Abstract:Auf ungewöhnliche Bindungszustände im 13C‐markierten Br3CLi (1) deutet das 13C‐NMR‐Spektrum bei – 100°C (THF‐Lösung) hin. Aus Verschiebungen und Kopplungen wird auf drei Spezies geschlossen: (1a) mit stark polarisierten BrC‐Bindungen, (1b) oder (1c), die aufgrund der schrittweisen Ablösung von LiBr verzerrt sind, und Dibromcarben (1d). (1a) und (1b) sind die ersten nicht ionisierten, auf der NMR‐Zeitskala monomeren Organolithiumverbindungen. magnified image
“…The order of the /0 and oÑci orbitals has no effect on the arguments presented In the mechanistic discussion. (11) The half-electron method is not well suited for the calculation of excitedstate radicals, and this Is not usually possible with our present version of the MNDO program. In this case, however,the radical can be obtained by use of a suitable starting geometry.…”
7147collapses to one identical with that calculated for initial attack perpendicular to the plane.The orbital situation outlined above is by no means uncommon, so that Skell's suggestion' that there may be more cases of excited-state radical generation is almost certainly correct. The search for further examples can, however, be greatly facilitated by preliminary screening of likely precursors by molecular orbital theory. Schleyer for support and encouragement and Dr. J. Chandrasekhar and Dr. P. Hofmann for helpful discussions. This work was facilitated by the cooperation of the staff of the Regionales Rechenzentrum Erlangen.
Supplementary Material Available: Summaries of M N D O resultsfor NCS and the two succinimidoyl radicals ( 3 pages). Ordering information is given on any current masthead page.
References and Notes(1) See Skell, P. S.; Day, J. C. Acc. Chem. Res. 1978, 7 7, 381, and references therein.(2) INDO calculations (Koenig, T.; Wielesek, A. Tetrahedron Lett. 1975Lett. , 2007 suggest that the uo radical may also be low in energy. We have been unable to calculate this species directly," but the reaction paths give only the s and ON radicals. (3) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899, 4907. Dewar, M. J. S.; McKee, M. L.; Rzepa, H. S. /bid. 1978, 700, 3607. (4) All radical calculations employed the half-electron method: Dewar, M. J. S.; Hashmall, J. A,; Vernier, C. G. J. Am. Chem. SOC. 1968, 90, 1953. Dewar, M. J. S.; Trinajstic, N. J. Chem. SOC. A 1971, 1220. (5) The MNDO geometry for NCS is in good agreement with the X-ray structure: Brown, R. N. Acta Crystallogr. 1961, 74, 711. (6) Howard, P. B.; Skinner, H. A. J. Chem. SOC. A. 1966, 1536.
“…The order of the /0 and oÑci orbitals has no effect on the arguments presented In the mechanistic discussion. (11) The half-electron method is not well suited for the calculation of excitedstate radicals, and this Is not usually possible with our present version of the MNDO program. In this case, however,the radical can be obtained by use of a suitable starting geometry.…”
7147collapses to one identical with that calculated for initial attack perpendicular to the plane.The orbital situation outlined above is by no means uncommon, so that Skell's suggestion' that there may be more cases of excited-state radical generation is almost certainly correct. The search for further examples can, however, be greatly facilitated by preliminary screening of likely precursors by molecular orbital theory. Schleyer for support and encouragement and Dr. J. Chandrasekhar and Dr. P. Hofmann for helpful discussions. This work was facilitated by the cooperation of the staff of the Regionales Rechenzentrum Erlangen.
Supplementary Material Available: Summaries of M N D O resultsfor NCS and the two succinimidoyl radicals ( 3 pages). Ordering information is given on any current masthead page.
References and Notes(1) See Skell, P. S.; Day, J. C. Acc. Chem. Res. 1978, 7 7, 381, and references therein.(2) INDO calculations (Koenig, T.; Wielesek, A. Tetrahedron Lett. 1975Lett. , 2007 suggest that the uo radical may also be low in energy. We have been unable to calculate this species directly," but the reaction paths give only the s and ON radicals. (3) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899, 4907. Dewar, M. J. S.; McKee, M. L.; Rzepa, H. S. /bid. 1978, 700, 3607. (4) All radical calculations employed the half-electron method: Dewar, M. J. S.; Hashmall, J. A,; Vernier, C. G. J. Am. Chem. SOC. 1968, 90, 1953. Dewar, M. J. S.; Trinajstic, N. J. Chem. SOC. A 1971, 1220. (5) The MNDO geometry for NCS is in good agreement with the X-ray structure: Brown, R. N. Acta Crystallogr. 1961, 74, 711. (6) Howard, P. B.; Skinner, H. A. J. Chem. SOC. A. 1966, 1536.
“…A low-temperature spectrum reveals that the dimeric solid-state structure is not retained in THF- d 8 solution. At −90 °C, a well-resolved 13 C 1:1:1 three - line coupling pattern due to 13 C− 6 Li scalar coupling is observed for the lithiated carbon of the monomer (the downfield shift of the 13 C NMR signal of the lithiated carbon in 2 relative to the δ ( 13 C(2)) of 3,3-dimethyl-1-(trimethylsilyl)cyclopropene, Δδ = 55.7, is similar to values observed for monomeric vinyllithium derivatives). ,20a,b The very large 1 J 13 C - 6 Li coupling constant of 17.6 Hz in 2 indicates high s-character of the C(α) orbital (C−Li bond): 20a,b,21 the NLMO C(α) hybridization 29 in (1-silylcyclopropenyl)lithium is sp 1.1 compared with the sp 1.7 hybrid used in bonding to hydrogen (in cyclopropene (Figure )). Coupling constants of monomeric organolithium compounds usually range between 10 and 16 Hz; larger magnitudes (16.3−17.2 Hz) have only been reported before for α-halogen-substituted organolithium compounds (carbenoids). 20a,b …”
Section: Resultsmentioning
confidence: 99%
“…Is Cyclopropenyllithium a Carbenoid? Carbenoids, R 1 R 2 C(Li)X (X= halogen, OR), are strongly rehybridized at the carbenoid carbon due to the presence of both an electropositive and an electronegative substituent at the same carbon (Chart ). 7g,, This results in large 1 J 13 C - 6 Li coupling constants (see above) as well as a decidedly elongated C−X bond. ,, Carbenoids easily react with nucleophiles, e.g. RLi, by metal-assisted nucleophilic substitution of the leaving group X (the substitution is already indicated by the elongated C−X bond; also, the lithium bridges the C−O bond in C(Li)O carbenoids and increases its ionicity 23b,c,e ). 22a,c,23d,e 1 Characteristics of 1-Lithiocyclopropene (and Derivatives) and Carbenoids…”
The reaction of 3,3-dimethyl-1-(trimethylsilyl)cyclopropene
with n-BuLi in the presence of 1 equiv of
tetramethylethylenediamine (TMEDA) affords
[3,3-dimethyl-2-(trimethylsilyl)cyclopropenyl]lithium (2).
While NMR
data reveal a monomer in tetrahydrofuran (THF) solution, 2
crystallizes as a dimeric TMEDA solvate,
(2·TMEDA)2.
The structure was determined by single-crystal X-ray diffraction
(crystal data: monoclinic, space group
P21/n, a =
1879.9(2) pm, b = 1035.0(2) pm, c =
2045.5(2) pm, β = 113.198(9)°, V =
3.6580(8) nm3, Z = 4,
[C28H62Li2N4Si2]). Although dimeric unsolvated
cyclopropenyllithium was computed (Becke3LYP/6-31G*) to have two
planar
tetracoordinate carbon
(R1R2CLi2) fragments,
(2·TMEDA)2 adopts a perpendicular
(“tetrahedral”) structure due to
lithium solvation and the steric crowding of the lithium ligands.
Lithiation at C(α) of the cyclopropenyl ring in
2
lengthens the vicinal and shortens the distal C−C bonds due to the
rehybridization at the lithiated carbon. This is
confirmed both by the natural localized molecular orbital carbon
hybridizations and by the large coupling constant,
1
J
13
C
-
6
Li
= 17.6 Hz, observed in THF solution (the usual range for
organolithium monomers is 10 and 16 Hz).
Despite the strong rehybridization and their relationship to the
C(Li)X halogen and C(Li)O carbenoids, the lithiated
cyclopropenes do not have carbenoid nature.
“…Zuschriften relative to the starting material 1-H (d C = 42.3 ppm): in case of 1-Li towards higher field (d C = 37.6 ppm), for 1-Na and 1-K towards lower field (d C = 42.9 and 42.6 ppm). [18,19] The increased 1 J PC coupling constant of approximately 70 Hz indicates ah igher scharacter of the PÀCb ond and an approximately sp 2 hybridization of the carbenoid carbon atom. This is in line with the magnetic equivalence of the phosphorus-bonded phenyl groups,which gave rise to diastereotopic sets of signals for 1-H. Thet hermal stability of the carbenoids in THF solution was studied by variable temperature (VT) NMR spectroscopy using the 31 P{ 1 H} NMR signal as probe.T hereby,c arbenoid decomposition upon warming could be traced by disappearance of the carbenoid signal (d P % 46 ppm) and the appearance of the signal of the decomposition product 2 (d P = 37.8 ppm), which is formed after MCl elimination.…”
As ar esult of the increased polarity of the metalcarbon bond when going down the group of the periodic table, the heavier alkali metal organyl compounds are generally more reactive and less stable than their lithium congeners.W en ow report ar everse trend for alkali metal carbenoids.S imple substitution of lithium by the heavier metals (Na, K) results in as ignificant stabilization of these usually highly reactive compounds.T his allows their isolation and handling at room temperature and the first structure elucidation of sodium and potassium carbenoids.T he control of stability was used to control reactivity and selectivity.H ence,t he Na and K carbenoids act as selective carbene-transfer reagents,w hereas the more labile lithium systems give rise to product mixtures. Additional fine tuning of the M À Ci nteraction by means of crownether addition further allows for control of the stability and reactivity.
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