for C,,H,,N,: C 80.8, H 5.1, N 14.1,found C 80.5, H 5.2. N 14.0; 'H NMR (500 MHz. C,D,. 25 C TMS): d = 8.68 (d,

1022 c, C H ~ ~ r l u g . g e s e ll s r h u f rbH, 0 -6 9 4 5 1 Weinheim, 1995 0570-0X33/9S/O909-1022 $ lO.OO+ ,2510 Angew. Chem. In t . Ed . Engl. 1995, 34 , N o . 9

7/30/2019 anie.199510211 Preparation, Structure, and Reactivity of 1,3,4- Triphenyl-4,5-dihydro-lH-l,2,4-triazol-5-ylidene, a

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COMMUNICATIONS2H . " J ( H H ) =7 .5 H L) , 7 . 35 (m , 2H). 7 .25 (br . t . 2H. ' J (H.H) =8 Hz), 7.21 (m.2H ) . 7 .06 ( t t , 1H . ' J ( H ,H ) =7 .4 H z and 4 J ( H .H ) = 1 . 2 Hz). 6.98-6.89 (m. 6H);I3C NM R (12.5.6 MH r, C,D,. 25C . TM S) 1241: 6 =214.6 (Cl) . 152.2 ( C 2 ) . 142.5(C IA ). 13Y.X (CIC) . 129.9 (C4B). 129.5 (C2B. C6B), 129.4 (C3A. C5A ). 129.1(C3C. C 5 C ) . 128.6 (C3B, C5B ). 127.9 (C4C ). 127.3 (C lB ) , 127.0 (C4A ). 126.7( C 2 C 3 C ' 6 C ) . 20 .2 ( C2A .C6A ) : lR ( K Br ) :C = 3046 ( w ). 1596 ( s ) , 1552 ( w ) . 1494(5). 1460( w ) . 1 4 4 8 ( m ) . 1359 (s ) . 1317( m) . 1307 ( m ) . 1215 (m) , 1152 ( m) . 1074 ( s ) ,Y87 (s). Y4Y ( m ) . 93 3 (m) . 914 (m). 782 (s).769 (s), 760 (5). 711 (s), 694 (s) c r n - ' ,MS (70 C V ) : 171 z ("A):297 (56) [ M I. 296 ( 7 3) [ M i - l ] . 194 ( 100 ) [ M +- P hN C] .180 (7). 104 (71. 91 (28). 77 (51) : calculated for CZ VH ,,N ,[M'] 297.1266, found297.1267. Received: December 27. 1994 (Z7590IE lGerman vers ion: At igeu. C/wm. 1995, 1/17. I 1 19Keywords: carbenes . cycloadditions . heterocycles . insertions

M c ~ i h n d i ~ n r g . Chetnie ( H ouh t v -Woyl),4th ed.. 1952-. Vol. E19b. 1989, pp.84 .W. Kirmse, Carhww, Curhet70ide i d orbenunulogu, 1st ed.. Verlag Chernie,Weinheim, 1969. p. 6 ff.a ) A . I g a u . H . Grutzmacher . A . Baceiredo, G. Bertrand, J. A m . Chem. Suc.1988, 110. 6463, b) A. Igau, A. Baceiredo, G. Trinquier, G. Bertrand, Angew.C/wtn. 1989. 101. 617: Atigew. Ch ~ t ~ i .n / . GI. f i g / . 1989, 28. 621a ) A . J Arduengo 111. R. L. Harlow. M. Kline. J. Am . CI7em. Suc. 1991, 113,361;b) A . J. Arduengo 111. H. V. R. Dias, R. L. Harlow, M . Kline. ibid. 1992,114. 5530.M. RegitL. Angeiv . Chcm. 1991. 103.691;Ange i i . .Clicwi. I n / .Ed . ngI. 1991.30.674.21) D. A. Dixon. K. D . Dobba. A. J. Arduengo, 111. G. Ber trand. J. Am . Ch r m .Snc 1991. 113. 8782; b) M. Soleilhavoup, A . Baceiredo. 0. Trentler. R.Ahlrichs. M . Nieger, G. Bertrand, ihid. 1992. 114. 10959.a ) G . Sicard.A. Baceiredo, G. Bertrand, J.-P. Major al, Angcw. Chem. 1984, 96,459; Anpeii. Chctn. 1171.Ed . Engl. 1984.23. 459; b) A. Baceiredo, G. Bertrand.JP M ajoral. G. Sicdrd. J. Jaud, J. G aly , J . A m . Chetn. SOC.1984, 106, 6088:C) R. Dagaili. (%em . rig. NeM'S 1994. 72(18) . 20 .a ) H.-W. W.inzlick. E . Sch ikor a, A n f i ~ w . hem. 1960. 72,494; b) H.-W. Wan-zlick. ihid. 1962, 74 , 129; An g r w . Cheni. Int. E d . Engl. 1962, I , 75; c) G.Scherowsk) , H. Matloubi , Liebigs A n n . C/7c,m.1978.98:d) H. Balli. H Griiner.R. Maul . H Schepp, He l v Cliini. A~,rrr1981, 6 4 . 648.The assignmcnt of the corresponding " C N M R signal was confirmed by I3Clabeling of the carhenc carbon atom CI .The geom etr) optim izations were carried out at the HF!SCF or MP 2 level oftheory with the parallel version of TURBOMOLE: S. Brode, H. Horn. M.Ehrig , D. Moldrup, J. E. Rice, R. Ahlrichs, J . Cotnp. Chem. 1993. 14 . 1142.Optimized SV - , SVP-. DZ- , DZ P- and T ZP bas is sets were used: A. Schaefer.H. Horn, R. Ahlrichs. J. Clirm. P/?ys.1992. 97, 2571. The chem ical shifts werecalculated by mcans oC t he G I A O method . J. G aus s , Chem . P h w . k i t . 1992,191. 614; J. ( 'hem. P/ZJ.S .993, 99. 3629.Crystal dat.1 for I : C2,H, ,N, , M, =297.36. crystal size 0.3 xO.4 x0.8 m m ,monoclinic . \pace group P2 J u (No. 14) , a =15.796(2), h =5.622(2). c =1 8 .8 3 2 (2 ) A ./ I =1 1 1 .3 8 6 (4 ) . F o r a ce l l v o l n m e V = 1 5 5 7 . 2 A 3 a n d Z = 4 t hedensity pL., ,L,,=1.268 gcrn- ' is obtaine d. Total num ber of electrons per unit cellF(O00)=624 The data was col lected on an Enraf-Nonius CAD 4 dif fractome-ter, 52/26 scans. 7 =293 K. graphite monochromator . A(Cu,,) =1.54179 A ,[ I =5.63 em I . 3899 measured a nd 3310 independent reflections ( h :- 9-18,k : 0-6./.0-?3). 480 observed ( />2u(I)) , , =0.017(14). Om,, =73.2 ' . Thes tructure wa s solved by direct methods (GENSIN. GEN TA N from XTAL 3.2.X T A L 3.2 R