3566 JONES, GOLLER,AND KAUFFMAN
The Journal of Organic Chemistry
Stereochemistry of the Addition of Methylcadmium and Methylzinc Reagents to 4-t-Butylcyclohexanone PAUL R. JONES, EDWIN J. GOLLER,‘~ AND WILLIAM J. KAUFFMAN’~ Department of Chemistry, University of New Hampshire, Durham, New Hampshire OS824 Received December 83, 1968 Reactivity and stereochemistry were used as criteria for comparison of the methyl reagents of Mg, Cd, and Zn toward 4-t-butylcyclohexanone. Characteristic differences in the behavior of all three were observed as a function of concentration and halide ion present. The greater amount of axial attack by Cd and Znre agents, compared with Mg reagents, is rationalized as arising from tighter, four-center transition states for the first two. The results fit a steric-approach control consistent with models proposed by Marshall and Felkin.
Although organocadmium and -zinc compounds are well known as reagents for the synthesis of ketones, very little has been established about their structures in solution. The mechanism of their reaction with acid chlorides, anhydrides, and other carbonyl groups is not understood. The observation by Kollonitsch2 in 1960, that in situ organocadmium reagents react rapidly and efficiently with simple carbonyl compounds, served to dispel the notion3 that these reagents are significantly less reactive toward aldehydes and ketones than their Grignard counterparts. I n turn, this has required a reassessment of the scope and mechanism of the reactions of cadmium and zinc reagents with the whole array of functional groups, which are attacked by Grignard and lithium reagents: aldehydes, ketones, esters, amides, nitriles, nitro groups, etc. One approach to an understanding of the reactions of cadmium and zinc reagents with various functional groups is to compare their behavior with that of the Grignard reagent. In view of the careful studies of the stereochemistry of the addition of methyl Grignard reagents to 4-t-butylcyclohexanone (1) reported in 19624 and 196Zj5we have undertaken a detailed investigation of the same reaction where the additive reagent is a methylcadmium or methylzinc compound. As shown in eq 1, the reaction is expected to lead to a mixture of
1
Me
I
?H
3 cis
2
trans
M = Li, Mg, Cd, Zn
trans- and cis-6methyl-4-t-butylcyclohexanols (2 and 3), in which the alcohol function is, respectively, axial and equatorial. From previous ~ o r k , it~ ,has ~ been convincingly demonstrated that magnesium reagents attack 4-tbutylcyclohexanone from the less hindered side with (1) (a) National Defense Education Act Fellow, 1966-1969. (b) National Science Foundation Trainee, 1966-1969. (2) J. Kollonitsch, J . Chem. Soc., A, 453 (1966). (3) H. Gilman and J. F. Nelson, Rec. Trau. Chem., 66, 518 (1936). (4) W.J. Houlihan, J . Ow. Chem., 27, 3860 (1962). (5) H. 0. House and W. L. Prespess, ibid., 80, 301 (1965). (6) The trans and cis notation is in reference to methyl and t-butyl groups.
preferential formation of the thermodynamic product, the trans alcohol. Additon of hydride from a variety of metal hydride reagents, on the other hand, occurs predominantly via axial approach. These observations have led to two divergent views on the controlling factors in addition: “steric-approach” vs. “productdevelopment” control.’ Marshall and Carroll8proposed a model for the transition state, by which one could estimate semiquantitatively the magnitude of the steric effect on the basis of the transition state bond lengths. From this model, it has been rationalized that 1,3 (diaxial) interactions are less important than 1,2 (equatorial axial) interactions in the formation of a C-H bond, while, with the longer C-C bond being formed during Grignard addition, the importance of the interactions is reversed. A corollary of this hypothesis is the prediction that, within certain rather narrow limits of bond distances, the amount of axial attack will increase as the transition-state bond distance decreases. Cherest and Felkin9 have pointed out the importance of considering torsional effects as well. Recent results on hydride reductionlo can best be explained with the Mashall and Felkin models.
Results Results are compiled in Tables 1-111. Experiments were carried out in such a way that the number of transferable methyl groups was held constant. Thus, for 1 mol of ketone, there were introduced 2 mol of (CH&Cd or (CH&Zn; 4 mol of CH3MgX, CH3CdX, or CH3ZnX.” I n a few instances olefin formation, caused by the presence of zinc or cadmium salts, was observed, but the amount was well below the level required to affect the ratio of alcohols (See Experimental Section). Several general observations can be noted from inspection of the tables. 1.-On the basis of the values of unchanged ketone in comparable experiments, the following reactivity series can be written: CH3Li, CH3MgX > (CH&Cd > (CH3)2Zn> CH3CdX, CH3ZnX. (7) W. G. Dauben, G. J. Fonken, and D. S. Noyce, J . Amer. Chem. Soc., 78, 2579 (1956). (8) J . A. Marshal1 and R . D. Carroll, J . O w .Chem., 30, 2748 (1965). (9) M. Cherest, H. Felkin, and N. Prudent, Tetrahedron Lett., 2199 (1968); M. Cherest and H. Felkin, ibid., 2205 (1968). (10) E. L. Eliel and Y. Senda, Abstracts of 156th National Meeting of the American Chemical Society, Atlantic City, N. J., 1968, ORGN-138; J. Klein, E. Dunkelblum. E. L. Eliel, and Y. Senda, Tetrahedron Lett., 6127 (1968). (11) Under these conditions the reaction can be considered to involve RIM or RMX but not RMOR‘. In separate experiments it was demonstrated that methyloyclohexyloxycadmium (and -zinc),C HsMOCaHu, was considerably less reactive than (CHs)zM toward cyclohexanone.
Vol. 34, N o . 11, Noclember 1969
4-t-BUTYLCYCL0HEXANONE-MGj
TABLE I REACTION OF an situ ORGANOMETALLIC REAQENTS I N ETHER WITH 4-t-BUTYLCYCLOHEXANONE
Entry
Reagent
Concn, M
Unchanged ketone,
trans alcohol,
%"
%b
trans/cis
53.8c 1 . 2 CI&hfgI 0.8 1 CHaMgI 0.1 1 61.7 1.6 1 61.@ 1 . 6 CHaMgBr 0.8 1 68.4 2.2 CHsMgBr 0.1 1 63.7c 1 . 8 CHaLi 0.8 (CHs)zCd(I,I)d 0.4 51.6e 1 . 1 7.5 (CHs)ZCd(Br,Br) 0.4 51.56 1.1 50 (CHs)zCd(I,Br) 0.4 53.5 1.2 5 (CHa)zCd(Br,I) 0.4 53.10 1.1 5 (CHs)&d(Br,Cl) 0.4 50.2 1.0 55 42.46 0.74 5 (CHs)zCd(I,Cl) 0.4 (CHs)*Cd(I,C1) 0.9 5 40.7 0.69 (CH~)ZC~(CH~L~,C 0 .~4I Z ) 99 ( C H ~ ) Z C ~ ( C H ~ L ~ , 0C.:4~ B ~99 ~) 37.76 0.61 0.8 CHsCdX(1,Cl) 90 (CH3)zZn(I,I) 0.3 20 46.5 0.87 44.4c 0.80 85 (CHa)rZn(Br,Br)J 0.3 (CHa)zZn(I,Br) 0.3 20 46.8" 0.88 44 0.8 (CH3)zZn(BrJC1) 0.3 97 38.3 0.62 (CII3)zZn(I,Cl) 60 0.3 (CH3)2Zn(I,Cl) 0.1 38.7 0.63 65 99 (CH3)zZn(CHaLi,ZnIz) 0 . 3 CHaZnI(1,I) 0.3 90 49. O e 0.96 CHaZnX(1,Cl) 0.3 99 a % = areak,tons/(areaketon, areatrans+ Eta alcohols) X 100. b Normalized %: % trans % cis = 100; yield of alcohols = 100 - yo ketone. cFor comparable results a t similar concentrations see ref 4. d Halogens in parentheses indicate, respectively, the methyl halide from which RMgX was prepared and the metal halide used for the exchange (eq 2). Result of at least two separate runs with a maximum deviation of =tl'%. J Reaction time was 8 hr, and 3 molar equiv of zinc reagent was used.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
+ +
2.-Reactivity of the reconstituted reagent is lower than that of the comparable in situ reagent and found to be dependent on the amount of halide present. 3.-Reagents were more reactive to addition when Mg12rather than MgBrz was present. 4.-Reactivity of cadmium and zinc reagents prepared from methyllithium was nil. 5.--Monomeric CH3MgX (0.1 M ) gives more trans alcohol, resulting from equatorial attack, than the corresponding associated species (0.8 M ) . At the same concentration, CH3MgBr shows a greater preference for equatorial attack than does CH3MgI. By contrast, there is no appreciable change in reactivity or stereochemistry when the concentration of (CH3)2Cd(I,Cl) is increased from 0.4 M to 0.9 M , or when the concentration of (CH3):!Zn(I,Cl) is decreased from 0.3 M to 0.1 M . 6.-Preference for axial attack follows the series: (CH&Zn > (CH&Cd > CH3llgX. 7.-Contrary to the Grignard reagents, in all cadmium and zinc reagents except R2R'I(I,C1), the stereochemistry of addition was independent of the halogens or of their source. For R2N(I,Cl), the amount of axial attack was significantly increased (translcis decreased). %-The stereochemistry of addition was essentially the same for comparable in situ and reconstituted reagents.
-CD,
AND -ZN
ADDITIONS3567
9.-Variation of the amount of magnesium salt in the reagent had little effect on the stereochemistry until it was reduced to 1 molar equiv relative to ketone (compared with 4 molar equiv in the in situ reagent). The effect was most pronounced in the prior coordination experiments and with magnesium iodide, where the relative amount of axial attack increased (translcis decreased). See Tables I1 and 111.
Discussion Three mechanistic pathways for the addition of dimethylcadmium and dimethylzinc to 4-t-butylcyclohexanone will be considered: addition of the Grignard reagent, present in small concentration in the reaction mixture (eq. 2 ) ; addition of the cadmium or zinc reagent by way of some six-center transition state; and addition of the cadmium or zinc reagent by way of some four-center transition state. An important consideration in any mechanism is the degree of association of the reagents. On the basis of earlier work,lZalbwe assume that the Grignard reagents in 0.1 M concentration and the cadmium and zincl2C reagents in 0.3-0.4 M concentrations are monomeric, while the 0.8 M solutions of Grignard reagent are polymeric. Magnesium halides present in the cadmium and zinc reagents (0.6-0.8 41) are undoubtedly polymeric. Our reagents were prepared from the Grignard reagent according to the stoichiometry represented in eq 2. Although it can be argued that RRIgX might be
present by reversal of eq 2, this is contrary to the general ob~ervation'~that a metal-metal exchange between organometallic and metal salt proceeds in the direction of formation of the less reactive organometallic. Qualitative tests support the conclusion that Grignard reagent is present in very low concentration if a t all. The familiar Gilman color testI4 for R N g X was negative in every experiment. The characteristic color of a charge-transfer complex between either 2,2'biquinoline or 1,lO-phenanthroline and Grignard reagent'5 was not observed with our reagents. Although the increased reactivity of cadmium and zinc reagents in the presence of magnesium halide would be compatible with attack by the Grignard reagent, our stereochemical results do not support this mechanism. Inspection of the tables reveals that both cadmium and zinc reagents lead to more cis alcohol (less thermodynamically stable),I6resulting from axial attack, than do the Grignard reagents at low concentration. Indeed, the zinc reagents, in all but one case, gave cis alcohol as the major product. The leveling effect of % trans alcohol with increasing MgBrz concentration shown in Tables I1 and I11 would not be expected if the added salt were shifting eq 2 to the left. (12) (a) E. C. Ashby, Quart. Rev., 81, 259 (1967); (b) M. Abraham and P. Rolfe, J . Organometal. Chem., 7 , 35 (1967); (c) I
