Indian Journal of Chemistry Vol. 44B, October 2005, pp. 2138-2148 Transition states for hydride and methyl 1,2-migrations in carbene rearrangements to alkenes: An AM1 SCF-MO study Peter G S Dkhar & R H Duncan Lyngdoh* Department of Chemistry, North-Eastern Hill University, Shillong 793 022, India Received 17 October 2003; accepted (revised) 24 May 2004 The AM1 SCF-MO theoretical method to investigate transition states for hydride and methyl 1,2-migrations in the rearrangement of 27 different singlet carbenes to alkenes or their analogues has been used. This study focuses only on qualitative trends regarding structural effects upon reaction facility, which include the effects of (a) bystander group(s) at the migration origin, (b) various substituent groups at the migration terminus, (c) varying ring size in cyclic carbenes, and (d) methyl migration instead of hydrogen. Calculated AM1 activation energies for the 1,2-shift are much larger than those calculated by more sophisticated regimes. However, the qualitative trends follow those obtained from highly accurate theoretical methods, and also follow the expectations of chemical intuition. Hydride migration is predicted to be kinetically favoured by branching at the migration origin, but thermodynamically preferred from electronegative atoms than from carbon atoms. Methyl migration is predicted to be kinetically less favourable than hydride migration. The transition state geometries are described in detail, all centering around a three-membered cyclic moiety in line with a concerted one-step mechanism for the rearrangement reaction. These are described as “early” or “late”, these descriptions being linked to the various structural features present. Keywords: Carbene-alkene rearrangements; hydride and methyl 1,2-migrations; AM1 SCF-MO method; transition state stability; “early” and “late” transition states IPC: Int.Cl.7 C 07 C Carbene rearrangements to alkenes by intramolecular may bear a substituent Y, whose structure also 1,2-migration have been much studied experi- influences reaction facility. Figure 1b considers an α- mentally1-17 and theoretically18-39. The migrating group methyl group undergoing 1,2-migration, when X may be hydride, methyl, alkyl, aryl or other groups. becomes C1MeB1B2. We here investigate hydride and methyl 1,2-shifts for Some experimental findings on carbene reacti- 27 different singlet carbenes using the semi-empirical vity. The reactions of carbenes9 in gas phase4 and in AM1 SCF-MO method40. The aim is to locate and solution5 include insertions into C-H single bonds14, characterise the transition states involved, as well as into C=C double bonds15, bond cleavages16 and to predict trends connecting structural and substituent various rearrangements including 1,2-migrations. factors to reaction facility. The 1,2-shift is assumed to Carbene 1,2-migrations have been reviewed1 with be a concerted, one-step process involving a three- regard to migratory aptitudes and bystander membered cyclic transition state for both hydride and assistance. Hydride migration has been regarded as methyl migrations, which can occur only in singlet more facile than methyl migration17. carbenes1. Some theoretical findings. Much theoretical work Figure 1a depicts the general case of hydride 1,2- has been done on various carbene systems18-38 from migration in a carbene1. The migration origin is the structural, stability and reactivity considerations. The 11,15,17,18,34 α-carbon C1, while the migration terminus is the landmark work of Schaefer’s group eluci- carbene centre C2. Bystander groups on the α-carbon dated the structure and stability of methylene in the are represented as B1 and B2 (both being hydrogen for face of experimental results. The singlet–triplet the unsubstituted methylcarbene), which may assist or splitting has been calculated by many workers27-34 for hinder the reaction, depending upon their structure. various systems. Theoretical work on carbene They may also migrate, competing with hydride. The reactivity includes insertion into saturated hydro- 35 36 whole group C1HB1B2 from which the hydrogen carbons , cyclopropanation (addition to alkenes) , 37 38 migrates is termed here as X. The carbene centre C2 cycloadditions and rearrangements including 1,2- DKHAR et al.: CARBENE REARRANGEMENTS TO ALKENES BY INTRAMOLECULAR 1,2-MIGRATION 2139 Figure 1 ⎯ Reaction course of (a) 1,2-hydride migration, and (b) 1,2-methyl migration during rearrangement of singlet alkylcarbenes to alkenes migrations. We concern ourselves here with intra- groups at the α-carbon, or replacement of the carbon molecular 1,2-migrations, much studied already by atom migration source by electronegative atom theoreticians18-26. sources (N, O, S and P); Scope of this study. We incorporate an over- b) incorporation of various substituents Y on the lapping total of 27 different singlet carbenes, 26 cases migration terminus C2 with varying electron-donating, being studied with regard to hydride migration, and 8 electron-withdrawing and steric effects; cases with regard to methyl migration. Acyclic c) effects of ring size and strain in cyclic carbenes; carbenes are described here as X-C-Y, where X and Y d) comparison of methyl migration with hydride are the two groups attached to the divalent carbene migration. centre. Cyclic carbenes are referred by their molecular Besides the effect of these factors upon kinetic and formulae. These all comprise 10 sets, Sets I to VIII thermodynamic favourability of the reaction, their being studied with regard to hydride migration, and effects on transition state geometries are also studied Sets IX and X with regard to methyl migration. here. This study aims only for qualitative comparisons concerning structural effects upon kinetic and Theoretical Methodology thermodynamic facility for hydride and methyl 1,2- We employed the semi-empirical AM1 SCF-MO migrations. Reproduction of accurate values on par method40 of the MOPAC 6.0 package to compute with higher level calculations or experiment is not energy-minimised structures and wave-functions for possible with semi-empirical methods. Instead, we 27 different singlet carbenes, their 34 products consider a large number of cases together to attempt (alkenes or their analogues) along with the 34 achieving meaningful qualitative trends. Such trends transition states for hydride and methyl migrations. may still be desirable, appealing to chemical intuition Various conformations were considered for the and the basic structural theory of organic chemistry. carbene reactant, the transition state and the alkene The hydride 1,2-migration of methylcarbene product in some representative cases to enable rearranging to ethylene is the prototype case here. selection of only the lowest energy conformers. Various structural features may alter the picture here, Transition states were located using the SADDLE which include: keyword (interpolating between the carbene reactant a) structural changes in the group serving as the and the olefin product), and identified as such by migration origin, like the presence of bystander diagonalisation of the force constant matrix to 2140 INDIAN J. CHEM., SEC B, OCTOBER 2005 confirm only one negative eigenvalue. We tabulated For such 1,2 shifts having small activation barriers, the activation enthalpy Ea and reaction enthalpy ΔHr the Hammond postulate predicts that the transition in kcal mol-1 and the single negative Hessian state would resemble the carbene reactant in energy -1 eigenvalue νi in mdyne cm . Note was taken of the and geometry (an “early” transition state). For large dihedral angle τ in the carbene reactant encompassing activation barriers, the concepts of “early” and “late” the carbene substituent atom Y, the carbene atom C2, transition states can still apply, being deduced from the α-carbon atom C1 and the migrating atom (α- the transition state geometries instead. We propose hydrogen H or α-methyl carbon Cme), as shown in that the ratio of the length of the old bond being Figures 1a and 1b for hydride and methyl migrations broken to that of the new bond being formed in the respectively. transition state gives an indication of the relative Geometrical parameters associated with the position of the transition state along the reaction triangular moiety of the transition state are portrayed coordinate. For hydride migrations, this ratio is given in Figures 2a and 2b for hydride and methyl by R1h/R2h, while for methyl migrations, the ratio is migrations respectively. Figure 2a portrays the R1c/R2c. Smaller values of this ratio indicate “earlier” general case of a substituted methylcarbene X-C-Y transition states, and vice versa. Note that these definitions of “early” and “late” are all only relative, whose α-hydrogen migrates from the C1 atom of and the values of the ratios used to define position on group X (C1HB1B2) to the carbene centre C2 having a substituent Y. Geometrical parameters include the the reaction coordinate are likewise not absolute. Rather than describing any particular transition state bond distances shown (R12, R1h and R2h) and the as being “early” or “late”, we can only say that one dihedral angles ϕ1, φ2 and φ3 encompassing the atoms case is “earlier” or “later” than another. Y-C2-C1-H, atoms Y-C2-C1-B1 and atoms Y-C2-C1-B2 in turn respectively. Figure 2b portrays the Abbreviations. Groups X and Y in an acyclic corresponding methyl migration in α-substituted carbene X-C-Y include the following groups carbenes, with the α-methyl group migrating from the (abbreviations in brackets) : methyl (Me), ethyl (Et), C (CH )B B group. Similar geometry markers apply 1 3 1 2 isopropyl (Pri), aldehyde (CHO), phenyl (Ph), here, given as the bond distances R , R and R , and 12 1c 2c trichloromethyl (CCl ), tert-butyl (But), amino (NH ), the dihedrals φ , φ and φ encompassing atoms Y-C - 3 2 1 2 3 2 phosphino (PH ), sulphhydryl (SH), hydroxy (OH), C -C , Y-C -C -B and Y-C -C -B respectively, C 2 1 me 2 1 1 2 1 2 me methoxy (OMe), fluoro (F), and chloro (Cl).