J. Org. Chem. 1983, 48, 614-615
614
(M = Ge(CH3)3), 83269-42-5; cis-I (M = Sn(CH3)3), 74089-88-6; 83269-44-7; trans-I (M = trans-I1 Sn(CH3)3), (M = 74089-89-7; Ge(CH3)3),cis-I1 83269-45-8; (M = cis-I1 Ge(CH3)3), (M = Sn(CHJ3),83269-39-0;trans-II (M = Sn(CH3)J,83269-40-3;cis-I11 (M = Ge(CHJ3),84454-70-6; trans-I11 (M = Ge(CH3),),8445471-7.
/
b=&i:yq
/ C H $ \ \ A q n T H1 S 2 70
(W,/2*15
Hsy,IZ=lZ
Hr)
Hz)
Geoffrey Wickham, William Kitching* Department of Chemistry University of Queensland Brisbane 4067,Australia Received September 16, 1982 HSo
I11
Electrophilic Substitution with Allylic Rearrangement (SE'), Syn Stereoselectivity in Sulfur Dioxide Insertion into Some
\
A
Cyclohex-2-en ylstannanes
Summary: Sulfur dioxide insertion into cis- and trans(5-alkylcyclohex-2-eny1)trimethylstannanesis demonstrated to be y regio- and syn stereospecific, in contrast to the y regiospecific but anti stereoselective trifluoroacetolyses of these stannanes.
Sir: In the previous paper' we demonstrated a pronounced anti stereoselectivity in trifluoroacetolysis of cyclohex-2enyl metallics and speculated that syn selectivity may be observed when participation by a nucleophilic appendage of the (electrophilic) reagent was possible. Sulfur dioxide insertion into carbon-tin bonds has been well studied and leads to 0-sulfinato derivatives2v3with complete rearrangement of the allylic moiety4 (eq 1). Second-order (CH3)3SnCti2CH=CHCH3
t SO2
-
C H3
I
CH2=CHCHSOSnJCH3)3
II0
AH
+o;
(1)
kinetics5 (for methanol solvent) have been demonstrated for some systems, and substituent effects5 and other feature$ support an electrophilic mechanism possibly involving significant 0.-Sn interaction in the transition state. We report that SO2 insertion into some (cyclohex-2eny1)trimethylstannanesproceeds with syn stereoselectivity (if not stereospecificity). The various stannanes (in CDC13) reacted rapidly and quantitatively with gaseous SOz to provide viscous oils or solids which were fully characterized as O-sulfinato monoinsertion products by analyses, IR spectra, and 'H, 13C, and '19Sn NMR spectra. Full details will be presented elsewhere. In the present context, the most notable feature of the 300-MHz lH NMR spectrum of the parent cyclohex-2-enylsulfinate (I) was the appearance of the proton shown in A at 6 2.71 ( Wl/2 = 15 Hz), indicative of a predominantly pseudoequatorial proton.6 (1) Wickham, G.; Kitching, W. J. Org. Chem., previous paper in this issue. (2) For a review, see: Kitching, W.; Fong, C. W. Organomet. Chem. Rev., Sect. A 1970, 5, 281. (3) Sulfur dioxide insertion into transition metal-carbon bonds has also been reviewed Wojcicki, A. Adu. Oganomet. Chem. 1974, 12, 32. (4) Fong, C. W.; Kitching, W. J. Organomet. Chem. 1970, 22, 107. (5) Fong, C. W.; Kitching, W. J.Am. Chem. Soc. 1971,93,3791. Fong, C. W.; Kitching, W. J . Organomet. Chem. 1973,59, 213. (6) For analogous examples see our discussion of the 'H NMR spectra of cyclohex-2-enyl metallica in; Wickham, G.; Young, D.; Kitching, W. J. Org. Chem. 1982, 47, 4884. Wickham, G. Ph.D. Thesis, University of Queensland, 1983.
0022-3263/83/1948-0614$01.50/0
Reaction of 71:29 trans-/cis-(5-methylcyclohex-2enyl)trimethylstannane'v6 provided an insertion product, the 13Cspectrum of which consisted of two sets of signals in the ratio of ca. 7525 (comparison of analogous signal intensities). The 300-MHz 'H spectrum showed (in part) two doublets for CCH, (6 0.94, J i= 6.4 Hz; 6 1.01, J i= 5 Hz), with the higher field signal being more intense (ca. 7030). The major sulfiinate showed a type-B proton signal \
/c
, ! \ so * s "
B at 6 2.70 (Wliz i= 1 2 Hz; 70%), with the minor sulfinate signal at 6 2.87 ( Wl i= 28 Hz). The major sulfinate, with the narrower type-b proton signal, must be trans (see 11) as this largely quasi-equatorial proton can experience no large uic-'H coupling. However, in the cis isomer (111)HI will experience a large vicinal coupling to H6a. This conclusion is supported by the C5chemical shift (25.28 ppm) in the major isomer (11)where y-compressional shielding (by S02Sn(CH3)3)can operate in the trans case but not in the cis (C5shift of 27.98 ppm). Repetition with a 70:30 trans/& stannane mixture provided a 66:34 trans/cis sulfinate mixture. Predominantly cis-(5-methylcyclohex2-eny1)trimethylstannane(&/trans ratio of 71:29) provided sulfiiates which on the basis of arguments presented above had a cis/trans ratio of 67~33.Repetition with 5941 cis/ trans stannanes provided 60:40 cis/trans sulfinates. These results (Table I) require syn insertion in each stannane isomer. Confirmation of allylic rearrangement in these cyclohex-2-enylstannane systems was demonstrated first by using appropriately 2H-labeled stannanes and direct 2H NMR analysis.'p6 Thus a 70:30 cis/trans mixture of (5methylcyclohex-2-eny1)trimethylstannane with the 2Hlabel ca. 58% at C3 (vinylic) and 42% at C1 (allylic) produced a 69:31 cis/trans sulfinate mixture with an 2Hdistribution of ca. 66% allylic (6 2.73)/33% vinylic (6 5.76). Second, (3,5-dimethylcyclohex-2-enyl)trimethylstannane was synthesized and on reaction with SO2 provided the tertiary allylic sulfinate regiospecifically, and 13CNMR analysis is consistent with stereospecific syn inserti~n.~ (Table I). (7) Determination of the cia or trans nature of tertiary allylic sulfinates is not straightforwardby 'HNMR (no type-A proton), but careful comparisons of the 13C NMR spectra of our collection of cyclohex-2-enyl sulfinates very strongly support the indicated stereochemistry. However, there is no doubt that complete allylic rearrangement occurs to yield the thermodynamically less stable tertiary allylic sulfinate.
0 1983 American Chemical Society
615
J. Org. Chem. 1983, 48,615-617 Table I. Stereoselectivity Accompanying Sulfur Dioxide Insertion into Cyclohex-2-enylstannanes ~
cis/
cis/
system
trans ratio 29:71 30:70 7 1 : 29
-
(Ch3
sulfinate
trans ratio
1
25:75 34~66 67:33
x
69:31
IaSnOpS
70:30
ordination by using methanol as the solvent were thwarted by an apparently rapid isomerization of the stannanes.1° Nevertheless, the present work demonstrates that the energy difference between syn and anti transition states is not so large that it cannot be manipulated by coordination, steric, and presumably other factors. Hence, it is not possible at this stage to formulate any generalization about the stereochemical outcome of a formally electrophilic y substitution of a a-allyl metallic. Full details of these and related studies with 4- and 6-substituted cyclohex-2-enylmetallics will be reported at a later date. Acknowledgment. This research was supported in part by the Australian Research Grants Committee, to whom we are grateful. High-field NMR spectra were obtained at the Brisbane NMR Center (Director, Dr. D. Doddrell; Operator-Consultant, Dr. P. Barron).
60:40 26:75
60:40 29:71 ICH3)3SnOZS
a
Based on 'H and I3C NMR spectra of the reaction mixRefers to (CH,),SnSO, relative to the 5-CH3.
tures.
(10)At equilibrium the favored isomer is trans-(5-methylcyclohex-2enyl)trimethylstannane, in which stabilizing u--T interaction is more effective. For another studv of raDid methanol-induced isomerization of organostannanes see: Leiuan, M. M.; Guillerm, G. C. R. Hebd. Seances Acad. Sci. Ser. C 268,858.
Scheme I
David Young, William Kitching* Department of Chemistry University of Queensland Brisbane 4067, Australia Received September 16, 1982
H
(W,,2f7
Hz)
The (5-tert-butylcyclohex-2-eny1)trimethylstannanes were synthesized and their configurations established as described in detail for the 5-methyl compounds.6 In particular, the cis stannane exhibited a vi~-"~Sn-'~C coupling (to C,) of 47.6 Hz, whereas the corresponding coupling for the trans isomer was 10 Hz, as required for the known dihedral angle dependence of this coupling and the conformations of these stannanetx6 The very clean reaction of these stannanes with SO2 provided the allylic sulfinates (Table I), exhibiting signals for type-A protons (300 MHz) at d 2.79 ( W1,, = 24 Hz) and 2.71 ( Wl = 7 Hz). As outlined above for the 5-methyl series, the s d n a t e with the narrower type-A proton signal must be trans. Additionally, the C5 chemical shift in the trans sulfinate, in which the shielding y-gauche effect of the S02Sn(CH3)3 moiety can operate, is to higher field (39.94 ppm) than the analogous signal (43.04 ppm) in the cis isomer in which the less significant (but slightly shielding)s y-anti effect operates. These data are summarized in Scheme I and collected in Table I. The above results contrast with the anti preference for trifluoroacetolysis and are consistent with the idea that accumulation of negative charge on oxygen, as the y-carbon-sulfur bond forms, permits stabilizing O-.Sn coordination in the presumed (cyclic) syn transition state.g Attempts to minimize the role of this intramolecular co(8)Rooney, R. p.;Evans, s. A. J . Org. Chem. 1980,45,180. (9)There is always the possibility that the "concertedness" of bond formation and bond cleavage in trfluorocetolysis and SO2insertion varies significantly, so that predictions based on "concertedness"are no longer valid. See: Anh, N. T. Chem. Commun. 1968,1089.
Electron-Transfer Reduction of Bis(gem -dihalocyclopropyl) Compounds. Evidence against the 1,4-Elimination-Type Remote Ionization of the Cyclopropylidene Radical Anion. Unusual Product Distribution as a Result of Reactions in the Region of Reagent Mixing Summary: The electron-transfer reduction of 4,4,8,8tetrachlorotricyclo[5.1.0.03~5]octane with a deficiency of potassium metal in an NH3/THF medium at -78 "C is reported. The only products of the reaction are starting material and the completely dechlorinated hydrocarbon, tricyclo[5.1.0.03~5]octane.No partially reduced products containing one, two, or three chlorine8 were detected. These results are explained by a model in which the reaction takes place in the region of reagent mixing rather than a remote ionization effect that had previously been proposed. Sir: The electron-transfer reduction of bidgem-dihalocyclopropyl) compounds is a reaction that presumably parallels the electron-transfer reduction of simpler gemdihaloalkanes. There have been several reports dealing with the mechanistic aspects of these reactions, and often these studies have revealed new and interesting chemistry.' There are data that illuminate some of the subtleties of the electron-transfermechanism, and there is now evidence for the cyclopropylidene radical anion2 Recently, Oku and co-workers proposed a new "1,4-elimination-type remote ionization effect" in the reduction of bis(gem-dihalocyclopropyl) systems to explain an unusual product ~~
(1)Oku, A.; Yagi, K. J. Am. Chem. SOC.1974,96,19661967.Oku, A,; Hino, T., Mataumoto, K. J. Org. Chem. 1976,40,695-699. (2) Sargent, G. D.; Tatum, C. M., Jr.; Kastner, S. M. J. Am. Chem. SOC.1972,94,7174-7176.
0022-326318311948-0615$01.50/0 0 1983 American Chemical Society
