TECHNICAL REVIEW
Skeletal and Metallotropic Rearrangements of Main Group Organometallic Compounds John J. Eisch Departmentot Chemistry. State Uoiversityot New York at Binghamton, Binghamton, New York 13901
he completed his un graduate work at A quette University and doctorate at Iowa S University with Henry man in 1956. After pi?stdoctoral study with Karl Ziegler at the Max P1,inch Institut fur Kohlenforschung in Mulheim, Ge rmany, and at the European Research Associates L aboratory in Brussels, Belgium, he was a member of the faculties of St. Louis, Michigan. and Catholic Uniuersitv. Since SeDtember 1972 he has been Chairman"and Professor of Chemistry at the State University of New Yorh at Binghamton. His research is chiefly concerned with structural and mechanistic organometallic and heterocyclic chemistry, in which areas he has published over one hundred papers and a book, "The Chemktry of Organometallic Compounds." ~
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Introduction The discovery of rearrangements in both organic and inorganic chemistry has been the cause of much initial frustration for the synthetic chemist. But these reactions that follow an unexpected course have served as a reminder that human expectations, even under the imposing name of chemical principles, remain fallible. Novel rearrangements thus have been the goad for many detailed mechanistic studies that have improved our grasp of how reactions occur. Such insights have guided the synthetic chemist in controlling the reaction path followed by his chosen reagents. Thus his initial frustration turns out to have been chemical opportunity in disguise. Rearrangements among organometallic compounds are numerous both as to type and occurrence ( I ) . With the broad structural classification of these compounds into uand a-bonded systems (2) one could subdivide the known rearrangements into isomerizing processes and derivatizing processes. In the former, a constant aggregate of atoms interconverts among various structural permutations; in the latter, an organometallic reagent (RM) of definite structure reacts with a substrate (A=B) to yield a derivative having an isomeric structure for the original organic moiety R or a different chirality at M. Several examples of isomerizing rearrangements, drawn from transition and main group metals, are given in eq 1-10.
(ref 5)
CHz
I I CH
CHa-C-BRx
-+
CH,-CH-CH,-BRz
I CH,
(4)
(ref 6 ) (5)
(ref 7) Ind.
Eng. Chem., Prod. Res. Dev., VoI. 14, No. 1. 1975 11
Me-fPCH2 Ni H2C.g
Et,P
f Ni\
,C-Me
EhP
(13)
CH?-CMe=CH,
-- - - - -coF,;e-,
,Fe
oc /I oc co oc
co (ref 16)
(ref 9) Me
(15)
(ref 17) It might be noted that the organometallic reagent, in certain cases, might be undergoing a rapid isomerizing process ( e . g . , fluxional character of allylic derivatives of alkali or alkaline earth metals or T-complexes of polyolefins with transition metals) prior to the derivatizing process (18).
11
R--.
*:
(ref 11)
.',0Et2
X\
-
R,
4
X
'.
*:
R,Mg..
'X*
\
,.0Et2
'
Mg
'OEtZ
The last example, that of the classic Schlenk equilibrium, is that of an isomerizing process occurring within a solvated associative aggregate (12). Illustrative of derivatizing rearrangements are the following (eq 11-15).
Furthermore, either type of rearrangement can be subdivided as to the changes undergone in the structure of R-M: (1) skeletal, in which the concatenation of atoms in R would change; (2) skin, in which the hydrogen or metal atoms would alter their sites of attachment on the same skeleton R ( e . g . , proto- or metallo-tropisms); and (3) orientational, in which a new, relatively stable, three-dimensional configuration would be imposed on either the R or the M center, while the same skeletal and skin relations would be maintained. Of the foregoing examples, eq 1, 3, and 10 would exemplify skeletal, eq 4, 5 , 6, 7, 8, 9, 11, 13, and 14 skin, and eq 2, 12, and 15 orientational processes. The copious amount of published research documents the high interest in organometallic rearrangements. Yet the variegated type of processes and the large numbers of metals make it highly unlikely that there are no new worlds to conquer. In the following discussion, we wish to survey recent results from our own laboratory bearing on the mechanisms and synthetic potential of certain rearrangements of main group organometallic compounds. As a convenient framework for the presentation, use will be made of the anthropomorphic classification of such rearrangements as skeletal, skin, and orientational. Factors in Isomerizing or Derivatizing Rearrangements Among the first recognized rearrangements of main group organometallic systems were the derivatizing process of benzyl Grignard reagents with formaldehyde (Tiffeneau reaction (19) eq 16) and the isomerizing processes
CBH~
CGHS
I CH,-Si-Cl I
R-Si
I -CH3 I
(ref 14) 12
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 1, 1975
in allylic metallics (eq 5) and in the composition of RMgX (eq 10). Even with the advent of modern spectroscopic tools (nmr, nqr), it has still not been possible to decide, in every case, whether an apparent derivatizing rearrangement (eq 17) proceeds via an isomerizing pathway (a) or not (b). If the structure of R-M can be defined before ex-
The highly polar and labile carbon-metal bond predisposes organometallic compounds to interesting carbon-skeletal and metallotropic rearrangements. Recent studies in our laboratory have explored the stereochemical and electronic factors in: (a) 1,2-, 1 3 - , and 1,4 shifts of organic or organoidal groups in main group organometallics; (b) base-catalyzed prototropic and metallotropic isomerizations; ( c j cyclizing isomerizations of open-chain organometallics; and (d) cis, trans interconversions of vinylic and cycloal kyl organometallics. The synthetic value of such rearrangements will be emphasized by discussing such reactions as: (a) the rearrangements of aryl-substituted hydrocarbons, ethers and amines; (b) the photochemical aryl migrations in aryl-aluminum and -boron systems, as well as the oxidative aryl migrations in tetraarylborate salts; (c) the 1, n shifts in silylalkyl carbanions; (d) the fostering and prevention of cis, trans isomerizations with organoaluminum compounds; and (e) the allylic rearrangements encountered with acenaphthenyl-, benzyl-, allyldihydropyridyl-, and allyl-metallic derivatives.
R'M
R-M
2 ' " R'-A-B-M patha
4P
LM
*
(17)
BJ
Donor solvents, especially of chelating character, such as 1,2-dimethoxyethane, N,N,N', N-tetramethylethylenediamine, and hexamethylphosphoric triamide (23), tend to favor the occurrence of both solvent-separated ion pairs (heterolysis) and electron-transfer processes (homolysis).
RQ MQ
path b posure to A=B, then the isolation of the isomeric derivative can be ascribed to a derivatizing process, occurring in statu reagendi (path b). The thermodynamic driving force for either kind of rearrangement obviously requires the formation of a structure of lower free energy. However, whether R-M will be able to form the most stable R'-M by isomerization, or the most stable adduct, R'-A-B-M, with A=B in a derivatizing process, is completely subject to kinetic factors. As a consequence, organometallic rearrangements will not always lead to the most stable products, but only to those products accessible by prevailing mechanistic pathways. This fact underlines the importance of understanding reaction mechanisms, if one hopes to harness such rearrangements for organic synthetic applications. The source of rearrangement reactivity is the generally polar and highly labile carbon-metal bond. The polarity of this linkage is not only greatly dependent upon the nature of the metal, but also on the hybridization state of the carbon (eq 18). Equally significant are the electronic
L, I
L2
4
L1 L2
L2 II
I11
characteristics of any ligands attached either to the carbon (L1) or metal (Lz) centers. Electron-attracting groups on carbon (L1 = F, RF, CsH5, C=O) and electron-donating groups on the metals (Lz = R, OR, NR2, or coordinated Lewis bases such as OR2, NR3, SR2, or PR3) would be expected to foster C-M bond heterolysis. Where the electropositivity of the metal or metalloid is low ( e . g . , Ge, B, Ga) and the bond rather weak (Hg, Cd, Sn), homolytic processes would become significant (I 111) ( 2 ) .However, a clean dichotomy into heterolytic and homolytic (freeradical) paths for rearrangements is not easily made, because carbanions can themselves unleash electron-transfer process (cf. the work of Russell (20) and Eastham (21) inter alios). Because carbon-metal bonds vary so in polarity, the reaction medium can have a profound, even decisive, influence on the ease of rearrangement. Highly ionic organometallics can exist as contact or solvent-separated ion pairs (eq 19) (22).
-
+ :D
D:M@
(19)
Even polar covalent organometallics are radically changed by donor solvents: ( a ) associative aggregates will be disrupted (eq 20)
and (b) the coordination number of the metal may increase (eq 21). (Ph,P),Ni
(Ph3P),Ni (ref 25)
+ Ph,P
(21)
Finally, then, there are certain basic mechanistic questions to be answered for any organometallic rearrangement. First of all, for an isomerizing process, does the reaction occur intramolecularily or intermolecularily? Secondly, is the principal pathway heterolytic or homolytic in character? Thirdly, do reactive intermediates, such as subvalent organometallics, (Rn-xM), organic elimination products (-CgE), carbenes, radicals or cyclic configurations, appear during the course of the rearrangements? Fourthly, from a standpoint of synthesis, how can the rearrangement or the nonrearrangement pathway be separately favored? The succeeding discussion will present some of our experimental attempts to address these questions. The variety and complexity of rearrangement mechanisms, however, mean that the mechanistic answers must be considered tentative. Skeletal Rearrangements These rearrangements can be suitably classified as substitution or addition processes. In the former, the nature of substitution can be specified as a l , n shift, where n is the terminus atom to which the R group moves (eq 22) (1).
In an addition process, a chain of atoms is interconvertible into a ring (eq 3 and 23). Skeletal Substitutional Processes. Many examples of substitution processes are known, such as the Wittig ether (A = 0, n = 2, C = R; reviewed by Schollkopf (26)), the Ind. Eng. Chem., Prod. Res.
Dev., Vol.
14, No. 1, 1975
13
w= Ch,
CH&1
Stevens (A = RzN+, n = 2, C = R; Stevens, et al. (27)) and the Grovenstein-Zimmerman (28) (A, C = R, n = 2; cf. Grovenstein, et al., ref 29) rearrangements. In further work on the last case, we were surprised to find that an esr signal is observed throughout the reaction, when conducted according to the conditions described by Grovenstein and by Zimmerman (eq 24) (30).
Although the reaction had been interpreted as proceeding via a carbanionic pathway (Grovenstein and Zimmerman), the esr signal does becloud this view. However, by generating IV in the absence of free metal (eq 25), we have shown that the sequence, IV V, depicted in eq 24 can be realized without the appearance of an esr signal (311.
-
dH2C1
(ref 30)
The cationic character of the pyridinium salt, in this situation, leads to a neutral, labile, but spectroscopically verifiable product. Pyridine derivatives also were used to test (30) whether an anionic mechan,ism were actually involved in the lithium metal-induced rearrangement (eq 24). In pitting 2- or 4-pyridyl groups against phenyl groups in a carbanionic process, one might expect that the pyridyl group would migrate preferentially, because the intermediate spiro anions (X, XI; charge on N ) are more readily formed than
XII .
VI
Thus, the occurrence of a genuine carbanionic process seems to obtain (eq 25), but one must be cautious about so concluding for metal-mediated reactions (eq 24). Such rearrangements between carbon centers, be they polar or radical, are intramolecular when R = C6H5 and partly intermolecular when R = C&,CHz in eq 22, as decisive labeling experiments have shown (32). Furthermore, for the intramolecular 1,z-phenyl shift, it is highly likely that a bridged dihydrophenyl anionic intermediate (VI) is involved. Where structural constraints prevent the attainment of such a spiro intermediate (VII), the rearrangement fails (eq 26; cf. the behavior of the open model VI11 eq 27).
X
XII
XI
In fact, the following exclusive rearrangements were observed for these competitive migrations (eq 29, 30) (30).
XI11 (C&)a-C-CH,Cl I
Li I
CH, CH2Cl VI1
@ \
XIV (266)
CH3 (ref 31)
The probability of such a spiro structure as an intermediate in these rearrangements was enhanced by trapping a similar structure from the following model reaction (eq 28) (30, 33). 14
Ind. Eng. Chern., Prod. Res. Dev.,Vol. 14, No. 1, 1975
Confronted with these contrasting migratory outcomes, one must seriously doubt whether a carbanionic process obtains for either or both cases (XI11 according with, and XIV seemingly incompatible with, such a view). A possible, unifying alternative is that the free metal-mediated rearrangements actually proceed via radical processes (31) (eq 31), where the superior reactivity toward radicals of a 2-pyridyl position over a benzene position would clarify the outcome of eq 29. Since the 4-pyridyl position is
somewhat less reactive (even without any quaternization on N) toward radicals than are benzene positions, the observed preference in eq 30 may reflect this tendency as compounded by the statistical weight of two phenyls groups
L but these results suffice to show that free metal-induced, 1,2-"carbanionic" shifts are not as well understood as has been supposed. T h e , previous copious work on the Wittig ether rearrangement, the 1,2-alkyl carbanionic shift from 0 to C, has left undefined the pathway for aryl migration (eq 32) Li
Q-p-[Qp]+-p 'CH
0-c H'
XVII
ic metallation (XXII), but no rearrangement, occurs, presumably because the carbazole skeleton constrains the system from assuming a bridging posture (eq 36) (37).
I
I
(26). Accordingly, experiments have been carried out to define the nature of such 1,2-aryl shifts (34).The fact that dibenzo[b, dlpyran undergoes a rapid rearrangement argues strongly against a bridging intermediate, which in this cyclic system should be formed only by much sacrifice of delocalization energy in XVI and much gain in strain in XVII (eq 33).
0
XXI
Even with similar derivatives of XX, the benzylic anion may be intercepted in other processes (in this case, insertion of ethylene stemming from the decomposition of THF), instead of undergoing rearrangement (eq 37) (31, 38). C6H5CH2\ / N-CH, C6H5
n-BuLi
CeH,CHLi\ / N-CH3 CsHb
\
XVI
. ,"C /+
CH2CH2Li
I
Oo
H
CH
C6HbCH
(33)
Moreover, a heterolytic fragmentation of such carbanions as XV and XVI into aldehydic and organolithium parts, followed by their alternative recombination (XVIII), does not seem to obtain, for added RLi is not able to scavenge the putative aldehyde. Therefore, a homolytic rupture, followed by radical recombination, (XIX) seems to be the most acceptable interpretation (eq 34)
/ \N-CH,
(37)
C6H5
Another type of shift that we have recently examined is a 1,3 organic group shift between carbon atoms. The impetus for this study was our incredulity toward a recent report (39) that reinterpreted the Ziegler-Zeiser reaction in the following manner (eq 38).
Li
XIX
R
The carbanionic rearrangement of tertiary amines (eq 35) bears kinship to the Stevens rearrangement, involving 1,2-alkyl shifts from cationic nitrogen to carbon (27). Again, although a few examples of alkyl shifts in such amines have been reported (35, 36), nothing has been reported on corresponding aryl shifts. Thus, it was of interest to find that the generation of a benzylic anion from benzyl(dipheny1)amine (XX) led to a smooth rearrangement (eq35) (37). In this case, there seems to be good reason to accept a bridging intermediate (XXI) as essential for rearrangement. In the closely analogous N-benzylcarbazole, benzyl-
I
Li XXIII
Li
xxrv
These workers claimed that the first-formed product was actually the 1,4 adduct XXIII, which could purportedly be trapped as its ethoxycarbonyl derivative; at higher temInd. Eng. Chem., Prod. Res. Dev., Vol. 14,
No. 1, 1975
15
peratures, however, XXIII was felt to undergo a 1,3-aryl shift to yield the usual product, Repetition of this Japanese work in our laboratory (40) and in another laboratory (41) showed, in actuality, that the purported XXIII was rea!ly XXIV; hence, no 1,3 shift had occurred. In reflecting on the possibility of finding a. genuine rearrangement in the quinoline system, we reasoned that a 1,3 shift of a 1,2-dihydro- into a 1,4-dihydroquinoline would be more promising, since the latter system is thermodynamically the more stable. Indeed, such a 1,3 shift was observed when 2-methylquinoline was allowed to react with allylmagnesium chloride in T H F (eq 39) ( 4 2 ) .
and a reagent thought to be sodium diphenylborate(1) or its dimer (eq 43) (46). NaB(C,Hj),
?Ja@&,,I-I~,12
+ C,H,-C,H,
(43)
The precise relationships between the rearrangements (eq 40-42) and the fragmentations (e.g., eq 43) are the subject of continuing studies. Yet, at least the fragmentation shown in eq 43 can also be considered as a 1,2 rearrangement, coupled with an ensuing fragmentation (eq 44).
MgCl
xxv At room temperature, XXV was formed in >90% yield, whereas in refluxing T H F XXVI became the principal product. Heating of separately generated pure XXV (from 2-allyl-1,2-dihydroquinolineand phenylmagnesium chloride in THF) showed that it did dissociate into 2-methylquinoline and allyl Grignard reagent; thus, the 1,3 shift in XXV to yield XXVI ensues principally by an intermolecular process. Further examples of 1,2 and 1,3 shifts we have investigated are those where the metal centers are regarded as parts of the rearranging skeleton, such as the photoisomerizations shown in eq 40 and 41.
XXVI
As in previous 1,2-aryl shifts from C to C or from N to C, the intramolecular bridging of the migrating group is revealed in the inertness of a cyclic analog: irradiation of sodium bis(o,o'-biphenyleneborate) for prolonged periods leads to no reaction ( 4 5 ) . Apparently the corresponding bridging structure XXVII would be too high in energy for an accessible photoreaction.
XXVII
XXVIII
(ref 4 3 , 44) H
I
J XXIX (ref 4 5 )
Both processes, which can be viewed formally as involving 1,2- or 1,3-phenyl shifts
are accompanied by significant secondary processes that produce subvalent metallic fragments. From triphenylaluminum, there is evidence that phenylaluminum(1) is also produced as a transitory reagent ( 4 3 ) . Analogously, sodium tetraphenylborate(II1) yields directly much biphenyl 16
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 1, 1975
The biaryl coupling-deboration shown in eq 44 can also be brought about by chemical oxidants, such as Ce(IV) or molecular iodine ( 4 7 ) . By studying the selectivity in biaryl formation for the oxidations of the potassium salts of the phenyl(tri-m-toly1)borate and the phenyl(tri-p-toly1)borate anions (eq 45), it has been possible to discern the operation of two different reaction mechanisms: (a) electron -transfer (with Ce(IV), Fe(II1) and DDQ), proceeding by way of unbridged radicals (XXVIII); and (b) electrophilic, involving attack of 12 or Brz and leading to a bridging transition state (XXIX). Again, the behavior of a cyclic K@%C&I5j3(C,H,CH,)
3
+
C6H5-C6HaCH3 CH3C6H4-C6H4CH3 (45)
analog was informative on the importance of bridging: since potassium bis(o,o'-bipheny1ene)borate underwent smooth biaryl coupling with Ce(1V) (leading, after protoboration, to o, 0'-quaterphenyl), but iodine did not yield any quaterphenyl, it is concluded that bridging is important in iodine oxidations (cf. XXVII and ,XXIX), but not in those with Ce(1V). Skeletal Addition Processes. A fair number of ringchain isomerizations of main-group organometallics has been studied, especially with Grignard and organolithium reagents ( e . g . , eq 1). In our laboratory, we have been interested in the process as a novel and possibly stereospecific method of cyclization. In a method related to the Ziegler cyclization shown in eq 3 but involving a skin rearrangement, 1-allylcyclohexene has been converted stereospecifically into cis-hydrindane by hydralumination (es 46) (48).
given in eq 4-6, 11, and 16. In our own work, carbanionic skin rearrangements have been shown to play an important role in the proton-abstracting behavior of radical-anions. The reaction of fluorene with lithium metal in THF to yield 9-fluorenyllithium and hydrofluorenes proceeds via proto- and metallotropic changes in the initially formed radical-anion XXXI (eq 49) (51, 52).
fluorene
XXXI
I
'H
1. Li 12, RH
i-Bu
6x*l-i.Bu -@ (46'
I
H
A
A related cyclization, in which two modes of reaction lead to nonorganometallic products, is that obtained by treating 3-(y-pyridyl)propyl chloride with magnesium (eq 47) ( 4 9 ) .
hydrofluorenes
Aluminotropic processes take place in such compounds as dialkyl(cyclopentadienyl)aluminum, whose nmr spectrum displays only one vinyl resonance down to -90" (53). Even though acenaphthenyl(diisobuty1)aluminum undergoes prototropic and aluminotropic rearrangements upon chemical reaction (eq 50), its nmr spectrum at room temperature in hydrocarbon solution is in good accord with the covalent structure XXXII H I
@Ea%
XXXII
+
(ref 55)
(50)
acenaphthene (ref5 4 , 56)
xxXI
c1 xxx
Time studies support the conclusion that XXX is a genuine addition isomerization of the Grignard reagent, while XXXI is formed only in the presence of the chloride. Finally, a cyclization that involves a subsequent reduction is observed at low temperatures with divinylsilanes and lithium metal. The carbanions so generated must be quenched with tert-butyl alcohol, in order to prevent polymerization (eq 48) (50). Skin Rearrangements Prototropic or metallotropic processes occurring during isomerizing and derivatizing rearrangements are common in organometallic chemistry and typical instances are
A portion of the aromatic proton resonances, displaying unusual shielding (
