Table 1. Effects of substitution on difluorocyclopropane precursors 9 (UB3LYP/6-31G*, gas phase, 298 K, Spartan'10).

VCP	R1	R2	ΔG‡B3LYP	ΔΔG‡B3LYP a
9a	2-Pyrrolyl	H	16.8	–8.5
9b	2-Furyl	H	21.0	–4.3
9c	4-Pyridyl-N-oxide	H	21.1	–4.2
9d	2-Thiophenyl	H	21.3	–3.8
9e	2-N-Boc-pyrroyl	H	21.4	–3.9
9f	5-Benzo[d][1,3]dioxolyl	H	23.0	–2.3
9g	H2C CH2 (vinyl)	H	24.5	–0.8
9h	3-Thiophenyl	H	24.6	–0.7
9i	2-Thiazolyl	H	25.3	0.0
9j	Ph	H	25.3	0.0
9k	Ph	Ph	25.8	+0.5
9l	HC C (alkynyl)	H	25.9	+0.6
9m	4-Pyridyl	H	26.4	+1.1
9n	2-Pyridyl	H	28.2	+2.9
9o	2,6-Dimethylphenyl	H	28.6	+3.3
9p	CN	H	29.3	+4.0
9q	Me	Me	31.8	+6.5
9r	–CH2(CH2)3CH2–		32.7	+7.4
9s	Me	H	33.9	+8.6
9t	C6H11	H	34.0	+8.7
9u	H	H	37.2	+11.9

^a ΔΔG‡B3LYP = (ΔG‡B3LYP9x) – (ΔG‡B3LYP9j).